copters.com

Short Shaft Failure

Although “Short Shaft Failure” is generally used when referring to a turbine helicopter,
a similar failure mode is possible with some piston helicopters as well. This depends on
how engine power is transmitted to the main rotor transmission.

In a turbine helicopter, there is generally an output shaft from the engine which is
connected through a freewheeling unit to the main rotor transmission. The freewheeling
unit is there to allow the main rotor system to continue to spin if the engine stops.
However, a failure of the freewheeling unit, or of the driveshaft, or any other components
between the engine output and the main rotor transmission input could prevent the engine
from being able to turn the main rotor.

Generally the pilot would identify a short shaft failure by his engine RPM tachometer
showing a rise in RPM (because the engine is suddenly unloaded) while simultaneously
seeing a decrease in his main rotor RPM (because it is no longer being powered by the
engine).

The recovery is to treat the failure as an engine failure. The pilot would enter autorotation
in order to keep the main rotor turning, and would also move the engine control to idle to
prevent a further overspeed of the engine.

Twin Engine Helicopters

A twin engine helicopter could experience a failure of the drivetrain between an engine
and the engine combining gearbox, in which case one engine would overspeed, but the main
rotor would continue to be rotated by the other engine. It could also experience a faliure
of the drivetrain between the combining gearbox and the main rotor transmission in which
case both engines would overspeed while main rotor RPM would decay. In practice, the possible
failure modes and indications would depend on the particular model of helicopter.