WW2 Bombers continuing on one of 2 or 4 engines, how would that work?

Question

I think I recall stories of damaged bombers making it home with only one of the original 2 or 4 engines functioning. But I visualize the plane being severely unbalanced if a single engine was propelling the plane and then simply going in circles or, worse, a tight spin.

My only guess as to how such a handicapped plane would be able to successfully fly is that the unbalance was somehow compensated for using the rudder and/or other control devices. But even then, it must have been pretty dicey and the plane might have trouble keeping up air speed sufficient to stay aloft.

Of course, perhaps my recollection is wrong and only a plane with a single non-functional engine out of 4 could actually fly successfully and if a two-engine plane lost one engine, then the plane would not be functional.

I wonder if modern two-engine jets somehow can fly on one engine and I vaguely recall such cases.

Answer 1

The typical case is that twin-engine airplanes can absolutely fly with one engine shut down. This being possible means that the extra engine provides redundancy; if such an airplane couldn't fly in that condition, the second engine would double the chance of a flight-ending engine failure occurring.

In order to keep the airplane controllable when one engine is at full power and the other is shut down, designers take steps like keeping the engines fairly close to the fuselage so that the "leverage" -- the moment arm -- of the asymmetric thrust is reduced. They also ensure that the tail surface and rudder -- which provides the force to counteract the yawing moment caused by the unbalanced forces -- is large enough to do so under any expected conditions.

From a pilot's perspective, the greater the airspeed, the more control authority the rudder provides, so in an engine-out case, you want to keep the airspeed high enough that your rudder input can balance out the asymmetric thrust. It's also possible to add a little bank toward the operating engine, which also helps to balance things so the airplane flies straight ahead rather than turning continuously toward the failed engine.

For a 4-engine aircraft that's lost 3 engines, the additional concern is a lack of power to keep the airplane aloft. Depending on the weight of the aircraft, a single engine might keep the airplane in level flight without losing airspeed, but more probably some amount of descent would be required to avoid losing speed (and, eventually, once the speed decayed enough, losing control as well). How far you can get would depend on all sorts of factors, such as how heavy you were to start with, how much altitude you have to work with, if the winds are pushing you along, how much other damage is adding drag to the aircraft, and so on. Maybe the B-17 can't get all the way back to England with 3 of 4 engines shot out, but if they can get to France, or to the English Channel, the odds of avoiding capture after bailing out get better.

If you had a light enough 747 or A380, you might maintain level flight with one engine, but at most weights you'd probably be trading altitude for airspeed. With two of four engines operating, you could hold level flight at some significantly higher weight, though probably well below max takeoff weight. Any 4-engine aircraft should be able to fly fine on 3 engines, since otherwise you'd be at the point where any (single) engine failure on takeoff is unsurvivable, and that's not an acceptable scenario.

Altitude is also a factor; losing an engine up high near the service ceiling may mean a "drift down" to an altitude at which max continuous power on the remaining engine(s) will hold level flight + maintain airspeed. If that altitude is above the terrain, then all is good; if it isn't then the crew will need to use the time & distance available while drifting down to head toward lower terrain, or (worst case) toward wherever they prefer for the forced landing to take place.

The max load you can carry might have to be reduced so that you aren't too heavy to keep flying with one engine out; pilots regularly practice (in the simulator) what happens if an engine fails right at takeoff, just after the point you can no longer stop the aircraft on the runway. If you (and the aircraft) can fly out of that scenario, all other cases are less demanding. How much weight you can have at takeoff, based on engine-out climb performance (and other criteria) is something that gets computed based on the particular conditions of "this" flight.

Left to its own -- i.e. if the pilot didn't make any control inputs to counter the asymmetric thrust, then the aircraft would continuously turn into the failed engine, and things wouldn't end well. But, part of the training required to be able to fly a multi-engine aircraft is learning how to do so even with an engine failure. That's been the case basically since aircraft with more than one engine started flying, and certainly continues today with 2- and 4-engine airliners.

Answer 2

Yes, you can fly an airplane home on one engine on a twin very easily. Multi engine pilots are trained to do that in an emergency situation.

The difficulties in operating such an airplane on only one engine is you have very limited climb performance and airspeed control. An engine failure causes a loss of at least 50% of total thrust available to you and 80% of your climb and maneuvering performance. It may also be the case that a multi engine airplane cannot maintain altitude or climb at all on a single engine, even when flown at the best rate of climb speed for single engine operations, Vyse. In certain cases of an engine failure during cruise at higher altitudes, the airplane cannot maintain cruise altitude and is forced to drift down to at or below the airplane’s single engine service ceiling. If your flight plan was predicated upon twin engine operations to clear mountains or other terrain taller than your single engine service ceiling, An engine failure could be very hazardous as you cannot maintain an altitude high enough to clear terrain. In wartime, this could be hazardous if you are trying to clear airspace defended by anti-aircraft guns and an engine failure causes you to drift down within range of them.

The Tony Scott film Top Gun features an inaccuracy in the climatic dogfight sequence near the end. There is a scene where enemy MiGs jump Iceman’s six and score hits on his F-14 with cannon fire. Iceman’s RIO inform him that they were hit in the right engine to which Iceman responds by shutting down the affected engine and remains in the fight. In reality, his F-14 has now lost at least 50% of its total available thrust and at least 80% of its climb and maneuvering performance. In the real world, the only choice Ice would have is to bug out of the fight and see if he could nurse his damaged Tomcat back to the ship - if, that is, the MiGs will let him leave without jumping his six again and blowing him out of the sky.

The other hazard in the event of an engine control is airspeed. Operations at lower airspeeds are demanding and can be very hazardous if you cannot or do not have proper control over your airspeed in these situations. There is a speed in multi engine aircraft known as Vmca at which the tailfin and rudder can no longer maintain directional control if an engine is inoperative. Flight at speeds below that with the good engine at maximum power will result in loss of control. If the airspeed is properly maintained at or above Vmca, the pilot can still maintain directional control using the rudder, if they maintain a slight bank angle of not more than 5° away from the good engine. It is also worth noting that flight at or close to Vmca only offers directional control. It is NOT a safe maneuvering speed for the aircraft and accidents have occurred when pilots have attempted to aggressively maneuver an airplane around Vmca.

Takeoffs are also a very risky time for an engine failure at low air speed As an airplane is operating at full power/thrust while trying to gain airspeed. Loss of engine power in a twin can cause loss of control easily during the takeoff roll and once airborne if the pilot does not react quickly and correctly the counter the situation. The typical procedure in twins is to use a takeoff decision speed, V1, which is the maximum speed at which the pilot may abort the takeoff roll and stop the airplane with runway remaining using braking alone. It is also the minimum speed, at which should an engine suddenly become inoperative, a pilot may continue the takeoff on only one engine and achieve the takeoff safety speed, V2, at least 35 ft above the departure end of the runway. It is worth noting as well that the decision speed V1 is well above Vmca/Vmcg, thus assuring directional control in the event of an engine failure. Pilots train for an emergency procedure involving an engine loss right at V1. This is known as a V1 cut. In jets the resulting thrust asymmetry is not as severe as it is with propeller, driven aircraft and much easier to manage given the performance guarantees inherent to the design of transport category airplanes.

Answer 3

I flew the P-3 Orion for the US Navy which is a 4-engine turbo prop. On almost every training flight I ever flew on we would practice flying, landing, and stopping with two engines simulated to be shutdown (both on the same side). This was accomplished by setting a known power setting that simulated having the propellers feathered (no thrust, and minimal drag).

It was not easy. Maintaining sufficient air speed was critical, because as others have said here controllability is critical with all the thrust coming from one side of the plane, and it is very easy to get in a position that you can't recover from. It took a ton of rudder input, and even with rudder trim, it was tiring.

In the real world, the worst thing that I ever had to deal with was a 3-engine landing, and it was one of the inboard engines, so almost not noticeable. Even with perfect conditions, a 2-engines out (on the same wing) landing would have elevated my heart rate a bit.

Answer 4

Here's part of the training manual for a (4 engine) C54 Skymaster, a WWII transport plane: Part 2

If you think you may ever find yourself with only one engine left, study the sections of this book on ditchings and emergency landings carefully. The proper procedure in this case is to start looking for a clearing.

There is also advice on flying with 2 engines:

With two engines out in a lightly loaded aircraft, it is possible to maintain altitude.

And with 3 engines:

Use roughly 10% less air speed [...] Always turn away from a dead engine if practical [...]Trim for unequal power by relieving the rudder pressure with the rudder trim tab.

I immediately though of the Skymaster because that did happen to my ancestor. They came into Midway with just enough altitude for the wheels to clear the runway.