DColeMAN From United Kingdom, joined Nov 2004, 274 posts, RR: 10 Posted (9 years 8 months 2 weeks 2 days 18 hours ago) and read 3343 times:
A couple of times I've seen aircraft arrive with the landing gear doors down as though there's no power to them and they're (in a way) just hanging there in the retracting and extending position. I wanted to ask is this always due to a hydraulics failure? Sometimes the aircraft has just taxied to the gate as normal and others have requested a full emergency landing because of this. Also, what causes this to happen?
Here are a couple of examples of what I'm reffering too -
DH106 From United Kingdom, joined Jun 2005, 627 posts, RR: 1
Reply 2, posted (9 years 8 months 2 weeks 2 days 17 hours ago) and read 3316 times:
When the hydraulic system that extends/retracts the gear fails the back up system to extend the gear is usually just a 'free fall' system where the gear's uplocks are removed and the gear just 'drops' into position under gravity and locks. The doors in this case are unpowered and remain open.
...I watched c-beams glitter in the dark near the Tanhauser Gate....
320tech From Turks and Caicos Islands, joined May 2004, 491 posts, RR: 5
Reply 3, posted (9 years 8 months 2 weeks 2 days 17 hours ago) and read 3290 times:
I beg to differ with LimaFox Tango. There is no hyd pump that specifically operates the doors.
I don't know about 757's (I'm sure it's the same), but on Dash-8's and A320's, when the aircraft lands with the gear doors down, it means the pilots have used the alternate gear extension. On the Dash-8, if a snag develops in the landing gear extension system, a caution light, "LG Unsafe" illuminates. The usual cause is either a Prox Sensor Electronic Unit (PSEU, the computer that controls the LG, amongst other things) failure, or a valve failure. The result, if you insisted on using normal extension, would be to have the doors fail to open in sequence, thus possibly jamming the gear. I've seen several Dash-8 snags like this.
I haven't seen any A320 snags, but it's probably not much different.
In the Dash, the pilot first opens a small overhead panel, which is connected to a bypass valve by a cable. He then pulls on a handle that's under the panel. The first notch opens the MLG doors, the second notch drops the gear by gravity. The nose gear is extended the same way, using a panel and handle on the floor.
The primary function of the design engineer is to make things difficult for the manufacturer and impossible for the AME.
NKP S2 From United States of America, joined Dec 1999, 1714 posts, RR: 5
Reply 5, posted (9 years 8 months 2 weeks 2 days 14 hours ago) and read 3237 times:
Quoting 320tech (Reply 3): There is no hyd pump that specifically operates the doors.
Well actually on the 757, there sorta' is. A DC motor driven pump will extend the doors and unlock the gear during alternate extension. Actually, when opening the L/G doors on the ground for maintenance, you hit a momentary contact switch that operates the pump. It will run till a pressure switch is satisfied.
Based upon the photo taken above, I'd say one of two things are occuring:
1) The L/G are lowered but the picture was snapped at the instant just before the sequence where the L/G doors shut.
2) The L/G was lowered via alternate extension due to some problem, and the A/C will land with the doors open.