Performance Instrumentation for Multiengine Safety

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Performance Instrumentation for Multiengine Safety

A. W. Hoadley

Many accidents in multiengine aircraft are caused by the pilot's inability to retain control and/or obtain maximum performance when an engine fails. In order to maintain control of the aircraft, the pilot must not allow the aircraft to stall with the asymmetric thrust caused by a failed engine. The strong yawing moment that results, coupled with the stall, will most likely result in a spin. Several training accidents have been caused by demonstrating Vmc (minimum controllable airspeed) at a sufficiently high altitude, where the lower engine thrust causes the Vmc to be below the stall speed of the aircraft. When the aircraft stalls before Vmc is reached, an unrecoverable spin can occur. Instrumentation designed to give the pilot a continuous display of the aircraft's stall margin (the percent of lift coefficient available but not being used) allows the pilot to know where the aircraft is relative to stall, regardless of its load factor, weight, or configuration. The same technology can be used to detect the sideslip of an aircraft with asymmetric thrust. An aircraft's best single engine performance is obtained when the sideslip is zero and the stall margin for best climb is flown. During an engine out emergency, the pilot would control the stall margin with pitch changes and the sideslip by adjusting the bank angle, while using the rudder to maintain directional control. The theory and hardware necessary to provide both stall margin and sideslip indications are discussed.