737 MAX Flight Crew Operations Manual
Flight Controls -
System Description
9.20.26
MN-FLT-OH-201
Asymmetry and Skew Detection and Protection
The Flap Slat Electronic Unit (FSEU) continuously monitors the position of wing
LE and TE high lift devices. If a device on one wing does not align with the
symmetrical device on the other wing, there is an asymmetry condition. A skew
condition occurs when symmetrical TE flaps do not operate at the same rate
causing the panels to twist during extension or retraction. Should a skew occur, the
FSEU automatically protects against roll by maintaining flap symmetry.
Trailing Edge Flap Position Indication
Wing TE position indications come from the FSEU. When the FSEU detects a TE
asymmetry or skew condition, the FSEU:
• closes the TE flap bypass valve
• displays a needle split on the flap position indicator
• shows position of left and right wing trailing edge flaps.
Leading Edge Device Improper Position Indication
Wing LE position indications come from the FSEU. When the FSEU detects a LE
device in an improper position, the LE FLAPS TRANSIT light remains
illuminated and one of the following indications is displayed on the LE DEVICES
annunciator panel:
• TRANSIT (amber) - Leading edge devices are in transit, or are not in
the selected position.
• EXT (green) - Leading edge slats and flaps in extend position, or are in
the selected position.
• FULL EXT (green) - Leading edge slats are in the full extend position.
• No lights illuminated - Leading edge devices are in the retract position.
Uncommanded Motion Detection, Protection and Indication
The FSEU provides protection from uncommanded motion by the LE devices or
TE flaps.
Leading Edge Uncommanded Motion
Uncommanded motion is detected when no TE flap position or autoslat command
is present and:
• two LE flaps move on one wing, or
• two or more slats move on one wing.
The FSEU shuts down the LE control and illuminates the amber LE FLAPS
TRANSIT light.
In addition, to prevent uncommanded motion from occurring on the LE devices
during cruise, the FSEU maintains pressure on the retract lines and depressurizes
the extend and full extend lines.
March 1, 2021
