737 Flight Crew Operations Manual
Flight Controls -
System Description
Boeing Proprietary. Copyright © Boeing. May be subject to export restrictions under EAR. See title page for details.
9.20.8 D6-27370-866-EGP
Stabilizer Trim Operation with Forward or Aft CG
In the event the stabilizer is trimmed to the end of the electrical trim limits,
additional trim is available through the use of the manual trim wheels. If manual
trim is used to position the stabilizer beyond the electrical trim limits, the stabilizer
trim switches may be used to return the stabilizer to electrical trim limits.
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
• flaps extended 0.05 to 14.5 units
• flaps retracted 3.95 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:
• STS Mach gain is fully enabled
between 100 KIAS and Mach
0.60 with a fadeout to zero by
Mach 0.68
• 10 seconds after takeoff
• 5 seconds following release
of trim switches
• Autopilot not engaged
• Sensing of trim requirement
September 26, 2013
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