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Aircraft Systems Topic 14.

Boeing 767 Flight Controls
Part 4: Leading Edge Slats, and Trailing Edge Flaps


High lift devices for takeoff and landing are provided each wing by 5 outboard and 1 inboard slat section, plus slotted Fowler trailing edge flaps. Additionally, the inboard ailerons droop with the trailing edge flaps.

Flap positions are UP, 1, 5, 15, 20, 25, 30. The 25 and 30 are landing flap selections.

Normal Operation

Under normal operations, the flaps and slats are operated by a single flap selector lever to the right of the thrust levers. Moving the flap lever to one of the flap detent's sends signals to 3 Power Drive Unit (PDUís), which in turn move the flaps and slats.

One PDU drives the inboard leading edge slats. Another drives the 5 outboard slat sections. A third drives the trailing edge flaps. Power to all is supplied from the centre hydraulic system


Flap/Slat position indicators

A cockpit dial with two pointers (left and right) shows the actual flap position. If left and right flaps/slats are moving correctly and in unison, the left point obscures the right point beneath it. The dial is used to identify the flap/slat position, during takeoff, approach and landing checks, along with the flap handle position itself.

  • In the UP position, both the leading edge slats and trailing edge flaps are retracted.
  • In position 1, the leading edge flaps are extended to the takeoff position. Trailing edge flaps remain retracted.
  • Moving the flap lever to the 5, 15, 20 detent's, drives the trailing edge flaps accordingly. The leading edges stay in the takeoff position.
  • Moving the flap lever to the 25 or 30 (landing flap) positions simultaneously moves the leading edge slats and the trailing edge flaps to the landing position.
Non-Normal operation

Alternate slat/flap movement is provided by electric motors, one connected to each of the PDUís. Normal hydraulic power is by-passed. Pressing either or both the leading edge or trailing edge alternate flap switches allows the pilot to select flap using the alternate flap selector knob, which is next to the flap position indicator dial.

Flap movement is much slower using the alternate (electric drive) system. It takes about 3 minutes to drive the flaps from the UP position to the 20 degree position.

Flap asymmetry and flap load relief protection are NOT provided when using the alternate system. The NORM position on the alternate flap selector switch is when normal hydraulic system operation is occuring.

Flap load relief system

Once the landing flap setting of 25 degrees has been selected, any over-speeding of the flap upper limit speed causes the trailing edge flaps to retract automatically to the 20 degree setting, thereby protecting the flaps from becoming over-stressed by air loads.The flap position indicator will show the change, but the flap lever position will NOT change, remaining in itís original position. Once the speed has reduced to below 25 flap limiting speed, the flaps automatically extend to the lever commanded position.





Flap/slat asymmetry protection

Should the leading edge slats, or trailing edge flaps on one wing move at a different rate to that of the corresponding unit on the other wing, the flaps/slats will cease to move any more under the normal hydraulic system. You could use the alternate system but it is not recommended, as there is no asymmetry protection afforded by the alternate system. Asymmetric flap/slat is a serious threat, as this can cause uncorrectable roll due to the different lift being developed by each wing. You may have to land with whatever flap was lowered when asymmetric flap first was sensed.

Leading edge or trailing edge disagreement

If either the flaps or slats are NOT driving toward their commanded position, a warning light illuminates, and an EICAS message appears. Slats and flaps can be moved using the alternate system.

I hope this mini-editorial assists you in the ATPL Aerodynamics and Aircraft systems examination.

The next editorial will look at hydroplaning, which is part of the same examination.

Until then, happy flying !

Rob Avery

ATPL Lecturer

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Marty says ... "Goodbye to GA".

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