Forum: aviation.stackexchange.com
Tags: <landing><landing-gear><engine-failure><electrical-system><apu-failure>
Title: Will the landing gear work mechanically in case of total power loss?
Question: 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.

Answers:
sents: text: 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 mainpower system fails.
text: There are numerous ways in which this is donedepending on the size and complexity of the aircraft.
text: Some aircrafthave an emergency release handle in the flight deck that is connectedthrough a mechanical linkage to the gear uplocks.
text: When the handle isoperated, it releases the uplocks and allows the gear to free-fall tothe extended position under the force created by gravity acting uponthe gear.
text: Other aircraft use a non-mechanical back-up, such aspneumatic power, to unlatch the gear.
text: The popular small aircraft retraction system ..... uses a free-fall valve for emergency gear extension.
text: Activated from the flight deck,when the free-fall valve is opened, hydraulic fluid is allowed toflow from the gear-up side of the actuators to the gear-down side ofthe actuators, independent of the power pack.
text: Pressure holding thegear up is relieved, and the gear extends due to its weight.
text: Airmoving past the gear aids in the extension and helps push the gearinto the down-and-locked position.
text: Large and high performance aircraftare equipped with redundant hydraulic systems.
text: This makes emergencyextension less common since a different source of hydraulic power canbe selected if the gear does not function normally.
text: In some small aircraft, the design configuration makes emergencyextension of the gear by gravity and air loads alone impossible orimpractical.
text: Force of some kind must therefore be applied.
text: Manualextension systems, wherein the pilot mechanically cranks the gearinto position, are common.
text: Consult the aircraft maintenance manualfor all emergency landing gear extension system descriptions ofoperation, performance standards, and emergency extension tests asrequired.
answer_details: author: aviation.stackexchange.com/users/790/Thunderstrike
score: 12
sents: text: The rulebook for transport category aircraft says 14 CFR 25.729 c)
text: Emergency operation.
text: 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)
text: The failure of any single source of hydraulic, electric, or equivalent energy supply.
answer_details: author: aviation.stackexchange.com/users/2341/Sports Racer
score: 5
sents: text: The F-16 has an "emergency blow-down" reservoir containing approximately 3,000 psi gaseous nitrogen.
text: Hitting a switch will release this pressurized nitrogen into the hydraulic system dealing with the landing gear.
text: Ergo, hitting that switch will extend the gear, even with the engine off.
text: So
text: yeah, the landing gear will still work in the event of a complete power failure.
text: It has backup systems.
text: In the older A-4 Skyhawk, the landing gear retracted up and forward.
text: 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.
text: I think that's a pretty slick design.
answer_details: author: aviation.stackexchange.com/users/2859/Meower68
score: 2
sents: text: Almost all aircraft have some sort of backup system for getting the gear down.
text: On the planes I have owned they were: Manual hydraulic pumps to pressurize the system with a car jack type handle.
text: You sort of pumped the gear down.
text: Obviously does not work too well if the problem with gear system is a no fluid (blown line or seal).
text: Mechanical crank (21 turns to get down, 2 more to lock) that drove the gears if the electric motor died.
text: Good workout.
answer_details: author: aviation.stackexchange.com/users/2953/JerryKur
score: 2
sents: text: The Twin Comanche I used for my AMEL rating had an interesting mechanism.
text: First you had to disconnect the electric motor, then you stuck this handle into a hole and crank it as far as you can.
text: 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.
text: Not something you get to try until you need it though.
text: Once you pull the disconnect it requires an A&P to restore the system to normal operation.
answer_details: author: aviation.stackexchange.com/users/49/Brian Knoblauch
score: 2
sents: text: All aircraft have some kind of backup system.
text: As mentioned before-
text: Skyhawks Landing gear gets sucked down.most of the other planes-
text: just fall down thanks to gravity.
text: 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.
text: As for the big jets- they have an amazing amount of backups.
text: And still- if in a 787 all your computers catch fire for some reason-
text: probably you wouldn't be able to extend your gear.
text: But then- even flying the plane would be a challenge, as the aircraft needs voltage for everything.
text: 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.
answer_details: author: aviation.stackexchange.com/users/20731/Stan
score: 1
sents: text: It all depends on the airplane in question.
text: Typically for small GA airplanes using an electro-hydraulic power pack for actuating the gear
text: , they’re held retracted by hydraulic pressure.
text: If the system fails, the pressure goes to zero and the gear just drops down due to gravity.
text: I have time in the DA-42 and DA-62 twins and those airplanes
text: use an alternate gear extension handle - basically an emergency valve which releases hydraulic pressure and allows the gear to drop down on their own.
text: Electrically powered landing gear can be typically lowered by means of a hand crank in the cockpit.
text: This is prominent on Mooneys and light Cessna twins.
text: 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.
text: 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.
text: It also removes the additional workload from emergency systems eg APU, RAT, etc. for more critical flight control functions.
text: Transport category airplanes often have multiple layers of redundancy ie multiple hydraulic systems and power sources to get the plane down safe.
answer_details: author: aviation.stackexchange.com/users/15637/Carlo Felicione
score: 0

Summary of the answers:
Almost all aircraft have some sort of backup system for getting the gear down.  Typically for small GA airplanes using an electro-hydraulic power pack for actuating the gear. Electrically powered landing gear can be typically lowered by means of a hand crank in the cockpit.  
There is usually a system that lowers the landing gear in an emergency. There can be an electrohydraulic pack, or a hand crank in the cockpit. Some bigger plans also have an emergency reservoir of compressed nitrogen for that job too.