There are 4 Emergency
(overwing) Exit Doors designed on the B737 which are protected to open by DC
activated electrical Flight Locks. The Locks prevent opening the doors from the
take off roll, during flight, until the landing roll out. The design of the
doors allow them to open outwards instead of being so called “plug type” doors,
where cabin pressure prevents them from opening. At higher altitude
(differential pressure) it would be possible to open the doors, when there
would be no Locks installed.
The Locks are energized by command of
the PSEU when the following required conditions are met to activate:
- Both thrust levers >53° TLA position or . .
- Ground/air sensors sense air
And:
- Three or more main entry/service doors closed or . .
- Either engine is operating (signal from the DEU´s)
- Both thrust levers >53° TLA position or . .
- Ground/air sensors sense air
And:
- Three or more main entry/service doors closed or . .
- Either engine is operating (signal from the DEU´s)
Electrical power for the LEFT Flight
Locks is taken from DC Bus 1, and the RIGHT Flight Locks from DC Bus 2, with a
common circuit breaker on P6-3 / E1. When electrical power is not available to
the Locks, they are designed to deactivate Fail Safe.
Good one to remember when loosing this
DC Bus 1 (LEFT Flight Locks) or DC Bus 2 (RIGHT Flight Locks).
When an Emergency Exit is not completely
closed or a failure of a Lock is detected during the takeoff roll or in flight,
the respective LEFT/RIGHT FWD/AFT OVERWING and DOOR annunciator light will
illuminate together with a MASTER CAUTION light.
When either Flight Lock has failed or a
fault is detected on the ground, the amber PSEU, OVERHEAD annunciator and
MASTER CAUTION light will illuminate when before takeoff thrust is set and
>30 seconds after touchdown.
