Light Twin Engine-Out Flying and Performance

A light twin is a small piston-engined airplane with an engine on each wing; the Piper Seminole and the Diamond DA42 are typical. It is the airplane of the multi-engine class rating, the MEP rating in Europe, and of much commercial flying in performance Class B: propeller airplanes with no more than nine passenger seats and a maximum takeoff mass of 5,700 kg or less. Its second engine is a real safety margin, but only for a pilot who knows how to use it. After a failure the airplane yaws and rolls toward the dead engine, loses most of its climb and, in some conditions, cannot climb at all.

This article covers what a light twin can still do with one engine inoperative (OEI). The aerodynamics of the yaw and roll are explained in asymmetric flight, and the certification of VMCA in minimum control speeds.

Engine failure in a light twin

Climb depends on excess power, the power available minus the power needed for level flight. When one of two engines fails, half the power available goes but almost none of the power required, so most of the excess goes with it. EASA exam material puts the loss of climb performance of a typical light twin at about 80 percent or more. The failed engine then takes more: a windmilling propeller is a large drag on the dead side, and the deflected rudder and any sideslip add their own.

The failure is most dangerous just after takeoff, when the airplane is slow, at high power and close to the ground. The Class A rules for transport airplanes consider an engine failure at every stage of flight, but the rules for multi-engined Class B airplanes consider one only above 300 ft above the takeoff surface (see performance classes and data). On a conventional twin, with both propellers turning clockwise as seen from the cockpit, the left engine is the critical engine, the one whose loss gives the larger yawing moment.

Identify, verify, feather

Drills differ between types and the flight manual always governs, but they share one order: fly the airplane, restore power, reduce drag, then find and feather the failed engine. The first steps need no identification; the last cannot quickly be undone. A failure after takeoff leaves no time for analysis, so the drill is practiced until it is a trained, rule-based response.

The engine gauges can mislead: the governor of a windmilling constant-speed propeller fines its blades to hold the selected rpm, so the failed engine's tachometer may look normal. The rudder pedals give the quicker answer, and closing the suspected throttle confirms it while the action can still be reversed.

Feathering follows at once. A propeller windmilling at a fine blade angle gives the most drag, a stopped one with flat blades less, and a feathered one, its blades edge-on to the airflow, the least. Light twin propellers are designed to fail toward feather: oil pressure holds the blades away from it, and selecting feather releases that oil so that a spring and the blade counterweights drive them fully coarse. Centrifugal latch pins, which keep the blades out of feather during a normal shutdown on the ground, engage once the rpm has decayed, so a propeller left to run down may no longer feather.

Afterward one alternator carries the whole electrical load, and the crossfeed, normally allowed only in level cruise flight, lets the live engine use the fuel on the dead side. A propeller synchronizer is switched off, because it would chase the failing engine's rpm.

VMC, VYSE and VXSE

Four speeds frame engine-out flying in a light twin, two of them painted on its airspeed indicator:

VMCA, the red line, is a control limit, not a performance speed. It is established with maximum takeoff power on the live engine, the most unfavorable (normally aft) center of gravity, takeoff flap, gear up and the failed propeller windmilling, and allows up to 5° of bank toward the live engine and a rudder force of no more than 150 lbf. With normally aspirated engines it falls with altitude while the indicated stalling speed hardly changes, so at altitude a twin slowed with asymmetric power may stall before it loses directional control. If control is being lost, the pilot reduces power on the live engine, which restores control at once, and lowers the nose to regain speed. VSSE, which FAA material calls the safe, intentional one-engine-inoperative speed, is a training margin above VMCA.

VYSE, the blue line, is marked for maximum weight at sea level. It gives the greatest climb, or the smallest descent, per unit of time and keeps a margin above VMCA and the stall. VXSE is slower and gives the most height per distance flown; it is used only while an obstacle must be cleared.

Both give their best only without sideslip. With the wings level and the ball centered, only a sideslip toward the dead engine balances the rudder's side force, adding drag and raising VMCA. Banking about 2° to 3° toward the live engine, with the ball about a third to half of its width out toward it, gives zero sideslip and the best single-engine climb; the VMCA demonstration allows no more than 5°.

View full schematic

For Class B twins the takeoff speeds are built on VMC, which may not exceed 1.2 VS1. For a non-commuter twin certified to CS-23 up to Amendment 4, VR is at least 1.05 VMC and 1.1 VS1, and the takeoff safety speed at the 50 ft screen at least the higher of 1.1 VMC and 1.2 VS1. With a VMC of 65 kt and a VS1 of 58 kt, 1.1 VMC is 71.5 kt and 1.2 VS1 is 69.6 kt: controllability, not the stall margin, sets the screen speed.

Single-engine climb performance

The climb that remains is small, and a windmilling propeller, extended gear or flap, sideslip, extra mass and density altitude all eat into it; a normally aspirated engine loses about 3 percent of its power for every 1,000 ft of density altitude. Extended gear even lowers VMCA slightly, by adding keel area, but its drag costs climb the airplane may not have.

Certification has not always promised any single-engine climb. Under the former CS-23.67, still taught in the syllabus, a twin of 2,722 kg (6,000 lb) or less whose VS0 exceeded 61 kt had to show a one-engine-inoperative gradient of at least 1.5 percent at 5,000 ft pressure altitude, feathered and clean. With a VS0 of 61 kt or less the gradient only had to be determined, and it could be negative. CS-23 Amendment 5 (2017) replaced these rules with objective-based ones, but airplanes already certified keep their original basis.

For commercial air transport, CAT.POL.A.340 sets minimum gradients for multi-engined Class B airplanes:

The takeoff flight path for obstacle clearance (CAT.POL.A.310) takes the all-engines gradient as 0.77 times its gross value up to the point where visual reference is expected to be lost and the engine is assumed to fail, then the one-engine-inoperative gradient up to 1,500 ft. Obstacles must be cleared by at least 50 ft vertically, or by a set horizontal distance.

Single-engine service ceiling

The single-engine service ceiling is the altitude at which the airplane, at maximum weight, can still climb at 50 ft/min with one engine inoperative; at the single-engine absolute ceiling the rate of climb is zero. The all-engines service ceiling uses 100 ft/min. EASA texts give these ceilings as pressure altitudes, while FAA material and some flight manuals, the Seminole's among them, give the single-engine service ceiling as a density altitude.

A twin whose single-engine service ceiling is 5,000 ft cannot hold a minimum en-route altitude that works out at 7,500 ft density altitude: after a failure it drifts down until the live engine's power balances the drag. In the cruise the pilot feathers, sets maximum continuous power, drifts down at the flight manual speed to the one-engine-inoperative ceiling and heads for an aerodrome within reach.

For Class B commercial flights, CAT.POL.A.315 makes this a route requirement: after an engine failure the airplane must be able to continue, at or above the minimum safe altitudes, to a point 1,000 ft above an aerodrome where it can land. The calculation starts no higher than the altitude at which the all-engines rate of climb is 300 ft/min at maximum continuous power, and uses the one-engine-inoperative climb gradient reduced by 0.5 percent, or the descent gradient increased by 0.5 percent (see one-engine-inoperative en-route performance).

Takeoff planning and the accelerate-stop decision

The accelerate-stop distance is the distance needed to accelerate to the speed at which an engine failure is assumed to be recognized, then stop. The aerodrome declares the accelerate-stop distance available (ASDA): the takeoff run available plus any stopway. A tailwind lengthens the distance needed, and so does a high density altitude, which raises the groundspeed and weakens a normally aspirated engine.

For most light twins this distance is not certified. Under CS-23 up to Amendment 4, the certification basis of most light airplanes in service, airplanes below the commuter category have no accelerate-stop requirement, so any flight manual figures are advisory and unfactored. Only commuter-category twins, with up to 19 passenger seats and 8,618 kg, have one, defined as for large airplanes. Planning with advisory data should credit no more than 50 percent of the headwind and at least 150 percent of any tailwind.

The Class B field length rule factors the gross takeoff distance. With no stopway or clearway, 1.25 times it may not exceed the takeoff run available, so a 480 m takeoff distance needs 600 m of runway. Where a stopway or clearway is used, the distance itself may not exceed the takeoff run available, 1.15 times it the takeoff distance available, and 1.3 times it the ASDA. None of this guarantees that a twin losing an engine near lift-off can either stop or climb away, so the decision is briefed before every takeoff. The FAA Safety Team's twin takeoff briefing card sets it out in this order:

Engine-out approach and landing

An engine shut down while the airplane holds its level comfortably is an urgency case, PAN PAN; if it cannot stay clear of terrain, the call is MAYDAY (see distress and urgency communications).

The single-engine approach is planned to avoid a go-around. The Class B one-engine-inoperative missed approach gradient, 0.75 percent at 1,500 ft, assumes the failed propeller feathered and gear and flaps up; the 2.5 percent baulked landing gradient with gear down and landing flap is for all engines operating. With gear and full flap down on one engine the airplane may not climb at all, so final flap is left until the landing is assured. On a twin whose gear is lowered by an electric hydraulic pump, the electrical load is reduced before the gear goes down, since one alternator now carries it all.

As power is reduced for the descent and flare, the live engine's yawing moment falls, so rudder trim and pressure come off progressively. The speed stays well clear of VMCA throughout, since a late go-around would demand full power on one engine at low speed. The wider procedures are in engine failure and engine fire.

Library articles are written for study and exam preparation. They do not replace your aircraft's approved documentation, your operator's procedures or the regulations themselves.