The Art of the Helicopter John Watkinson - Karatunov.net

Engines and transmissions 257
The pilot can isolate the ramp actuators and the brake/steering circuits in case of
damage or failure so that utility power remains available. The brake system contains a
small accumulator so that limited brake operation is still possible after a utility failure.
The hydraulic display panel shows for each system the level in the reservoir, the oil
pressure and temperature. There are warning lights for each of the four pumps and
warning lights operated by excessive pressure drop across the filters.
6.34 HUMS
Health and Usage Monitoring Systems are an important development that promises
improved safety and running costs. The use to which helicopters are put varies tremendously.
This means that the wear and fatigue suffered are difficult to assess simply
from a flight hour timer. As a result components have to be conservatively lifed so
that no failures occur whatever the circumstances. This means that a machine used in
an undemanding role has to replace parts that are still serviceable at the same rate as
a machine in arduous service that has worn the parts out. The solution is to assess
the actual conditions of service of each machine so that the degree of wear on major
components can be predicted. In this aspect of HUMS, sensors are located at strategic
points and the stresses on the machine are recorded.
The unacceptability of failure leads to the regular replacement of parts. However,
not all parts fail instantly. Many components deteriorate gradually and do not actually
fail. In others failure is preceded by symptoms that are often present for some time
before the failure itself. For example, a failure may result from a crack. Cracks tend
to start slowly and propagate at increasing speed as the remaining material lessens. If
it is possible reliably to detect those symptoms, an early warning of a failure can be
given. For example, gears and bearings make slight characteristic vibrations when they
are working normally, but these will change in the case of a defect. The presence of a
crack will alter the way a device vibrates. By analysing the vibrations from a gearbox,
it is possible to determine if a part is failing and even to suggest which part. Given that
most helicopter flights are quite short, if the vibration analysis is sufficiently sensitive
the failure will be anticipated sufficiently in advance that the flight can be completed.
A more sophisticated analysis could determine the rate of deterioration and suggest a
precautionary landing.