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LESSON 7 Solar T62 Auxiliary Power Unit.
TEXT ASSIGNMENT Reference Text AL0993, paragraphs 6.1-6.12.
LESSON OBJECTIVE To enable you to describe the T62 APU, how it operates, and its various components.

Chapter 6



The SOLAR T62 auxiliary power unit (APU) is used in place of ground support equipment to start some helicopter engines. It is also used to operate the helicopter hydraulic and electrical systems when this aircraft is on the ground, to check their performance. The T62 is a component of both the CH-47 and CH-54 helicopters -- part of them, not separate like the ground-support-equipment APU's. On the CH-54, the component is called the auxiliary powerplant rather than the auxiliary power unit, as it is on the CH-47. The two T62's differ slightly.

This chapter describes the T62 APU; explains its operation; discusses the reduction drive, accessory drive, combustion, and turbine assemblies; and describes the fuel, lubrication, and electrical systems.


The T62 gas turbine engine auxiliary unit consists of a combustor, turbine, reduction drive and accessory drive assemblies, engine accessories, plumbing, and wiring. The engine has a single shaft with the compressor and turbine rotor mounted back-to-back.

The T62 develops approximately 70 shaft horsepower. It has its own fuel control unit, hydraulic starter motor, ignition unit, and reduction gear drive. Operating time for the APU is maintained separately from aircraft engine time by an hour meter mounted on the APU. The T62 used on the CH-47 differs slightly from the one used on the CH-54. The accompanying table IV gives the particulars for each engine.

Table IV. T62 Engine Leading Particulars.
Table IV

Both models of the T62 are shown in figures 6.1 and 6.2.

T62T-2A Used on the CH-47
Figure 6.1. T62T-2A Used on the CH-47.

T62T-16A Used on the CH-54
Figure 6.2. T62T-16A Used on the CH-54.


The T62 gas turbine engine consists of three major sections: the reduction and accessory drives, the combustor, and the turbine sections as shown in figures 6.1 and 6.2. Air is drawn into the inlet of the engine when the hydraulic starter rotates the compressor during the starting cycle. After the engine is started, air continues to be drawn into the compressor by the power produced by the engine. The air is compressed and directed into the combustor; fuel is introduced through six vaporizer tubes and is burned. During the starting cycle, fuel from the start nozzle is ignited by a spark plug. When the APU reaches 90 percent speed, a speed switch opens which closes the start fuel solenoid valve, shutting off fuel flow to the start fuel nozzle. The hot expanding gas flows through a turbine nozzle and to the turbine assembly. Power is extracted by the turbine rotor and is then transmitted to the reduction drive assembly.


The T62 is equipped with protective devices that shut the APU down if any of the operating limitations are exceeded. If the oil pressure drops below approximately 6 psi, the low oil pressure switch shuts off the fuel solenoid valve, which stops the flow of fuel to the engine. When this occurs the light marked "Low Oil Press" will be illuminated on the instrument panel in the cockpit.

Overspeeding the APU is prevented by an overspeed switch, set at 110 percent. When this limit is exceeded, the overspeed switch opens and shuts off the fuel flow to the engine. This causes the overspeed (OVSP) light to illuminate on the instrument panel.

Hot starting of the APU is prevented by the high exhaust-temperature switch, which shuts the engine off if the engine is too hot. Like the low oil pressure and overspeed switches, it stops the fuel flow to the engine. When the operating temperature is exceeded, the high exhaust temperature light, marked "HIGH EXH TEMP," in the cockpit will be illuminated.

If any of these conditions occur and the APU shuts down, the control switch in the cockpit must be moved to the STOP position before attempting to restart the engine.


The major difference between the T62T-2A and the T62T-16A is the accessory drive housing. A comparison can be made by looking at item 31 in figures 6.3 and 6.4. Reduction of high engine rpm is accomplished by using a single stage planetary gear reduction. Three fixed-center planetary gears are driven by the externally splined pinion of the rotor shaft, item 7. An internally splined ring gear transmits the drive from the planetary gears to the output shaft. External gear teeth on the output portion of the shaft drive an upper intermediate gear and oil pump gear. The intermediate gear drives the accessory section. The accessory drive assembly consists of accessory gears and the intermediate gear. These gears drive the hydraulic starter,the fuel control, and a speed switch and tachometer generator.

T62T-2A Cutaway Engine Assembly
Figure 6.3. T62T-2A Cutaway Engine Assembly.

T62T-16A Cutaway Engine Assembly
Figure 6.4. T62T-16A Cutaway Engine Assembly.


The annular combustor assembly consists of a housing, liner, and nozzle shield, items 15 through 17 in figure 6.3. The assembly is secured to the turbine housing by a V-clamp, item 19 in the figure. Six self-tapping screws support and center the liner in the housing. Fuel for the combustor is supplied by six equally spaced fuel vaporizers connected by a circular manifold on the combustor housing. A combustion chamber drain valve is installed at the 6 o'clock position on the combustion housing.


The turbine assembly consists of the rotor, air inlet, diffuser, and nozzle assemblies. These items are shown in the airflow diagram in figure 6.5.

Airflow Diagram
Figure 6.5. Airflow Diagram.

The centrifugal compressor rotor and radial inflow turbine rotor are bolted to the aft end of the rotor shaft. A single forward ball bearing and an aft roller bearing support the shaft. These are items 8 and 9 in figure 6.3 and 6.4. The air inlet assembly serves as a structural support between the combustor assembly and the reduction drive. The cylindrical, contoured casting is open at both ends and is equipped with a wire mesh screen to cover the intake duct. Intake air passes through the inlet assembly and into the impeller. The air then passes through the diffuser to the combustion section where it flows between the outer walls of the housing and combustion liner. At the end of the liner, the flow reverses direction and enters the combustion chamber. After combustion, the gas exits at the forward end of the combustion liner through a nozzle and the turbine wheel.


The T62T-2A and T62T-16A have identical fuel systems. The fuel system is illustrated in figure 6.6. The system consists of an inlet filter, fuel control, six main fuel injectors, a start fuel nozzle, main and start fuel solenoid valve, a fuel pressure switch, and the necessary plumbing. Fuel for the APU is supplied from the same source that supplies the engines.

Fuel Control Schematic
Figure 6.6. Fuel Control Schematic.


The accessory drive gear drives both the fuel pump and the acceleration control assembly to deliver fuel to the engine at approximately 300 psi. Fuel is directed through the inlet filter, in the fuel pump, through the outlet filter with the fuel pump, and into the governor housing. A relief valve installed in the governor housing bypasses excess fuel from the pump outlet to the pump inlet.

During engine starting, fuel is forced through internal passages to the fuel pressure switch. Normally closed, the fuel pressure switch opens on increasing fuel pressure at 100 to 120 psi. When open it actuates the start fuel solenoid valve and the main fuel solenoid valve to the open position, and completes the ignition circuit. Combustion takes place in the combustor when atomized fuel from the start fuel nozzle is ignited by the spark plug.

At 90 percent speed, fuel flow to the start fuel nozzle is cut off. The main fuel injectors continue to supply fuel to the combustor. Fuel flow to the main fuel injectors is increased in direct proportion to increasing air pressure, thus eliminating the possibility of over temperature or compressor surge during acceleration.

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 15,000 feet.

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.

Lubrication System Schematic
Figure 6.7. Lubrication System Schematic.

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.

The low oil pressure switch, normally open, closes on increasing oil pressure to 5 to 7 psi. When the switch contacts close, the low oil-pressure circuit is deenergized. At rated engine speed, a drop in oil pressure below 5-7 psi opens the low oil-pressure switch contacts, and, through electrical circuitry, closes the main fuel solenoid valve and shuts down the engine.


Circuitry for ignition and engine electrical accessories is included in the electrical system. The system includes the ignition exciter, spark plug, speed switch, tachometer generator, thermocouple, and hour meter. The engine harness-assembly connectors mate with receptacles on the junction box, oil pressure switch, ignition exciter, speed switch, and tachometer-generator. Descriptions of the components in the electrical system are given in the following subparagraphs.

a. Ignition exciter. The input voltage to the ignition exciter is 10 to 29v dc. Input voltage is converted to an intermittent high-energy current which is directed to the spark plug for ignition. Minimum spark rate is two per second at 14 volts.

b. Spark plug. The shunted surface-gap type spark plug is threaded into a boss in the end of the combustor. The plug furnishes the spark necessary for initial ignition of fuel during the starting cycle. Ignition is terminated by the No. 1 switch in the speed switch when the engine attains 90 percent rated speed.

c. Speed switch. Mounted on the rear accessory drive pad in tandem with the tachometer is the speed switch. Inside the switch housing, two flyweights are mounted to the speed switch drive shaft. The flyweights move an actuating plate held in position by two springs of unequal strength. The speed switch is two switches in one housing. The No. 1 switch is set at 90 percent rated speed, and the No. 2 switch is set at 110 percent rated speed. As rpm is increased, the force of the lighter and then the heavier spring is overcome, allowing the actuating plate to actuate the No. 1 switch (90 percent speed). When the No. 1 switch is actuated, the start fuel and ignition systems are shut off. When the No. 2 switch is actuated at 110 percent speed, the main fuel solenoid valve is closed, and this shuts off fuel to the engine.

d. Turbine exhaust thermal switch. A thermocouple projects into the exhaust gas stream at the aft end of the combustor. If an over temperature occurs,the contacts normally closed in the turbine exhaust thermal switch will open, close the main fuel solenoid valve, and shut the engine down.

e. Tachometer generator. The APU tachometer generator is mounted in tandem with the speed switch on the accessory drive assembly. A synchronous rotor in the tachometer generator produces three-phase ac voltage proportional to the speed at which the rotor is turning. This voltage is transmitted to a tachometer indicator in the cockpit which indicates the engine speed in percent of rated rpm.

f. Hour meter. Engine operating time is recorded by the hour meter attached to the engine. The meter operates on 24v dc supplied by the aircraft bus.


The T62 is used on the CH-47 and CH-54 helicopters as an auxiliary power unit. The engine consists of a combustor, turbine, and reduction and accessory drive assemblies. Both the compressor and turbine rotor are mounted on the same shaft. A fuel control, hydraulic starter motor, and ignition unit are mounted on the engine. Operating time for the APU is maintained separately from the aircraft engine time by an hour meter mounted on the APU.

The T62 is equipped with protective devices that shut the APU down if any of the operating limitations are exceeded. If the oil pressure, rpm, or exhaust temperature limits are exceeded, a light on the instrument panel will be illuminated and the APU shut down.


Lesson #7 Practice Exercise
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