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C-130 pressurization schedule


wildweasel_pt
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Hi guys

A question arose the other day. We usually fly with max differential on our pressurization system. We set 1000' on the dial and then keep it until reaching max diff. That occurs around 19000' (don't have my dash-1 handy). We started discussing the benefits of that versus the stress on the fuselage. Anyone of you already discussed this subject, or is there even any guidance regarding the fact that we may fly with lower cabin pressure in order to ease on the fuselage? I know that the dash1 doesn't say anything about it but its common sense that the same number of cycles in fuselages that suffer max diff or "medium" diff will be different.

Thanks

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Making sure I understand the pressure controller is set at 1000 or field altitude (if higher than 1000) and stays there until Max Diff, Right?

If so the pressure controller should limit to a Max of 15.16, and once Max Diff is achieved the cruise altitude is set.., right?

The issue I see is that once the higher cruise Alt is set your pressure will decrease because the pressure controller is asking for a higher cabin Alt. This in turn will "cycle" the aircraft once again as pressure bleeds off and then returns to Max Diff for the new Alt.

If you set the higher cruise altitude at say 10,000 feet you are not going to cycle the aircraft to and then from Max Diff because you are not at Max Diff, you will meet your cruise Alt and Max Diff at the same time.

If at that same 10,000 feet you set 500 feet higher than your cruise Alt in the pressure controller you will not achieve Max Diff and prevent the outflow valve from riding the Max Diff mode and stay in the Isobaric (aircraft lower than selected Pressure controller altitude) This will help prevent the continuious cycling of the aircraft at Max Diff. This 500 feet can bee any number you want or you can say limit the aircraft to 10 Hg, in which case you run the chart and come up with a pressure controller (Alt) setting to give you the desired Hg.

You could also do this to set a cabin altitude...

Limit the "cycles" by never getting to Max Diff in the first place.

Edited by NATOPS1
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WHen i say that we operate at max diff is when we fly at 18000' with 0' cabin press for instance or in our case 20000' with 1000' cabin. Thats like 90% of the time. The thing is that we could be flying at 8000' cabin with 20000' on altimeter, that would reduce the diff to 8,5 that would be almost half of the stress on the fuselage. Im saying this because i think that the civilian companies use 8000' as a reference for cabin alt. And eventhough they fly higher i think that they only start pressurizing upon reaching 8000' alt, so if they fly a shorter leg that doesn't make them go to F350 but F200 for instance they don't operate on max diff, they would be operating at something like the 8,5 i talked about before. But this is all to know if there isnt't anything regarding this kind of operation issued by Lockheed and to know if everyone does it like that around the globe. At least i know that we don't have nothing ragarding fuselage pressurization cycles

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We teach 500' above field alt for landing so the aircraft is depressurized before landing and 500 above cruise alt to stay in isobaric....

We do it 1000' because our homebase altitude is 46', otherwise will be the 1000' above elevation :D. The last step on the Before Landing Checklist is always the Air Conditioning Master Switch - No Press at 1000' (above field elevation that we brief on the approach plate). Hmmm ok.... I thought that there would be some guidance regarding this subject. There isn't afterall.

I agree with you RZHill. For confort, unless you have a emergency depress that its gonna mess up all your good plans...:D

Thanks guys

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