CMEL - 80% loss of performance when losing an engine

CC268

CC268

Final Approach
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CC268
I plan to talk to my CFI about this more, but I was reading about how a loss of an engine equates to about a ~80% decrease in performance. So you've lost 50% of your power, but about 80% of your performance.

Here is my layman explanation of why that is. Let me know if this is completely wrong.

"Most of the power of one engine is being used to simply maintain level flight. The other engine contains most the extra power/thrust required to climb. So, when you lose an engine, you've lost the majority of your climb ability."

Any feedback/help is appreciated. Thanks.
 
Drag! Rudder deflection, angle of attack, windmilling propeller,
 
How about this for oversimplification...it takes X horsepower to maintain level flight. Everything above that is available for climb. With one engine inop, we lose 80% of what’s “above that”.

Just to put some made-up numbers on it...you have two engines, each of which produce 200 hp. It takes 150 to maintain level flight. That leaves 250 for climb. Lose one engine, only 50 hp is available for climb. 80% loss.

Of course, the factors that @Grum.Man noted make it much more complex than a simple horsepower equation.

Note that while making up numbers like this, the AFH says you can lose “up to 80% or more” performance decrease, or something equally committing. ;)
 
That's why the skymaster and the Adam were such a great idea ;-)
 
Plus the crap in the pants changes the w&b. (Single thinker)
 
80 seems a little high depending on the plane, but more or less I’ll buy it, especially in trainer twins
 
As has been mentioned already, when they say 80pct losses, they are addressing surplus power, not total. The power required to maintain level flight with two engines operating is a sunk cost. No free lunch in life. Tankering a second engine is worse than tankering fuel.

The figure is probably a good approximation for underpowered twins, of which most trainers belong to.
 
80% is probably a good figure for a lot of jets, too...for a lot of the high, hot, heavy performance scenarios that we train, we can make in excess of a 20% climb gradient on two engines, but take one away and we can struggle to make 3-4%.
 
So the extra losses are because of asymmetric thrust. You have all the thrust a few feet off of centerline, and extra drag from the prop on the other side. So this is creating quite a bit of moment to create yaw. This has to be offset by control surfaces, in particular the rudder. So you are now in a fairly dirty configuration with the rudder practically acting as a flap, in terms of creating drag. And that the rudder now has much reduced ability to create yaw towards the live engine, you now have a substantial impact to maneuverability.
 
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Welcome to Twin Time: Multi Engine Training Kindle Edition
Link: http://a.co/d/injZua6
 
I inserted a word to make it easier to accept when studying for the test. "80% loss of climb performance".
 
Ben Diss said:
I inserted a word to make it easier to accept when studying for the test. "80% loss of climb performance".
Click to expand...

That may make the statement inaccurate. Some twins may not have any climb performance on one engine.
 
midwestpa24 said:
That may make the statement inaccurate. Some twins may not have any climb performance on one engine.
Click to expand...

I was referring to an FAA test. It's not supposed to be accurate.
 
Here's what the FAA actually says in the AFH...
Sea level rate of climb performance losses of at least 80 to 90 percent, even under ideal circumstances, are typical for multiengine airplanes in OEI flight.
Click to expand...
 
Grum.Man said:
Drag! Rudder deflection, angle of attack, windmilling propeller,
Click to expand...

Not a twin driver, but I thought proper technique with an engine out was a slight coordinated bank into the good engine? And while there is still some drag, the failed prop is feathered, not windmilling, isn’t it?
 
Blatham489 said:
Not a twin driver, but I thought proper technique with an engine out was a slight coordinated bank into the good engine? And while there is still some drag, the failed prop is feathered, not windmilling, isn’t it?
Click to expand...

That is correct but bank alone will not overcome the dead engine, it still takes a ton of rudder. Also yes, the engine will eventually be feathered but it doesn't explicitly state at what point you have 80% loss in performance. During the initial engine failure and during the process of identifying the dead engine propeller will be windmilling. Not until you get get to the secure part of the check list does the prop get feathered. When caught off guard in a real emergency it can take a surprising long time to get to the feather stage.
 
80% is rough. Sometimes it’s quite a bit higher. I believe some senecas (1 maybe) have closer to 90% loss.

Remember this sage advice. If in a light twin:
1. Plan on the aircraft unable to maintain altitude. So start looking for a close alternate and go straight there.
2. Fly the airplane always. Don’t get caught up on all the procedures until you are firmly controlling the plane. Basically no point in feathering if you’ve lost control and are going to die. So fly first feather second.
3. The operative engine will take you to the scene of the crash. Accept this. Because if it does happen, that’s a bad time to be thinking about “saving” the plane. Let insurance take care of that. Accept the crash now and learn about surviving and paying up your insurance. So if it happens you can focus.
4. DO NOT attempt a singe engine go around in a light twin. Hell its hard to do in a powerful jet with resources. In a piston twin your gonna botch it. Just accept the outcome of a botched approach. That will be in your favor more than a botched SE go around.

Not the best explanation but you can figure it out. Pilots are smart right?
 
Blatham489 said:
Not a twin driver, but I thought proper technique with an engine out was a slight coordinated bank into the good engine? And while there is still some drag, the failed prop is feathered, not windmilling, isn’t it?
Click to expand...
Asymmetric thrust requires rudder defection, which is drag. The amount of rudder deflection is reduced by (for certification purposes) up to 5 degrees of bank into the good engine, which is not generally optimum performance...that usually happens at 2-3degrees bank for light twins. You can actually get to the point where the bank will overcome the asymmetric thrust without rudder input, but that’s usually upwards of 7-8 degrees bank, and is VERY uncomfortable.
 
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