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In case of a total power failure in all the aircraft systems like engine failure and APU failure, would it be possible to use mechanical means (Manually) to open the landing gear bay door and deploy the landing gear through mechanical means?

I know it’s possible to glide the flight if the engines failed. But, wondering how they land.

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Not a real example, but this comes from the FAA, including some nice drawings around page 13-20:

The emergency extension system lowers the landing gear if the main power system fails. There are numerous ways in which this is done depending on the size and complexity of the aircraft. Some aircraft have an emergency release handle in the flight deck that is connected through a mechanical linkage to the gear uplocks. When the handle is operated, it releases the uplocks and allows the gear to free-fall to the extended position under the force created by gravity acting upon the gear. Other aircraft use a non-mechanical back-up, such as pneumatic power, to unlatch the gear.

The popular small aircraft retraction system ..... uses a free-fall valve for emergency gear extension. Activated from the flight deck, when the free-fall valve is opened, hydraulic fluid is allowed to flow from the gear-up side of the actuators to the gear-down side of the actuators, independent of the power pack. Pressure holding the gear up is relieved, and the gear extends due to its weight. Air moving past the gear aids in the extension and helps push the gear into the down-and-locked position.

Large and high performance aircraft are equipped with redundant hydraulic systems. This makes emergency extension less common since a different source of hydraulic power can be selected if the gear does not function normally.

In some small aircraft, the design configuration makes emergency extension of the gear by gravity and air loads alone impossible or impractical. Force of some kind must therefore be applied. Manual extension systems, wherein the pilot mechanically cranks the gear into position, are common. Consult the aircraft maintenance manual for all emergency landing gear extension system descriptions of operation, performance standards, and emergency extension tests as required.

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    $\begingroup$ in other words "it depends on the aircraft". Of course you can always attempt a belly landing, and then there's aircraft with fixed landing gear. $\endgroup$ – jwenting Mar 12 '14 at 9:56
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    $\begingroup$ There's also the possibility that the ram air turbine will generate enough power to lower the gear without resorting to mechanical means. $\endgroup$ – David Richerby Mar 12 '14 at 11:20
  • $\begingroup$ @DavidRicherby i'm going to expand on the answer a little later with examples if i got time. $\endgroup$ – Thunderstrike Mar 12 '14 at 11:56
  • $\begingroup$ Gimli Glider used gravity drop according to Wikipedia $\endgroup$ – orique Mar 12 '14 at 17:14
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    $\begingroup$ LOT B763 at Warsaw on Nov 1st 2011 landed gear up because a popped circuit breaker prevented alternate extension. Proves that in large planes some electricity is needed even for the alternate extension. But then electricity is needed to keep the hydraulics pressurized too, that's why there is the RAT. $\endgroup$ – Jan Hudec Mar 12 '14 at 21:25
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The rulebook for transport category aircraft says

14 CFR 25.729

c) Emergency operation. There must be an emergency means for extending the landing gear in the event of—

(1) Any reasonably probable failure in the normal retraction system; or

(2) The failure of any single source of hydraulic, electric, or equivalent energy supply.

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    $\begingroup$ I'm not sure total power failure counts as either, though. $\endgroup$ – a CVn Jul 11 '14 at 11:09
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Almost all aircraft have some sort of backup system for getting the gear down. On the planes I have owned they were:

  • Manual hydraulic pumps to pressurize the system with a car jack type handle. You sort of pumped the gear down. Obviously does not work too well if the problem with gear system is a no fluid (blown line or seal).
  • Mechanical crank (21 turns to get down, 2 more to lock) that drove the gears if the electric motor died. Good workout.
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The Twin Comanche I used for my AMEL rating had an interesting mechanism. First you had to disconnect the electric motor, then you stuck this handle into a hole and crank it as far as you can. Then you shift it to a second hole (that becomes usable after the first one is used) and crank on it again until it stops.

Not something you get to try until you need it though. Once you pull the disconnect it requires an A&P to restore the system to normal operation.

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The F-16 has an "emergency blow-down" reservoir containing approximately 3,000 psi gaseous nitrogen. Hitting a switch will release this pressurized nitrogen into the hydraulic system dealing with the landing gear. Ergo, hitting that switch will extend the gear, even with the engine off.

So yeah, the landing gear will still work in the event of a complete power failure. It has backup systems.

In the older A-4 Skyhawk, the landing gear retracted up and forward. So, in the event of a hydraulic failure, if you could get the gear doors open, the airflow past the aircraft in flight would "drag" the gear down and aft and lock it into position. I think that's a pretty slick design.

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