There are several instances where we see the landing gear getting stuck before landing. In these kind of situations, can the crew, or one of the pilots, go down to the landing gear area and solve the issue?
On older (WWII-era) large planes, the crew had access to a manual crank to lower the gear if the automatic extension/retraction system (usually a linear actuator powered electrically) failed. I don't know of any that had a gravity-based manual system; I'm sure a few bomber crews would have appreciated something like that after having to crank the handle to get a disabled main gear locked. Here are the manual crank ports for the B-17, on the forward bomb bay bulkhead with red arrows indicating the direction to crank:
The B-17 had a retractable tail gear as well, so there was a third manual crank in the tunnel to the rear gunner's position (but if there were multiple gear failures, the main gear would be the priority).
On more modern airliners with hydraulic extension/retraction, manually cranking the gear into place isn't as much of an option (especially since any access to a crank linkage would be in either the passenger cabin or the cargo hold). For these aircraft, the gear are designed with a "manual extension" or "gravity release". This basically disconnects the gear strut from its uplocks and the hydraulic cylinder, allowing gravity to pull the gear down and then momentum (or a spring mechanism) will engage the downlocks. Here is a diagram of the 737's manual gear panel on the right-hand wall of the flight deck:
The 747 has a manual nose gear extension system in the avionics compartment below the first-class cabin. The 747's main gear are much more complex, but it is still possible to let gravity pull them into place, by using a set of "alternate gear extension" switches on the overhead panel to release each gear from the hydraulics and uplocks:
It doesn't always work; a Virgin Atlantic 747 pilot was forced to make an emergency landing with the outboard right main gear stuck in a half-extended position:
No, there isn't.
I don't think any aircraft has access door from inside to the main wheel well. Some aircraft do have access hatch via the nose wheel well, but since the wheel well is unpressurised and the cabin is pressurised, it would not be possible to open the hatch in flight and even if it was possible (after depressurising the aircraft at low altitude), trying to do anything there with the gear door open (which it would have to be obviously) would be extremely dangerous and the chance of success would not be big, because a man has much less strength than the hydraulics that already tried to get the gear out and failed.
Any aircraft with retractable landing gear is required to have an alternate extension mechanism that can be used to extend it if the primary system operating the gear fails, which was already well explained by KeithS. But when that also fails, there is no way to access the gear to attempt to unjam it.
Aircraft are designed with all sorts of emergencies in mind and even landing with partial gear is not a big deal. Sure, it will do some damage to the plane, possibly even to the point that it's not economical to try to fix it, but that's what insurance is for, and risk of injury is minimal. For recent examples of gear mishaps see:
- Jetblue A320 at Los Angeles on Sep 21st 2005, nosewheel tilted 90 degrees for landing and Vueling A320 at Sevilla on Apr 20th 2011, nose wheels tilted 90 degrees on landing
- Wizz A320 at Rome on Jun 8th 2013, left main gear did not extend and Alitalia A320 at Rome on Sep 29th 2013, right main gear did not extend
- LOT B763 at Warsaw on Nov 1st 2011, forced gear up landing
All occupants of those aircraft were OK, only 3 minor injuries in one of them from the evacuation.
Also in some cases the gear fails when the aircraft touches down, so nobody could do anything about it, because there is no indication it's going to happen beforehand. If it's just that, the risk of injury is still small. Example:
which also ended without injury.