Low Side Governor Failure

A governor is a device or system which controls engine power output
such that the main rotor maintains a constant RPM. When the pilot raises
collective, putting more pitch on the blades, and thus increasing the power
required, the governor system would add more fuel to the engine such that
the extra power required is exactly balanced by extra power output. The
result is that even though power required goes up and down, the rotor RPM
remains constant.

Some piston helicopters have a correlation system which is a mechanical
way of adjusting power output as the collective is raised and lowered.
A governor, on the other hand, uses a closed loop system to sense the
RPM and adjust the fuel control to maintain the desired RPM.

A low side governor system failure would be one in which the governor
either allows the engine output to go to zero, or to a lower power setting
than is required by the current collective pitch angle.

Some helicopters, such as the Robinson, allow the pilot to manually adjust
the throttle even when the governor is activated. In this case, the pilot
could simply increase the throttle manually by reference to the tachometer,
and thus work around the governor failure.

Other helicopters, such as the Bell, have a position on the throttle or
flight condition lever in which the governor is in charge of engine power
output, and in this position the throttle can not be manipulated by the
pilot. For instance, in Bell helicopters, you roll the throttle all the way
on to activate the governor. In this position, the throttle is already all
the way on, so the pilot has no way of commanding more power than what the
governor selects. In a helicopter with that sort of a system, the only choice
the pilot would have is to reduce the collective until the proper RPM is
attained. This might be at a fairly high power setting or a fairly low one,
depending on how the governor has failed. If the governor has failed at a low
enough setting, the pilot may have no choice but to autorotate. If the governor
has failed at some moderate power setting, it may be possible to maintain
powered flight at a lower speed, or with a slow descent.

High Side Governor Failure

A high side governor failure is is just the opposite of a low side failure. In
this case the engine is being commanded to put out too much power. The indication
to the pilot will generally be high engine RPM and torque, as well as high rotor RPM.

The Robinson governor can also be overridden in this situation by rolling the throttle
to a lower power setting, thus the Robinson can easilly be recovered from a high side
failure. Generally the pilot would just turn off the governor in this situation and
rely on the throttle correlator (mechanical) to help him maintain proper RPM.

Most turbine helicopters will allow the pilot to retard the engine power output using
the throttle or power lever. Thus a high side failure can generally be handled by
rolling off throttle or pulling back on the engine condition lever until RPM is restored
to the green range. In an extreme situation the pilot might have to turn off the fuel
in order to shut down the engine.