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.