Multi-Engine Ground School Part 3: Performance and Limitations

All right, guys, I got a few questions for you.

So I'm going to ask the questions, give you a few seconds to think about it, and we're going to go through these, okay? Question and answers.

All right, question is, when we lose one engine on a multi-engine aircraft, How much does our performance get decreased by? When we lose a single engine, aircraft performance decreases by how much? And it should be noted that for what we're talking about right now, we're going to define the word performance as excess thrust or excess horsepower, okay? Basically, whatever extra power you have available to climb, all right, for any given situation.

So when one engine is lost, aircraft performance is decreased by how much? What would you say? 30%, 40%, 50%? 50% is usually a good go-to number, right? I mean, we got two engines.

We lose one of them.

Well, 50%, we just lost, right? In fact, we are losing between 80% and 90% of our performance when we lose one engine on a multi-engine aircraft.

Well, on a twin-engine aircraft, I should say.

Okay? Okay? 80 to 90%.

Well, why is that? Well, let's think about that.

We're basing our performance on excess horsepower, okay? Again, power that's available to allow us to climb.

Let's just take our Apache, for instance.

We have two Lycoming O360 engines on our Apache, okay? Each one has 180 horsepower.

That gives us a total horsepower of 360 between the two.

Now, these are just raw, rough numbers.

This is not published anywhere.

This is not necessarily even accurate.

But let's just say for a given situation, we need 160 of those horsepower just to maintain level flight.

in our current loading condition, our current density altitude, we're using 160 horsepower to maintain level flight.

If we have 360 horsepower available, well, that gives us 200 extra horsepower to be able to climb, do whatever we want, right? 200 horsepower worth of excess.

And that's our performance, okay? Now, we lose an engine, right? Left engine goes, whoops, bye-bye, see you left engine.

We just lost 180 horsepower.

Now we only have an additional, or we only have 180 left, right? We just lost half of our horsepower.

We have 180 left.

We need 160 of that to maintain level flight, which leaves us only 20, 20 horsepower for excess power, for excess performance, right? That just got decimated by 90%, from 200 down to 20, okay? So 90% loss of performance when we lose that one engine.

Of course, there's other considerations as well, the amount of drag that's now being produced by that engine, especially before we actually get the propeller feathered and all that, okay? So 80 to 90% loss of performance when we lose a single engine.