quinta-feira, 12 de junho de 2025

Air India Flight 171 B787-8 VT-ANB – POSSIBLE FAILURE EVENT on Slat Autogap and Slat Pregap

 




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.



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