What does the F-16 (perhaps other fighters as well) use to decide when to deploy leading edge flaps? I would assume angle of attack, airspeed, stick deflection, or some combination of the three, are the main factors.


4 Answers 4


Technically, F-16 and "similar fighters" have leading edge flaps (LEF) (or droop flaps), rather than slats. The difference is that they don't form a gap between themselves and the main surface when deflected. This makes them usable at high speeds, whereas slats are typically used for low-speed takeoff/landing.

For fighters, the most important use for them is to extend the range of usable angles of attack (AoA), which is important for manoeuvring as well as for takeoff/landing.

For F-16, @Mr R posted some details. I have some info for Soviet fighters of the same generation.


LEFs have two positions, up and down (20°). They are deployed:

  • When landing gear is down (including takeoff and landing).
  • Automatically in flight when AoA > 9° and M < 0.8 (Mach number).
  • On manual override by the pilot.


LEF on Su-27 don't have fixed positions and constantly track AoA in flight. Maximum deflection 30°.

They are automatically deployed to 3/4 of travel when landing gear is down for takeoff or landing.

The pilot has a manual override (down-auto-up selector).

For both aircraft, if LEF fail, the allowed AoA is limited to 10°, which is roughly half the normal (depending on speed).


According to a F16 forum..

Those are the LEF's (leading edge flaps). They are there to produce extra lift to the wings during high AOA and low airspeeds. The reason they are up on the ground is they are wired to the left and right main WOW (weight on wheels) switches. They are schedule to -2 degrees on the ground. The only time they move on the ground is during FLCS self test or during LEF ops checks. In flight they are scheduled as a function of AOA and airspeed.

OR AoA and Mach Number (rather than airspeed).

  • 3
    $\begingroup$ and they actually move up (to form a diamond shape) at supersonic speeds $\endgroup$
    – DeepSpace
    Apr 27, 2021 at 19:25
  • $\begingroup$ Same thing happens for flaperons, which means that go up in same way at Mach speed to avoid Mach tuch phenomenon. $\endgroup$
    – George Geo
    Jul 22, 2021 at 2:53

Early F-16 LEF schedule according to NASA TP-1538: NASA F16 LEF

It's basically 1.38 times the Angle of Attack, with a lead-lag filter, and then minus 9.05 times Qc/Ps. Qc/Ps is a rough indication of mach number. So what this control logic actually did is that, the LEF deflection increases with AOA, and decreases with mach number.

If WOW or reaching supersonic speed, the LEF is deflected 2 deg upwards.

For Su-27, the main source is chapter 8.7.8 in the SU-27SK flight manual. Here's a rough summary:

In FLIGHT mode, the LEF are deflected in the range of 0-30° at Angles of Attack of 1°-15°. The relation is basically 2 x AOA.

In TAKEOFF and LANDING mode, the LEF are deflected by a fixed angle of 23°.

When IAS ≥ 860 km/h or M ≥ 1.05, the LEF are retracted. When flight speed decreases, the automatic scheduling of LEF turns back on at IAS ≈ 790 km/h or M ≈ 0.98.


For the F-16 only talk, the wings must be at an angle of attack more than 9 degrees slower speed or more if you are making a turn but no more than Mach 0. 95 above this value is upward 2 degrees and locked. For the F 16 with TO /Landing gains(landing gear and trailing flaps _flaperons _at 20 degrees down, the Leading Edge Flaps are 15 degrees down - on speed - if not will vary depending on the AOA or weight on wheels = upward 2 degrees ).

  • 6
    $\begingroup$ What is your source for this information? $\endgroup$
    – Ralph J
    Aug 4, 2021 at 13:21

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