737 Flight Crew Operations Manual
Warning Systems -
System Description
Boeing Proprietary. Copyright © Boeing. May be subject to export restrictions under EAR. See title page for details.
D6-27370-866-EGP 15.20.7
Proximity Switch Electronic Unit (PSEU)
The PSEU monitors the following systems:
• takeoff configuration warnings
• landing configurations warnings
• landing gear
• air/ground sensing.
The PSEU, its sensors, and its input signals are monitored for internal faults.
When designated faults are detected, a PSEU light on the aft overhead panel
illuminates, and the OVERHEAD system annunciator light and MASTER
CAUTION lights illuminate. The PSEU light can be reset following a
maintenance BITE check or repair of the cause of the fault.
YK981
The PSEU light and OVERHEAD system annunciator do not illuminate for a
single fault in certain redundant systems, also known as a "simple fault."
However, if the system annunciator panel is pushed for recall, then the PSEU light
and OVERHEAD system annunciator will illuminate for a simple fault; in this
case, resetting the MASTER CAUTION system extinguishes the PSEU light.
YK982 - YW274
The PSEU light, OVERHEAD system annunciator and Master Caution illuminate
automatically for a single fault in certain redundant systems, also known as a
"simple fault." However, in this case these lights will illuminate only after a
landing. The PSEU light extinguishes when a parking brake is set or when both
engines are turned off.
The PSEU light is inhibited:
• in flight
• when the thrust levers are advanced toward takeoff power
• for 30 seconds after landing.
Mach/Airspeed Warning System
Two independent Mach/airspeed warning systems provide a distinct aural
warning, a clacker, any time the maximum operating airspeed of Vmo/Mmo is
exceeded. The warning clackers can be silenced only by reducing airspeed below
Vmo/Mmo.
The airspeed indicator displays red and black warning bands indicating maximum
and minimum allowable airspeeds. The ends of the amber bands indicate
maximum and minimum maneuver speeds.
When either an overspeed condition or a system test occurs, the ADIRU transmits
a signal to the aural warning module, sounding the clacker. The system can only
be tested on the ground.
September 15, 2016
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