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11.  Engine Pressure Ratio (EPR) Transmitters and Indicators
A.  The EPR transmitters convert the turbine discharge pressure (Pt7) and the compressor inlet pressure (Pt2) into a ratio and generate three-phase electrical signals corresponding to pressure changes in the engine. The EPR signals are applied to both the EPR indicators and the autothrottle computer. The EPR transmitters are located at STA 570 on the right and left side of the airplane in the air conditioning bay.
B.  In the EPR mode, the autothrottle computes an EPR error by comparing the actual EPR (received from the EPR transmitters) and the target or command EPR from the PDC. This EPR error is then used to generate engine thrust control outputs.
C.  Two EPR indicators are located on the pilot's center instrument panel. Each indicator displays the ratio of turbine discharge pressure to compressor inlet pressure, and is used as a primary thrust setting instrument since EPR varies proportionally with developed thrust. The actual EPR is displayed by the pointer as well as by a high resolution digital drum counter readout. The drive system for these two readouts uses synchro positioning signals from the engine mounted EPR transmitter.
D.  Target or command EPR is indicated by a reference marker (bug), which, under certain operational modes, is driven from the performance data computer. When controlled from the PDC, the bug displays optimum EPR setting for maximum economy of operation in the particular flight mode in use.
E.  Failure of the EPR transmitter, or indicator, results in a failure flag appearing over the upper digital readout of the EPR indicator. Failure of the PDC, or the indicator's associated servo drive circuits, results in a failure flag appearing over the lower digital readout of the EPR indicator.
12.  Flap Position Sensor
A.  The flap position sensor is part of the flap position indicating system. Two outputs are available from the sensor, one to the flap position indicating system (Ref Chapter 27), the other to the autothrottle computer. The flap position is used in calculating safe flight speeds during various cruise and approach conditions.

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Oct 20/85  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  22-31-01 Page 13 

13. Operation
A.  Functional Description (Engagement and Warning Lights)
(1)  
An autothrottle engage (A/T ARM/OFF) switch is provided on the AFCS MCP for autothrottle engagement. The A/T ARM switch is electrically isolated from all AFCS components and is wired directly to the autothrottle.

(2)  
The autothrottle is engaged by moving the A/T ARM switch to the ARM position. The switch will hold in this position if the autothrottle computations are valid and all autothrottle circuit conditions, including various interlocks, are satisfied.

(3)  
Any of the following will cause disengagement:

(a)  
Moving the A/T ARM switch to the OFF position.

(b)  
Operation of either thrust-lever mounted A/T disconnect switch.

(c)  
Autothrottle fault and validity monitoring detects a fault.

(d)  
Two seconds after the squat switch operates on landing.

 

(4)  
When the autothrottle is disengaged, with the exception of landing, flashing red warning (A/T P/RST) lights in the Flight Mode Annunciator are turned on. The warning lights may be extinguished by any of the following:

(a)  
Moving the A/T ARM switch to the ARM position.

(b)  
Momentarily pressing either thrust-lever mounted A/T disconnect switch.

(c)  
Momentarily pressing either flashing warning light.

(d)  
By the squat switch until airspeed is less than 64 knots.

 

(5)  
The warning (A/T P/RST) lights will continuously illuminate steady red during the ground maintenance built-in-test (BITE).

B.  Functional Description (Mode Annunciation)
(1)  Mode annunciation is by Flight Mode Annunciator (FMA) and is driven directly by the autothrottle. The following modes are annunciated on the FMA:
1.
ARM  4. RETARD

2.
EPR  5. PDC SPD

3.
SPEED  6. THR HOLD

C.  Functional Description (Autothrottle Mode Operation)
(1)  
Take off Operation

(a)  
The take off mode is prepared for use by first engaging the PDCS in a take off page with the airplane on the ground. The FMA will show TO for the PDC. Moving the A/T ARM switch to the ARM position arms the autothrottle for take off and the FMA will show ARM for the autothrottle. The autothrottle cannot be engaged on the ground unless the PDC is in a take off mode or A/T BITE page has been selected during self-test.

(b)  
　
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