Does an airplane fly less or more efficiently after a mid-flight engine failure?

Question

When a multi-engine airplane loses an engine mid-flight, does it consume more or less fuel per distance travelled during the remainder of the flight with fewer engines operative? In other words, is the maximum range extended or reduced?

And for what reason (if that's an answerable question)?

As far as cars are concerned, 4-wheel-drive is less efficient than front-wheel-drive, see:

• https://engineering.stackexchange.com/questions/26704/is-it-true-that-four-wheel-drive-is-less-efficient-and-if-so-why
• https://mechanics.stackexchange.com/questions/48012/where-is-extra-fuel-consumption-in-rwd-and-awd-cars-dissipated

I am basically asking the same question for airplanes.

In case it matters, please distinguish between aircraft engine types in your answer.

Answer 1

In almost all cases, the maximum range is reduced.

This is because the in-operative engine causes substantial drag. In addition, it causes an asymmetric flight condition that requires deflection of control surfaces and flying at an unusual attitude -- all which cause more drag than nominal cruise flight.

It is not entirely an apples-to-apples comparison because the damaged aircraft will likely also reduce its flight speed. Drag will be reduced, but flight time will also be increased. Most commercial aircraft strive to fly at slightly faster than the best range speed. Fuel efficiency (pounds per mile) is very important, but we'll take a small penalty because we also value our time (get there fast). A common rule is to fly beyond the best range speed at a speed where your fuel efficiency is 99% of the best efficiency.

So, this means that an aircraft in an emergency could get a very small efficiency improvement by slowing down slightly. Not enough to overcome the added drag.

There were some propeller aircraft that could increase their range by shutting down one or more engines. They could feather their propeller to minimize the drag of the non-operating engine (jet engines don't have this option). And then they could slow down as well.

There are also some three-engine helicopters (think Coast Guard Rescue) that shut down one engine in cruise and then start it back up when they get to the rescue site.

In cruise, with three engines operating, they operate at a relatively low throttle setting -- where they aren't as efficient as they could be. So, by shutting one down, the other two can operate at a more efficient point. This is particularly true when they're flying at a best endurance point vs. a best range point (this is slower and lower throttle).

In cruise, if one engine were to fail, you have time, power, and margin to get the third engine fired up. As you get to the rescue site (or when you find the rescue site), you start up the third engine to maximize redundancy during the rescue hover -- where you have no time or altitude for a failure.

Answer 2

Fuel consumption for OEI is significantly higher per mile.

Refer to this question for an example of fuel flow: Does one-engine cruise consume less fuel than two-engine cruise?

There's an 8% reduction in fuel flow per hour, but a 16% loss in airspeed, and that's assuming the same altitude. The altitude for single engine cruise will be lower, which makes matters worse.

It's in part the extra fuel reserve required for OEI operation that determines ETOPS range limits.

Answer 3

In a vehicle with 4WD, you have one engine driving all the wheels. This drivetrain contains gears, universal joints etc. that have friction. In this case, disconnecting drive to two of the wheels reduces friction which reduces fuel consumption.

In an airplane, each engine is (generally) a self-contained unit. You don't have one engine driving multiple propellers. You do reduce friction losses in the engine itself when you shut down an engine, but those savings tend to be smaller than the extra drag you produce by having an unpowered propeller or fan in the airstream.

There are exceptions:

1. the HS Nimrod (a 4-engined maritime patrol aircraft) could be operated on two engines. The aircraft would be flown at low speeds during the patrol, and at those speeds it was more efficient to have two engines running at full throttle, instead of running four engines near idle.

2. The Fairey Gannet used the same principle. It had a Double Mamba turboshaft (i.e. two Mamba engines in a single casing) driving contra-rotating propellers. One of the engines could be shut down. Again, this was used for maritime patrol where it had to operate at low speeds for long periods.