Tristar L1011 APU STL6-73

PT6 derived APU gas turbine

Commissioning project; Tristar L1011 APU (ST6L-73) mounted in operational position

L1011 APU Description


ST6L-73 (shaft-turbine 6 series 1973?) An unusual design of auxiliary power unit that was developed from the very successful Pratt and Whitney PT6 turbo-prop engine. The PT6 is probably the World’s most prolific and successful engine in its class and powers the equally successful Beach 200 series of aircraft. Faced with the high starting power demands of the new Rolls Royce RB211-22 engine installed in the Lockhead Tristar L1011 aircraft, the manufacturer Hamilton Standard must have selected this powerful turbo-prop for development as an APU.


The ST6L-73 engine consists of the basic PT6 two shaft engine core. It’s made up of a 3-stage axial compressor and a single stage centrifugal compressor driven by a single stage axial turbine. This forms the “gas generator” section of the engine. An annular reverse-flow combustion chamber is employed that is fitted with 18? duplex atomising burner nozzles. A second free power turbine is fitted that would have driven the propeller via a reduction gearbox. Instead a different gearbox is fitted that drives a number of accessories. The free turbine is also connected directly to a “Load” compressor wheel consisting of a large single centrifugal impeller. The load compressor air inlet is fitted with variable inlet guide vanes that form a type of “throttle” to control the available airflow from the compressor.


The two-shaft layout is unusual, most APU designs are of a single shaft variety with load compressors and accessories all on the one shaft. The Rover AAPP MK10501 is a similar but smaller unit comprising of the two-shaft layout with a driven load compressor. Also a Boeing 503 series unit was built with a two shaft load compressor arrangement. The free turbine drives two accessories in addition to the load compressor. A high-speed fan is used to blow air through an oil cooler unit and a high-speed oil-cooled alternator is also driven at 12,000 rpm. The alternator is a 400Hz four pole machine and so is required to be driven at this speed in order to provide a constant frequency.


The ST6 fuel system consists of a re-worked version of the hydro-mechanical system that would have been used with the turbo-prop. An integrated system is supplied by a gear type pump and makes use of compressor delivery air to pressurize a control circuit to meter fuel in to the engine via a number of valves. The original engine power lever is locked in place at full gas-generator speed.


The engine load compressor and alternator must be driven at constant speed and so a governing system is fitted to the engine. This consists of an electronic system fed with speed and temperature sensors mounted in the engine. A special electrical spill valve is connected to the air control circuit in the fuel system and is driven with a signal from the electronics. A control loop is formed that keeps the turbine speed constant, regardless of electrical or compressed air loads. Additional protection and safety circuits are also built in to the electronics as is the aircraft cock-pit instrumentation.

A second electronic control loop is also used to control the airflow in to the load compressor. A pneumatic actuator alters the angle of a set of guide vanes that are mounted in the load compressor intake.  Bleed air from the main engine compressor provides the force to move the actuator and a second electrically operated air spill valve regulates the position of the vanes in conjunction with electronic control system. Feedback of the vane position is provided by a potentiometer sensor mounted with the actuator mechanism.


A condition occurs for main engine starting where the vanes are fully open and the system runs at maximum capacity (max mode) to provide the greatest possible air flow. To crank the RB211 to starting speed requires some 500 or so “air horsepower” to be delivered from the APU.



PT6 derived APU gas turbine

ST6L-73 Specification/Characteristics

Power Output...........….……………………………………………………….………….…………...635 Hp
RPM..............….…………………………………………..…….................NG 37,000 rpm, NF 33,000 rpm
Compressor………………………………………………..……….…3 stage axial followed by 1 stage radial

Load Compressor………………………………………………..…….single stage radial with variable IGVs
Load Compressor Air Mass Flow..……………………………..……….385 Lbs/Min @ 41 PSI (Inc 45Kw)
Overall Pressure Ratio..……………………….………………………………………………………..….7:1
Combustion Chamber..……………………………..……......Reverse Flow Annular with 14 Duplex Burners
Compressor Turbine..….……………………………………………………………….....1 Stage Axial each
Power Turbine..…………………………………………………..........1 Stage Axial with reduction gearbox

Layout.....................….……………………………………..….......Two Spool with Accessory Gearboxes Starting................….………………………………………………………………….........Starter Generator
Ignition......….……………………………………….………...............Two  Igniters with High Energy Plugs
Fuel System….……...................Gear pump with Hydro-mechanical and Spill-Valve Electronic Fuel Controls, Electronic Protection System.
Oil System......….……...........Return System with Pressure and Scavenge pumps, Engine Mounted Tank and Oil cooler
Weight...........….………………………………………………………………………….................800 Lbs
Applications......….……………………………………………….………………...........APU L1011 Airliner

Similar PT6 Turbo-prop King Air, PC12.


L1011 APU ST6L-73 PT6

Underside view showing air intake door in open position

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Gas Turbine Jet Engine L1011 Tristar Aircraft

Plug and Play instrument panel system for ST6L-73 APU