OPERATIONS MANUAL

Flight Controls
Stabilizer Trim:
The aircraft pitch attitude is trimmed by changing the horizontal stabilizer’s
angle of incidence. Hydraulic systems 2 and 3 move the trim mechanism. If
only one system is pressurized, the trim moves at half rate. The trim system is
controlled through normal and alternate control channels. When the normal
channel is in use, the trim rate is airspeed dependent: the rate is high when the
airspeed is low. When the alternate control is applied, the trim moves at the
highest possible rate. Trim rates, trim limits, protections, and other functions
are controlled by two stabilizer trim & rudder ratio changer modules (SRMs)
labeled as SRM L and SRM R.
The normal channel is linked with the trim switches in the pilots’ control
wheels; the alternate channel is linked with the alternate trim switches on
the aisle stand. The stabilizer will not move when two pairs of trim switches
are pushed in opposite directions, or when the normal trim switches and the
control columns are pushed in opposite directions.
When the normal trim switches are pushed during single autopilot operation,
the autopilot will disengage. During multi-channel autopilot operation, the
normal trim switches are disabled and the autopilots will not disengage. The
alternate trim switches always override the trim commands of the autopilots,
but will not disengage the autopilots.
The control system includes an automatic speed trim function which improves
the airspeed stability in certain flight phases. Typically, when the airspeed rises,
the aircraft will increase pitch on its own and will climb, which in return will
decrease the airspeed and thus restabilize it. Under certain conditions, however,
the aircaft is not able to induce sufficient nose up momentum on its own. To
compensate this problem, the speed trim function will—to a certain degree—
move the stabilizer in the nose up direction when the airspeed rises. Speed trim
is active when all of the following conditions are true:
+ 20 seconds have passed since liftoff.
+ Pilot is not trimming the stabilizer.
+ Autopilots are disengaged.
+ Flap lever is not set to 25 and not set to 30.
+ IAS is between 120 and 220 kt.
+ SRM L and hydraulic system 2 are operative; or SRM R and hydraulic
system 3 are operative.
Freighters also have a Mach trim function that compensates effects caused
by the shorter, freighter specific upper deck. Mach trim starts at Mach 0.86,
leading to a stabilizer position change of 0.5° at Mach 0.9 (too small to be
noticable in the simulator).
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