Stabilizer Position Indication and Green Band
Stabilizer position is displayed in units on two STAB TRIM indicators located inboard of each stabilizer trim wheel. The STAB TRIM indicators also display the TAKEOFF green band indication.
The trim authority for each mode of trim is limited to:
.
Main Electric Trim
YA613 - YA809
.
flaps retracted 4.30 to 14.5 units
YC153 - YC413, YK101 - YK104
.
flaps retracted 3.95 to 14.5 units
YD651 - YD655
.
flaps retracted 3.90 to 14.5 units
.
flaps extended 0.05 to 14.5 units
.
Autopilot Trim 0.05 to 14.5 units
.
Manual Trim -0.20 to 16.9 units. The green band range of the STAB TRIM indicator shows the takeoff trim range.
An intermittent horn sounds if takeoff is attempted with the stabilizer trim outside the takeoff trim range.
Speed Trim System
The speed trim system (STS) is a speed stability augmentation system designed to improve flight characteristics during operations with a low gross weight, aft center of gravity and high thrust when the autopilot is not engaged. The purpose of the STS is to return the airplane to a trimmed speed by commanding the stabilizer in a direction opposite the speed change. The STS monitors inputs of stabilizer position, thrust lever position, airspeed and vertical speed and then trims the stabilizer using the autopilot stabilizer trim. As the airplane speed increases or decreases from the trimmed speed, the stabilizer is commanded in the direction to return the airplane to the trimmed speed. This increases control column forces to force the airplane to return to the trimmed speed. As the airplane returns to the trimmed speed, the STS commanded stabilizer movement is removed.
STS operates most frequently during takeoffs, climb and go-arounds. Conditions for speed trim operation are listed below:
.
Airspeed between 100 KIAS and . Autopilot not engaged
Mach 0.5 . Sensing of trim requirement
.
10 seconds after takeoff
.
5 seconds following release of
trim switches
Copyright . The Boeing Company. See title page for details.
9.20.8 June 1, 2005
Flight Controls -System Description
Pitch Control Schematic
YA721, YA722
Copyright . The Boeing Company. See title page for details.
August 15, 2001 9.20.9
Flight Controls -System Description
YA613 - YA687, YA801 - YK104
Copyright . The Boeing Company. See title page for details.
9.20.10 September 26, 2003
Flight Controls -System Description 737 Flight Crew Operations Manual
Stall Identification YA613 - YA687, YA801 - YK104
Stall identification and control is enhanced by the yaw damper, the Elevator Feel Shift (EFS) module and the speed trim system. These three systems work together to help the pilot identify and prevent further movement into a stall condition.
During high AOA operations, the SMYD reduces yaw damper commanded rudder movement.
The EFS module increases hydraulic system A pressure to the elevator feel and centering unit during a stall. This increases forward control column force to approximately four times normal feel pressure. The EFS module is armed whenever an inhibit condition is not present. Inhibit conditions are: on the ground, radio altitude less than 100 feet and autopilot engaged. However, if EFS is active when descending through 100 feet RA, it remains active until AOA is reduced below approximately stickshaker threshold. There are no flight deck indications that the system is properly armed or activated.
As airspeed decreases towards stall speed, the speed trim system trims the stabilizer nose down and enables trim above stickshaker AOA. With this trim schedule the pilot must pull more aft column to stall the airplane. With the column aft, the amount of column force increase with the onset of EFS module is more pronounced.
Copyright . The Boeing Company. See title page for details.
Flight Controls -System Description 737 Flight Crew Operations Manual
Yaw Control
Yaw control is accomplished by a hydraulically powered rudder and a digital yaw damper system. The rudder is controlled by displacing the rudder pedals. The yaw damping functions are controlled through the stall management/yaw damper (SMYD) computers.
Rudder YA613 - YC413
The rudder provides yaw control about the airplane’s vertical axis. The A and B FLT CONTROL switches control hydraulic shutoff valves for the rudder and the standby rudder.
Each set of rudder pedals is mechanically connected by cables to the input levers of the main and standby rudder PCUs. The main rudder PCU is powered by hydraulic system A and B. The standby rudder PCU is powered by the standby hydraulic system. At speeds above approximately 135 kts, hydraulic system A pressure to the rudder PCU is limited by approximately 50%. This function limits full rudder authority in flight after takeoff and before landing.
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