12.one-engine-inoperative-performance. One Engine Inoperative (OEI) Performance
When a multiengine airplane loses one engine, the remaining engine does not provide half of the airplane's normal performance — it provides far less. The loss of one of two engines results in a loss of approximately 80 percent of climb performance, not 50 percent. This counterintuitive figure is the single most important performance concept the multiengine pilot must internalize, and it explains why so many engine-failure accidents occur during takeoff, initial climb, and go-around.
Why the 80 Percent Figure
Climb performance depends on excess thrust horsepower — the power available beyond that required to maintain level flight. In cruise, an airplane uses only a fraction of total available power, so excess power (and therefore climb capability) is large relative to power required. When one engine fails:
- Total power available is cut in half (50 percent loss).
- Power required to maintain altitude actually increases because of the drag from the windmilling or feathered propeller, asymmetric thrust, deflected control surfaces (rudder, aileron), and the sideslip used to counter yaw.
- The remaining excess power — the only power that produces climb — collapses to roughly 20 percent of its two-engine value.
For example, an airplane with a 500 fpm two-engine rate of climb may achieve only about 100 fpm single-engine, and many light twins at gross weight on a hot day will achieve a negative single-engine climb gradient.
Service Ceilings
Multiengine performance charts list two distinct ceilings:
- All-engines service ceiling — the density altitude at which the airplane can still climb at 100 fpm with both engines operating.
- Single-engine service ceiling — the density altitude at which the airplane can climb at 50 fpm with the critical engine inoperative and feathered, the operating engine at full available power, gear and flaps retracted, and the airplane flown at V_YSE.
- Single-engine absolute ceiling — the altitude at which single-engine rate of climb is zero.
Above the single-engine service ceiling, the airplane will drift down to that altitude after an engine failure. Pilots flying near or above this altitude must understand that an engine failure means a controlled descent to a lower cruise altitude, terrain permitting.
Factors That Degrade OEI Performance
Single-engine climb is extraordinarily sensitive to configuration and technique. Each of the following can erase what little climb capability remains:
- Windmilling propeller on the failed engine (vs. feathered) — can reduce climb by 150–300 fpm.
- Landing gear extended — typically costs 150–250 fpm.
- Flaps extended — even a small flap setting can be the difference between climbing and descending.
- Bank angle — wings level or with insufficient bank into the operating engine increases drag and V_MC; a bank of approximately 2°–3° into the operating engine, combined with a half-ball rudder deflection toward the operating engine, minimizes drag and yields best performance.
- Airspeed deviation from V_YSE (blue line) — even 5 knots fast or slow significantly reduces climb.
- Density altitude, weight, and CG — each adversely affects performance just as in single-engine airplanes, only more so.
V-Speeds Associated With OEI Operations
- V_MC (red line) — minimum control speed with the critical engine inoperative. Below V_MC, directional control cannot be maintained.
- V_SSE — safe, intentional one-engine-inoperative speed, used by manufacturers for training engine cuts; provides margin above V_MC.
- V_XSE — best angle-of-climb speed with one engine inoperative (most altitude per horizontal distance).
- V_YSE (blue line) — best rate-of-climb speed with one engine inoperative (most altitude per unit time). This is the airspeed that yields the published single-engine climb rate.
The Decision
Because OEI climb performance is marginal at best, the multiengine pilot must make a continuous mental commitment during takeoff:
- Before reaching V_MC and a positive rate, an engine failure means close both throttles and land straight ahead.
- After liftoff with gear up, V_YSE, and positive climb, continued flight may be possible — but only with prompt identification, verification, and feathering of the failed engine, immediate cleanup of gear and flaps, and precise airspeed and bank control.
Sample OEI Climb Calculation
If a light twin's POH lists a two-engine rate of climb of 1,500 fpm and a single-engine rate of climb of 250 fpm at sea level, standard day, gross weight:
- Loss of climb performance = (1,500 − 250) / 1,500 = 83 percent.
At higher density altitudes the percentage worsens until single-engine climb becomes zero at the single-engine absolute ceiling. Pilots must consult performance charts for actual conditions — pressure altitude, temperature, weight — and never assume book numbers without proper configuration and technique.
Understanding that the airplane is essentially a single-engine airplane with a great deal of extra weight and drag following an engine failure is the foundation of safe multiengine operation.