Month: May 2026

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