Principles of Flight

Stall and Buffet in Practice - ATPL 081 — Principles of Flight | Pilot Assessment Training

Stall and Buffet in Practice

Stall and Upset · Lesson 6 of 6•9 min read

The module's physics is complete; this closing lesson is operational. It walks the situations where buffet or stall actually finds aircraft in service: the crew's own errors, the weather at both ends of the altitude band, and the automation traps, with the mitigation for each. And it finishes with the certified vocabulary of the subject: what officially counts as a stall warning, and what counts as a stalled aircraft, because the two demand different responses, and the exam quotes these lists nearly verbatim.

Pilot-induced situations

Inappropriate take-off configuration — take-off performance assumes the planned high-lift configuration, and the leading edge is the critical half: with slats or leading-edge flaps not extended (or asymmetric), the wing's stalling angle is far lower than the schedule assumes: rotation to the normal attitude can stall the wing at lift-off, and the warning systems, set for the planned configuration, may speak only as it happens. The mitigation is procedural: configuration checklists and the take-off configuration warning, treated as absolute.
Steep turns — the load-factor lesson in the wild: at 45° the stall speed is 19% up, at 60° it is 41% up, and a distracted tightening of the final turn spends margin invisibly. Mitigation: bank limits (and the discipline to go around rather than tighten), plus speed additives where manoeuvring is anticipated.
Go-around at TOGA (take-off/go-around thrust) with underslung engines — engines mounted below the wing push from below the centre of gravity: full go-around thrust produces a strong nose-up pitching moment just as the aircraft is slow, light at the end of a flight, and trimmed nose-up for the approach. Unchecked, the pitch attitude can run up toward the stall in seconds. Mitigation: expect the pitch-up, fly the pitch target positively (push as needed, retrim), and where appropriate use reduced go-around thrust.

Check you understood

Three questions from this lesson — one straightforward, one that applies it, one that pushes a little further. Nothing is recorded, and you can draw a new set as often as you like.

1. Warm-up
'Stall warning' means a natural or synthetic indication, approaching the stall, that may include:
2. Applying it
Taking off with the slats not extended (contrary to the planned configuration) risks:
3. Pushing it
With unreliable airspeed indications, the immediate protection against stalling is to:

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Environment: high altitude

Thunderstorms in the ITCZ (the intertropical convergence zone, the equatorial storm belt) — its storms tower to and beyond cruise levels, so they cannot be climbed over: their tops carry severe turbulence and icing into the very band where the buffet margins are thinnest. Mitigation: lateral deviation, early and generous.
Jet streams — the strong wind shears at a jet stream's edges produce turbulence and abrupt speed changes; a few knots of shear-induced excursion can consume half of a thin high-altitude buffet margin. Mitigation: anticipate from the forecast, and cross shear zones at appropriate speeds.
Clear-air turbulence (CAT) — the same threat with no visual warning at all, striking at cruise where the previous lesson's corridor is narrowest. Mitigation: turbulence penetration speed, seat-belts policy, and when margins have thinned, the previous lesson's escape: descend to where the corridor widens.

Automation and indication traps

Inappropriate autopilot climb mode — an autopilot in a mode that commands a trajectory without protecting speed (vertical-speed mode is the classic case) will trade speed for the commanded path without complaint: as thrust runs out at altitude, the speed decays quietly toward the low-speed buffet while the crew sees a perfectly steady climb. Mitigation: climb in speed-protecting modes at altitude, and monitor the speed trend as a primary instrument.
Loss of, or unreliable, airspeed indication — with the airspeed lying (iced probes the classic cause), a crew chasing the false number can stall a perfectly healthy wing or overspeed it; the aeroplane itself has not changed at all. Mitigation: recognise the disagreement, fly the memory items (known pitch attitude plus known thrust setting produce a known, safe performance) and work the checklist to identify the reliable source.

Any airspeed, any attitude, any altitude

One sentence from the module's start deserves its own certification-endorsed repetition, because every scenario above is an instance of it: a stall depends on one quantity only, the angle of attack: so a stalled condition can occur at any airspeed, any attitude, and any altitude. Under load factor the wing meets its critical angle at speeds far above the 1g figure; in an upset the attitude tells nothing about the airflow; and at cruise altitude the Mach-limited wing stalls at an indicated speed that would be perfectly safe below. The corollary is the recognition discipline that follows: identify the stall by its signs, never by assuming the speed, attitude or altitude protect against it.

What happened in the pastAir France Flight 447 — the South Atlantic, 1 June 2009

In cruise at FL 350 over the mid-Atlantic on the night of 1 June 2009, an Air France A330 temporarily lost consistent airspeed indication: ice crystals had blocked the pitot probes. The autopilot and autothrust dropped out and the flight controls reverted to alternate law, its stall protections gone. The pilot flying answered with rapid roll inputs and a sharp, sustained pull; the aircraft climbed toward 38,000 ft shedding energy, the stall warning sounded continuously for 54 seconds, buffet set in, and the trimmable stabiliser wound to 13 degrees nose-up and stayed there.

The A330 reached its apogee near 38,000 ft and stalled, and it stayed stalled for three and a half minutes: angle of attack above 35 degrees, descending at 10,000 ft/min with the nose held up. Neither pilot ever named the stall aloud; the report describes a crew that never connected the lost speeds, the warning and the buffet into the one diagnosis that mattered. The warning logic itself played a part: below 60 kt of measured airspeed the angle-of-attack data is ruled invalid and the stall warning falls silent, so in the depths of the stall the system behaved backwards — a push toward recovery revalidated the data and set the warning off, while a further pull silenced it. The aircraft struck the sea with 228 people aboard; there were no survivors.

The findings drove changes still visible in every type rating: the pitot probes were replaced under airworthiness directives cited in the report itself, the industry's upset and stall recovery training was rebuilt for the high-altitude case, and the recommendations asked for an angle-of-attack indication in the cockpit and a stall warning that keeps working at very low measured speeds. The rule this lesson closes on is the one the report keeps arriving at: the stall is a matter of angle of attack alone, recognised by its signs (continuous warning, buffet, a descent that will not arrest), never argued away because the altitude, the attitude or the aircraft's protections make it seem impossible.

Stall and Buffet in Practice

Pilot-induced situations

Check you understood

Environment: low altitude

Environment: high altitude

Automation and indication traps

Any airspeed, any attitude, any altitude

The certified language: warning against stalled

What the exam asks