Brief Background of the Incident

Mayday Season 22 Features an Episode "Terror over Michigan" (Episode 6) based on the events of TWA Flight 841. The Episode revolves around a Boeing 727 cruising at 39000ft, Suddenly Loses 34000ft altitude in about a minute.

The episode depicts a shortcut (deviation from the SOPs) which involves pulling a circuit breaker leading to prevention of automatic extension of slats when the flaps are deployed (Flaps 9, I guess). The investigation talks about this procedure used sometimes to improve the performance of the aircraft while in cruise (which involves extension of flaps but NOT slats)

The episode takes a course where the circuit breaker pulled was accidentally pushed back again which caused the unintentional extension of the Slats. The aircraft starts rattling, on realizing the issue, the flap lever is pulled back to retract them but during retraction of both slats/flaps, Slat #7 remains extended in an unusual position leading to the uncontrollable descent (disruption of airflow over Slat #7). The documentary states at about 5000ft, the upset had caused the hanging Slat #7 to break off the aircraft, making it controllable again.

My Question

Does the sophistication built into present day airliners like the Boeing 737-800, A320neos, A350s, etc. have any protection feature to prevent Extending the Flaps but NOT SLATS during cruise? If they exist, Can they be overridden in the Flight Deck?


1 Answer 1


There are protections against selecting flaps and slats during cruise in modern airliners.

For example, on the Airbus A380 there is a function to prevent extension of flaps 1 during cruise conditions:


In case the FLAPS lever is inadvertently moved from 0 to 1 during cruise, the slats/flaps cruise baulk function will maintain the slats and flaps in their fully retracted position. In addition, the ECAM will trigger the F/CTL FLAP LEVER NOT ZERO warning.

The slats/flaps cruise baulk function activates, when the indicated airspeed is greater than the VFE of CONF 1 +2.5 kt, or the aircraft is at an altitude of more than 22 000 ft.

The slats/flaps cruise baulk function is valid only for FLAPS lever position 1. The selection of any other slats/flaps setting (2, 3, or FULL) will override the cruise baulk function, and allow slats' and flaps' extension.

(Airbus A380 FCOM 27 Flight Controls - Slats/Flaps)

While this function is only available for flaps in CONF 1, the normal flap load relief system (FLRS) is active for CONF 2, 3, and FULL.

Similarly, on the Boeing 777 any selection out of the UP position is inhibited during cruise conditions:

To protect against inadvertent deployment during cruise, flap and slat extension from the UP position is inhibited when speed is more than [777-200] 250 [777-200ER and 777-300] 265 [777-200LR, 777F, and 777-300ER] 275 knots or altitude is above approximately 20,000 feet.

If the flap handle is moved out of UP while the flaps are inhibited, LOAD RELIEF displays.

(Boeing 777 FCOMv2 9.20.19 - Flight Controls - System Description)

For most systems, there is a way to override it in the flight deck because any failure of this system would otherwise create a high risk. Possibly, this could be done by pulling a circuit breaker, but I'm not sure if either of the above systems have a dedicated circuit breaker (not documented in the FCOMs). It might also be possible to override these systems by removing air data sources since the systems are based on knowing the airspeed. This would however create more problems.

The Airbus A350 automatically uses a small amount of differential flaps during cruise to reduce wing bending (for details, see this answer).

  • $\begingroup$ Thank You! But I have one question. According to the A380s FCOM (You quoted above) The selection of any other slats/flaps setting (2, 3, or FULL) will override the cruise baulk function, and allow slats' and flaps' extension.. Now the question is, if I set flaps 2 or 3 or even FULL during cruise, can i prevent the extension of slats along with it? In the accident aircraft, I see the circuit breaker prevents extension of slats when flaps are deployed. $\endgroup$ May 3 at 23:55
  • 1
    $\begingroup$ @AtheeshThirumalairajan If you really want to, you can do it. On the A380, the slats are driven by an electric motor or the green hydraulic system. The flaps are driven by the green or yellow hydraulic system. The electric motor can almost certainly be disabled with a circuit breaker. If you now disable the green hydraulic system (turn off engine driven pumps and electric pumps), the flaps could still extend while the slats would be unable to move. This is crazy and reckless of course, but technically possible. $\endgroup$
    – Bianfable
    May 4 at 6:04

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