FAILURE on Slat Autogap and Slat Pregap
The
slat autogap function is only available in primary mode when the slats are in
the middle position [FLAPS lever set to 1 position] and the airspeed is below
225 KIAS. At a high angle of attack [seconds before the crash the plane has been
seen increasing the nose pitch up], 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 [but the nose pitch up
was kept]. 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.
Flaps
and Slats
The
flaps and slats are high lift devices that increase wing lift and decrease
stall speed during takeoff, approach, and landing.
The
airplane has an inboard and an outboard flap on the trailing edge of each wing,
and one inboard and five outboard slats on the leading edge. A two-position
Krueger flap provides a seal between the inboard slat and the engine nacelle on
each wing.
In
the flaps 1 position, only the slats move. Flaps 5, 15, and 20 are takeoff flap
positions. Flaps 25 and 30 are landing flaps positions. Flaps 20 is used for
some non-normal landing conditions.
Flap
and Slat Sequencing
When
the flap lever is in the UP detent, all flaps and slats are commanded to the
retracted position. Moving the flap lever aft allows selection of flap detent
positions 1, 5, 15, 20, 25 and 30. The flaps and slats sequence so that the
slats extend first and retract last.
Starting
from flaps UP, selection of flaps 1 commands the slats to move to the middle
position. The flaps remain retracted.
Selection
of the flaps 5, 15, and 20 positions commands the flaps to move to the position
selected. The slats remain in the middle position.
Selection
of flaps 25 commands the slats to move to the fully extended position. The
flaps do not move.
Selection
of flaps 30 commands the flaps to extend to the primary landing position.
During
retraction, flap and slat sequencing is reversed.
The
mechanical gate at the flaps 20 detent prevents inadvertent retraction of the
flaps past the go-around flap setting. The mechanical gate at flaps 1 prevents
inadvertent retraction of the slats past the middle position.
Flap
and Slat Modes
There
are three modes of flap and slat operation:
•
primary
•
secondary
⚫
alternate
In
the primary mode, the flaps and slats are controlled together and positioned
using center hydraulic system motors. Autogap and flap load relief operate in
the primary mode.
The
secondary mode is automatically engaged when any of the following conditions
occur:
⚫
center hydraulic system failed, or
·
flap or slat primary control failure, or
primary
mode fails to move the flaps or slats to the selected position, or
•
control surfaces travel at less than 50% of the normal hydraulic rate, or
⚫
uncommanded flap or slat motion is detected, or
•
flap or slat disagree is detected
In
the secondary mode the slats and flaps are controlled separately and can be
positioned by hydraulic or electric motors. For example, if the slats hydraulic
control fails, the flaps are still driven hydraulically but the slats are now
powered electrically. Pilot control is through the flap lever but operation in
secondary mode is limited to flaps 20 by non-normal procedures.
The
three-position alternate flaps selector extends and retracts the flaps and
slats. The flaps and slats extend simultaneously, but slat retraction is
inhibited until the flaps are up. Alternate mode flap and slat extension is
limited to the slats middle position and flaps 20. Asymmetry and uncommanded
motion protection, slat autogap and pregap, and flap and slat load relief are
not available in alternate mode.
The
alternate mode must be manually selected. Slat and flap operation time in the
secondary and alternate modes is greatly increased.