.
slats extend to the extend position.
When the FLAP lever is moved beyond the 5 position the TE flaps extend to the commanded position and the LE:
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flaps remain at the full extended position and
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slats extend to the full extended position. The LE devices sequence is reversed upon retraction. Mechanical gates hinder inadvertent FLAP lever movement beyond flaps 1 for
one engine inoperative go–around and flaps 15 for normal go–around.
Indicator lights on the center instrument panel provide overall LE devices position status. The LE DEVICES annunciator panel on the aft overhead panel indicates the positions of the individual flaps and slats.
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Flight Controls -System Description 737 Flight Crew Operations Manual
Flap Load Relief
The flaps/slat electronics unit (FSEU) provides a TE flap load relief function which protects the flaps from excessive air loads. This function is operative at the flaps 30 and flaps 40 positions only. The FLAP lever does not move, but the flap position indicator displays flap retraction and re–extension.
When the flaps are set at 40, the TE flaps:
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retract to 30 if airspeed exceeds 163 knots
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re–extend when airspeed is reduced below 158 knots. When the flaps are set at 30, the TE flaps:
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retract to 25 if the airspeed exceeds 176 knots
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re–extend when airspeed is reduced below 171 knots. YC411, YC413
The FLAP LOAD RELIEF light illuminates when the TE flap load relief function is activated.
Autoslats
At flap positions 1, 2 and 5 an autoslat function is available that moves the LE slats to full extended if the airplane approaches a stall condition.
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 5 are selected, the LE slats are in the extend position. As the airplane approaches the stall angle, the slats automatically drive 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.
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 provides 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)
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.
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Flight Controls -System Description 737 Flight Crew Operations Manual
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.
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Flight Controls -System Description 737 Flight Crew Operations Manual
Leading Edge Devices and Trailing Edge Flaps Schematic
YA613 - YC154, YD651 - YK104
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