Engine Failure in Cruise: Driftdown versus MSA
Twin-engine jet at mass 62,000 kg cruising FL350 in ISA+10. The right engine fails. OEI driftdown data from the AFM, net gradients, are: FL350 minus 1.6%, FL330 minus 1.0%, FL310 minus 0.4%, FL290 plus 0.2%. The airway MSA is FL300 and high ground of 28,500 ft lies within 30 NM ahead. Driftdown speed gives 420 kt TAS and GS. Fuel remaining is 8,400 kg; diversion fuel to a lower-MSA airport is 4,200 kg; final reserve is 1,500 kg; 45 minutes holding at FL150 burns 1,200 kg. A lower airway with MSA FL220 lies 25 NM left of track. ATC has not yet issued a descent. The crew is in IMC and the terrain warning database is current.
1. Determine the driftdown level-off altitude and whether the aircraft can maintain the airway MSA of FL300. Show your reasoning.
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Interpolate where the net gradient is zero: at FL310 it is minus 0.4%, at FL290 plus 0.2%, so zero occurs about FL297, one-third of the way from FL310 to FL290. The driftdown level-off altitude is therefore about FL297, not FL300. The airway MSA is FL300, so the aircraft cannot maintain MSA by about 300 ft. Terrain is 28,500 ft, so the level-off altitude still gives about 1,200 ft above the high ground, but that does not satisfy the published MSA in IMC. The crew must not simply level at FL297 and continue on the airway; they need a lower MSA route or a turn away from high ground.
2. What immediate actions and ATC requests should the crew make?
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Fly the driftdown speed, secure the failed engine, and declare an emergency or urgency to ATC. Request an immediate descent and a turn to the lower airway with MSA FL220, which is 25 NM left of track. If ATC cannot clear the turn immediately, state the MSA problem and ask for priority; do not wait until level-off. Monitor terrain, fuel and engine parameters. Consider the diversion airport if the lower airway is not quickly available. Do not attempt to hold altitude at the expense of speed or stall margin.
3. Why must the crew use net rather than gross driftdown gradients?
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Use net gradients, not gross. Net performance already includes a degradation allowance for pilot technique, turbulence and aircraft variation, and it is the value that must clear terrain and MSA. Gross gradient would be more optimistic and could mislead the crew into thinking the aircraft can maintain FL300. The driftdown level-off and terrain clearance must be based on the net data; if the net path cannot maintain MSA, the crew must change route or altitude. Fuel and diversion decisions should use the net descent profile and the resulting time to the lower airway.