Coffin Corner in Mountain Wave Turbulence
A swept-wing business jet is cruising at FL430, ISA+5, at Mach 0.80 and 19,500 kg. The AFM buffet chart for this weight shows high-speed buffet onset at M 0.82 and low-speed buffet onset at 205 kt IAS, which corresponds to about M 0.70. The crew is using the autopilot in altitude hold; current IAS is 225 kt. The aircraft enters moderate mountain-wave turbulence. The first gust produces +1.4 g, then a -0.3 g unload. The autopilot responds with a nose-up trim input, and within 20 seconds IAS decays to 210 kt while Mach remains 0.81. The crew sees no stall warning yet, but the Mach trim annunciator flickers. The destination is 90 minutes away, and the minimum safe altitude in the area is FL240. The aircraft has enough fuel to divert.
1. Explain why this aircraft is in a coffin-corner situation. Quantify the effect of the +1.4 g gust on the low-speed buffet margin and explain why the 210 kt IAS / M 0.81 combination leaves almost no usable speed band.
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The 1 g low-speed buffet onset is 205 kt IAS. For a gust load factor n = 1.4, the buffet/stall speed rises approximately with sqrt(n): 205 × sqrt(1.4) = 205 × 1.183 = 243 kt IAS. At 210 kt the aircraft is therefore already below the estimated 1.4 g buffet boundary, even though the 1 g margin was 5 kt. The autopilot's nose-up trim increases AoA and load factor, making this worse. At the same time, M 0.81 is only 0.01 below the high-speed buffet onset of M 0.82. The usable band is bounded below by an AoA-limited low-speed buffet and above by a Mach-limited high-speed buffet; at FL430 the IAS band between them is very narrow. A nose-down input raises IAS but also raises Mach, while a thrust reduction lowers Mach but also lowers IAS. That is the coffin corner: the two margins cannot be restored simultaneously without changing altitude.
2. Recommend an immediate action sequence. Explain why neither a simple thrust reduction nor a simple nose-down input alone is sufficient.
Read the worked answer
Disconnect the autopilot and take manual control with small, smooth inputs. Reduce thrust toward idle and lower the nose to stop the IAS decay while keeping Mach below M 0.82; accept a descent. A sensible target is M 0.78 with the nose trimmed for a shallow descent, using speedbrakes only if the Mach trend cannot be controlled without increasing AoA. Descend toward FL350 or FL300. At lower altitude the IAS for a given Mach is higher because density is greater, so the low-speed buffet margin opens up; the high-speed limit also becomes easier to respect because the aircraft is further from the Mach-limited ceiling. Do not chase the altitude hold or make a large nose-down push, which could overspeed the Mach or unload the wing abruptly. Once clear of the wave and at a lower level, set the turbulence penetration speed and review fuel and diversion. A thrust reduction alone would reduce Mach but further reduce IAS; a nose-down input alone would restore IAS but push Mach toward the high-speed boundary. Only a descent changes both margins.