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Every experimental amateur-built airplane has to do its first few dozen hours of flying inside a defined geographic test area. That’s a Phase 1 requirement, written into the aircraft’s operating limitations and signed off by the FAA’s Designated Airworthiness Representative (DAR) when the airplane is issued its airworthiness certificate. You can fly anywhere you want inside the area; you can’t fly outside it until Phase 1 is complete.

I went into the conversation with my DAR with an ambitious proposal: twelve airports scattered across central Virginia, several of them anchoring the boundary of the test area and all of them on the approved landing list — places I could put the airplane down on a runway if anything went wrong. The DAR took a look and came back with a constraint from the FAA: the approved landing list had to be reduced to no more than six airports. Twelve was too many; six was the cap.

So we kept the boundary pentagon and pared the landing list down. Two of those concepts are now separate:

The boundary — the pentagon that encloses the test area — is defined by five waypoints:

• KFRR — Front Royal-Warren County (NW corner)
• KHEF — Manassas Regional (NE corner, my home base)
• KXSA — Tappahannock-Essex County (SE corner)
• KFVX — Farmville Regional (S corner)
• KSHD — Shenandoah Valley Regional / Staunton (W corner)

The landing list — the six airports where I’m actually approved to put the airplane down — is six airports, three of which happen to also be on the boundary:

• KHEF Manassas (boundary + landing)
• KFVX Farmville (boundary + landing)
• KSHD Shenandoah Valley (boundary + landing)
• KHWY Warrenton-Fauquier (interior landing)
• KCJR Culpeper Regional (interior landing)
• KCHO Charlottesville-Albemarle (interior landing)

All six show up on the map as green dots. The remaining two boundary corners — KFRR (Front Royal) and KXSA (Tappahannock) — are red dots: they help define the shape of the test area, but I’m not approved to land there.

The constraint behind all of this came from a rule of thumb the DAR worked through with me: at the airplane’s cruise speed (~160 kt), I should be able to reach a runway within roughly a 30-minute leg. Thirty minutes at 160 knots is 80 nautical miles, and the six landing airports we kept are positioned so that no point inside the pentagon is more than that 80-nm window from one of them. That’s a reasonable margin while the engine is still breaking in.

The map at the top of this post overlays all ten Phase 1 flights so far on that pentagon. You can see how much of it I’ve actually been using: the first six flights (F1–F6) stayed close to KHEF — local pattern work and the early shakedown profiles. Starting with F7 I pushed out toward the Casanova / Culpeper practice area in the western part of the pentagon, and F8 / F9 / F10 all worked roughly in the same neighborhood. Most of the pentagon’s southern reach is still unflown.

There’s a lot of test card left to fly inside this shape, and plenty of room to do it.

I owe the blog about six flights. Some of that is because I’ve been heads-down on the actual flying, some because every flight in this stretch turned up at least one new thing worth investigating, and some because the bigger investigations — the CAN bus rewire, the AHRS situation, the CHT data — kept demanding their own dedicated posts.

So rather than try to retroactively write six separate Flight N posts, here’s a single catch-up running from Flight 5 through Flight 10, hitting the headline from each. The dedicated deep-dive posts cover the analysis side; this is the chronology.

The Numbers

Eight engine hours of additional flight time across six flights. About 900 nautical miles flown. ~107 gallons burned. From ~6 engine hours coming into this batch to ~12.4 out of it. Phase 1 on this airplane is a flight-test-card program, not a fixed-hours one — we work through the required test cards rather than counting down to a hard hours limit — but the experimental Lycoming IO-540 with the SDS electronic ignition would put a 40-hour floor on Phase 1 even if we were counting hours. So 12.4 engine hours is early in the program, not midway.

Flight 5 — April 19. The Last Flight of “Phase 1A.”

1.4 engine hours · 21.3 gal · 155 nm.

Flight 5 was a per-cylinder CHT survey at altitude — methodical, mostly uneventful, and the last flight in what I now think of as Phase 1A: the batch of early flights that locked in the questions we’d spend the next month answering. I used roughly 13 gallons out of the left tank and landed needing a top-up — a fine reminder that I needed to start trusting the totalizer more and the gauges less.

The other thing F5 made clear was that the ADAHRS deviation problem was real and reproducible, not a Flight 1 fluke. PFD #1 tumbled on the takeoff roll, percentage-deviation values were spiking high, and the pattern matched the four flights before it. Time to stop flying and start fixing.

The Maintenance Gap — April 19 to May 14

Twenty-five days on the ground for what turned out to be three separate jobs:

• CAN bus rewire — completed May 13. The Garmin G3X harness was both over-length (~96.5 ft total vs Garmin’s 66 ft max) and built with the wrong wire (standard aircraft shielded twisted pair instead of 120 Ω spec cable). New harness: 57.7 ft of Carlisle IT CAN24TST120(CIT). Full writeup is here.
• Dynamic prop balance — went from 0.57 IPS down to 0.01 IPS. That’s a ~57× reduction. The engine cowl is now noticeably calmer in the air.
• Left fuel-gauge float reoriented — the left tank gauge had been reading frozen high since first fueling. Root cause turned out to be the float-arm wire positioning the float too close to the tank’s interior ribs / baffle, where it would get physically stuck. The 2025 Van’s plans update shows a different way to bend the float-arm wire that orients the float more laterally and gives it more clearance to move up and down. Implementing that new bend pattern appears to have fixed it — the left gauge now tracks correctly through the readable band. (Full write-up to follow in a future post.) The right-side gauge is still stuck up (same root cause suspected, same fix likely needed), still on the to-do list.

Three problems addressed, one bench-flight worth of confidence restored, and we were back at the runway.

Flight 6 — May 14. First Flight Back, and a Surprise.

1.3 engine hours · 19.1 gal · 149 nm.

F6 was deliberately a low, slow shakedown — peak ~2,070 ft, just enough to verify everything still worked after a month of wrenching. The CAN bus data was immediately, dramatically clean (zero protocol errors across the whole flight; pre-rewire flights had several hundred Bus-Off events apiece). That part was a win.

The surprise was carbon monoxide. The G3X CO Guardian had read essentially zero on every one of Flights 1 through 5. On Flight 6, with nothing changed about the engine, exhaust, or cabin sealing other than the avionics rewire, cabin CO peaked at about 7 ppm. Not alarming in absolute terms — the working alarm threshold is around 35 ppm — but a new signature that hadn’t been there before. The leading suspect, which I’ll come back to in a future post, is the firewall heat doors not seating fully closed.

For context on what those PPM numbers mean as the values climb on later flights, here’s the standard CO exposure scale:

And here’s what the G3X CO Guardian actually measured in the cabin across all ten flights:

The story the chart tells: nothing on F1 through F5 (a few 1–3 ppm blips, mean below 0.5), then a clear new pattern starting at F6, peaking at 10 ppm on F7 during the slow-flight stalls and at 9 ppm on F8 with the wing roots taped. F9 looks essentially clean — but it was a benign autopilot-tuning profile. F10 is back to small numbers under a short, more aggressive profile.

Flight 7 — May 15. Out of the Pattern. First Stalls. First Airspeed Cal.

1.7 engine hours · 19.4 gal · 169 nm.

Flight 7 was the first time I really left the immediate KHEF area. Climbed to about 8,800 ft west of the field and ran a clean-configuration stall series at light forward CG: about 1 kt per second deceleration, minimum airspeed ~58 KIAS before heavy buffet, no clean nose break yet. Wing roots were still bare (the wing-root grommet seal had been a contender for the CO source at this point) and cabin CO peaked at 10 ppm during the slow-flight portion of the flight — confirming that whatever was leaking was somehow correlated with high angle of attack or low airspeed, not just temperature.

Flight 7 also gave me a clean airspeed-calibration dataset I hadn’t planned on. Over about 22 minutes at ~7,000 ft and ~148 KIAS, I’d flown a mix of straightish legs and curving legs while working other test cards — not a single deliberate 360° turn, just a happy combination of headings that, taken together, ended up giving the GPS ground-speed vector enough azimuth coverage to support the GPS circle method. With 1,336 samples spread across the full compass and a circle-fit RMS of 2.4 kt, the result said the pitot-static system is essentially clean in cruise — position error +0.9 kt at ~148 KIAS. One cal point in the books; more to come at other airspeeds.

Flight 8 — May 16. Aggressive Stalls, and the Autopilot Wants to Dive.

1.1 engine hours · 12.0 gal · 97 nm.

I went back up to ~8,500 ft on F8 with more aggressive intentions: deeper stalls, more flap deployment, slower minimum airspeeds. Minimum logged IAS was 45.9 kt at about 7,900 ft, which is well into the “we’re not in the certified envelope anymore” territory and exactly what Phase 1 is for. Wing roots were taped on this flight to test the wing-root-as-CO-source theory.

Cabin CO peaked at 9 ppm anyway — actually with the highest mean CO reading of any flight to date. Wing roots, ruled out. The leading edge of the diagnosis shifted to the firewall heat doors. (More to come in a dedicated CO post; the investigation is still active.)

And then there was the autopilot. First time I engaged the AP in this airplane, it commanded a nose dive. Pitch channel sense was reversed — the AP was simply trying to hold pitch level, but with the servo direction wired backwards every correction it made was the wrong one, so it just kept pushing the nose over into a dive. We disengaged immediately, hand-flew the rest of the flight, and changed the G3X pitch servo direction from “normal” to “reversed” on the ground after landing. Configuration error, not a hardware failure, but a useful reminder of why we test these things at altitude before relying on them.

Flight 9 — May 17. Autopilot Tuning. And a Visit From an Old Friend.

1.7 engine hours · 23.3 gal · 230 nm.

F9 was an autopilot tuning flight: 1.7 hours, max GPS altitude ~9,100 ft, mostly hand-flown to get up to altitude and then about 60% of the cruise samples on the autopilot. Confirmed the pitch-reversal fix from F8 — the AP now correctly climbs when it wants to climb. Worked the roll gain from 0.5 up to 0.7 and back to a settled 0.6, with the roll servo max torque bumped from 15% to about 40%. Roll axis is now tracking well; pitch tuning is up next.

The fun part of F9 was the air-to-air. Harry T and Bob H (Bob’s Cessna N8EM, “8 Echo Mike”) took off about 15 minutes behind me and joined up in the practice area west of Casanova / Culpeper. I worked at about 8,500 ft, they worked at about 3,000 ft, and we relayed live updates on the AP tuning progress on the air-to-air “fingers” frequency, 123.45 MHz. Visual contact. First time flying N997CZ alongside Bob’s airplane. The kind of flight where the data takes a backseat to the smile.

Two data-related notes from F9: cabin CO was essentially zero for the whole flight (the profile was too benign for the AoA-correlated leak signature to express itself, but worth logging). And the 11 autopilot-stabilized wings-level legs at ~8,500 ft / ~145 KIAS that came out of the tuning work happened to produce a perfect second airspeed-calibration data point: position error +0.5 kt, matching the F7 GPS-circle result almost exactly. Two cal points, two methods, two days apart — the pitot-static system is clean in cruise.

Full airspeed-calibration writeup — methodology (constant-IAS GPS circle, multi-leg wind triangle), per-leg data tables, and the F7/F9 cross-validation — is available as a PDF download below.

F9 was also the first confirmed mid-flight tank switch. Post-flight refueling receipt was 22.1 gal vs totalizer 23.3 gal (1.2 gal / 5% — within fuel-flow calibration tolerance), with the right tank taking about 8–9 gallons. Working tank split for this flight: ~13.5 L / ~8.5 R.

Flight 10 — May 20. The First Configuration Change.

1.0 engine hours · 11.7 gal · 104 nm.

F10 was the first deliberate configuration change to the airframe since the maintenance gap. The Cyl 1 front air dam was removed before the flight to see whether more cooling air would lower Cyl 1’s persistently-hottest CHTs (and at what cost to the cylinders downstream of it). About an hour of engine time, max GPS altitude ~11,080 ft (a new high for this airplane), warm day with cruise OAT around 71 °F.

I’ll be honest: I didn’t check the weather before this one. It was a beautiful blue-sky day with a few white puffballs of fair-weather cumulus, and I was preoccupied with the air-dam mod. Departing Manassas to the west I could see some thunderstorm activity well to the north — at the time it looked like it was on the far side of Dulles, possibly out near Leesburg, and based on that I felt comfortable climbing west and away from it to do the test profile.

Up at 10,000–11,000 ft and turned around looking back east, the situation had changed. The storm had organized and was starting to encroach on Dulles and the corridor between Dulles and Manassas. Decision time was easy: hightail it home. I came back to Manassas and they had me land from the north to the south with the thunderstorm just behind my tail. Wheels down, taxi clear, into the chocks, shut down, debrief in the cockpit — all of that with the sky behind me turning increasingly serious.

Maybe one or two minutes after engine shutdown I opened the cabin door. The gust front hit exactly then. It took everything Harry and I had between us to keep the cabin door from being ripped off the hinges by the wind, and then it took everything we had to push the airplane back into the hangar against the gusts. The RV-7’s tow bar lives in the other hangar; we made it work, but N997CZ needs its own tow bar. Added to the parts list.

Headline result of the actual test, OAT-adjusted, F9 → F10: Cyl 1 max CHT dropped 33 °F. About 23 °F of that drop is attributable to the air-dam change beyond the cooling the other cylinders got on the same flight. Strong, clean signal. The downside, also predicted: Cyl 5 max went UP +9 °F — that’s the rear cylinder on the right bank, downstream of the redirected airflow. It’s a “canary” reading on whether the air dam was protecting Cyl 5’s climb cooling. F11 will tell us whether F10 was noise on a small sample, or whether we need a partial air dam to keep Cyl 5 happy.

Full CHT trend analysis across all 10 flights is its own dedicated post (the short version: the cylinder ranking has held invariant across all 10 flights, real break-in is visible in the climb maxes but essentially absent in cruise, and I was leaning too early per Lycoming’s SI 1427C).

Where We Are Now

12.4 engine hours in, with plenty of Phase 1 test cards still to fly. The headline open items going into F11 and beyond:

• AHRS still tumbling on the takeoff roll every flight. CAN bus is clean, vibration is essentially eliminated (0.01 IPS), so the GSU 25C hardware itself is the leading suspect. I’ve just sent Garmin a fresh writeup closing the loop on the CAN bus rewire result and the prop balance result, alongside a 10-flight analysis showing the AHRS tumble pattern is identical before and after both fixes — and asked them once again about Service Bulletin SB 2144 and the potential need to swap or recall the two GSU 25C units on N997CZ. Waiting to hear back.
• Cabin CO investigation continues. Firewall heat doors are the leading hypothesis; tape-test still to fly.
• Cyl 5 climb cooling — F11 will tell us whether the air-dam removal was a free win or whether we owe Cyl 5 a partial fix.
• Airspeed calibration at slow-flight speeds (~80, 100, 120 KIAS) — only the cruise point has been mapped so far.
• Right fuel-tank float still stuck up. To-do.
• Pitch autopilot tuning — roll is settled; pitch gains are next.
• Flight 5 dedicated post — never going to happen at this point; consider this paragraph the substitute.

Next post will probably be the CO investigation, depending on which one closes out first. As always, thanks for following along.

— Jim

If you’ve been following the N997CZ journal, you’ll know that cylinder-head temperatures have been on my mind since the very first takeoff. Flight 1’s Cyl 1 peaked over 450°F before I’d even cleared the pattern, and from that moment on, every flight has carried with it a small cardiac event for me when I look down at the engine page on the G3X.

After ten flights and about 13 engine hours, I’ve finally pulled all of that CHT data into one place, run some regressions across it, and learned that a lot of what I thought I was seeing wasn’t quite right. This is the story of what the data actually shows — and where my initial interpretation broke down once I added one more variable.

Why CHTs Matter on a New Engine

For anyone new to all of this: cylinder-head temperature is the temperature of the metal at the top of each combustion cylinder, measured by a thermocouple right at the spark plug boss. Lycoming’s redline on the IO-540 is 500°F. The general guidance for break-in is to keep CHTs comfortably under 420°F during cruise and well clear of redline during climb, but more importantly to let the engine work: high power, full-rich mixture, no babying. The piston rings are seating against the cylinder walls during these first 25 to 50 hours, and that process needs combustion pressure and heat to happen properly.

The trade-off is that you’re running an engine hot, and you’re learning its baffling system, its idiosyncrasies, with no historical data to compare against. So you watch. And you write down what you see. And, in my case, you eventually pull it all into Python and start asking questions.

The Method: One Regression Per Cylinder, OAT Normalized

CHTs depend strongly on outside air temperature. A 70°F day produces materially hotter cylinders than a 40°F day, all else equal, just because the cooling air arriving at the baffles is hotter. So before I could see a meaningful “break-in” trend across ten flights spread over five weeks of spring weather, I needed to normalize for OAT.

I pulled the cruise window from each flight’s G3X log (filtered to RPM ≥ 2400 and altitude relatively steady), grabbed the OAT (°F) and the per-cylinder CHTs (°F), and fit a simple two-term regression for each cylinder:

CHT_cruise = a + b · OAT + c · flight_number

b tells me the OAT sensitivity (°F of CHT per °F of OAT — engineering rule of thumb is around 1.0). c is the break-in slope: how fast that cylinder is cooling off as the engine accumulates hours, after controlling for OAT.

I also kept the raw max CHTs flight-by-flight (peak during the climb), since climb is where the cooling system runs nearest its capacity and where any improvement in heat transfer would show up most clearly.

What I Thought I Was Seeing

The first fit was across all ten flights, F1 through F10. The numbers looked beautiful:

• Cyl 1: c = −2.75 °F/flight. Hottest cylinder. Cooling fastest.
• Cyl 2: c = −1.52 °F/flight.
• Cyl 4: c = −1.31 °F/flight.
• Cyl 6: c = −1.33 °F/flight.

That fits the classic break-in story so well. The hottest cylinder breaking in fastest. Rings seating. The cruise temps trending down at a measurable rate. I wrote it up, called the family in to admire the regression coefficients, and started drafting a post about how my engine was breaking in nicely.

Then I added one more variable.

The Thing That Broke the Story

The variable was fuel flow. I’d been logging it the whole time, but I hadn’t pulled it into the CHT analysis. The thought was: what is each flight’s mean cruise fuel flow, and does it correlate with CHT in any obvious way?

Here’s what it looked like:

Look at the step between F6 and F7. Mean cruise fuel flow dropped from 19.83 to 15.68 gph — a sharp single-flight transition — and fuel economy stepped from 8.11 to 9.61 nm/gal (+18%). Every F7+ flight runs leaner than every F1–F6 flight, without exception.

That step is exactly what it looks like: it’s where I started leaning the mixture. F1 through F6 were flown with the mixture at full rich, per the typical break-in playbook. By F7 — about 10 engine hours in — I’d decided I had enough data to start running a little leaner in cruise, and the fuel-flow numbers tell the story.

Now look at what that did to CHTs. The F6 → F7 comparison, OAT-adjusted: cruise CHT went UP +10°F from F6 to F7 (352°F → 362°F across the cylinder array). That’s the textbook leaning signature — pulling the mixture leaner moves combustion peak temperatures closer to the cylinder head, and head temps respond.

In other words, my break-in regression across F1 through F10 was fitting a smooth line through what is actually two different operating regimes. The “cruise CHT is dropping” trend wasn’t break-in; it was 60–70% the F7 leaning step pulling the F7+ flights down toward a different equilibrium. The regression couldn’t tell the difference.

It gets worse. Flight 10 had another step change: I pulled the air dam in front of Cyl 1. That’s a physical change to the cooling baffling. Another shift in the operating regime, layered on top of the leaning regime.

Re-fitting on the Clean Range

Once you know about a regime change, you fit around it, not across it. So I re-ran the regression on F1 through F6 only — the clean, full-rich, no-air-dam-mod range — and the picture changed entirely:

• Cyl 1 c = +0.29 °F/flight (was −2.75 in the all-flights fit)
• Cyl 2 c = +0.37 (was −1.52)
• Cyl 3 c = +0.35 (was +0.10)
• Cyl 4 c = +0.02 (was −1.31)
• Cyl 5 c = −0.02 (was −0.86)
• Cyl 6 c = −0.67 (was −1.33)
• Mean c = +0.06 °F/flight (was −1.28)

Cruise CHT break-in, in the full-rich regime, is essentially zero. The cylinders are not measurably cooling at cruise as the engine accumulates hours. The earlier “Cyl 1 broke in fastest at −2.75°F/flight” was about 60–70% the F7 leaning step plus the F10 air-dam mod. The actual break-in signal in cruise data, in the regime where break-in is supposed to be happening, is statistical noise.

And the OAT sensitivity that I’d been wondering about (the all-flights fit said b ≈ +0.25, way below the engineering rule of thumb of 1.0) snapped back into shape when I limited it to the clean regime: b ≈ +0.89 °F CHT per °F OAT in F1–F6 only. The rule-of-thumb is roughly right for this engine in full-rich cruise; the previous low number was suppressed by mixing in the leaning regime.

Where Break-In Actually Shows Up

So is my engine breaking in at all? Yes — but the signal lives in maximum CHTs (climb conditions), not in cruise.

That’s a −21°F drop over six flights for the hottest cylinder, in conditions where the cooling system is running near its capacity. That’s the break-in signature. Climb is where the engine is making the most heat per unit of cooling air; that’s where any improvement in heat transfer (or any small reduction in friction work) is going to be most visible.

The lesson: cruise has so much cooling margin that ring-seating doesn’t move cruise temps in any measurable way. If you want to see break-in in your CHT data, look at the climb maxes, not at cruise medians. I’d been looking at the wrong column.

Lycoming SI 1427C and the Leaning Mistake

While I’m being honest about what I got wrong: Lycoming Service Instruction 1427C explicitly says do not lean during break-in. I leaned at Flight 7, which was around 10 engine hours of total operation — earlier than the manufacturer recommends. The good news is that F8 and F10 don’t show any distress signature (no CHT excursions, no rough running, no oil consumption surprises I can see), so it’s probably fine.

But the cleaner path for the remaining ~12 to 15 hours of break-in is to go back to full rich at high power, and let the cylinders finish seating against the cylinder walls without me changing the operating point underneath them.

There’s a second piece of Lycoming guidance worth flagging: their actual gold-standard test for “is break-in complete?” isn’t CHT at all. It’s oil consumption. When per-hour oil add stabilizes at a low and consistent rate, the rings have seated. I haven’t been tracking oil consumption nearly carefully enough through these first 10 flights, and that’s a habit I’m going to fix going forward.

The F10 Experiment

I mentioned the F10 air-dam removal in passing — that deserves its own paragraph because the result is interesting.

Cyl 1 has consistently been the hottest cylinder on this engine, and the cylinder layout (Lycoming IO-540, right bank front to rear: 1 → 3 → 5; left bank front to rear: 2 → 4 → 6) puts it at the front of the right bank, where you’d expect it to be getting the freshest, coolest induction air. So why is it always the hottest?

One hypothesis was that the small air-dam baffle in front of Cyl 1 was redirecting too much cooling air past it (toward Cyl 3 and Cyl 5 deeper in the bank) and not onto it. So I pulled the air dam before Flight 10 to see what happened.

The result, OAT-adjusted, F9 → F10 deltas:

• Cyl 1 cruise: −32°F. Cyl 1 max: −33°F. That’s about 23°F of extra cooling on Cyl 1 beyond what the other (control) cylinders saw. Strong, clean air-dam-removal signal.
• Cyl 3 (right middle, downstream of the redirected air): no penalty. Cruise tracked the control group; max actually dropped slightly.
• Cyl 5 (right rear, downstream): the canary. Cruise tracked the control group. But max Cyl 5 went UP +9°F while every other cylinder went down 0–10°F. That’s a 12–18°F relative warm-up in the high-stress climb regime, on the cylinder that’s furthest downstream on the right bank.

So the air dam was doing real work — keeping enough air flow over to the back of the right bank to keep Cyl 5 happy during climb. F11 will be the test. If Cyl 5 max stays elevated relative to the trend, that’s a real climb-cooling cost and I’ll need to think about a partial air dam or a different baffle modification. If Cyl 5 pulls back into line, F10 was noise on a small sample size.

What I’d Do Differently

A few takeaways from this round of analysis, mostly aimed at past-me:

1. Don’t lean during break-in. Lycoming says so, and it confounds your data in addition to whatever it does to your engine. I should have stayed full-rich for the first 25 hours.
2. Always split your regression at known regime changes. Adding fuel flow as a variable, or just visually inspecting it for step changes, would have caught this before I started believing the −2.75 °F/flight number. A regression doesn’t know about operating regimes; you have to tell it.
3. For break-in, watch max CHT, not cruise CHT. That’s where the cooling system is closest to its limits, and that’s where any change in heat transfer will show up. Cruise has too much margin to be a useful break-in metric on this engine.
4. Track oil consumption rigorously. It’s the actual answer to “is break-in done?” — and I’ve been treating it as a footnote.
5. Single-flight changes are not break-in. F10 looked great on Cyl 1, but it was one data point against a known air-dam-removal mod. F11/F12 will tell us whether that’s the new baseline or a fluke, and whether the Cyl 5 climb-cooling cost is real.

A Footnote on How This Analysis Got Done

I want to be honest about the workflow here, because it’s something I’m thinking about a lot lately. I built the original “Cyl 1 is breaking in fast” regression in an afternoon, and I was ready to ship it. The reason it didn’t ship that way is that I was talking through the result with Claude — Anthropic’s AI assistant — and it kept asking annoying questions like “have you looked at whether fuel flow changes across these flights?” and “are you sure the operating point is the same?” The fuel-flow analysis that broke the story open was prompted by exactly that kind of push-back.

I’ll write a longer post on this workflow at some point, but the short version: AI-assisted data analysis is genuinely useful for catching the things a tired homebuilder is about to ship. It’s also a useful collaborator for sanity-checking the math, generating the chart, and writing the script that drops a fuel-flow column into an existing dataframe. None of which is a substitute for thinking about the data carefully — but it’s a force multiplier on the thinking.

What’s Next

F11 and F12 are going to be the air-dam follow-up flights. The plan: go back to full-rich mixture, fly a representative climb-and-cruise profile, see what Cyl 1 cruise (does it stay near 354°F, or does it drop further — that distinguishes mod-only effect from continuing break-in) and Cyl 5 max (does it stay elevated, or pull back in line) do. And start logging oil add events more carefully so I have the actual break-in metric Lycoming cares about.

If you’re working through Phase 1 on a new engine yourself and you’ve been staring at CHT numbers wondering what they’re telling you, I hope this is useful. The headline I’d leave you with: watch the maxes, not the means; don’t change two things at once if you can avoid it; and if your data tells you a beautiful story, ask it what’s missing before you publish.

Posted from N997CZ flight test program — see the CAN Bus Rewire post for the most recent system-level fix, and the First Flight post for the origin of the CHT story.

If you’ve been following the N997CZ build journal, you know the first five flights have been a mix of exhilarating milestones and humbling detective work. Flight 1 was a dream. Flight 2 sent me home early with erratic oil temperature gauges. Flight 3 gave me two solid hours and flaps for the first time. Flight 4 found an alternator belt slipping. But threaded through all of them was something I kept noticing and not fully understanding: in every single flight, the PFD #1 artificial horizon has tumbled upside down at least once.

That’s not a nuisance — that’s a serious ADAHRS event. Once I started pulling the deviation data from all five flights and laying them side by side, the pattern became impossible to ignore. I posted the data to the Van’s Air Force build thread and the community response was immediate, deep, and directly useful. This post is about what the data shows, what the community has experienced, what Garmin tech support confirmed, and my current working theory: this is primarily a vibration problem — with CAN bus wiring issues thrown in as a secondary layer.

What Does “ADAHRS % Deviation” Actually Mean?

N997CZ actually carries three ADAHRS units, all integrated into the G3X system. The G3X Pilot’s Guide confirms that the G3X supports up to three ADAHRS sources, with the G5 functioning as a full participant in the cross-comparison and miscompare monitoring via CAN bus — not a standalone backup, but a third integrated ADAHRS. The G3X uses two GSU 25C units — ADAHRS #1 and ADAHRS #2 — plus the Garmin G5, all cross-checked against each other. Both GSU 25Cs are mounted on the sub-panel directly forward of PFD #1. The G5 sits in a separate location, which makes its deviation data a valuable third data point — if its profile looks different from the GSU 25s, it helps localize where the disturbance is worst. That data is being pulled and will be added in a follow-up.

Before diving into the charts, an honest caveat: Garmin has not publicly disclosed what % deviation actually measures internally. We asked. They didn’t specify. So everything below is interpreted with that uncertainty in mind. It could be a cross-check residual between the two GSU 25 units, or something internal to each unit — perhaps related to Kalman filter covariance, or how much raw IMU data agrees with the filter’s predicted state. Without Garmin’s definition, we can’t be certain.

Working assumption: % deviation is a proxy for disturbance severity — higher is worse, sustained high values indicate a real problem. Garmin tech support declined to specify what maximum acceptable values should be. The 999% hard pegs have not been confirmed as caused by CAN bus dropouts specifically. Garmin’s guidance: fix the CAN bus first, then re-evaluate what remains.

Hypothesis on the 999% spikes: These hard pegs likely occur during takeoff as RPM sweeps upward from idle. The rising forcing frequency sweeps through the natural resonant frequency of the ADAHRS mount — where vibration transmission into the sensor peaks. Once RPM stabilizes at cruise, the deviation drops to an elevated-but-lower baseline. This is consistent with classic mass-spring-damper resonance behavior and with community reports of problems being RPM- and power-setting-dependent.

Several things can drive this, in rough order of likelihood for a new RV-10: an unbalanced or un-clocked prop; GSU 25 mount resonance amplifying vibration at one sensor location; loose lower cowl hinge pins transmitting vibration into the firewall; and CAN bus dropouts causing the cross-check math to fail and peg at 999%.

Five Flights of Data

Blue = ADAHRS #1 | Teal/green = ADAHRS #2. A flat, quiet trace near zero is healthy. What you’ll see instead is large % deviation in both sensors from takeoff to touchdown across all five flights — sustained throughout every flight, not isolated spikes.

Flight 1 — April 11, 2026 (First Flight, 127 nm)

The very first flight and already there’s a clear problem. Both sensors show elevated deviation from shortly after takeoff through landing. ADAHRS #2 is consistently worse than #1, with a hard 999% peak around 13:00:38. The sustained asymmetry between the two units throughout the flight points to a physical difference in what each sensor location is experiencing. The horizon tumbled on this flight — PFD #1 only.

Flight 2 — April 17, 2026 (Abbreviated, 68 nm)

Identical story to Flight 1. Large deviation in both sensors from takeoff to landing, #2 running consistently worse than #1. The repeatability across two flights makes it clear: this is not random noise. The horizon tumbled again.

Flight 3 — April 18, 2026 (Two Hours West, Flaps First Time)

The two-hour flight shows the same sustained high deviation. What’s notable is a concurrent peak where both sensors reach similar values (229% and 233%) at the same moment — suggesting the disturbance is either broad enough to affect both locations equally, or the CAN bus dropouts confirmed by Garmin are contributing.

Flight 4 — April 18, 2026 (Alternator Belt Discovery)

Large deviation in both sensors across the whole flight, #2 running consistently worse again. Flight 4 was when we discovered the alternator belt was slipping, though the ADAHRS deviation pattern is consistent with every other flight.

Flight 5 — April 19, 2026

Both sensors show large sustained deviation from takeoff to landing, same as every prior flight. The consistent, high deviation across all five flights is itself the story: this is not a problem that comes and goes — it is present in every flight, start to finish.

Coming next — G5 data: The Garmin G5 is mounted in the main instrument panel, further aft than the two GSU 25Cs on the sub-panel. The sub-panel is tied directly to the firewall via a fore/aft rib, making it a near-direct receiver of engine vibration. The main panel is at the end of a longer structural path with more joints and compliance. If the G5’s deviation runs lower than the GSU 25s — which early recollection suggests — that would directly implicate the sub-panel’s structural coupling to the firewall. That data is being extracted and will be added in a follow-up.

What the Van’s Air Force Community Has Experienced

I posted the data to the N997CZ build thread on VAF and the response was both immediate and sobering: this is a well-known problem, and multiple experienced builders have been down this exact road.

The GSU 25 Mount Location Is the Central Issue

Both GSU 25s are mounted on the sub-panel in the upper-left corner — close to the display, accessible, tidy. But multiple builders reported this location is problematic on the RV-10, and several noted a frustrating catch: Garmin’s ground-based vibration test passes in essentially every position. The only meaningful test is in flight at full power.

The community consensus is that stiffening the sub-panel — adding 0.063 angle stock, doublers, or bracing — doesn’t reliably solve the problem. Making the bracket stiffer just makes it a better conductor of vibration rather than isolating it. Multiple builders who resolved the problem did so by relocating the GSU units entirely: some to the back of the GDU display (despite Garmin’s manual discouraging this), others to a remote aft mount.

The community’s blunt summary: ground vibration tests are a waste of time for diagnosing this. You can’t simulate full power and flight conditions on the ground, and the GSUs will pass the test in any position — then fail in flight. The only data that matters comes from the air.

The Cowling Hinge Pin Fix (An Unexpected One)

One builder shared a fix I wouldn’t have thought to look for: the standard Van’s lower cowl piano hinge vertical pins. Their ADAHRS problem persisted through a sub-panel stiffener and a GSU hardware swap under the Garmin service bulletin. The breakthrough came when someone noticed the lower cowl vertical pins allowed a tiny amount of flex — detectable by hand-pressing the sides of the cowl against the firewall. Replacing those pins with slightly oversized ones resolved the AHRS drift over 30+ hours of subsequent flying.

The diagnostic tell was clean: the ADAHRS ground test passed with the cowl off and failed with the cowl on. N997CZ already uses heavier-than-standard pins, but I’ll be checking the lower vertical pins specifically — it’s a low-cost check worth doing early.

Garmin Service Bulletin SB 2144 — Both My Units Are Affected

Garmin issued SB 2144 specifically addressing GSU 25 units susceptible to “acoustic noise energy” — their language for vibration-induced sensor degradation. The bulletin was traced back to COVID-era supply chain issues that resulted in different MEMS components being used in a production run of units. Both of my GSU 25Cs fall within the affected serial number range.

Some builders had both units replaced under SB 2144 and reported clean results. However, others found the hardware upgrade alone wasn’t sufficient — it took the cowl pin fix or a mount relocation on top of the new units. The SB appears to be a necessary step, but not necessarily sufficient on its own.

Van’s Service Bulletin SB-00028 — Rubber Isolators

Van’s issued SB-00028 (currently RV-12 specific) directing owners to assess ADAHRS performance and, where issues are found, install rubber isolators on the GSU mounts. This directly contradicts Garmin’s guidance to mount to “the stiffest part of the airframe.” The community notes the irony, and the flight experience of multiple builders suggests Van’s got it right.

The Stiff vs. Soft Mount Debate

The intuition for stiff mounting is that it keeps the sensor stable relative to the airframe. But that logic breaks down when the airframe itself is vibrating: a stiff connection transmits vibration energy directly into the sensor. The natural frequency of a mounted system is f_n = (1/2π) × √(k/m). You want f_n well below the engine’s excitation frequencies — which means lower stiffness k and higher mass m. Soft rubber isolators reduce k; adding mass to the isolated platform reduces f_n further.

A useful analogy: mechanical vibration systems map directly to RLC electrical circuits — mass to inductance, spring stiffness to 1/capacitance, damping to resistance. A low-pass filter attenuating high-frequency noise is doing the same job as a soft, heavy vibration isolator. The back-of-GDU solution likely works through both added mass and reduced stiffness in the mount path, arriving at isolation inadvertently.

Even Aft Mounts May Need Additional Reinforcement

One builder who relocated to Van’s designated aft ADAHRS mount (behind the baggage compartment) still had significant vibration issues during Phase 1. The eventual solution was adding substantial metal reinforcement to the mount structure. Their full troubleshooting documentation is in a dedicated VAF thread: GSU-25 Remote Mount Vibration Issue — worth reading before deciding on a relocation strategy.

What Garmin Tech Support Confirmed

CAN Bus Dropouts Confirmed in Flights 1–3

Garmin confirmed intermittent CAN bus dropout signatures in the first three flight logs. What causes the 999% hard pegs specifically has not been established — the dropouts are a confirmed concurrent issue, but attributing those particular readings to the bus alone would be premature. Garmin’s guidance: fix the CAN bus first, get clean logs, then evaluate what vibration problems remain.

CAN Bus Length: ~87 Feet vs. 66-Foot Maximum

Garmin’s maximum recommended CAN bus length is 20 meters (66 feet). My estimate puts the total run in N997CZ at ~87 feet — about 21% over spec. Beyond that length, signal reflections and attenuation degrade bus integrity, especially with non-spec cable.

Wrong Wire: Standard Twisted Pair Instead of 120Ω Controlled-Impedance

The CAN bus requires 120Ω characteristic impedance. Standard aircraft twisted pair (~$1/ft) doesn’t reliably hit that spec, causing signal reflections that look like dropouts at the receiver. Garmin specifically recommends Carlisle IT P/N CAN24TST120(CIT) or GigaFlight P/N GF120-24CANB-1 (~$7/ft). Estimated rewire cost: $420–$560 in cable for a 60–80 ft run.

Two Problems, a Ranked Action Plan

Problem 1 — Fix First (per Garmin): CAN Bus. Wrong wire spec, over-length run, confirmed dropouts. Resolve this completely before evaluating what vibration problems remain.

Problem 2 — Address After CAN Bus Is Clean: Vibration. The tumbling horizon and escalating deviation pattern across five flights make it clear something is wrong — the clean-CAN-bus baseline will tell us how much of it is vibration.

1. Measure CAN bus run length precisely before ordering wire.
2. Rewire CAN bus with Carlisle IT CAN24TST120(CIT) or GigaFlight GF120-24CANB-1. Verify 120Ω termination at both ends.
3. Re-fly and pull fresh logs — establish a known-good CAN bus baseline.
4. Dynamic prop balance — highest-yield vibration fix, lowest cost. Also explore prop re-clocking.
5. Contact Garmin re: SB 2144 — both GSU 25Cs are in the affected serial number range.
6. Inspect lower cowl vertical hinge pins for any play. Replace with oversized pins if flex is detectable.
7. Evaluate GSU mount options — rubber isolators on current sub-panel location, or relocation to GDU back-panel or aft Van’s mount with reinforcement.
8. Post-fix flight test — five clean flights, deviation at baseline, horizon that stays put.

I’m cautiously optimistic that this is all solvable — the VAF community has collectively worked through every one of these failure modes, and there are clear paths forward. But I want to earn that optimism with data, not just a plan.

If you’ve dealt with ADAHRS deviation on your own build, or have experience with the SB 2144 units, I’d love to hear from you in the comments or over on the VAF thread. This community has already saved me multiple trips down dead ends and I’m grateful for every one of those replies.

N997CZ is a homebuilt experimental aircraft (Van’s RV-10). This post documents personal experience during Phase 1 flight testing. Nothing here constitutes maintenance or airworthiness advice. Consult your avionics manufacturer and A&P/IA for guidance specific to your aircraft.

If Flight #3 was the calm, confidence-building morning session, Flight #4 — same afternoon — reminded me that experimental test flying keeps you humble. We got up, noticed something electrical wasn’t quite right, came back down, pulled the cowl, and figured it out. That’s the program, and the Garmin G3X makes this kind of detective work so much easier.

I’m sharing the full data and photos below in case anyone else has been down this road with their IO-540 alternator installation — and I’d genuinely love to hear your thoughts in the comments.

The Setup: Two Flights in One Day

Flight #3 happened early that same morning at KHEF — a roughly two-hour session that went really well. First flap deployments, west-side practice area, calm winds. You can watch the Flight 3 footage here, which covers that morning session right before this one:

By evening I was back at the airport for a second bite at the apple. The controller on duty hadn’t seen our experimental test program before, so we had a good chat on the radio about the practice area setup. During the run-up I noticed the mag drop on each side — left conventional magneto and right SDS CPI-2 electronic ignition — was running around 160 RPM per side, a bit more than I’m used to from my RV-7 background.

Mag drop of ~160 RPM per side with a conventional mag on the left and SDS CPI-2 on the right on an IO-540. Does that sound about right for this combination? I’d really appreciate a comparison point from other builders running this setup.

The Takeoff Smell — and the First Clue

Takeoff on 16L was otherwise normal. Almost immediately after rotation I caught a mild burning smell — subtle, but on a 4th-flight experimental you don’t ignore subtle. I knew we had a small burn spot on the cowl interior from an earlier exhaust pipe contact (now protected with aluminum foil tape and the pipe repositioned), so it might have been residual. But then the electrical picture started to change.

Normally Alternator 1 carries the bulk of the load and Alternator 2 handles residual backup loads. On this flight, Alt 1’s ammeter dropped into single digits — well below what the aircraft was actually drawing. Battery 1 was starting to discharge. Cycling the ALT 1 switch would briefly coax it back, but it wouldn’t hold normal output.

What the G3X Data Shows

This was a short flight — about 13 minutes airborne, max altitude 2,031 ft MSL, max IAS 166 kts. But the electrical log tells the story clearly. Bus 2 held solid at 14.1–14.2V throughout; Bus 1 steadily lost ground as Amps 1 faded from 28A to just 4–5A while Battery 1 discharged.

The decision to return was easy: no imminent emergency — Battery 2 and Alt 2 were healthy, and Battery 1 still had reserve — but continuing to discharge an unknown cause wasn’t prudent at this stage of testing.

Engine Temps: CHTs and Oil Temperature

While the electrical issue was the main story, I was watching engine temps closely too. CHTs peaked at 441°F on CHT-1 this flight, with cylinders 1, 2, 5, and 6 still running warmer than 3 and 4 — a consistent pattern since Flight #1 that seems to be gradually improving with each flight. Cylinders 3 and 4 are now comfortably below 350°F in cruise, which is encouraging.

Oil temperature came in at 215–220°F at taxi-in after landing — higher than I’d like, though this was after a high-power takeoff and immediate return to land with very little cruise cooling. We also found the oil cooler airflow door wasn’t quite at 90° open (the winter-operation door should be fully open in summer), so we straightened it. We’ll see if that makes a difference on the next flight.

The Post-Landing Find: Alternator Belt

After landing we pulled the cowl. The culprit was immediately apparent: noticeable slack in the alternator belt. Not a broken belt, not a failed regulator — just insufficient tension. Both the main lower pivot bolt and the smaller tensioning bolt were intact and the tensioning bolt was still safety-wired, but the belt had clearly stretched or the alternator had shifted slightly.

We loosened both bolts, used a screwdriver to lever proper tension back into the belt, re-tightened everything, and re-safety-wired. The intermittent charging behavior in flight — briefly recovering when I cycled the switch, then dropping off again — makes perfect sense in hindsight: the belt was marginal and slipping under load. I’m relieved it wasn’t a chafed or burned wire. Battery 1 went on the wall charger overnight.

The belt seemed fine at install and through the first three flights. Did it stretch after initial heat cycling? Is there a recommended belt tension spec or a “check after first N hours” item for the Lycoming IO-540 alternator drive I should be following? Would love to hear from anyone who’s been down this road.

Oil Cooler Door — Another Small Fix

As noted above, we found the oil cooler winter-operation door sitting at slightly less than 90° open, potentially restricting airflow to the cooler across all previous flights. We adjusted it to fully open. With the shorter flight profile and high-power takeoff, the 220°F oil temp is plausibly explained by the flight itself rather than restricted airflow — but the door is now confirmed fully open and we’ll have cleaner data going forward.

The Landing: A Real Improvement

Not all problems this flight. The full-flap landing at the end of Flight #4 was noticeably better than the steep close-in approach at the end of Flight #3. In Flight #3 I was at virtually full aft stick at roundout with nothing left in reserve — the RV-10’s limited elevator authority at forward CG with full flaps on a steep approach is a real thing.

For Flight #4 I used a much shallower approach angle, carried a touch more power, kept the nose slightly higher, and touched down at about 64–65 knots indicated. The flare felt natural and in-control. This will be the approach profile going forward.

Note on CG: Flying a fairly forward CG for these initial flights — intentional for early test stability — with ballast in the rear seats to nudge it back slightly. Limited elevator authority at forward CG + full flaps + steep approach is a documented RV-10 characteristic. Curious how others have managed approach technique in this configuration during early flights.

Items Still on the List

AHRS 1 tumbling on takeoff: PFD-1’s artificial horizon tumbles consistently when I apply takeoff power — every single flight. PFD-2 stays solid so it’s not a safety issue right now, but I want to understand it. Vibration? Connector seating? Has anyone seen this in a G3X installation?

CHT temperatures: Still watching closely. Peak temps hit 441°F on CHT-1. The hotter cylinders (1, 2, 5, 6) seem to be gradually trending down with each flight as the rings seat. Hoping to see all six under 400°F in level cruise before long — does that sound like a realistic expectation at ~4.5 engine hours, or should I be looking at baffling adjustments?

Fuel float gauges: Still getting hung up at the full position. Not urgent since the totalizer is primary, but the mechanical float/wire fit issue inside the tanks needs to be addressed eventually.

What’s Next

Before Flight #5: re-verify alternator belt tension and confirm Alt 1 charging properly on a ground run. Assuming that checks out, the goal is continued envelope expansion — more time at altitude, more cruise data, and continued CHT monitoring as the engine breaks in.

Your turn: If you have experience with alternator belt tension on Lycoming IO-540 installations, SDS CPI-2 mag drop numbers, G3X AHRS tumbling on takeoff, CHT break-in patterns, RV-10 approach technique with forward CG, or oil cooler management — please drop a comment below. Whether you’re an RV builder, an A&P, or just following along, your input is genuinely welcome here.

Please join the discussion or send feedback here: VAF Thread — RV10 N997CZ Takes to the Skies

After the abbreviated 28-minute Flight 2 and a bit of detective work on the oil temperature wiring, Flight 3 felt like a different airplane — or at least a different experience. Early morning, calm winds, the west side of the field, and a solid two-hour session in the air. It went well enough that I’ve been smiling about it all day. I want to share the data and some of the moments that stood out, and I’m genuinely curious to hear from anyone who’s been down this road with an RV-10 or a similar build.

Getting the West Side — Again

For Flight 2, Manassas Approach had me confined to the east side of the field at only 1,400 feet MSL, which made for a cramped pattern and more stress than I wanted on a second experimental flight. This time, I coordinated early and came out just after 6:30 AM, and the controllers were fantastic. They gave me the west side at 1,800 feet MSL and — when traffic allowed — let me run the full length of the north-south pattern. When arrivals or departures needed the airspace, they’d keep me north of the railroad tracks or south of them as needed. It was a smooth, professional collaboration and made the whole flight feel much more manageable.

All told, we flew 20.5 gallons out of the left tank, then switched to the right and burned another ~20.5 gallons there — roughly 41 gallons total confirmed by both the fuel totalizer and the fuel truck after landing. The fuel float gauges are a different story (more on that below), but the totalizer was spot-on.

CHTs: Trending in the Right Direction

The cylinder head temperatures were the thing I was most anxious about going in. Flight 1 had a spike past 450°F; Flight 2 saw all six cylinders over the red line briefly. So I was watching the CHT traces like a hawk on departure.

Good news: after the initial climb, everything settled down, and the cruise numbers were genuinely encouraging. Cylinders 3 and 4 spent most of the flight below 350°F — which felt almost cool by comparison to the first two flights. Cylinders 1, 2, 5, and 6 held in the 380–390°F range at 89–93% power. The peak for the entire flight just barely touched 435°F, and only briefly at the start of the climb.

For those who’ve flown IO-540s through break-in — does this trajectory look about right to you? I’ve been running full rich at near-full power. Would love to hear if anyone has observations on what to expect from here.

Oil Temperature: Night and Day vs. Flight 2

This one made me happy. You’ll recall that the erratic oil temperature readings in Flight 2 were ultimately traced to a poorly crimped wire connection at the oil temperature probe — one that could literally be pulled free by hand once we unwrapped the wire bundle. We re-did the crimp, and the proof is right here in the data.

Oil temp settled in around 200–209°F for the duration of the flight and never wandered. Oil pressure ran 75–89 PSI throughout. After two flights with that anxious eye on the gauge, seeing a clean flat line in the green was genuinely satisfying.

One thing to note: At the end of Flight 4 (same day, evening flight), oil temp crept up to 215–220°F after a high-power takeoff and immediate landing. We also discovered the oil cooler airflow door wasn’t sitting at a full 90° open position — it had been slightly restricting flow during all previous flights. We’ve corrected that and will watch the temps closely next time. More on Flight 4 in a future post.

RPM and Power

The RPM and power traces look about as clean as you could ask for. We held 2,600–2,690 RPM through most of cruise, operating in the 85–93% power range. There’s a noticeable step-down in the middle of the flight where I reduced power for a bit — I was experimenting with different power settings to see how the CHTs responded — before coming back up to near-full power for the remainder of the cruise portion.

The Highlight: First Flap Deployment

This was the part I’d been looking forward to. With two hours of relatively calm airspace and a stable aircraft, Flight 3 was the first time I had enough room to slow down and actually try the flaps — something I hadn’t been comfortable attempting in the tighter, lower patterns of the first two flights.

I stepped through the detents methodically: reflex to the first position (about 3° down from reflex), then to the 15° intermediate, then all the way to full flaps. Each step brought a noticeable pitch change and a mild need for roll trim, but the airplane handled it gracefully. The RV-10 felt remarkably steady through all the configurations — more planted than I expected, honestly.

The first full-flap landing came at the end of Flight 3. We were cleared in from 1,800 feet on a tight, close-in approach — I was running 85 knots, the descent angle was steep, and the CG was fairly far forward (with a couple of cinder blocks in the back seats to nudge it rearward, but still nose-heavy). As I’ve read about in the RV-10 community, the elevator authority at that configuration was noticeable — stick all the way back into my lap for the flare. The landing was fine, but it was an education. On Flight 4 that evening I used a shallower approach with a bit more power on, and that felt considerably better.

Fuel Floats: A Minor Mystery

Both fuel float gauges have been reading high throughout these flights — stuck at 25 gallons on the left and 24 gallons on the right even when the tanks actually hold 30 gallons each. My working theory is that the float arm is bent just slightly too close to one of the fuel tank ribs, causing it to hang up. A quick thump on the tank after landing from Flight 3 dropped the left gauge from its pegged “full” reading down to 11 gallons — right where it should have been given the fuel burned.

Has anyone tackled a stuck float in an RV-10 tank? I’d rather not pull a tank if there’s a less invasive approach. Any tricks for getting better clearance on the float arm without a full disassembly? Would love to hear how others have dealt with this.

Quick Summary of Flight 3

Overall, Flight 3 felt like a turning point. The crimp fix eliminated the oil temperature anxiety, the CHTs are trending cooler with each flight, we got the preferred practice area on the west side of the field, and I finally got to try the flaps. The airplane is performing well and the data is looking progressively more like what I hoped to see. There’s still plenty to sort out — the stuck fuel floats, the AHRS that tumbles on every takeoff roll, and the alternator issue that cut Flight 4 short — but the trajectory feels right.

Thanks as always to the Manassas tower team for being so accommodating. Flying experimental in Class Delta isn’t always easy to coordinate, and they’ve been genuinely helpful.

Your turn: If you’ve built or flown an RV-10 (or any IO-540-powered aircraft through early flight test), I’d genuinely love to hear your perspective in the comments. Whether it’s about CHT break-in, flap behavior, float arm fixes, or anything else that caught your eye in the data — all input welcome. Next up: a post on Flight 4 from that same evening — a shorter flight that ended with a loose alternator belt and more lessons learned.

Please join the discussion or send feedback here: VAF Thread — RV10 N997CZ Takes to the Skies