Ambient air pressure varies inversely with altitude. The altitude compensator has an aneroid
bellows assembly that reacts to changes in ambient air pressure. It thereby reduces fuel flow to the fuel
injectors during acceleration if the engine is operated above sea level, up to a maximum altitude of
A minimum fuel flow needle, installed in the fuel passage that bypasses the governor, allows
a small amount of fuel under pump pressure to flow to the fuel injectors when fuel flow is reduced by
the governor. The engine speed is controlled throughout the operating range of 100 to 102 or 105
percent rpm by the flyweight assembly, in the governor housing.
The combustor drain valve is mounted on the bottom of the combustor housing. This valve
drains unburned fuel from the combustor during engine shutdown after a false or aborted start.
Figure 6.7 shows a schematic of the lubrication system. The pump draws oil out of the sump
through an oil passage and into the housing. Oil under pump pressure enters the bottom of the filter
housing, passes through the filter element, and flows out of the housing through a passage in the filter
cap. A relief valve in the filter assembly opens at a differential pressure of 15-to 25 psi. This allows oil
to flow from outside the filter element, through a passage in the filter element cap, to the filter outlet
passage. If the filter element becomes clogged, this valve will open and allow oil to bypass the filter.
From the filter, oil is forced into a passage to the pressure relief valve and to four oil jets.
The oil jet ring which encircles the high speed input pinion contains three of these jets, and sprays oil to
the points where the high speed input pinion meshes with the three planetary gears. One jet directs a
spray between the end of the output shaft and the high speed pinion to create a mist to lubricate the
rotor shaft bearings. The remaining gears and bearings are lubricated by air-oil mist created when oil
strikes the planetary gears and high speed pinion.
System pressure is maintained at 15 to 25 psi by a pressure relief valve. The valve regulates
pressure by bypassing excessive pressure directly into the reduction drive housing. The bypassed oil
strikes the inside surface of the air inlet housing, thus aiding in cooling the oil. Bypassed oil returns to
the sump by gravity flow through an opening in the bottom of the planetary gear carrier.