Single Engine Climb Performance

In general, when a twin engine aircraft experiences an engine failure, climb performance is reduced by 80 to 90 percent. This is because climb performance is based on excess power. Our Seneca has a total of 440 HP available. For example’s sake, let’s assume that the aircraft needs 40% of available power to maintain altitude at low cruise (175 HP). That leaves 60% available for climb (265 HP). Now if we loose an engine, that’s 220 HP no longer available, leaving 45 HP available for a climb. That’s about an 85% performance reduction.

According to Part 23 of the Federal Aviation Regulations, any reciprocating engine twin of more than 6,000 pounds max takeoff weight and/or Vso more than 61 knots must be able to demonstrate a single engine climb performance gradient of 1.5 percent. For aircraft type-certified before February 4, 1991 , the required climb rate is expressed in feet per minute of at least .027 Vso2.

This climb rate is determined under the following conditions:
  • 5,000 feet MSL @ standard temperature
  • Landing gear retracted
  • Flaps retracted
  • Propeller of inoperative engine feathered
  • Maximum power on operating engine
  • Zero sideslip condition
Since Vso on our Seneca is 64 knots, it is subject to meet this climb gradient requirement. We can figure out the vertical speed required mathematically by converting our airspeed in knots to feet per minute and then multiplying that result by the proportion of 1.5 feet vertical per 100 feet horizontal:

Keep in mind that this is with a maximum power, minimum drag configuration. If an engine failure at low altitude occurs, it is imperative that drag is reduced immediately.

Also, keep in mind that if an aircraft does not meet the requirements to be subject to demonstrate a single engine climb gradient, all the manufacturer has to do is determine what the single engine climb rate will be at 5,000 feet MSL. This number may be negative! Not all aircraft are required to climb at all on a single engine!
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