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LEFT FLIGHT CONTROL COMPUTERSYNC UNTIL MODE ENGAGEINERTIAL ALTITUDE FROM IRS WITH BARO CORRECTIONLATCHED WHEN VERTICAL POSITION ENGAGED
Autopilot/Flight Director System Pitch Control Signal Flow Schematic
A
757
MAINTENANCE MANUAL
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ú ú 18 Page 27 ú Jan 28/02
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LEFT FLIGHT CONTROL COMPUTERSYNC UNTIL MODE ENGAGEINERTIAL ALTITUDE FROM IRS WITH BARO CORRECTIONLATCHED WHEN VERTICAL POSITION ENGAGED
Autopilot/Flight Director System Pitch Control Signal Flow Schematic
A
757
MAINTENANCE MANUAL
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GUN 001-050, 052-999 ú 22-12-00
ú ú 16 Page 28 ú Jan 28/02
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757 MAINTENANCE MANUAL
2) At a radio altitude of 5 feet a logic zero is sent to the AND and OR gates in the IN-AIR logic circuit. The logic zero is put through a 2 second delay. The NAND gate outputs a logic one (IN-AIR) 2 seconds after the airplane passed the 5 foot point. The IN-AIR logic one enables the SPD ENG and disables the 8 degree pitch-up bar command (TAKEOFF ATT ENG). This adjusts the 8 degree pitch-up command to the set pitch altitude in 2 seconds to keep the takeoff speed.
(b) Takeoff Mode Control Signal Flow and Monitoring 1) The TAKE OFF SPD ENG provides flight director pitch commands using speed schedules for a safe climbout speed. The roll channel provides a versine compensation gain schedule to provide pitch corrections for an airplane bank angle greater than |1 degree. a) The versine signal is modified by the versine compensation gain schedule as a function of flap position and impact pressure (IAS).
b) Increased flap position is directly proportional and increases the signal. An increase in impact pressure is inversely proportional and decreases the signal.
(c) Speed Through Elevator 1) Speed Through Elevator Command Processing a) The Speed Through Elevator (STE) function computes elevator commands to capture and hold the selected speed. It is normally used with autothrottle to achieve airspeed and rate of climb control. The alpha floor calculation computes elevator commands to capture and hold minimum speed (130% of stall speed) for angle of attack control. The A/T mode predicts the elevator response necessary to compensate for throttle change and adds this to the other two signals.
b) Command selection normally uses STE signals but automatically selects alpha floor signals when necessary to prevent flying below minimum speed. These signals produce the FLCH proportional command in degrees of pitch attitude.
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757
MAINTENANCE MANUAL
c) Three flight modes use Speed Through Elevator. The takeoff mode, Flight Level Change (FLCH) mode, and go-around mode use the speed through elevator function. The FLCH mode combines with throttles to reach a new flight altitude. In the go-around mode, throttles are used to gain altitude and maintain speed.
2) Takeoff Mode (Speed Engage)
a) The takeoff mode is a F/D only mode. The speed through elevator function provides a smooth transition from rotation bar bias to speed control when in the air.
3) Flight Level Change Mode
a) In FLCH, the pitch channel controls the elevators to capture and maintain the selected IAS/MACH. The throttles are used to control rate of climb or descent. With the A/T armed, throttles control vertical speed based on the test altitude change requested. With the A/T off, the pilots control the throttles as required. Without any change of thrust, capturing a new speed results in vertical speed change.
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