OEI Performance


So you're flying along in your big, bad new twin and, oops, one engine suddenly gives up the ghost. How much of the airplane's power have you lost? 50%? Right. How much of the airplane's climb performance have you lost? 50% again? Bzzzt! The correct answer is between 80-90%. Here's an example to illustrate.

Imagine a twin with two engines each producing 200 horsepower. Assume total level-flight thrust horsepower required is 175. With both engines operating normally, there is an excess of 225 hp. When one engine is inoperative, that excess thrust is reduced from 225hp to 25hp, or an 88% reduction.

Here's an illustration showing the excess power available with both one and two engines working. Remember, climb is determined by excess power over power required.

power

Many people are surprised to learn that there is no requirement for a light twin (6,000 lbs or less with a stall speed of 61 knots or less) to maintain altitude or climb in any configuration with one engine inoperative. The single-engine rate of climb at 5,000 feet MSL must simply be determined, it does not have to be a positive number!

It is, therefore, very important to check the performance charts for your airplane with a given set of conditions (density altitude, weight, etc) before launching in order to fully understand the airplane's performance should an engine be lost in a given phase of flight.

On that topic, allow me to briefly mention service ceilings applicable to multi-engine planes:

Service Ceilings (Both Engines):
1) All-engine service ceiling - reached when aircraft climbs at 100 fpm
2) Absolute service ceiling - when aircraft can no longer climb

Service Ceilings (Single Engine):
1) Single-engine service ceiling - reached when aircraft climbs at 50 fpm
2) Absolute service ceiling - when aircraft can no longer climb