787 Flight Crew Operations Manual
DO NOT USE FOR FLIGHT
Flight Controls -
System Description
Copyright © The Boeing Company. See title page for details.
D615Z003-TBC 9.20.21
Flap/Slat Load Relief
In the primary mode, the flap load relief system protects the flaps from excessive
air loads. If flap airspeed placard limits are exceeded with the flaps in the 15
through 30 position, LOAD RELIEF is displayed and the flaps automatically
retract to a position appropriate to the airspeed. Load relief retraction is limited to
flaps 5.
When airspeed is reduced, the flaps automatically re–extend as airspeed allows.
Re–extension is limited to the commanded flap position.
If a flap overspeed exists, load relief prevents flap extension beyond the 5, 15, or
20/25 positions until airspeed is sufficiently reduced. Flap load relief is available
only in the primary mode. The EICAS flap display indicates an in–transit flap
condition and shows actual flap position. The flap lever does not move during flap
load relief operation. Load relief for slats is not required in the primary or alternate
modes.
Slat load relief is available in the secondary mode. If airspeed exceeds 225 KIAS
with the slats fully extended, they retract to the middle position and LOAD
RELIEF is displayed. For all lever positions except Up, the slats are driven to their
extend position when the airspeed is below 225 KIAS.
To protect against inadvertent deployment during cruise, flap and slat extension
from the UP position is inhibited when speed is more than 260 KIAS or altitude is
above approximately 20,000 feet. This inhibit is only available in primary and
secondary flap and slat modes.
Slat Autogap and Slat Pregap
The slat autogap function is only available in primary mode when the slats are in
the middle position and the airspeed is below 225 KIAS. At a high angle of attack,
autogap fully extends the slats to increase the wing camber, thus increasing the lift
and margin to stall. The slats return to the middle position after the angle of attack
decreases. The autogap trip threshold is a function of AOA, airspeed and flap
position.
In the secondary mode the system is too slow to respond to an autogap request, so
a pregap function exists. The slats automatically move to the fully extended
position from the middle position when the flap lever is not UP and airspeed is less
than 225 KIAS. The slats remain in the extended position until the flap lever is in
the UP position or airspeed is above 225 KIAS.
February 15, 2010
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