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The Art of the Helicopter John Watkinson - Karatunov.net

The Art of the Helicopter John Watkinson - Karatunov.net

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Engines and transmissions 257<br />

<strong>The</strong> pilot can isolate <strong>the</strong> ramp actuators and <strong>the</strong> brake/steering circuits in case <strong>of</strong><br />

damage or failure so that utility power remains available. <strong>The</strong> brake system contains a<br />

small accumulator so that limited brake operation is still possible after a utility failure.<br />

<strong>The</strong> hydraulic display panel shows for each system <strong>the</strong> level in <strong>the</strong> reservoir, <strong>the</strong> oil<br />

pressure and temperature. <strong>The</strong>re are warning lights for each <strong>of</strong> <strong>the</strong> four pumps and<br />

warning lights operated by excessive pressure drop across <strong>the</strong> filters.<br />

6.34 HUMS<br />

Health and Usage Monitoring Systems are an important development that promises<br />

improved safety and running costs. <strong>The</strong> use to which helicopters are put varies tremendously.<br />

This means that <strong>the</strong> wear and fatigue suffered are difficult to assess simply<br />

from a flight hour timer. As a result components have to be conservatively lifed so<br />

that no failures occur whatever <strong>the</strong> circumstances. This means that a machine used in<br />

an undemanding role has to replace parts that are still serviceable at <strong>the</strong> same rate as<br />

a machine in arduous service that has worn <strong>the</strong> parts out. <strong>The</strong> solution is to assess<br />

<strong>the</strong> actual conditions <strong>of</strong> service <strong>of</strong> each machine so that <strong>the</strong> degree <strong>of</strong> wear on major<br />

components can be predicted. In this aspect <strong>of</strong> HUMS, sensors are located at strategic<br />

points and <strong>the</strong> stresses on <strong>the</strong> machine are recorded.<br />

<strong>The</strong> unacceptability <strong>of</strong> failure leads to <strong>the</strong> regular replacement <strong>of</strong> parts. However,<br />

not all parts fail instantly. Many components deteriorate gradually and do not actually<br />

fail. In o<strong>the</strong>rs failure is preceded by symptoms that are <strong>of</strong>ten present for some time<br />

before <strong>the</strong> failure itself. For example, a failure may result from a crack. Cracks tend<br />

to start slowly and propagate at increasing speed as <strong>the</strong> remaining material lessens. If<br />

it is possible reliably to detect those symptoms, an early warning <strong>of</strong> a failure can be<br />

given. For example, gears and bearings make slight characteristic vibrations when <strong>the</strong>y<br />

are working normally, but <strong>the</strong>se will change in <strong>the</strong> case <strong>of</strong> a defect. <strong>The</strong> presence <strong>of</strong> a<br />

crack will alter <strong>the</strong> way a device vibrates. By analysing <strong>the</strong> vibrations from a gearbox,<br />

it is possible to determine if a part is failing and even to suggest which part. Given that<br />

most helicopter flights are quite short, if <strong>the</strong> vibration analysis is sufficiently sensitive<br />

<strong>the</strong> failure will be anticipated sufficiently in advance that <strong>the</strong> flight can be completed.<br />

A more sophisticated analysis could determine <strong>the</strong> rate <strong>of</strong> deterioration and suggest a<br />

precautionary landing.

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