Oddball King Air Maintenance Question

[jergar999]

This is probably more of a maintenance question, but maybe there are some mechanics on this board, or pilots who have encountered this before. We are having a weird pressurization problem on a BE-10. During flight, if the environmental is turned on (manual or auto) or the blower is turned on (auto or high) the cabin climbs and the aircraft will not hold max differential. Turn them off, and the cabin descends back down to the selected cabin pressure. It is possible to fly up to FL180 while keeping the cabin pressure below 10000 with the environmental on, with it off we can go up to FL250 (we don't go higher than that, so I don't know if FL310 is possible). The following maintenance has been performed:

Inspected and tested pressurization controller, outflow valve, safety valve, flapper, both flowpacks, bleed air from engine to cabin and associated valves, all to no avail. Our mechanic has contacted the company DOM, Beechcraft, Pratt, Yingling, and Stevens Aviation in an attempt to gain some insight and no one has ever heard of anything like this before. If you believe the schematics, the only thing the pressurization and environmental systems share is flowpacks, which tested perfectly.

I was just wondering if any King Air guys, mechanics or pilots, had ever encountered this before, and what was done to remedy it. We are scratching our heads.

 

[dogg]

2 things come to mind.1) when you turn on the env. possibly the airplane thinks it is on the ground. 2) common wire bundle somwhere with a cross short, maybe in the belly. Make sure that on the ground during test that no other part of the pres. system is getting a spurious signal........... sounds like a tough one good luck

 

[jergar999]

Thanks for the response, the company betting pool currently favors an electrical problem.

 

[Torqued]

A quick arm chair guess is that the pressure vessel has a leak. When additional bleed air load is required to operate the packs, not enough air flow is left available to maintain cabin pressure. A buildup of small leaks or a single larger leak can do it. Try checking the seals at wiring cable feed throughs, door seals, control cable seals where they exit the pressure vessel, emergency hatches, antennas, etc. Looking back at what recent maintenance was done just before the problem arose may point in the right direction.
A simple way to find it would be for someone to light up a cigarette and follow the smoke. If that's a non starter, maybe some incense (or whatever is handy but not explosive). Hold whatever you light up a few inches from possible leaky areas and if there's a leak, the smoke will make a straight line for it.
Better if this is done during a ground run, where walking around the cabin is not a problem. There's usually a way to bypass the squat switch to pressurize it on the ground.

Good luck

 

[skylane58]

We had a similar problem in a B200. What it ended up being was a combination of problems. We found that the electro magnet in the emergency bypass was bad, so it leaked there. After that didn't fix the problem completely, helped but not cured, we turned to the packs. Both of them were turned way down, so a "quik" replace of those cured the problem. I think that they replaced a few other odds and ends parts but those were the major ones.