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What would happen if, during cruise at a high gross weight, one of these airplanes suffered a double engine failure:

  1. With one failed engine on each side (leaving one working engine on each wing)?
  2. With both failed engines on the same side (leaving the aircraft with two working engines on the left wing and none on the right wing, or vice versa)?

I know that airplanes with two engines can still fly with only one working engine (by using the rudder to counter the yawing moment created by asymmetric engine thrust, placing the aircraft into a sideslip).

Airbus A380 with four engines

Boeing 747-400 with four engines

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    $\begingroup$ I have a sneaking suspicion @Terry (a user on here) will be able to answer for the early 747 variants. Hopefully he sees this one. $\endgroup$ – Dave Jan 9 at 1:49
  • $\begingroup$ There should be no reason why they can't - any airplane can manage on 50% of thrust, but as with a twin jet with a single engine failure, half power isn't enough to keep cruising at 37000 ft and they'll have to descend to a lower altitude. It's done at a speed that gives the lowest descent rate and is known as a "driftdown". $\endgroup$ – John K Jan 9 at 2:02
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    $\begingroup$ @JohnK No, it's done the other way. You only assume a single engine failure, regardless of how many engines the plane has. If a plane needs x thrust to fly safely at some mass, and has n engines, then they put in engines with x/(n-1) thrust. This is why the A340 is famously lethargic, it has less of a margin over minimum thrust. It's also why the BAe 146 has 4 engines, the short-field performance is based on 3/4 of maximum thrust, not 1/2. $\endgroup$ – user71659 Jan 9 at 4:29
  • $\begingroup$ I realize the the performance assumptions are single point of failure not double, for the purpose of climb and altitude performance, just saying that a 4 engine jet should be able to stay in the air at some altitude on 2, like a piston twin can stay in the air on half its power but can barely climb. On the other hand, with the thrust line with 2 on one side running so far outboard, maybe there isn't enough rudder authority? $\endgroup$ – John K Jan 9 at 14:29
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    $\begingroup$ A single engine failure in a 4-engine aircraft is an "event" but not an emergency. A 2 engine failure is an emergency and the pilots will put the aircraft down at the earliest opportunity. Even a 2-engine out on one wing isn't an "unflyable" aircraft, but they aren't going to continue on with 2 engines out. USA Today "Ask a pilot" 2 engine failure and 747 2 engine crash $\endgroup$ – Ron Beyer Jan 9 at 15:51
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First, my experience was with 747-100/200 aircraft. However, I'm reasonably certain that the subsequent variants of the 747 would perform just as well, and I would be surprised if the A380 did not as well.

If during cruising with full load....

"Full load" is a little ambiguous. If what is meant is MTOW (max takeoff weight), note that by the time you reach cruising altitude, you're considerably lighter than MTOW.

But let's say you leveled at FL310 or thereabouts, and shortly thereafter two engines failed. Discounting catastrophic failures that leave things hanging out in the wind that generate a lot more drag than a simple inoperative windmilling engine, you should be able to fly as far as your fuel will take you, although common sense and the regulations would dictate finding the nearest suitable airport.

You won't be able to maintain your 4-engine cruising altitude. You'll drift down until you reach an altitude that you can maintain.

You'll be able to fly whether the two failures are both on one side or one on each side, but both on one side is the more problematical. We used to practice that in the sim. They always put both failures on one side to make it more interesting. The drag from the rudder necessary to counter the adverse yaw from the operating engines is significant, and as you start reducing the power for the landing, judging how much rudder to decrease and when is an interesting dance. You would have run in lots (maybe all) the rudder trim. How much of it should you take out and when. Take it all out too soon and your leg is going to get awfully tired.

While a double failure at cruise is handleable, it's easy to come up with scenarios that are not. If you had a double failure on takeoff just after V1, I doubt you'll make it. V1 and the balanced field length are predicated on one engine failing, not two.

Take a look at https://thepointsguy.com/2017/03/engines-boeing-a-747-can-fly-on/, the relevant part is:

After almost 15 minutes in a controlled gliding descent, the crew coaxed one engine back to life, allowing them to significantly reduce their rate of descent. A few minutes later, a second engine came back on line with partial power, allowing the aircraft to enter a slow climb.

It's also worth noting, I think, that when you're doing a 3-engine ferry in a 747, you're depending on it being able to continue flying on two in case of an engine failure during that ferry flight.

A final thought about the last part of your question:

controlling the rudder makes the airplane fly sideway.

To the best of my knowledge, the use of the proper amount of rudder to counter asymmetrical thrust does not make the airplane fly sideways. However, I am not an aerodynamicist. Perhaps one of the many here qualified in that area can comment.

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    $\begingroup$ You are right; with full rudder the aircraft will most likely fly at some sideslip angle, but still mostly ahead. The side force from the rudder needs to be balanced somehow, and the only effective surfaces that can do that are the wing (shallow bank, wing with live engines low) and the fuselage (in a sideslip). Since you get more yawing moment from the rudder when flying with some bank angle, I would expect that with two dead engines on one wing only banked flight will be trimmable. $\endgroup$ – Peter Kämpf Jan 9 at 20:34
  • $\begingroup$ "Full Load" mean that the aircraft is loaded at maximum load minus the consumed fuel. Or another word, when the airplane is loaded approaching their load. $\endgroup$ – AirCraft Lover Jan 10 at 13:30
  • $\begingroup$ Perhaps the best wording is sideslip as @PeterKämpf said. $\endgroup$ – AirCraft Lover Jan 10 at 14:02
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Terry already provided the practical side! A similar question was asked already, regarding the Airbus A380. Thanks to DeltaLima: https://aviation.stackexchange.com/a/11583/1084

Basically it is a requirement for certification, that a B747, A380 or A340 is still flyable with two engines off.

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