Tag: slow flight

N997CZ — Flight 13: The Stall Block

Ground track for N997CZ Flight 13 over the Phase 1 test area
Ground track for Flight 13 over the Phase 1 test area.

Every flight test program has a day where you stop sneaking up on the stall and just go fly it. For N997CZ, that was Flight 13. The slow flight and first banked stalls on Flight 12 had shown the airplane was honest at high angle of attack; this flight was about systematically documenting where the wing quits — clean, with flaps, and with power on — and doing all of it with a lot of sky underneath.

It was also the flight that finally set the airplane free of round numbers: a new program-high altitude of 11,520 ft MSL (12,069 ft GPS).

The Numbers

Date 2026-06-07
Engine time ~2.2 hr
Distance 233.3 nm (GPS path)
Fuel used 24.1 gal (totalizer)
Engine hours 16.2 → 18.3
Max altitude ~11,520 ft MSL (12,069 ft GPS) — new program high
Cruise OAT +67 °F (warm day)
Loading solo, light weight
Cruise mixture leanest yet — ~11.5 gph

The Card Deck

Flight 12 only got partway through the stability cards before the slow-flight work used up the morning, so this flight’s deck rolled the leftovers in with the stall series — eight cards in fly order: area set-up, steady-heading sideslips, spiral stability checks, then the pre-stall set-up and recovery drill, and finally the stalls themselves — 1-g clean, full-flap, and power-on. (The last card, accelerated/turning stalls, didn’t get flown before fuel and the warm afternoon said go home — it carries forward.)

The sideslips and spiral stability points were flown and are sitting in the log waiting for their own data reduction — a future post once the numbers are out.

The full 16-page deck — a briefing script page for each card plus a boxed, cut-out kneeboard card for the cockpit — is here: Flight 13 test cards (PDF). Here’s one from the deck, the 1-g clean stall:

FT-13-5 kneeboard card — 1-g clean stall, document the break
The FT-13-5 kneeboard card — 1-g clean stall, document the break.

The Stall Block

Flight 13 altitude and indicated airspeed vs time, with the stall block shaded
Altitude and indicated airspeed vs time — the shaded block is 33 minutes of stall work, ending with the climb to the program-high 11,520 ft MSL (12,069 ft GPS).

The profile chart shows the structure: climb out of Manassas, stability work around 10,000 feet, then a sustained 33-minute stall block — that dense comb of deceleration spikes in the red trace, each one a deliberate walk down to the break and a recovery. Toward the end of the block the airplane steps up and up, topping out at 11,520 ft MSL (12,069 ft GPS) for the power-on series, then a long descent home.

The headline numbers, all flown solo at light weight:

Series (engine-verified) Break Notes
Power-off, early series 52–58 KIAS idle/low power, ~10,000 ft — likely the clean (flaps-up) cards, per fly order
Power-off, later series ~48 KIAS three events, 48.0–48.7, idle/low power — likely the full-flap cards, per fly order
Power-on (departure) 55–59 KIAS 16–18″ MAP / ~2,500 RPM, +14 to +18° pitch — verified from engine data

A word on how those rows are labeled, because honesty matters more than tidiness. The flap-position sensor is one of the program’s open squawks — this flight’s log records raw counts from −521 to +9, which is to say, nothing usable — so no row above can claim a data-verified flap setting. What the data does verify is power: the engine channels cleanly separate five power-on breaks (16–18 inches of manifold pressure, 2,500 RPM, nose up +14° or more) from seven power-off breaks at or near idle. The flap attribution rests on the card fly order — the clean stalls were scheduled before the full-flap ones — plus a physics cross-check: flaps lower the stall speed, and sure enough the later power-off series broke a good seven knots slower than the earlier one. Likely, not proven, until the new flap sensor goes in.

The numbers also pass the placard test once you account for weight. The placarded stall speeds at gross are about 61 knots clean and 52 with full flaps; stall speed scales with the square root of weight, and this was one pilot and partial fuel, far below gross. Scale the placard numbers down and you land almost exactly where the airplane did: high-50s breaks for the (likely) clean series, 48 for the (likely) full-flap series. The G-trace adds one more correction — the breaks happened partially unloaded, 0.74–0.88 G, which reads lower still. And through all of it the airplane behaved: buffet warning, a straight-ahead break, conventional recovery, every time.

FlySto’s pitch trace makes the hour of stall work visible at a glance — every tooth in that comb is a nose-up walk toward the break and the nose-drop after it, and the tall +14 to +18° peaks late in the block are the power-on series, right on the card’s target attitudes:

FlySto pitch chart for Flight 13 — repeated pitch-up/break cycles through the stall block
FlySto’s pitch chart for Flight 13 — every tooth in the comb is one stall cycle; the +14–18° peaks are the power-on series.

One more layer from Flight 11’s calibration work: at these speeds the airspeed indicator reads about a knot and a half low, so calibrated break speed is closer to 49–50. Every knot of that bookkeeping matters when these numbers eventually set the approach speeds.

Can You See the Buffet in the Data?

The pre-stall buffet is unmistakable in the seat — the airframe starts talking to you well before the break. A fair question is whether the data logger hears it too. The answer: yes, faintly. The G3X logs at 1 Hz, and real airframe buffet shakes at several cycles per second or more, so the log catches only an aliased, heavily muted echo of what the pilot feels. But it’s there: in eleven of the twelve stall events, the roughness of the normal-acceleration trace climbs to two to five times its smooth-air baseline in the final seconds before the break.

One Flight 13 stall entry: IAS decay, G-trace roughening in the buffet, the break, and the recovery
One stall entry from the block — airspeed bleeding down, the G trace fraying in the buffet window, the break, and the recovery.

The chart above is one entry from the block: airspeed bleeding down, the G trace starting to fray in the shaded buffet window, the drop at the break, and the firm recovery pull after. The amplitudes look small — a few hundredths of a G — but remember the sampling: the cockpit experience is a much louder version of what survives into a once-per-second log.

Engine: Leanest Cruise Yet, and a Plot Twist in the CHTs

The mixture story keeps marching: median cruise fuel flow was ~11.5 gph, the leanest of the program — a long way from the 18–23 gph full-rich break-in flights.

FlySto CHT chart for Flight 13: all six cylinders vs time with color bands
FlySto’s CHT chart for Flight 13 — one climb-out peak brushing the caution band, then a busy green-band sawtooth through the stall work.

The FlySto chart shows the whole flight: one hot moment on the climb-out where the pack peaks just over 420 °F, brushing the caution band, then a busy, healthy green-band sawtooth as the stall work cycles power up and down for an hour. The plot twist is which cylinder topped the chart: cylinder 2 at 422 °F — not cylinder 5, the climb-cooling canary from the last three flights (it logged 419). One warm-day flight isn’t a trend, but the ranking shuffle is noted and goes in the watch file.

The CO Detector Read Zero — Which Is Suspicious

Cabin CO logged 0 ppm for the entire flight. Taken at face value that would be the best result of the program — but every slow-flight-heavy flight from 6 through 12 showed at least a few ppm, and this was the most aggressive high-AoA profile yet. A sensor that suddenly reports perfect silence on exactly the profile that always made it talk is more likely asleep than victorious. Verifying the detector is on the squawk list; until then, this flight’s CO data gets an asterisk, not a trophy.

Also On This Flight

  • A freshly overhauled attitude unit went into the panel the day before this flight — and through an hour of stalls and high-AoA work, the attitude display never so much as twitched. By my count that makes this the first flight in the airplane’s life with no attitude-horizon tumble — the cross-source roll disagreement that hit ~136° the day before stayed under 10° through all of it, and it has stayed that way since. (The story has a sequel brewing: the #2 attitude source has started miscomparing against the newly healthy #1.) The full saga, and the data behind it, is getting its own post.
  • Manual flying practice: about an hour of the flight was hand-flown — stall work is hand-flying by definition, and the autopilot got the cruise legs.

Squawks

  • Accelerated/turning stall card not flown — carried forward to a future sortie.
  • CO detector read zero all flight — verify it’s actually alive before trusting the result.
  • Flap position indication still broken — and it stings more on a flight like this one, because it leaves the stall-speed-versus-configuration record resting on the card sequence and pilot recollection instead of data. Replacement sensor is on the parts list.

Bottom Line

Flight 13 closed out the bread-and-butter stall series: power-off breaks walking down from the high 50s to 48 knots indicated as the flaps (most likely) came out, power-on breaks in the high 50s — all of it right where the placard predicts once you do the weight math — honest manners throughout, and a new program-high 11,520 ft MSL (12,069 ft GPS). The engine ran its leanest cruise yet, cylinder 2 stole the hot-cylinder crown for a day, and the CO detector’s perfect silence earned suspicion instead of celebration. One card carries forward, and the stability data is in the can awaiting analysis. The envelope is opening up.

Engine time after Flight 13: 18.3 hours.

N997CZ — Flight 12: Slow Flight at 9,500 Feet

Ground track for N997CZ Flight 12 over the Phase 1 test area
Ground track for Flight 12 over the Phase 1 test area.

Flight 11 was a marathon — two and a half hours of precision circles for the airspeed calibration. Flight 12, six days later, was the opposite kind of flight: shorter, higher, and slower. The plan was to start working through the stability test cards, and the headline act was a slow-flight series flown higher than this airplane had ever been.

The Numbers

Date 2026-06-06
Engine time ~1.5 hr
Distance 156.5 nm (GPS path)
Fuel used 18.5 gal (totalizer)
Engine hours 14.8 → 16.2
Max altitude ~9,090 ft MSL (9,535 ft GPS) — highest of the program (so far)
Cruise OAT +58 °F (mild morning, ~13 °F warmer than F11)
Autopilot ~94% of airborne time (more on that below)
Config 7 qt oil; burn well under one tank, no tank switch

Climb High, Fly Slow

The idea behind doing slow flight way up high is simple: altitude is recovery margin. If the airplane is going to do something rude at high angle of attack, I’d rather it do it with 9,000 feet underneath me than 3,000.

Flight 12 altitude and indicated airspeed vs time, with the slow-flight series shaded
Altitude and indicated airspeed vs time — the shaded block is the slow-flight series at 9,100–9,500 ft.

The profile chart shows the shape of the flight: climb out of Manassas, work in the mid-8,000s, then push up to the program-high 9,090 ft MSL (9,535 ft GPS) and settle into about thirteen minutes of slow-flight work between 9,100 and 9,500 feet. The red trace tells the story — a string of deliberate decelerations, each one walked down slowly, held, and recovered, over and over. The slowest stabilized point was about 56 KIAS at 9,145 feet.

That block wasn’t all straight-ahead slow flight, either. Working down the stability card deck — the FT-12/FT-13 card set is here (PDF) — the points included longitudinal static-stability checks at 55% power (trim the airplane, displace 10 and then 20 knots either side of trim, and watch how it comes back), flaps-up power-off stalls banked both ways at 10°, 20°, and 30° of bank, and a set of power-on stalls at 55% power. The power-on stalls turned out to be the interesting ones — more on that next.

Remember from Flight 11 that the airspeed indicator reads a knot and a half or so low down at these speeds — so true calibrated speed at that 56-knot point was around 58, and true airspeed at that altitude meaningfully higher still. The margins were real, which is the whole point of doing this work upstairs.

The Autopilot Kept Tapping Out

One genuinely interesting system behavior surfaced during the slow-flight series: the G3X autopilot repeatedly dropped offline with a “Fail / Inertial miscompare” annunciation, then re-engaged — about four minutes of dropouts across seventy airborne minutes, and the trigger was specific: the power-on stalls. It wasn’t just the autopilot being cautious, either — the attitude horizon itself tumbled several times during those stalls. In the data, the disagreement between attitude sources reached roughly 136° of roll — the worst cross-source split of the entire program. Outside the high-AoA block the autopilot was rock solid; it flew about 94% of the airborne time.

“Inertial miscompare” doesn’t mean the system knows which attitude source is lying. Per the G3X installation manual, data from the two ADAHRS units is continuously cross-compared and a disagreement is simply annunciated — the system flags the mismatch and lets the pilot sort it out. (Interestingly, the G5 standby only joins the comparison if the system has degraded to a single ADAHRS, so there’s no three-way vote while both primary units are alive.) Figuring out which unit is actually bad happens one level down: each ADAHRS runs its own internal integrity monitoring, and a unit that loses confidence in its own solution drops it and re-aligns — at which point the system switches over to the survivor. Long-time readers will recognize this as the ADAHRS-deviation theme that’s been running since the first five flights — it gets provoked by exactly this kind of high-AoA, low-airspeed maneuvering. The decision coming out of this flight’s debrief: change out AHRS #1 and see what the data says. The replacement unit went into the panel that same day. That story deserves its own post.

Carbon Monoxide: Best Slow-Flight Result Yet

This one is encouraging. Every slow-flight-heavy flight since the CO detector went in has shown cabin CO in the 5–10 ppm range, consistent with the firewall heat-door leak theory. Flight 12 was the most slow-flight-intensive profile yet — and CO stayed benign: peak 5 ppm, mean 0.5.

Cabin CO across all twelve flights
Cabin CO across all twelve flights — Flight 12 is the cleanest slow-flight result since the detector went in.

Two honest caveats. First, the log shows a scary-looking 250 ppm spike — that’s the CO detector’s power-up warmup transient, recorded in the first ~50 seconds on the ground with the engine off, decaying steadily as the sensor warms. It’s a self-test artifact, not cabin air. (The trend script now auto-discards it.) Second, since I still haven’t flown the deliberate tape-the-heat-doors test, a quiet flight doesn’t tell me the leak is fixed — just that this profile, on this cool morning, didn’t pull much exhaust in. The isolation test stays on the list.

The Cylinder 5 Canary: Holding, Not Climbing

The air-dam saga continues, with a hopeful data point. Recap: before Flight 10 I removed the cooling air dam in front of cylinder 1, which fixed #1’s chronic heat but started costing cylinder 5 in the climb — its temperature-adjusted peak climb CHT marched 401 → 410 → 427 °F across Flights 9–11.

FlySto CHT chart for Flight 12: all six cylinders vs time with normal/caution/limit color bands
FlySto’s CHT chart for Flight 12 — all six cylinders (raw temps) with the comfort bands painted behind them.

Here’s the whole flight at a glance, as FlySto renders it from the G3X log — all six cylinders, raw temperatures, with the comfort bands painted behind them. The climb-out spike just after takeoff is the hot moment of the flight, topping out around 407 °F, and everything spends the rest of the sortie comfortably in the green. You can even see the slow-flight series as that choppy stretch in the middle, and the brief dip to ~260 °F right before it — the power-off deceleration entries.

Flight 12: 408 °F. Still the hottest cylinder in the climb, but it fell back to roughly the Flight-10 level instead of continuing to rise — helped, no doubt, by climbing into cooler air up high. Meanwhile cylinder 1 keeps enjoying its freedom: coolest jug on the engine, with cruise temperatures (OAT-adjusted) down around 327 °F. Cruise CHTs across the board were the lowest of the program — the reward for flying high in cool air. Verdict: the canary is alive but still in the mine. Watching.

Fuel: The Totalizer Earns Its Keep Again

A small satisfying footnote. The totalizer said the flight burned 18.5 gallons; the fuel truck that topped the airplane off twenty minutes after shutdown delivered 18.30 gallons. That’s agreement to 0.2 gal, and it keeps the program-long truck-versus-totalizer reconciliation within a fraction of a gallon. The fuel-flow system has earned trust the fuel-quantity gauges haven’t (the right float is still stuck — see squawks).

Cruise mixture ran richer than Flight 11 — about 15.5 gph against 14.5 — mostly because I spent the flight flying maneuvers instead of babysitting the red lever.

Squawks

  • Flap position sensor broke again — the May 31 workaround (remounting the worn pot to ride the healthy end of its range) didn’t hold, and this time the failure ran the other way: it wouldn’t let the flaps come up. On Flight 11 the same sensor had refused to put them down, forcing a no-flap landing. Two flights, two opposite refusals — enough. The fix chosen: reconfigure the VP-X so the flap switch works as momentary — hold the switch and the flaps drive up or down, no listening to the position sensor at all. (Since completed, with Dan’s laptop hooked to the VP-X.) The replacement for the 20-year-old Ray Allen pot stays on the parts list.
  • VP-X pitch and roll trim speeds need to come down — trim runs too fast.
  • AHRS attitude tumble / inertial miscompare under the power-on stalls — see above; next step chosen: swap AHRS #1.
  • Right fuel-quantity float still stuck (known item; totalizer is the authority).
  • Heat-door CO isolation test still not flown — benign CO this flight is encouraging but inconclusive.

And two items came off the list before this flight, in a Friday repair session: the upside-down left magneto switch from the Flight 11 squawks, and the remaining flap close-out work.

Bottom Line

Flight 12 took the airplane higher than it had ever been and slower than it usually flies, in the same thirteen minutes. The slow-flight handling was honest, the CO behaved, the cylinder 5 canary stopped climbing, and the totalizer matched the fuel truck to a couple tenths. The one system that complained — the autopilot’s inertial miscompare under high AoA — is the same attitude-source thread this airplane has been tugging since flight one, and it was about to get a lot more attention.

Engine time after Flight 12: 16.2 hours.