Emergency procedures will differ from aircraft to aircraft and from operator to operator.
Generally for an OV-10 Bronco (twin-prop):
Quote:
GEAR EMERGENCY EXTENSION
Landing gear handle - DOWN, below gear limit speed.
Airspeed - REDUCE TO 120 KIAS.
Increase "g" if required to lock main gear.
Quote:
LANDING WITH UNSAFE GEAR
UNSAFE GEAR INDICATION IN COCKPIT
OR
MAIN GEAR DOWN WITH UNSAFE NOSE GEAR
Stores - JETTISON.
Shoulder harness - LOCKED.
Nose gear STEER button - DEPRESS.
Just prior to touchdown:
Condition levers - FEATHER & FUEL SHUT-OFF.
BATTERY - OFF.
Land straight ahead. Stop the aircraft and install gear pins.
MAIN GEAR COCKED WITH NOSE GEAR UP OR DOWN
Gear - UP.
Stores - JETTISON.
Shoulder harness - LOCKED.
BATTERY - OFF.
Condition levers - FEATHER & FUEL SHUT-OFF, prior to touch-down.
But let's go to something with a jet: F-16
Quote:
LG FAILS TO EXTEND
If LG handle cannot be lowered normally:
1. DN LOCK REL button Depress and lower LG handle (N.I. in F4)
If LG handle still cannot be lowered:
2. ALT FLAPS switch Extend.
3. Go to ALTERNATE LG EXTENSION page E-4
If Landing Gear lowers and one or more LG indicates unsafe:
1. LG Handle Cycle and monitor LG handle warning light
and WHEELS down light.
If LG handle warning light illuminated when the LG handle was lowered,
then went OFF and tests good, or if WHEELS down light operated
normally:
2. Speedbrake Verify opening is less than 43°
3. Land Normally
If LG handle warning light did not illuminate or remained illuminated
after LG handle was lowered, and if one or more WHEELS down light did
not illuminate: when the LG handle was lowered, then went OFF and
tests good, or if WHEELS down light operated
4. Go to ALTERNATE LG EXTENSION
Quote:
ALTERNATE GEAR EXTENSION
1. LG Handle DOWN
2. Airspeed 190 Kts MAX. (if practical.)
3. ALT GEAR handle PULL
If Landing Gear indicates safe:
4. Land normally.
5. Stop straight ahead on runway.
If landing indicates unsafe:
4. Stick Apply alternating G forces (-1.0 to +3.0G) to free LG
If Landing Gear indicates safe:
5. Land normally.
6. Stop straight ahead on runway.
If landing Gear still indicates unsafe:
5. Speedbrake Verify Opening is less than 43°
6. Go to Landing with LG UNSAFE/UP page E-5
Quote:
Other considerations:
• Alternate LG extension can be used up to 300 Kts. However, the NLG
may not fully extend until 190 Kts. Time above 190 Kts should be
minimized in case there is a leak in the pneumatic lines.
• Do not depress the ALT GEAR button while pulling the ALT GEAR
handle. This action may preclude successful LG extension.
• If possible get a visual confirmation of LG position.
• Consider a go-around capability in the event the brakes are found to
be inoperative after touchdown.
• NWS is not available following alternate gear extension.
Quote:
LANDING WITH LG UNSAFE/UP
If conditions are not favorable:
1. Refer to EJECTION (TIME PERMITTING) page F-6
To accomplish the landing:
1. Retain empty fuel tanks and rack.
2. Armament Jettison
3. TANK INERTING switch TANK INERTING (N.I. in F4)
4. AIR REFUEL switch OPEN
5. FCR OFF
6. ST STA/ HDPT/ECM power OFF
Other considerations:
• Prior to landing with any of the LG unsafe or up, consider the airfield
facilities, crosswind component, Runway conditions.
• If time permits, delay landing until external fuel tanks are empty. If an
immediate landing is required, jettison all external fuel tanks.
• Failure to depressurize the tanks significantly increases the probability
of tank explosion and fire
• Delay placing the AIR REFUEL switch to OPEN until all external tanks
are empty.
There are quite a few emergency checklists for all sorts of aircraft on the internet.