One Engine at FL410: Driftdown and the Diversion Decision
FL410, M0.80, gross weight 62,000 lb, ISA+12, 280 nm from destination. The right engine fails and is secured; single-engine service ceiling at this weight and temperature is FL280. Terrain along the route has a grid MORA of 21,000 ft and a 14,000-ft ridge 40 nm ahead. Driftdown to FL280 takes 11 minutes and burns 600 lb extra. Fuel at failure is 8,200 lb. Destination requires 4,500 lb including reserves at single-engine speeds and has 600 overcast, 2 sm, 9,000-ft runway. The alternate, 60 nm behind, requires 3,000 lb and has CAVOK and an 11,000-ft runway. Low-speed buffet at FL410 is about M0.74; you are at M0.80.
1. What is your immediate action, and why is attempting to hold FL410 the wrong choice?
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Initiate driftdown immediately at the single-engine driftdown speed, turn toward the diversion field, and tell ATC you are descending. Do not wait for a revised clearance to leave FL410.
The airplane cannot hold FL410 on one engine; the ceiling is FL280. Attempting to hold altitude puts the airplane near the thrust-limited ceiling, where the speed band between low-speed buffet at about M0.74 and the high-speed limit narrows quickly. At M0.80 with one engine you have only about 0.06 Mach of margin to the low-speed buffet, and a small distraction, a temperature rise, or an ATC delay erodes it. If the speed does decay, lower the nose and accept the descent rather than pulling to hold altitude.
Terrain is not a reason to stay high: FL280 clears both the 21,000-ft MORA and the 14,000-ft ridge with margin.
2. Do the fuel and terrain numbers support continuing to destination or diverting to the alternate?
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Fuel at FL280: 8,200 - 600 = 7,600 lb.
- To destination: 7,600 - 4,500 = 3,100 lb surplus.
- To alternate: 7,600 - 3,000 = 4,600 lb surplus.
Both are legal, so fuel does not make the decision. Weather, runway, and single-engine approach considerations do.
Destination: 600 overcast, 2 sm, 9,000 ft, 280 nm ahead. A single-engine approach to low weather means a real probability of a missed approach with no second engine and little fuel margin left for a second attempt and a diversion. Alternate: CAVOK, 11,000 ft, 60 nm behind, 4,600 lb surplus.
The alternate is the lower-risk choice because it removes the weather variable entirely, and the fuel surplus is large enough that flying 60 nm backward costs nothing in legal terms. Continuing to destination is defensible only if the approach is comfortably within single-engine minimums and the crew is confident of a landing on the first attempt.
3. ATC asks you to maintain FL410 for three minutes for crossing traffic. What do you say and why?
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Say you are unable to maintain FL410 and are descending now; if the situation is urgent, declare it. A three-minute level-off at FL410 on one engine pushes the airplane further into the thrust-limited region, where speed decays toward the low-speed buffet while you have no excess thrust to recover it. The margin at M0.80 is already thin, and the delay occurs at the worst possible moment, before the driftdown is established.
The controller needs a clear statement, not a negotiation: 'Unable to maintain FL410, one engine, descending through FL390, request heading 270.' Traffic separation is the controller's problem to solve; staying at an altitude the airplane cannot hold is yours. Once established in the driftdown, you can accept a lower altitude or a route change that does not require level flight at high altitude.