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Has there ever been an uncommanded lowering of the landing gear on a large commercial jet? I could not find any instance of it in a preliminary google search. Could a simple short-circuit cause such a thing?

I think this would actually be pretty bad at cruising conditions, because the airplane is going much faster up there, than on approach. But the air is also much thinner so I can't be sure.

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    $\begingroup$ IIRC (so take this with a large handful of salt), jetliners cruise at something like 300-400 KIAS, and land at maybe 100-120 KIAS. Since it's indicated airspeed that would be of interest here, it stands to reason that lowering of the landing gear would still be much worse in cruise than near landing. $\endgroup$
    – user
    Commented May 27, 2018 at 14:29
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    $\begingroup$ @MichaelKjörling, I am rather confident jetliners cruise below 300 KIAS. The Vmo is usually 320 or 330 knots indicated, but the Mmo becomes the limit above around FL270 and pushes it below 300 knots indicated. $\endgroup$
    – Jan Hudec
    Commented May 27, 2018 at 22:24
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    $\begingroup$ @JanHudec Yes, very much so. High 30's for a typical cruise altitude, Mach .78 ballpark for a 737, you could have 450 knots +/- TRUE airspeed, but 250ish knots INDICATED. Dropping the gear there would be LOUD, but within its operating limits. With millions of commercial airline flights per year, the answer to "has there ever" is surely "yes". Noteworthy enough to get its own internet article, doubtful. $\endgroup$
    – Ralph J ♦
    Commented May 28, 2018 at 4:27
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    $\begingroup$ @JanHudec It's certainly possible. Like I said, apply a large handful of salt. $\endgroup$
    – user
    Commented May 28, 2018 at 7:07
  • $\begingroup$ I have never heard of uncommanded gear extension/retraction. My guess is no it has never happened but a very thorough database search would be needed to confirm/disconfirm that. $\endgroup$
    – kevin
    Commented May 28, 2018 at 7:33

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The answer to your question is, in fact, "yes" - but the circumstances involved demonstrate just how difficult it is to make an aircraft's landing gear deploy without being commanded to do so.

In 1985, CI006, a 747SP, experienced an uncommanded flameout of its #4 (right outboard) engine (something that particular engine was quite prone to doing) while cruising at FL410 en route from TPE to LAX. An attempted relight failed, and a lack of corrective flight control inputs combined with a poorly-designed autopilot caused the airplane to decelerate and gradually roll into a steep bank despite the autopilot's attempts to counter the thrust asymmetry; the captain eventually noticed this and disconnected the autopilot, but made no flight control inputs of his own.

This, unsurprisingly, resulted in an immediate loss of control, and the aircraft rolled into a steep inverted dive, during which it exceeded mach 1 for an indeterminate period of time, plunging 30 kilofeet before the captain was able to orient himself when the aircraft broke out of the clouds at 11 kft and pull out of the dive, eventually levelling the aircraft at 9,600 feet. During the dive and subsequent pullout, the aircraft was subjected to aerodynamic loads exceeding +5 Gs,1 which, in addition to causing severe structural damage to the aircraft (including tearing away large parts of the aircraft's horizontal tail), ripped off the body gear doors, broke the support brackets holding the body gear uplock hooks in place, and forced the aircraft's left and right body gear into the down-and-locked position. (The added drag from the extended body gear [and from the aircraft's 29-kft maximum gear-down altitude] forced the flight to divert to SFO, having insufficient fuel to reach LAX with almost half of the aircraft's landing gear hanging out in the airstream.)

So, yes, it has happened... during an over-5-G pullout from a 30-kft supersonic dive in a 747.

1: The highest normal acceleration recorded on the aircraft's FDR was +5.1 Gs during the pullout from the dive; however, the absolute maximum G-forces encountered by the aircraft are unknown, as the extreme G-forces exerted on the aircraft, and the FDR, caused the latter to fail to record properly for large portions of the dive and pullout.

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    $\begingroup$ “despite the autopilot's attempts to counter the thrust asymmetry”—the thing is that autopilot doesn't know how to do that. It controls only pitch and roll and yaw damper independently controls rudder to damp the dutch roll only, so it won't apply the rudder as is needed for outboard engine failure. $\endgroup$
    – Jan Hudec
    Commented Oct 22, 2019 at 20:52
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    $\begingroup$ as far as I know all autopilots, by design and very intentionally, don't handle asymmetric thrust. It is something nobody wants to have in an autopilot. Airbus will even indicate how much rudder trim should be added, but it requires pilots to do it. It would also have to be a special function, because autopilot does not normally have any reason to manipulate rudder, and nobody wants that either. $\endgroup$
    – Jan Hudec
    Commented Oct 23, 2019 at 9:26
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    $\begingroup$ @JanHudec: The A300/A310's autopilot does use the rudder to compensate for engine-failure-generated asymmetric thrust (see the footnote on page 19/33 of this AAR), and I would be surprised if they had removed that autopilot feature in their later airliners (given Airbus's automation fetish). $\endgroup$
    – Vikki
    Commented Nov 10, 2019 at 1:55
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    $\begingroup$ there is similar note in A320 materials that says it will indicate the correct amount of rudder trim, but won't add it. Which does not mean it does not compensate anything—the normal law will keep wings level, and the yaw damper will probably add some rudder, but the plane will be in slight skidding turn. Regarding deleting features, keep in mind that the A320 control system was completely designed from the ground to be consistent and simple. A lot of things are completely different from A300. $\endgroup$
    – Jan Hudec
    Commented Nov 10, 2019 at 11:45
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    $\begingroup$ That's pretty impressive. Is this the max speed achieved by a 747 that made it to a runway? $\endgroup$
    – Organic Marble
    Commented Sep 11, 2020 at 16:25
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Only thing I can find is on the X-15 rocket plane. Expansion of the airframe caused the nose gear to come unhooked and deploy at Mach 4.2. The air friction overheated the tires, damaging them and causing them to disintegrate on touchdown. The pilot was still able to make a safe landing.

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  • $\begingroup$ "The air friction tires and caused them to disintegrate on touchdown." – Are you missing a verb near the beginning of that sentence? $\endgroup$
    – Sophie Swett
    Commented Feb 1, 2020 at 14:09
  • $\begingroup$ @TerranSwett I’m missing something, but I don’t remember what I was trying to say. $\endgroup$
    – TomMcW
    Commented Feb 1, 2020 at 23:54
  • $\begingroup$ @TomMcW: How's it look now? $\endgroup$
    – Vikki
    Commented Sep 11, 2020 at 0:48
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    $\begingroup$ Question asks specifically for "a large commercial jet" $\endgroup$
    – Organic Marble
    Commented Sep 11, 2020 at 16:24
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Typically it takes two points of failure for it to happen on a jetliner: the gear uplock to fail, and the door uplock to fail.

I found incidents where landing gear doors fell off jetliners, and only one incident where the gear uplock failed on an Airbus A320, but the door held it inside.

The crew had to slow down to a safe 'gear extended' speed.

A320 flight crew experiences a nose gear uplock failure climbing through FL200. Nose gear doors remain closed but aircraft is slowed to 220 KTS to comply with overspeed warning and ECAM logic (37000feet.com).

'L/G GEAR UPLOCK FAULT' is one of the abnormal procedures in the A320 flight crew operating manual. The first two items is to keep the landing gear down and a max speed of 280/.67.

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