Step Climb Above the Buffet Ceiling
Twin-jet cruising at FL360, mass 205,000 kg, ISA+15, headwind 50 kt. The flight manual gives optimum altitude 34,100 ft, maximum altitude for 1.3 g buffet 37,200 ft, and a recommended turbulence penetration speed of M0.76. ATC offers FL380. Fuel on board is 15,200 kg; required trip fuel is 9,800 kg, alternate fuel 2,100 kg, contingency 1,000 kg, final reserve 1,500 kg. Moderate turbulence is forecast at FL380, and the PF wants the higher level for fuel savings. The aircraft is not ETOPS-critical on this sector and has no performance-limited obstacle ahead. The current speed is M0.82. The crew must decide whether to accept FL380, stay at FL360, or request a lower level.
1. Calculate the fuel margin over destination alternate and final reserve, and decide whether FL380 is acceptable. Explain using the buffet limit.
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Fuel margin is 15,200 minus 9,800 trip, minus 2,100 alternate, minus 1,000 contingency, minus 1,500 final reserve, which equals 800 kg. That is positive but small. The altitude limit is more restrictive: maximum altitude for 1.3 g buffet is FL372, and FL380 is above it by 800 ft. At FL380 the aircraft would have insufficient buffet margin, especially in moderate turbulence. Therefore FL380 is not acceptable even though the fuel figures are legal. The optimum altitude is FL341, so the efficient choice is a lower level, not a higher one. Request FL340 or FL360 and recompute fuel.
2. What cruise altitude and speed would you request in the turbulence, and why?
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Stay at or below FL360 and slow from M0.82 to the turbulence penetration speed M0.76, or the AFM value, while keeping a margin above VLS and buffet onset. Do not climb into FL380. In turbulence, maintain wings level, avoid large thrust changes, and accept the fuel penalty. If the crew stays at FL360, monitor the buffet margin and Mach; if it deteriorates, request a lower level. The priority is stall/buffet margin and structural protection, not fuel saving.
3. If ATC cannot give FL340, what is the safe option?
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If ATC cannot give FL340, FL360 remains acceptable because it is below the FL372 buffet limit, provided the crew slows to the turbulence speed and monitors margin. If even FL360 becomes marginal, request a block or lower level; if unavailable, divert or amend the route. Do not accept FL380. The 800 kg fuel margin is not large enough to justify a prolonged hold at a poor level; plan a fuel stop or diversion before the margin disappears.