Engine failure at cruise: drift-down and destination decision
A twin-jet is cruising at FL360, 640 nm from destination, with gross weight 62,000 kg and 8,400 kg of fuel. One engine fails and the crew drifts down to FL240, the single-engine cruise altitude for the current weight and temperature. Single-engine cruise at FL240 gives TAS 390 kt and fuel flow 2,350 kg/h. The destination forecast is good, and its alternate is 140 nm beyond destination. Company fuel policy requires 30 minutes holding at 1,400 kg/h, 5% contingency on trip fuel, and 250 kg for approach and landing. Along the drift-down and onward route, MORA is 21,500 ft and grid MSA is 22,000 ft. A closer airport is 210 nm away, has a 3,200 m runway and good weather, but no engineering support. The captain must decide whether to continue to destination, divert to the closer airport, or hold.
1. Compute the fuel required to destination and alternate, and decide whether continuing is feasible.
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Use single-engine figures from the failure point.
- Drift-down FL360 to FL240: allow 600 kg.
- Destination: 640 nm / 390 kt = 1.64 h; 1.64 × 2,350 = 3,856 kg.
- Alternate: 140 nm / 370 kt = 0.38 h; 0.38 × 2,350 = 889 kg.
- Contingency: 5% of (3,856 + 889) = 237 kg.
- Final reserve: 30 min at 1,400 kg/h = 700 kg.
- Approach and landing: 250 kg.
Total = 600 + 3,856 + 889 + 237 + 700 + 250 = 6,532 kg. Fuel available is 8,400 kg, leaving about 1,868 kg (about 48 min at 2,350 kg/h). Continuing to destination is feasible on fuel, provided the destination weather and single-engine approach minima remain acceptable.
2. Explain the obstacle-clearance check during drift-down and whether FL240 is acceptable with MORA 21,500 ft and MSA 22,000 ft.
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At FL240 the aircraft is 2,500 ft above MORA 21,500 ft and 2,000 ft above grid MSA 22,000 ft. MSA provides terrain/obstacle clearance within 25 nm of the navaid, normally 1,000 ft (2,000 ft in mountainous terrain); MORA provides similar clearance. Being above MSA is therefore a reasonable first check, but MSA is not a route clearance. The drift-down net flight path must clear each obstacle by the required margin (1,000 ft, or 2,000 ft in designated mountainous terrain), so the obstacle data along the actual track must be checked. If the aircraft cannot hold FL240 and drifts lower, or if terrain rises above the MSA, a route change or diversion is required. On these figures FL240 is acceptable.
3. What operational factors could make the closer airport the better choice despite the fuel available for destination?
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Fuel to the closer airport: 210 nm / 390 kt = 0.54 h; 0.54 × 2,350 = 1,269 kg, plus 600 kg drift-down, 700 kg final reserve, 250 kg approach and about 63 kg contingency = about 2,882 kg. That leaves over 5,500 kg, so fuel is not the deciding factor. The closer airport may be better if destination weather is doubtful, if a single-engine approach at destination is not suitable, or if a passenger or medical diversion is needed. Continuing to destination is reasonable if its weather is good and engineering support is available; the closer airport has no engineering support and a shorter runway, so it is a contingency, not an automatic choice.