LOT 16 was a 767 which experienced a hydraulic leak, resulting in the failure of the landing gears and the proceeding belly landing shown below:
Why did the backup system(s) fail? I was under the impression that a gravity/freefall backup is mandatory.
Gravity/freefall backup (or more precisely some backup) is mandatory, but in large aircraft running pulley from cockpit to the wheel wells to release the uplock (the thing that keeps the gear retracted) mechanically would be too complicated or the force required to operate it too large, so the uplock still needs to be released electrically.
Unfortunately the circuit breaker that protects the circuit responsible for releasing the uplock was "popped" (open), so the uplocks didn't release and the gear didn't fall down.
The crew was trying to troubleshoot the issue for 80 minutes and consulted maintenance over radio, but because the checklists for either central hydraulic system (which does the normal gear extension) failure or alternate gear extension did not mention the circuit breaker (C829) neither crew nor maintenance thought of checking it.
It is not possible to find why and when the breaker popped. They tested the alternate extension afterwards and it worked, but the breaker covers several other circuits and there could have been transient problem somewhere or it could have been maintenance error or some other reasons.
You can read excerpt from the report on The Aviation Herald.
Shortly after the evacuation, a team from the Polish State Commission for Aircraft Accident Investigation arrived, and discovered that the C829 circuit breaker, which protects a number of systems including the alternate landing gear extension system was "popped". The C4248 breaker for the alternate landing gear remained closed.
After the plane was lifted off the runway, the C829 circuit breaker was closed, and the landing gear was extended, using the alternate system.
