Flying your twin safely

by Donald Talleur

Flying light twin-engine airplanes safely is more than just a matter of handling one more engine and the extra systems that may be associated with it. Understanding concepts such as critical engine, minimal controllable airspeed (Vmca), and single engine performance are also important. While it is true that most light twin-engine airplanes are not significantly harder to fly than a high performance single, twins can become much harder to fly with the loss of an engine or other critical system.

The term "critical engine" elicits several different definitions depending on who you ask, but traditionally, we define it as the engine that, due to it’s failure, would have the most adverse affect on aircraft control and or performance. So, if we do not depart from tradition, the left engine on most twins would be the critical engine. The reason for this is simple. If the airplane has both clockwise rotating propellers, then the thrustline of each engine is to the right of each engine. This puts the left engine’s thrustline inboard between the engine and the fuselage and the right engine’s thrustline outboard between the engine and the right wingtip. If the left engine were to fail, the right engine creates a large yawing tendency due to effective thrust being farther out from the fuselage. A failure of the right engine would require less control effort since the left engine’s effective thrust is acting closer to the fuselage. Naturally, a twin with counter-rotating propellers, such as a Beechcraft Duchess or Piper Seminole, does not have critical engine when considering the traditional definition.

Not being one to question the conventions of our industry, I sheepishly promote the following definition for critical engine. Why not think of the critical engine not only as the engine that most adversely effects control but also as one that may adversely affect critical aircraft systems. Even though control issues may not arise during an engine failure, such as with the Beechcraft Duchess, it may still be important to consider what emergency situations may arise from inoperative systems as a result of the engine shutdown. Those of you that fly the Duchess are saying "but the Duchess has dual everything!" and you’re right. But not all twins are equipped so that the other systems continue to function after an engine failure. The Piper Aztec is one such example. On the Aztec, the hydraulic pump, which controls gear and flap extension, is on the left engine. Failure of the left engine or the hydraulic pump places the pilot in a difficult situation. Approximately 50 strokes of a manual pump handle are required to extend or retract the gear and 15 to activate the flaps. The pumping becomes increasingly difficult as the gear reach full extension or retraction. In this case, the left engine is not only critical in terms of control (during an engine failure) but also because of the extra systems emergencies created by an engine failure. Luckily, most manufacturers have gone to electrical driven hydraulic pumps that run independently of the airplane’s engines. It is important to consider what systems may be negatively affected by an engine failure in addition to understanding which engine failure will create control difficulties.

Vmca, or minimum control speed, single engine, is the minimum speed at which a pilot can expect to recover directional control of the airplane within 20 degrees of heading change and then maintain straight flight with not more than five degrees of bank when one engine fails. This speed is designated by a red radial line on the airspeed indicator. However, the specified Vmca speed has some conditions that go with it. First, it’s a sea-level value, airborne and out of ground effect. Secondly, both engines are producing full power just prior to a sudden engine failure. Thirdly, the centre of gravity is at it’s most rearward allowable position. Next, the flaps are set for takeoff, then the landing gear is retracted and, the dead engine’s propeller is windmilling or feathered if you have automatic feathering. This makes a worse case scenario for control should one engine fail, and since most of the time you will not have met all of these conditions, the actual Vmca will be lower than the red radial line. The reasons for the lower Vmca are straightforward. Most of us do not fly at sea level. As a result, lower air density means less engine performance and less thrust to yaw the aircraft should one engine die suddenly. The pilot may not be flying with full power. Anything less than full power will decrease Vmca. The C of G is not usually at its full aft position. Any position forward of full aft effectively lengthens the control arm or distance between the rudder and the point which the aircraft yaws about, subsequently increasing the rudder’s control effectiveness. Better effectiveness means you can fly to a slower speed and still maintain directional control. Flaps and landing gear, if extended, may have a stabilizing effect, once again lowering Vmca. If the engine failure is quickly and correctly diagnosed, the dead engine can be feathered quickly. Less drag on the dead engine side will lessen the need for rudder input to maintain directional control.

It is important to remember that at certain altitudes the aircraft will actually stall prior to reaching Vmca. As a result, it is critical to recover at the first sign of a stall since the simultaneous occurrence of a stall and Vmca loss of directional control could easily lead to a spin.

The last area I’d like to address is that of twin-engine performance. It is a well-known fact that the loss of one engine will produce a greater than 50 per cent loss of performance. Pilots also need to be aware that light twin-engine aircraft are not certified to maintain altitude after an engine failure. This means the pilot must have a plan of action in mind at all times in case of an untimely engine failure. Accelerate-stop performance is also of critical importance since you should not attempt to continue a takeoff after experiencing an engine failure. Likewise, should the engine fail after takeoff, knowing the single-engine climb performance will allow you to determine what your options are for continued flight. Just because one engine is still running does not mean that you will make it back to the airport. Be prepared for an off field landing at all times just like you would be if flying a single engine aircraft.

Twin-engine flying can be an enjoyable experience if you take time to prepare for the flight and exercise safe flying practices. Knowing your critical engine, understanding Vmca and single-engine performance are good steps towards that goal.