How easy is it for a twin donked a/c flying on only one to fly a perfect hold at holding speed at FL70 up to FL150 turning either into or away from the shut off engine maintaining perfect level?
For a "light" twin (A319/20/21, B737) it shouldn't be a problem. The smaller aircraft are also certified to land at their max take-off weight so an immediated return is also possible. For a B777 or A330 it's a different matter. They will still be able to maintain a "positive flight path" but will be dumping fuel to improve performance and get below max landing weight. Obviously if they have an uncontained fire, they're going to return at more than MLW and pray everything works out.
An A320 can maintain about FL180 on one engine, so the hold at FL 70 is not an issue (apart from the fact that you're only on one engine
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A4