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Autothrottle Control Panel

Technical Information

Catalogue No: PM35296
Picture Type: Rochester Photo Negative
Topic: Engine Control
Title: Autothrottle Control Panel
Platform(s): Boeing 747 
Date: 28 Jan 1981
Width (mm): 127
Height (mm): 102
Copies: 1
Location: Negatives Cabinet PM ("54/" Negs) [RAA Office]

This picture is of the version linked in Related Items but there is a very similar unit identified as N1 LIM Mode above the right hand knob. N1 is related to the engine thrust. Other Auto Throttle Units are labelled EPRL for Engine Pressure Ratio Limit which again is an indication of the thrust being developed by a turbofan engine.

An autothrottle (automatic throttle, also known as autothrust, A/T) is a system that allows a pilot to control the power setting of an aircraft's engines by specifying a desired flight characteristic, rather than manually controlling the fuel flow. The autothrottle can greatly reduce the pilots' workload and help conserve fuel and extend engine life by metering the precise amount of fuel required to attain a specific target indicated air speed, or the assigned power for different phases of flight. Autothrottle and AFDS (Auto Flight Director Systems) can work together to fulfill the whole flight plan.

There are two parameters that an Autothrottle can maintain or try to attain: speed and thrust.

In speed mode the throttle is positioned to attain a set target speed. This mode controls aircraft speed within safe operating margins. For example, if the pilot selects a target speed which is slower than stall speed, or a speed faster than maximum speed, the autothrottle system will maintain a speed closest to the target speed that is within the range of safe speeds.

In the thrust mode the engine is maintained at a fixed power setting according to the different flight phases. For example, during takeoff, the Autothrottle maintains constant takeoff power until takeoff mode is finished. During climb, the Autothrottle maintains constant climb power; in descent, the A/T reduces the setting to the idle position, and so on. When the Autothrottle is working in thrust mode, speed is controlled by pitch (or the control column), and not by the Autothrottle. A radar altimeter feeds data to the Autothrottle mostly in this mode.

The Autothrottle Computer Unit is part of the Autothrottle system designed for the Boeing 747. FFRATS (Full Flight Regime Auto Throttle System). The system controlled engine power at the press of a button, even applying and aligning the power settings of the four engines during take-off, provided, that is, the computer had been fed with the correct information such as air temperature and atmospheric pressure beforehand. The system comprises the Autothrottle Computer and the Autothrottle TAT/EPR Limit Mode Select Panel made by Marconi Elliott Avionics and also a TAT/EPR Limit Computer and various sensors, panels and displays. There were two variants; the Total Air Temperature/Engine Pressure Ratio TAT/EPR version was for aircraft fitted with Pratt & Whitney JT9D or Rolls Royce RB211 engines and the Total Air Temperature/N1 TAT/N1 was for General Electric CF6 engines but operation was essentially the same.

 The equipment was designed to optimise speed control and throttle activity from take-off, through cruise to approach and landing. The objective was to reduce pilot workload, improve the fuel economy and enhance engine life. This system operates in three main modes:-

·         EPR Mode (Engine Pressure Ratio)  is used in take-off, climb maximum continuous thrust and go-around). This ensures that the engine with the highest EPR indication acquires and maintains the limit value (less any selected decrement). All four engines are compared and that with the highest EPR is chosen as the controlling unit. The flight crew can balance the thrust controls to equalize the EPR for all the engines.

·         Mach Hold Mode  is used during cruise and maintains the Mach number existing at the time of mode engagement.

·         Speed Select maintains the selected airspeed and is used for descent, holding approach and landing. A bias function compensates for gusts during the approach.

There are secondary modes such as Minimum speed protection, Flap speed limit protection and EPR limit protection. The modes are selected on the Autothrottle-TAT/EPR Limit Mode Select Panel. The Computer takes inputs from the TAT/EPR Limit Computer, the Central Air Data Computer, Angle of Attack and Airspeed.

FFRATS had a number of operational shortcomings and was replaced and retrofitted on all 747 aircraft by PMS (Performance Management System) which continually monitored weight, atmospheric conditions and fuel consumption.

The computers are used on  different airframe/engine variants and contain some modules unique to each variant. The Autothrottles were fitted to the Boeing 747-200 and -300 aircraft and a range of engines were used; Pratt Whiney JT9D-7 or -70, GE CF6-50 or RollsRoyce RB211-524 The Autothrottle was also fitted to the  SP version of the Boeing 747 which had  P&W JT9D-7 engines although a single SP aircraft had Rolls Royce engines (GE engines were not fitted to any SP aircraft).
The airframe/engine combination is described by the last three digits of the Boeing Part No. Thus -110 is for the -200 aircraft fitted with P&W JT9D-70 engines. The -155 and -153 was for aircraft fitted with the GE CF6-50 engines (Thanks to Alan Dedden for updates. He was a designer of the Autothrottle analogue computing))

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