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.
9.20.20 D6-27370-866-EGP
Alternate Extension
In the event that hydraulic system B fails, an alternate method of extending the LE
devices and extending and retracting the TE flaps is provided.
The TE flaps can be operated electrically through the use of two alternate flap
switches. The guarded ALTERNATE FLAPS master switch closes a flap bypass
valve to prevent hydraulic lock of the flap drive unit and arms the alternate flaps
position switch. The ALTERNATE FLAPS position switch controls an electric
motor that extends or retracts the TE flaps. The switch must be held in the DOWN
position until the flaps reach the desired position. No asymmetry or skew
protection is provided through the alternate (electrical) flap drive system.
When using alternate flap extension the LE flaps and slats are driven to the full
extended position using power from the standby hydraulic system. In this case the
ALTERNATE FLAPS master switch energizes the standby pump and the
ALTERNATE FLAPS position switch, held in the down position momentarily,
fully extends the LE devices.
Note: The LE devices cannot be retracted by the standby hydraulic system.
September 23, 2010
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