@lomapaseo:
dunno about other planes, on the 737 though there are four ground/air sensors on the main landing gear and at least one on the nose gear and they are monitored by the PSEU system on the ground. If one fails you will get a non-dispatchable fault. Had that happen a few times lately with sensors that were just out of range, one was quite funny as it caused the PSEU warning to come on in left turns and switched it off again in right turns.
And if all those sensors fail i still can open reverse if i want to, after all below 10ft RA it is possible. It is not a recommended procedure in normal flight operations of course.