Something that may have jumped out at you early in multiengine ground school is this quote in the AFH 13-1:
Multiengine and single-engine airplanes operate differently during an engine failure. In a multiengine airplane, loss of thrust from one engine affects both performance and control. The most obvious problem is the loss of 50 percent of power, which reduces climb performance 80 to 90 percent. In some cases after an engine failure, the ability to climb or maintain altitude in a light-twin may not exist. After an engine failure, asymmetrical thrust also creates control issues for the pilot. Attention to both these factors is crucial to safe OEI flight.
So how do we get such a big performance drop from a 50% power cut? The answer lies in large part in what I like to call the “gravity tax:” to fight gravity, you need a basic power input to provide enough forward movement to get sufficient lift to counteract the weight of the aircraft. If I give you a Seminole in the traffic pattern with both engines out, you won’t be starting from zero, you’ll be starting from -1000 FPM (I’m rounding numbers here for illustration purposes). That big red box there is your gravity tax:
Multiengine and single-engine airplanes operate differently during an engine failure. In a multiengine airplane, loss of thrust from one engine affects both performance and control. The most obvious problem is the loss of 50 percent of power, which reduces climb performance 80 to 90 percent. In some cases after an engine failure, the ability to climb or maintain altitude in a light-twin may not exist. After an engine failure, asymmetrical thrust also creates control issues for the pilot. Attention to both these factors is crucial to safe OEI flight.
So how do we get such a big performance drop from a 50% power cut? The answer lies in large part in what I like to call the “gravity tax:” to fight gravity, you need a basic power input to provide enough forward movement to get sufficient lift to counteract the weight of the aircraft. If I give you a Seminole in the traffic pattern with both engines out, you won’t be starting from zero, you’ll be starting from -1000 FPM (I’m rounding numbers here for illustration purposes). That big red box there is your gravity tax:
Now let’s say we throw in a single engine that’s worth +1200 FPM: you add the red -1000 GT and blue +1200 E1 and you get a net +200 FPM CLB1, so now you’re barely eking out a climb:
Now let’s say we toss in a second engine, E2 for another +1200 FPM. Now our net climb is +1400:
That second engine is essentially pure climb performance, because we’ve already paid the gravity tax with the first engine. In essence, we have a “breakeven engine” and a “climb engine.” If we rearrange those climb and descent blocks and put them next to one another, we can see the attribution of each engine’s work: the first one fights gravity and then provides a bit of climb, while the second is pure climb power. If you take that second engine away, you lose 1200 FPM of your total 1400 FPM climb, for a total loss of 1200/1400=86%:
Now let’s say we climb a couple thousand feet and our power drops off a bit. Now the first engine is breaking even or even negative, while the second is the climb engine:
You only need about 17% (1000/1200) performance penalty to completely erase OEI climb performance, which is why your single-engine climb rate becomes negligible a few thousand feet up, depending on temperature. This becomes important when planning approaches: if you lose an engine near your FAF intercept altitude, the windmill drag will make your climb performance net negative, meaning you have little or no time to troubleshoot before you’ve drifted too low. Plan and brief this ahead of time.
high-performance airplanes
When you fly airplanes with more performance, you still pay the gravity tax, but the percentage of climb performance lost when OEI is less than the 85% in light twins. That said, it’s always going to be more than 50%, because the gravity tax eats into the first engine’s performance:
V1 cuts and departure cuts still require a prompt rudder application, verification of reserve thrust, and attention to profile, but, depending on the type, there’s typically less urgency to complete the OEI drill in a few seconds or risk going into the trees.