Failure modes for suspension forks, air and coil An air suspension fork on one of my bikes starts showing its age: noticeable play, scratches on stanchions etc. It has not become less smooth and does not leak air so far, however.

I am starting to wonder how safe it is to continue using it, and what will happen when it "fails".

Possible outcomes of a non-catastrophic suspension fork failure that I can imagine are:


All air leaks out and it compresses fully, becoming very short
All air leaks out and it expands fully, becoming at its full extension
Air air blows out, making stanchions loose in the legs
The fork just locks at random compression position.
Stanchions become so loose with so much play that it becomes noticeable at braking and steering.


What of these options are real, both for air forks and coil forks? Are there other failure scenarios resulting from a fork's old age but not a crash?
Basically the air and oil seals get worn and leaky. You'll get less air spring pressure, droop and increasingly poor damping. 

Of your options, 2 and 3 are not realistic. If the air spring pressure drops to zero (relative to ambient atmospheric pressure), there is no force to extend the fork. With no air spring pressure the stanchions will not slide freely on the legs because the oil based damping system is still working.