Level Turn at FL350: Which Buffet Is It?
A 20 000 kg twin-jet is holding at FL350 at M0.78. Wing area is 55 m², CLmax 1.40 and MMO 0.82. For this mass the performance manual gives a clean 1 g stall of 125 kt IAS, valid at any level. ATC asks for a 30° bank orbit; shortly after roll-in the crew feels a light, high-frequency buffet and a faint airframe rumble. The captain's first reaction is that the aircraft is on the edge of the stall and calls for thrust and back pressure. The first officer points out that the IAS is 240 kt. The aircraft is then cleared to hold in a 45° bank to remain inside the protected area, and the buffet becomes clearly stronger. Temperature is ISA+8, the air is smooth, the autopilot is off and the autothrottle is in MACH hold.
1. Using the figures given, calculate the load factor and the accelerated stall speed in the 30° and the 45° bank. What does the result say about the origin of the buffet?
Read the worked answer
Load factor in a level turn is n = 1/cos(bank):
- 30°: n = 1/cos30° = 1.155; speed multiplier sqrt(n) = 1.075 → Vs = 125 × 1.075 ≈ 134 kt IAS.
- 45°: n = 1/cos45° = 1.414; multiplier 1.189 → Vs ≈ 149 kt IAS.
At 240 kt the aircraft is about 1.6 times the 45° accelerated stall speed, roughly 90 kt of margin. A low-speed (AoA) stall buffet is therefore impossible: buffet onset normally appears at about 1.05–1.10 Vs, and here even the accelerated stall is 90 kt away.
The only boundary that can be reached at 240 kt IAS and M0.78 is the high-speed boundary. Local flow over the upper surface has exceeded M1.0, a shock has formed and is oscillating, and the separated flow behind it produces the rumble. This is Mach buffet, not a stall warning, and the correct response is the opposite of a stall response.
The teaching point is that the same IAS gives the same stall margin at any altitude, because both the stall speed and the IAS scale with the square root of density. The Mach number at which the wing starts to shock, however, is fixed for the wing and does not move with altitude. At FL350 a normal cruise IAS therefore sits very close to the high-speed boundary, and the crew's instinct based on IAS was misleading.
2. Why did increasing bank to 45° make the buffet worse? Explain the coupling between load factor, lift coefficient and the high-speed buffet boundary.
Read the worked answer
At constant mass, altitude and Mach, the lift coefficient is set by the need for the vertical component of lift to equal weight: L cos φ = W, so CL = 2W/(ρ V² S cos φ). Going from 30° to 45° raises 1/cos φ from 1.15 to 1.41, a 22% increase in CL, all of it obtained by increasing angle of attack because speed was held in MACH hold.
A higher CL means a stronger suction peak near the leading edge, higher local velocities over the upper surface, and therefore a lower freestream Mach at which the local flow reaches M1.0. The buffet-onset Mach for a given wing falls as CL rises, typically by a few hundredths of a Mach number for a 20% CL increase and more sharply near the boundary. The high-speed boundary is therefore not a fixed vertical line at MMO; it is a curve of Mach against CL, and MMO is only one point on it. That is why M0.78 produced nothing at 30° but a clear buffet at 45°.
Pulling back, as the captain proposed, has the same effect: it raises AoA and CL and pushes the boundary down again. The same logic explains why the coffin corner narrows in a turn - the low-speed boundary rises with bank (Vs ∝ sqrt n) while the high-speed boundary falls with the higher CL needed.
3. State the correct crew action and the reasoning behind each element. Why would adding thrust and back pressure have been counter-productive?
Read the worked answer
The aircraft is behind the high-speed buffet boundary, so everything that reduces CL at the same lift, or reduces Mach, moves it back inside.
- Relax the back pressure and lower the nose slightly - reduces CL and the local Mach over the wing. This is the primary action and it is the opposite of the stall recovery drill, which is why identifying the buffet correctly mattered.
- Reduce bank to 25–30°. The CL penalty of the turn is removed. If ATC needs the orbit, ask for a larger radius or a different fix; a 45° orbit at this mass and Mach is outside the buffet margin.
- Reduce Mach - retard thrust, and if the buffet persists extend speedbrakes and descend. At a lower level the same Mach needs a lower CL because density is higher, so both boundaries move apart.
- Do not increase thrust. For a high-speed buffet, thrust raises Mach and makes the situation worse; the 90 kt low-speed margin means there is no risk at all in slowing down.
- Report the event, note the level and the conditions in the technical log, and amend the holding speed for the remainder of the flight. A buffet encounter at altitude should also prompt a check of mass against the cruise chart, because a heavier aircraft reaches the boundary earlier.