OPERATIONS MANUAL

Landing Gear
System Overview: (continued)
Gear retraction and extension
Hydraulic system 1 actuates the nose and body gear; hydraulic system 4
actuates the wing gear. In case of low hydraulic pressure, all gear can be
extended by gravity.
Air-ground relay system
Various sensors are installed in the landing gear framework. They detect,
among other things, whether the nose gear strut is compressed, whether
the body and wing gear trucks are tilted, and whether the body gear is
down and locked. The proximity switch electronics unit (PSEU) evaluates
the sensor signals and accordingly energizes or relaxes certain relays.
These relays inform a multitude of other aircraft systems—such as FMCs,
autobrakes, and so on—about whether the aircraft is on the ground or in
flight. In case of a power loss, some relays relax to the ground position,
while other relays relax to the air position, depending on the respective
worst-case option; for example, the air-ground relay that unlocks the thrust
reversers will, in case of a power loss, relax to the air position, so that
the reversers remain disabled in flight. They will then be disabled on the
ground as well. But this option is safer than enabling the reversers in flight.
PNF
In the simulator, the virtual Pilot Non-Flying (PNF) may set the gear lever
to the OFF position after takeoff when climb thrust is set and the gear is up,
and may set the autobrakes when descending through 15000 ft. The PNF can
be deactivated on Instructor > Situation > Human > Pilot by clearing the
checkbox Performs silent tasks.
SYSTEM
ANALYSIS
In the simulator, touchdown vertical speeds are indicated on Instructor >
Analysis > Miscellaneous. There is one indication for the V/S at the CG
position, recorded at initial aircraft ground contact; and another indication
for the V/S at the nose gear, recorded at initial nose gear touchdown.
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