I've noticed that Boeing 737-800's cockpit has a 3-position landing gear lever. During takeoff, the pilot moves the lever from DOWN to UP and then to OFF position. What's the point of the OFF position? Why not simply have UP/DOWN positions?
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5$\begingroup$ Although the question asks about the 737, this setup is used on all "older" Boeing craft, back to the 707. $\endgroup$– BowlOfRedCommented Aug 17, 2015 at 8:30
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2$\begingroup$ Just for complementing the Danny Beckett's answer: "In the event of turbulence the gear handle is selected back to UP to provide hydraulic up pressure to reduce structural loads on the mechanical uplocks." [Source: pprune.org/archive/index.php/t-351692.html] $\endgroup$– ClaudiCommented Aug 17, 2015 at 12:19
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$\begingroup$ @Claudix a number of people also replied saying they haven't heard of this before. $\endgroup$– Danny BeckettCommented Aug 17, 2015 at 12:21
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$\begingroup$ @DannyBeckett, yep, I just posted this comment to know what people from stackexchange thought about. $\endgroup$– ClaudiCommented Aug 17, 2015 at 12:28
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$\begingroup$ This arrangement can also be found on the 747. $\endgroup$– Burhan KhalidCommented Jan 27, 2016 at 7:32
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3 Answers
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With the gear handle in the UP position, the retraction side of the landing gear actuators remain pressurized. The gear will stay in the full up position, however they will not be hanging on the mechanical locks designed to hold them in the up position. They've simply hit the full up mechanical stop.
When the handle is placed in the OFF position, hydraulic pressure is removed from the retract side of the gear actuators, allowing the gear to settle down and onto the mechanical uplocks, which are essentially hooks that just hold the gear up.
There are a couple reasons why it's beneficial to select OFF. One is that if they remained pressurized, then the gear are essentially hanging in space, at a mechanical stop provided by the actuator. Hydraulic actuators are delicate components, and these weren't designed to hold landing gear for an extended time in the retracted position. Imagine the loads imposed on the actuator in turbulence for example, as the gear are loaded vertically with each bump. Removing hydraulic pressure allows the gear to hang on the mechanical uplocks that were specifically designed to hold them, without putting additional wear on the actuators.
The other reason is that gear actuators are major users of hydraulic fluid volume, and the actuators and the lines that supply them are a potential for leakage. If they were to remain pressurized for a several hours long flight, that increases the amount of pressurized lines and components that can leak and cause a complete loss of the hydraulic system.
Note that if you look at more recent Boeing airplanes such as the 777, 787, and 747-8, you will not see an OFF position on the gear handle, only DN and UP. The OFF function is done automatically on these models. After you select UP and the gear are all fully retracted, there's a short time delay and the system automatically depressurizes the retract side of the actuators. It removes one After Takeoff checklist item for the pilots to do.
To answer some of the questions posed in the comments: If you selected OFF while the gear were still in transit in some intermediate position between up and down, they would simply freefall into the down position since you've removed the retraction power. If there's a leak in the landing gear actuator or its associated hydraulic lines, it would cause a loss of that hydraulic system, and all other users of that system would be failed. The 737 has two (and a half) hydraulic systems. The half is a standby hydraulic system that's very limited in its function, but I digress.
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1$\begingroup$ I'm not sure it will "freefall". We are speaking of a significant weight whose abrupt displacement (freefall to from almost entirley up to down and lock position in less than few seconds) may be bad for flight control. I would think the design include drains such that if no power is applied, slow displacements of hydraulic fluid (and therefor displacement of gear retraction mechanism) and only those slow displacements are allowed. This way, gravity extension of landing gear may take several tenth of seconds but is smoother. $\endgroup$– Manu HCommented Aug 21, 2015 at 14:32
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$\begingroup$ not too sure. it would decrease the chances of getting gears locked by the help of lateral movements in case of double hydraulics failure $\endgroup$– userCommented Feb 10, 2016 at 10:35
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1$\begingroup$ If an actuator or hose let go, you wouldn't lose the whole associated hydraulic system. There are fuses that limit how much fluid you can lose through the gear actuation system. Now if you were to open a hole in the system upstream of the fuses, then maybe. As for manual extension; that takes several seconds from full up. $\endgroup$ Commented Jul 1, 2017 at 4:49
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$\begingroup$ @ManuH: I thought that landing gear were specifically designed to be able to freefall, so that, if the gear hydraulics fail, the gear can still be locked in the extended position in preparation for landing by having it reach full extension forcefully enough to engage the latching mechanism? $\endgroup$– VikkiCommented Apr 27, 2018 at 18:01
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$\begingroup$ @Sean I read somewhere (unable to find the reference now) that the pilot may shake the plane if the locking mechanism do not engage. Yet, there can be a good compromise between slow enough to avoid abrupt mass displacement and fast enough to engage locking mechanism. $\endgroup$– Manu HCommented Apr 27, 2018 at 19:31
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When UP or DOWN is selected, the hydraulics pressurize in the appropriate direction.
Once the gear is up and the mechanical locks are keeping the gear locked in place, OFF can be selected to disconnect the hydraulic power.
answered Aug 16, 2015 at 18:19
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5$\begingroup$ Is it also true that you could move the lever to OFF with the landing gear in an intermediate position? Would there ever be a situation where you'd want to do this? $\endgroup$ Commented Aug 16, 2015 at 23:48
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16$\begingroup$ -1. this answer does not answer the question at all. It merely describes the operation. The question was what is the "point" of having an "off" position? Why did they build it that way? No other MFG seems to need it as they are all(mostly) either up or down only. It is easy for a system to automatically sense up or down and not require pilot interaction for it so why did Boeing think differently on this particular model? $\endgroup$– Steve HCommented Aug 17, 2015 at 2:20
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6$\begingroup$ Why can
OFFnot be inferred by byUP? What would be a situation where you would want your landing gear to beUPbut notOFF? $\endgroup$ Commented Aug 17, 2015 at 3:34 -
$\begingroup$ @SteveH, not limited to this particular model. Is present on 707 and probably just continued to be used on models since (but not on 777). I imagine folks didn't trust the "automatic sense" bit as much back in 707 days and just put it under pilot control. $\endgroup$ Commented Aug 17, 2015 at 8:26
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6$\begingroup$ If there was no off position the control system would be responsible for shutting down the hydraulic power when the gear reached its end stop. If there was a hydraulic leak, I'm guessing the gear would stop moving and the control system would pump all the hydraulic fluid out through the leak. Does anyone know if that circuit serves only the gear, or does it also serve more vital parts (like control surfaces)? $\endgroup$ Commented Aug 17, 2015 at 13:16
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I worked on the 737 Landing Gear system design in the 90s and can try and answer the question from memory. All Boeing Commercial airfraft used mechanically positioned hydraulic selector valves until the 777, 787 and now the 737 NG starting about 3 years ago (now electrically commanded). The cockpit handle drives a quadrant which drives a cable loop to a 3 position spool valve in the MG wheel well. So it can't be automatically depressurized by an electronic control system. Additionally there are several reasons to depressurize it in flight, although that question was heavily debated during the 777 system design.
The Nose Gear hydraulic pressure line which connects to the selector valve in the MG wheel well runs through the pressurized cabin compartment below decks. If it sprung a pin hole sized leak at high pressure it would eventually mist the cabin air making it uncomfortable to breath. Skydrol is nasty stuff to get in your lungs. Been there. So depressurizing it makes a leak less likely (in theory) and if it did leak at low pressure it would not form a mist.
The landing gear hydraulic supply is common to other flight control systems and so the hydraulic pressure droops from system fluctation would add fatigue cycles to the landing gear system if the valve remained in the UP position in flight.
On the other question; leaving the gear handle up in rough weather is news to me but there could be some airlines that chose to do so because the nose gear has a tendency to unlock in flight if it bounces hard enough. It doesn't have an uplock hook but instead has a reinstating overcenter lock brace. So I could see it being done. Also, if both Main gears are deployed at full cruise and do not come back up, this is a big deal. On the Seattle Honolulu run, it would be a tough swim due to the drag fuel burn issue.
Note: On the 777 there are two selector valves. One for the nose and one for the main. So the line running to the nose is pressurized in flight. Or at least it was when I worked the system in the early 90s.
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2$\begingroup$ Good info - thanks for adding a perspective that is new for even those of us who fly the 737's -- I'd never considered the idea of the line inside the pressurized cabin & the misting issue, though it makes complete sense as soon as you explain it. Welcome to Av.SE, and please check back often. The Boeing jets get a lot of discussion here, and the history you've seen will be a great addition to the board! $\endgroup$– Ralph J ♦Commented Aug 29, 2019 at 5:51