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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.
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.
The rulebook for transport category aircraft says
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.
Almost all aircraft have some sort of backup system for getting the gear down. On the planes I have owned they were:
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.
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.
All aircraft have some kind of backup system. As mentioned before- Skyhawks Landing gear gets sucked down. most of the other planes- just fall down thanks to gravity.
A lot of the airplanes (including military and some civilian)- have a handle to open the bays mechanically (like in the 737) and the gear falls down and locks automatically with a mechanism that is independent of all hydraulic or electricity.
As for the big jets- they have an amazing amount of backups. And still- if in a 787 all your computers catch fire for some reason- probably you wouldn't be able to extend your gear. But then- even flying the plane would be a challenge, as the aircraft needs voltage for everything. In this case, there is another backup for flying the plane using a special connection of direct wires from the stick to the flight control surfaces, powered by backup generators.
It all depends on the airplane in question.
Typically for small GA airplanes using an electro-hydraulic power pack for actuating the gear, they’re held retracted by hydraulic pressure. If the system fails, the pressure goes to zero and the gear just drops down due to gravity. I have time in the DA-42 and DA-62 twins and those airplanes use an alternate gear extension handle - basically an emergency valve which releases hydraulic pressure and allows the gear to drop down on their own.
Electrically powered landing gear can be typically lowered by means of a hand crank in the cockpit. This is prominent on Mooneys and light Cessna twins.
Larger airplanes and jets may make use of emergency reservoirs of compressed nitrogen called blowdown bottles to provide emergency hydraulic pressure and extend the gear. Typically the landing gear retraction jacks can be isolated from the main hydraulic system via a cockpit switch to prevent total loss of hydraulic fluid in the event of a leak. It also removes the additional workload from emergency systems eg APU, RAT, etc. for more critical flight control functions.
Transport category airplanes often have multiple layers of redundancy ie multiple hydraulic systems and power sources to get the plane down safe.
