Not my turf but may be gCapt engine department may leave some comments here.
Wow! That must have been exciting!
I’ve had that style of start air valve hang open on me twice. In both cases we had indications of the problem on the MOP and automation. We had a drop in cylinder pressures and exhaust temps that approached the deviation alarm. We also got a fire alarm off the detector above the ME but that wasn’t immediate. The water mist system did not activate.
In both cases the stuck valve was caused by the bolts that connect the start air valve stem to the low pressure (pilot) piston. The bolts backed out and jammed against the top cover of the start air valve. Each successive opening would cause them to back out a little more until the valve could no longer close. It could still open fully, it would just close less and less.
The first time it happened we didn’t know what was going on so we shut down the ME and had our assist tugs drag us to anchor. The second time we recognized the situation and cut fuel to the cylinder and continued to a nearby anchorage under our own power.
In my first instance we scorched the paint on the start air piping pretty good. The piping got hot on the second case but not hot enough to scorch anything.
In the afternath I found two different MAN procedures in the same maintenance manual for affixing the bolts. One used lock washers, one used lock wire, neither of which our parts lists indicated. We reached out to MAN and they sent us an updated work card specifying a certain LocTite product. We had no further issues after that.
This engine is an MC-C so a little different design on the SA valves and system and no MOP to give them the real time engine data.
This is not a new or unheard of event. MAN has put out service letters covering this in the past:
The issues I described having with the start air valves predated those SL’s by a couple of years and it was on an ME-C. I’ve probably got pictures on my old computer of what we found as we took the failed valves apart. It didn’t take very long for the valve to stick once the bolts loosened, the grade marks on the bolt heads were still visible.
Checking the drain lines is one of many little jobs that can easily be overlooked.