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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.

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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 at 7:47
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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 at 10:19
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Well. The term squat switch is self explanatory. Thanks – Fabrizio Mazzoni Mar 20 at 10:34
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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. – Brian Drummond Mar 20 at 19:01
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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 at 21:57
up vote 44 down vote accepted

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 a oleo strut which is compressed by the weight of the aircraft. Besides airborne / ground detection, the struts absorb the vertical energy during touchdown.

enter image description here

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.

enter image description here

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

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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 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 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 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 at 15:51
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@mins " it cannot be opened while driving" - don't bet your own money on that. youtube.com/watch?v=JA9piRHcQUE (and you can easily find more video evidence). – alephzero Mar 20 at 21:27

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, 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.

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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 at 8:06
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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 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 at 13:01
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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 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. – Malvolio Mar 21 at 19:05

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

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So THAT's why the gear lever has a wheel on the end of it! – TomMcW Mar 21 at 1:01
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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 at 3:43

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