737 MAX Flight Crew Operations Manual
Flight Controls -
System Description
9.20.24
MN-FLT-OH-201
Autoslats
Autoslat operation is normally powered by hydraulic system B. An alternate
source of power is provided by system A through a Power Transfer Unit (PTU) if
a loss of pressure is sensed from the higher volume system B engine driven pump.
The PTU uses system A pressure to power a hydraulic motorized pump,
pressurizing system B fluid to provide power for the autoslat operation. (Refer to
Chapter 13, Hydraulics, Power Transfer Unit)
At flap positions 1, 2, 5, 10, 15, and 25 an autoslat function is available that moves
the LE slats to full extended if the airplane approaches a stall.
The autoslat system is designed to enhance airplane stall characteristics at high
angles of attack during takeoff or approach to landing. When TE flaps 1 through
25 are selected, the LE slats are in the extend position. As the airplane approaches
the stall angle, the slats automatically begin driving to the full extended position
prior to stick shaker activation. The slats return to the extend position when the
pitch angle is sufficiently reduced below the stall critical attitude.
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.
March 1, 2021