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Cargo AC Low Flow/Pressurization Problem


coolfry73
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Ok, so here is what we got. 65 model acft (E model moded to HC-130P). Ok, crew has repeatedly reported acft will not pressurize to maintain 5K alt @ 20K agl (please don't just respond the acft can't pressurize to this std because our second plane here is doing great same model). When we check out system here is what we get. With GTC we can only get 7 in HG (our normal std is approx 8.5 in HG to meet std). HOwever, the cargo compartment (because we pressurize in full hot) just will not get hot and the flow is almost nill in both hot and cold. I have pressurized many planes and it should be very hot there and know you should be able to feel the air moving.

Here is what we have checked and changed. Have R2'd two CC flow control and SOV and valve, reads approx half to a little more open in full hot with GTC. Have leak checked CC and FD heat exchangers. Checked visually the butterflies on both aux vent valves with cabin pressurized and there is no leakage. Have visually checked safety and outflow valve butterflies and they are full closed with no leakage. Have R2'd a CC turbine. Found the H2O separator exit duct cracked so R2'd that duct. Have leak checked cabin and found some normal leaks but nothing that is crazy.

This check fails in pressurization manual mode with the GTC pushing about 40PSI and once under pressure drops to about 35PSI on the gauge. I have physically checked the underfloor heat SOV and it is closed but opens when demanded. Also checked the diverter valve to make sure it was operating, and it is. Also checked temp control valve butterfly operation and no binding there either. Checked ducting all the way up RH side of acft into cabin, no leakage. Pulled duct and inspected mixing chamber and flapper valve and saw no binding either.

I just can not figure out why there is so low flow, it won't get hot in the cargo, and then we can't get pressurization to meet. I believe that pressurization is either causing the low flow (and I can't isolate it) or the low flow is causing the pressurization malf. Please throw me any thoughts on this subject. Thanks

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clogged sock on those packs equals low flow. also the fcsov should not fully open on the ground. it is altitude compensated. dont tweek it or it will have even more trouble at altitude. what does it do with engine bleed air. get higher with an engine and do some decay checks. will let you rule out leakage. had same if not worse probs on a 1500 series coast guard bird. biggest cause was the cc aux vent. external indicator showed closed but i had to know....opened the duct and sure nuff, the butterfly was 'loose' on the shaft. motor was driving as directed, but butterfly was doing what it wanted. when doing decay check, when packs get cut off have someone outside stick hands and face into the pack intake. if they are feeling lots of air, aux vent is hosed. i know you said you checked already. just a thought.

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Well, I have to guess that your temp control valve actually says "HOT" when you are attempting to pressurize. If you care to check and see if both butterflies move, it may save you time later. If they are moving and the rest of your description is accurate, then I would suspect that your GTC is giving you "some air," but it is not giving you as much as it should. Since your pressure is staying at 35 PSI, that is where I'd look. We have a GTC checkout workcard that requires that the GTC - with both packs running - drop to a much lower valuve. The CC has the bigger pack, so I'd guess your GTC should be dropping to about 30 PSI.

I wish you luck.

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Disconnect your CC AC temp control box and install jumper wires (B to E, and C to F).

This will give you DIRECT control of the temp control valve...(WITHOUT the PROTECTION of the temp control box and duct sensors. See if the CC AC will go HOT... Just pulse the temp switch so as NOT to overheat your ducts.....

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Have you tried seeing what happens with airflow in Aux Vent? If you hook a cart up to the intake and run Aux Vent and still have no airflow, the check valve flapper could is probably hanging up on the inside of the ducting. It won't necessarily show when you pull the ducts but might still hang up when they are bolted back together due to some minor misalignment. It doesn't take much for the flapper to hang up!!

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  • 2 weeks later...

Could a fault in the pressurization control panel cause this problem? There are (I think) two rubber diaphragms and rubber check valves in the box, its a good chance its 45 years old

I know a lot of times low or no flow can be fixed with an adjustment, or R2 of the control panel.

Those things are really really temperamental and its almost impossible to find someone who has the "magic fingers" to tune it golden. We used to get AWADS birds to pop the safety valve, then the magic tech rotated back to the states and we had to get used to 9 or 10 hg on the birds instead of 15.6 :(

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  • 2 months later...

I can't speak to how things should operate for a grnd checkout, but a GTC issue has no bearing on your original problem. I have been battling a very similar issue for the past 2 weeks in flight, and after a CND, an r2 press cont, an r2 outflow valve, MX has finally taken my advice and looked at the aux vent valves. The CC aux vent valve was intermittent, and must have been cycling closed and not-so-closed in flight, which led to a fun sortie with me recycling the outflow valve every 5 min or so to battle the climb/descent rates as the aux vent valve changed position. I've also had them simply stick, which means no flux, but a much lower max in/hg differential.

My advice is the same as keepemflying's, check em out, hope it works for ya.

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