# Which cruise altitude is prefered in engine-out operations of Boeing airplanes?

On Engine Out Operation in Boeing Airplanes, which altitude is preferable, the OPT Altitude, the MAX Altitude or the RECOM Altitude of the ENG OUT CRZ Page?

You'd typically drift down to your max single-engine altitude, which means essentially that max continuous power on the operating engine is sufficient for level flight at that altitude. Higher than that, you have a slow descent going in order for the thrust you have to maintain the recommended driftdown airspeed.

In general, there wouldn't be any advantage to descending lower than this until you need to do so for approach and landing, so you'd probably remain at that max altitude. As you burn off fuel, your max engine-out altitude would slowly increase so (hypothetically) you could actually step-climb up a little bit if you were going to be at that new Single Engine cruise altitude long enough, but in practice you'd need to lose the engine a LONG way from landing before the idea of step-climbing up to a higher SE cruise altitude would make practical sense.

The "Max" altitude is only reachable with the thrust of 2 engines, and the "OPT" altitude is probably well above what your max SE altitude is anyway -- it's optimum for cruising with both engines operating, after all. Granted, a sufficiently high Cost Index and/or strange winds can pull the Optimum altitude down, but in 99% of the cases, Optimum is going to be well above Max Single Engine altitude.

• max single-engine altitude - in a 747? ;) – Simon Jul 24 '15 at 22:16
• Yeah, it's not un-heard-of for a 747 in an ultra-long-haul op to step or cruise climb on 3 engines. – UnrecognizedFallingObject Jul 24 '15 at 22:23
• @UnrecognizedFallingObject My (jokey) point was that Ralphs answer only addresses twins. It ignores the 747 when the OP asks for "Boeing". – Simon Jul 24 '15 at 22:55
• @Houba In the case of having lost an engine, you will have declared an emergency so you'll probably be cleared for whatever altitude or block of altitude you need. You may well also turn out of the tracks so that you can petform the gradual driftdown descent clear of traffic. But "not descending" is hardly an option regardless of ATC when that motor quits. You're correct that temp & winds are used in the FMC calculations that give you ECON + driftdown speeds & the various altitudes. – Ralph J Jul 25 '15 at 12:16
• @RalphJ The difference between the OPT and the RECMD Altitude for Single Engine and All Engine is predicted wind. I worked with some airplanes with the Honeywell Pegasus FMC, and you can uplink or entered wind prediction for each waypoint, on the Active Flight Plan, with 4 different altitudes for each waypoint. WIth that information the FMC calculates the recommended Altitude based on wind. – aiglesiasn Jul 28 '15 at 15:45