demisability

Demisability is the engineered capacity of an aircraft powerplant to be safely shut down and remain inert during flight without creating a hazard to the aircraft or crew. The term applies most directly to multi-engine aircraft where the loss of one engine need not be catastrophic. A demisable engine must be capable of being cleanly secured: fuel cut off, ignition disabled, and propeller feathered if applicable, with no risk of windmilling damage, fire, or unplanned restart.

The concept is rooted in certification requirements for twin and multi-engine aircraft. Regulatory authorities, including the FAA, mandate that an aircraft be controllable and maintain altitude on remaining engines if one powerplant fails or is shut down. Demisability ensures that intentional shutdown mid-flight, whether due to mechanical failure, fire, or system malfunction, does not introduce new failure modes. The engine must not seize, shed debris into the airframe, or allow fuel to pool in nacelles where it might ignite.

Design and Maintenance Implications

Achieving demisability requires specific design features: robust engine mounts that can handle asymmetric thrust, adequate cooling flow through feathered propellers, sealed fuel systems that prevent after-drip, and reliable ignition cutoff. Maintenance technicians verify demisability through ground runs and functional checks of fuel shutoff systems, propeller feathering mechanisms, and engine bearing lubrication during wind-down. Corrosion or mechanical slack in these systems degrades demisability and can ground an aircraft.

The term itself reflects aviation's precision language: "demise" here means controlled cessation, not destruction. An engine that cannot be safely shut down mid-flight, or that cannot safely remain shut down, is not demisable and presents an unacceptable risk profile. This distinction is especially critical in single-engine-inoperative (SEI) operations and emergency procedures.

Demisability is not a property of the engine alone but of the engine-airframe-system integration. A robust powerplant may fail to be demisable if the aircraft's fuel system allows carburettor ice to form after shutdown, or if nacelle design traps heat. Conversely, thoughtful system design can make even marginal engines sufficiently demisable for certification. It remains a cornerstone of multi-engine aircraft safety doctrine.