Can the landing gear be pulled up while the plane is on the ground?

Question

If a pilot makes a mistake and inadvertently touches the gear up lever, will it actually activate while on the ground and make the plane drop onto its belly?

I suppose that there would be some sensor to prevent this, but I would like clarification on that.

On 747-100/200 aircraft there's a sensor setup that determines whether the aircraft is on the ground. When the sensor says the aircraft is on the ground, a metal flange is positioned in the vertical slot that the gear handle must move through to raise the gear. The flange physically prevents upward movement of the gear handle. When you take off, you can hear the flange retract when the gear is off the ground, and you can then move the gear handle up through it's vertical slot. — Terry, Mar 20 '16 at 07:47
Probably a duplicate of: How does the squat switch work? but the existing answer could be improved easily. A search on "squat switch" retrieves good images, like this one, cited in a comment of the linked answer. — mins, Mar 20 '16 at 10:19
Without interlocks, raising the landing gear on the ground was an important plot point in Nevile Shute's novel "No Highway". This could be written off as artistic licence, except that his "day job" was aircraft design. — user_1818839, Mar 20 '16 at 19:01
I recall a story of a fighter pilot who accidentally landed with gear retracted and then tried to cover up his mistake by flipping the switch on the ground- unfortunately for him there was enough pressure remaining in the hydraulics to cause the aircraft to lurch up on the gear. Probably 1970s give or take a decade or two. — Spehro Pefhany, Mar 20 '16 at 21:57
@SpehroPefhany - somehow I doubt very much that landing a fighter, or any aircraft, with the gear retracted could be "covered up" by putting the gear down once all the excitement had subsided. — Bob Jarvis - Слава Україні, Mar 21 '16 at 02:35
@BobJarvis The idea being that the pilot could claim he was not at fault because the gear failed to deploy, of course. — Spehro Pefhany, Mar 21 '16 at 02:59
This just makes me think of the time where Homer Simpson did exactly this - "I keep telling you I'm not a pilot!" "And I keep telling you, you flyboys crack me up!" — Nuclear Hoagie, Mar 21 '16 at 08:39
@SpehroPefhany I can't imagine a military pilot, or any pilot for that matter, not being acutely aware that the FDR would always tell the true story. I don't doubt that you recall this story... I'm just not so sure I'm ready to believe that it's true. — J..., Mar 21 '16 at 10:36
@J Way out of my area of expertise, but I don't think all F-111-era or earlier military planes had much in the way of flight data recorders. Of course it's possible the story (in Popular Science or whatever it was) was inaccurate and/or anecdotal. In any case, a failure analysis would have shown there was nothing wrong with the aircraft so the point is valid. — Spehro Pefhany, Mar 21 '16 at 16:52
The following link shows a fighter plane gear retracting DURING takeoff, resulting in a long slide. However, I don't know if it was mechanical failure or pilot error. https://www.youtube.com/watch?v=-Cunaek9W24 — James, Mar 21 '16 at 19:06
I've heard of a show-off pilot ordering the gear up before taking off, trusting the switch to not actually pull it up until after takeoff. (The idea was to pull it up the instant the plane was airborne.) Unfortunately, there was a little hump in the runway... — Loren Pechtel, Mar 22 '16 at 05:30
I used to work at an aircraft works, and we could watch the pilots taking off and landing. Once a test pilot was practicing take off, wheels up, loop round, wheels down, land and roll, take off... of course the time came when he forgot to put the wheels down and just slid along the ground. We looked at each other and said "well, there went our Christmas bonus." — RedSonja, Mar 22 '16 at 13:56
Anecdotally, I have heard of a Delta 727 that allegedly had this happen during maintenance. As I understand the situation, power had been removed from the aircraft and in that timeframe the gear handle was raised to perform some sort of maintenance check. The handle wasn't tagged out, neither was power, and another mechanic came into the cockpit, turned power on and immediately turned on hydraulic pumps. I've heard that story told a few times from different people but don't know if it's actually true. — Frank, Jun 01 '17 at 19:25

score 55 · Accepted Answer · edited Jun 01 '17 at 19:09

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There is a weight sensor which senses if the plane is on the ground. This sensor prevents gear retraction while the plane is still on the ground. Failure of this sensor would prevent gear retraction after takeoff.

If you note closely, the landing gears (even the non-retractable ones) are not connected using a simple metal pole; rather, there is an oleo strut which is compressed by the weight of the aircraft. Besides airborne / ground detection, the struts absorb the vertical energy during touchdown.

I recall some decades ago engineers experimented obtaining the plane's gross weight by installing weight scales to each landing gear (as opposed to just an air/ground detection). The readings were found to be inaccurate and they soon abandoned the idea.

Landing gear stories cannot be complete without mentioning this incident in 1990:

A training captain of a Saab 340 was betting with his students that the weight-on-gear mechanism would prevent gear retraction while on the ground. On the accident airplane type, the mechanism would lock the gear handle, but the lock can be overridden if the pilot manually pull out and move the handle. The instructor confidently pulled out the handle and to his surprise, the hydraulics started to move and the gears were retracted while the plane was still on the ground.

(image source)

The aircraft was written off. This incompetent instructor pilot was killed 11 years later in another accident.

edited Jun 01 '17 at 19:09

msanford

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answered Mar 20 '16 at 05:39

kevin

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I wouldn't call the Saab pilot incompetent on the basis of the totaled 340 alone. My curiosity has done damage to lots of equipment, and I encourage that same curiosity in my children. Imagine the look on his face when the gear started retracting! – dotancohen Mar 20 '16 at 14:36
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@mins: As a software developer, I would file that under "Things nobody ever considered might happen, not even QA". Automated and dynamic smart test suites might catch those issues in software and CAD today, as they can dynamically explore all the code and component paths. But on a mechanical system designed in the late 70's and early 80's, I can see how it slipped through. – dotancohen Mar 20 '16 at 15:11
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@dotancohen Despite the training captain's vast experience and reputation, his competence was quickly brought to question after the deadly accident of Crossair 3597. Throughout his career he has failed multiple checks and examinations even after repeated attempts, and graded "below average" on many occasions. – kevin Mar 20 '16 at 15:38
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@kevin: That brings the competence of his superiors into question as much as his own. Who let him hold the stick with that many issues? – dotancohen Mar 20 '16 at 15:51
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@dotancohen you're encouraged to read the final report of the crash to find out more (= You may start at page 20. – kevin Mar 20 '16 at 15:56
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It is possible to override this mechanism (at least on some planes - I'm no expert, just remember this from working on simulation for aircraft repair), which is sometimes done when servicing the landing gear. Needless to say, the plane should be placed on servicing jacks first. Jacking up a plane (especially a large one) is no small task - on the ones I was working with it can take most of a day - so it is not a common procedure. – Darrel Hoffman Mar 20 '16 at 18:07
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@mins That capability would almost certainly be needed for maintenance. Even in maintenance, though, they shouldn't need to retract while the squat switch is in the ground position (I'd assume the jacks lift the gear off the ground before any attempt is made to retract the gear, since otherwise the tires would be damaged.) Also, a condition that prevents the gear from retracting while the engines aren't running could be a problem in the case of something like US Air 1549. – reirab Mar 20 '16 at 18:49
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@reirab: No offense, but a car hood is also required to open for maintenance, but it cannot be opened while driving, even if I pull on the opening lever, there is a security. Considering the price of an aircraft... this makes me think sometimes aviation is incomprehensibly archaic, just like for voice/data recorders and their ridiculous ULB, compared to the cost of recent SAR operations. – mins Mar 20 '16 at 19:16
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@mins " it cannot be opened while driving" - don't bet your own money on that. https://www.youtube.com/watch?v=JA9piRHcQUE (and you can easily find more video evidence). – alephzero Mar 20 '16 at 21:27
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@mins That's why I said the squat switch would still be a reasonable mechanism for a lockout. Just not the engines running. You generally don't want the engines running when you're doing maintenance on the landing gear... especially wing-mounted jet engines. But, yes, much of aviation technology is indeed quite a bit older than the technology in most cars. There are two main reasons for this: getting any kind of changes designed and certified for an aircraft is very expensive and most aircraft are simply much older than most cars. – reirab Mar 20 '16 at 22:22
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@mins: Car hoods can open while driving. I've had it happen to me. Though I'm surprised that the hydraulics can produce enough force to pull the main gear sideways while the weight is on the wheels. – jamesqf Mar 21 '16 at 05:40
@jamesqf and afephzero: This shouldn't be possible on cars sold in France (and probably in the EU), there is a security on the hood itself that must be operated manually (so you need to do two actions in that order: In the car pull up the lever, then go to the front and release another lever. I never heard of a default. – mins Mar 21 '16 at 09:01
Another Re: Car hoods - I left mine open, but with the safety catch latched. Two miles down the road at 50 MPH the safety catch was not enough. I was very, very confused. And now I'm paranoid about it happening again so I driving down the road I always look at the end of the hood to see if it's popped up at all. – Wayne Werner Mar 21 '16 at 15:02
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@mins: Yes, the safety catch is supposed to prevent the hood from opening while driving, but it doesn't always work. Like the squat switch is supposed to prevent the gear from operating while there's weight on wheels... – jamesqf Mar 21 '16 at 17:12
@jamesqf: "it doesn't always work", for me it works and I see no reason it would be faulty. Sorry but I can't buy the hood that opens while driving if nobody tampered with it. This hood story is just an illustration of whether gear retraction process can be improved, I think everyone interested has got the picture. Of course cabin doors can open in flight, and fuel tanks can leak, everything can fail exceptionally, and in this case corrective actions are required. – mins Mar 21 '16 at 18:28
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It's sort of interesting that the Saab 340 was written off. In the picture, it looks like it settled down nicely. I wonder what the damage was and why it couldn't be economically fixed. Was the structure compromised just by its weight resting on solid ground like that? – Dronz Mar 22 '16 at 00:42
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@Dronz: You may ask this question in a separate post (but ensure you do not ask for opinions but facts) – mins Mar 23 '16 at 18:56
@dronz I doubt the aircraft "settled down nicely.' It wasn't landing at the time. Most likely it tipped to one side or the other in the process (think what happens when someone walking with two crutches loses traction and falls) and damaged one or both wings, or key portions of the airframe, landing gear mounts or maybe even the wing spars to the point where the aircraft was not economically repairable. – Juan Jimenez Apr 09 '17 at 15:19
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@mins "I see no reason why it would be faulty." Um... What? A fault means something is wrong with it. Anything can foul up and make a situation go sideways. Nothing is perfect. – T.J.L. Apr 03 '20 at 12:21
@T.J.L. With due respect, the well-worn risk zero doesn't exist is an obviousness nobody will challenge. It's true an aircraft engine will eventually land in my garden if time is not counted. In reality, the chance such event can happen within a lifetime is quasi-null, it's acceptable to say "I see no reason why it would fail". – mins Apr 03 '20 at 16:24
@mins "I see no reason" and "It is not likely" are two entirely different statements. – T.J.L. Apr 03 '20 at 17:59
@T.J.L.: Ah I see, then let's say, "I see no statistically significant possibility for it to fail" – mins Apr 03 '20 at 18:24
The only question still left: did The Simpsons actually inspired this bet, or vice versa did they hear about the incident and put it in the show? – beefeather Apr 07 '20 at 19:15

score 20 · Answer 2 · edited May 03 '22 at 17:32

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This incident was "half on ground" during takeoff:

Last year, the copilot of a Dash 8 applied erroneously the gear lever during takeoff as the rear gear had still ground contact. This resulted in a tail strike. The aircraft bounced back on runway 9 of Saarbrücken's airport, slipped some hundred meters and will be written off as well.

See the report of the German aviation authority (English Edition of The Report), pages 60-76. It turns out that the Dash 8 has a weight-on-wheels sensor only on the front gear. The report says that the design responsible called this to "comply with the design logic".

Differently from the incident @kevin mentioned, the gear doors were already closed as the fuselage touched ground.

edited May 03 '22 at 17:32

Atheesh Thirumalairajan

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answered Mar 20 '16 at 20:52

Horst Grünbusch

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This makes you wonder if the designers ever saw a plane take off. You'd expect that the landing gear designers would know which wheels land first and take off last. – MSalters Mar 21 '16 at 08:06
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@MSalters, I think this falls under dotancohen's comment on kevin's answer - nobody in design or QA thought of the possibility of that during test. They are humans, after all... – FreeMan Mar 21 '16 at 12:53
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@FreeMan: Reviewing engineering plans is a learned skill. If I review a fellow engineer's idea for a squat sensor on one wheel, I know what failure mode it's tyring to prevent: "retract gear while there is still weight on the gear". I then ask myself, "what does fail mean in this context?" The answer would be that there is still weight on the gear, and yet it's retracted. How can that happen? The weight isn't on the front wheel with the sensor. Then it follows the weight is on the other wheels. When is the plane on its rear wheels? At takeoff. So what happens if I then retract the gear? Oops – MSalters Mar 21 '16 at 13:01
I'm not some sort of supergenius, but I know there are two general failure modes for any mechanism: Type I, not working when you're supposed to work, and Type II, working when you are not supposed to. Sometimes, one mode is much less important than the other (and, without knowing much about planes, I would guess it's better to refuse to retract than to retract wrongly), but if one of them doesn't matter at all, make a mechanism that never does anything or always does something. @MSalters chain of reason strikes me as obvious and obviously correct. – Michael Lorton Mar 21 '16 at 17:50
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@Malvolio: There's also a generally nasty case of "stuck half way" which you need to consider. E.g. gear retracted, but gear bay doors refuse to close. Or worse: gear started to fold, and cannot carry the planes weight anymore, but did not full retract. The squat switch is supposed to be binary, but real physics tend to be analog. What if the sensor shorts out and doesn't give a reading? And since it's close to a wheel, what if the wheel/brake catches fire? It would be quite dangerous if you have a fire and then have the gear collapse. As a QA engineer you have to consider everything. – MSalters Mar 21 '16 at 18:58
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@MSalters, I'm a programmer not an aeronautical engineer, so the problems of analog don't come up in my own life, but a friend of mine is a private pilot and once found himself in exactly the situation you describe: one gear wheel partially retracted, neither a gear-down nor gear-up landing was possible. His solution was to open the door (in flight!) and snag the dangling wheel with the tow-handle and pull it the rest of the way up. After that, it engaged properly and would lower and lock in place. – Michael Lorton Mar 21 '16 at 19:05
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@Malvolio programmers may not have to deal with analog situations that much, but nondeterminism can crop up in the most unlikely of places... https://gcc.gnu.org/bugzilla/show_bug.cgi?id=323#c43 – JAB Mar 21 '16 at 21:13

score 8 · Answer 3 · answered Mar 20 '16 at 21:49

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Safety standards have improved since the 1940s, but soon after a £1m restoration project on a WWII Spitfire, somebody blocked the runway at a local airport by demonstrating this design flaw. Apparently the pilot got confused about which of two levers retracted the flaps, and which retracted the landing gear.

The accident happened just before sunset, so I don't think there are any good pictures available on the web that show the damage.

http://www.bbc.co.uk/news/uk-england-leicestershire-22474805

https://assets.digital.cabinet-office.gov.uk/media/5422ed3640f0b6134200014f/Vickers_Supermarine_Spitfire_MK_XIX__G-RRGN_5-2013.pdf

answered Mar 20 '16 at 21:49

alephzero

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So THAT's why the gear lever has a wheel on the end of it! – TomMcW Mar 21 '16 at 01:01
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@TomMcW Well, you have half an excuse for not knowing your right hand from your left when flying that particular Spit. It has a Griffon engine instead of the more common Merlin, so the prop rotates the opposite way from what you might expect. – alephzero Mar 22 '16 at 03:43

Can the landing gear be pulled up while the plane is on the ground?

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