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Pressurisation In Fighter Planes  
User currently offlineAF1624 From France, joined Jul 2006, 657 posts, RR: 0
Posted (1 year 11 months 2 weeks 4 days 19 hours ago) and read 6617 times:

Hello all,

I have a question that's been bothering me for some time now.

How does the pressurisation of fighter planes work ?

In a commercial airliner, the pressurisation system basically pressurises the cabin while the aircraft "gently" or not so gently climbs. Still, it's climb rates mostly under 3000 ft per minute, giving only a cabin altitude climb of about 500ft per minute which allows the passengers to not have their ears popping all the time.

In extreme scenarios, an empty 747 climbing very fast won't pressurise quick enough so the pilots will don the oxygen masks during the climb - but that doesn't solve the ear pop issue.

Some fighter planes can climb from 3000 to 30000 feet in two, three, four minutes. How does the cabin stay pressurised in such situations ? Isn't it extremely painful for the pilot ? And what about stress to the pressure vessel itself, being that the pressure varies so quick ?

Can someone explain ? Thanks !


Cheers
11 replies: All unread, jump to last
 
User currently offlinelegs From Australia, joined Jun 2006, 239 posts, RR: 0
Reply 1, posted (1 year 11 months 2 weeks 4 days 19 hours ago) and read 6612 times:

Ear pain is something that fighter pilots have to be very cognizant of when doing high rate of climb manuevers. I was reading a report just last week where a back seater decided the slight head cold he picked up over night wasn't an issue, and neglected to mention it to the pilot during the brief. Problem was, they were slated to be doing 'combat' climbs and dives. He reported severe ear pain and the sortie had to be terminated.

As for the pressurization, fighter cockpits are essentially pressurised the same as airliners, however the much, much smaller volume allows the system to hit its mark far quicker than an airliner. Some fighters Ive tested can hit their pressurisation differential mark on the order of 15 seconds or less using ground air (at much lower flow than with engines running). The cabin altitude rate of change is generally managed by controlling the outflow valve, to keep it to an acceptable level, so the cabin altitude will lag behind the planes altitude during rapid chages (I can't remember the actual numbers, sorry)

The big difference is, at altitude fighter cockpits aren't pressurised to the same pressure differential, and in combat mode even less so. This is done to keep the pressure differential low in case the cockpit is compromised during combat, not doing this could incapacitate the pilot (very bad in a combat scenario!)

In order to keep the pilot oxygenated, fighter pilot breathing regulators generally have three modes of operation:
1) Normal - pilot is fed a mix of cabin air and 100% oxygen on demand (when he breathes in)
2) 100% - pilot is fed 100% (or about 90% in the case of OBOGS equipped planes) on demand
3) Combat - pilot is fed 100%(or 90%) at pressure, forcing the pilot to breathe 'backwards'

[Edited 2012-09-06 01:16:29]

[Edited 2012-09-06 01:18:38]

User currently offlineAF1624 From France, joined Jul 2006, 657 posts, RR: 0
Reply 2, posted (1 year 11 months 2 weeks 4 days 18 hours ago) and read 6601 times:

That's very interesting, thanks!

In combat mode, does that mean that the pilot is basically "force-fed" oxygen (doesn't have to breathe in), forcing him to consciously breathe out ? Is that a way for him to handle all the G-forces ?



Cheers
User currently offlineautothrust From Switzerland, joined Jun 2006, 1595 posts, RR: 9
Reply 3, posted (1 year 11 months 2 weeks 4 days 17 hours ago) and read 6561 times:

Quoting AF1624 (Thread starter):
Some fighter planes can climb from 3000 to 30000 feet in two, three, four minutes.

The Typhoon can climb from ground to 14000meter in less then 70 seconds(max 315m/sec or 1040 feet per second). Also F-22 and Typhoon have a higher service ceiling then most other fighters. (45000 feet)

That must be sure a challange for the pressurisation systems.



“Faliure is not an option.”
User currently offlinelegs From Australia, joined Jun 2006, 239 posts, RR: 0
Reply 4, posted (1 year 11 months 2 weeks 4 days 17 hours ago) and read 6554 times:

I don't know if the pressure supply is to help handle the forces, but more to work in concert with the methods pilots use to fight off the effects of G-forces (straining their core, and upper legs) and the G-suit to help restrict blood flow away from the torso. Pressure breathing is pretty strange, not only do you have to strain to breathe out, you have to use the same muscles and your mouth and tongue to regulate breathing in. It can take a bit of getting used to.

It's also enabled at high alittudes just like an airliner mask, to keep the pilots O2 saturation up.


User currently offlineMax Q From United States of America, joined May 2001, 4435 posts, RR: 19
Reply 5, posted (1 year 11 months 2 weeks 2 days 20 hours ago) and read 6204 times:

Quoting AF1624 (Thread starter):


In extreme scenarios, an empty 747 climbing very fast won't pressurise quick enough so the pilots will don the oxygen masks during the climb - but that doesn't solve the ear pop issue.

That is complete nonsense. Why on earth would the Pilots need to don oxygen because of a fast climb ? The pressurisation system can easily cope with that scenario.


If this was true the aircraft could not have been certified !



The best contribution to safety is a competent Pilot.
User currently offlineAF1624 From France, joined Jul 2006, 657 posts, RR: 0
Reply 6, posted (1 year 11 months 2 weeks 19 hours ago) and read 5903 times:

Quoting Max Q (Reply 5):
That is complete nonsense. Why on earth would the Pilots need to don oxygen because of a fast climb ? The pressurisation system can easily cope with that scenario.


If this was true the aircraft could not have been certified !

No, it isn't. Or so I was told anyway - and I'm pretty sure I wasn't told bullshit.

An empty 747 (for a placement flight for example) can do such climbs.

For example, AF crews would usually do that when taking off from Paris to Chateauroux, where they do training. The distance is very small so the aircraft is close to empty (no pax, almost no fuel). And no, the pressurisation system would not cope with that scenario. Again, so I was told.



Cheers
User currently offlinetitanmiller From United States of America, joined May 2006, 89 posts, RR: 0
Reply 7, posted (1 year 11 months 2 weeks 15 hours ago) and read 5897 times:

Quoting AF1624 (Reply 6):
And no, the pressurisation system would not cope with that scenario. Again, so I was told.

There shouldn't be any issue whatsoever. If the outflow valves are kept closed by the pressurization system, as the aircraft climbs there is no place for the trapped air to go which means that it can't help but pressurize. The other side of the issue would be climbing so fast that the air can't escape fast enough, but this too is impossible.

The aircraft I fly on can change cabin pressure at up to 2,000 feet per minute. With a maximum cabin differential of 8.6psi and an 8,000' cabin altitude, this corresponds to a flight altitude of 43,000'. In other words, you'd have to climb or descend at a sustained rate of over 10,000ft/min before the system "gets behind" and even then it isn't a big deal because all that means is that your cabin altitude is a bit behind schedule.


User currently offlineAF1624 From France, joined Jul 2006, 657 posts, RR: 0
Reply 8, posted (1 year 11 months 2 weeks 15 hours ago) and read 5897 times:

Quoting titanmiller (Reply 7):
There shouldn't be any issue whatsoever. If the outflow valves are kept closed by the pressurization system, as the aircraft climbs there is no place for the trapped air to go which means that it can't help but pressurize. The other side of the issue would be climbing so fast that the air can't escape fast enough, but this too is impossible.

The aircraft I fly on can change cabin pressure at up to 2,000 feet per minute. With a maximum cabin differential of 8.6psi and an 8,000' cabin altitude, this corresponds to a flight altitude of 43,000'. In other words, you'd have to climb or descend at a sustained rate of over 10,000ft/min before the system "gets behind" and even then it isn't a big deal because all that means is that your cabin altitude is a bit behind schedule.

Wow, then I stand corrected. My apologies to Max Q for being stubborn. I really thought the person telling me the above was a reliable source.



Cheers
User currently offlineMax Q From United States of America, joined May 2001, 4435 posts, RR: 19
Reply 9, posted (1 year 11 months 1 week 6 days 22 hours ago) and read 5854 times:

Quoting AF1624 (Reply 8):

Wow, then I stand corrected. My apologies to Max Q for being stubborn. I really thought the person telling me the above was a reliable source.

Thanks, you can't believe everything you hear you know.


And I apologize for my slightly 'over the top' response



The best contribution to safety is a competent Pilot.
User currently offlineAAR90 From United States of America, joined Jan 2000, 3471 posts, RR: 47
Reply 10, posted (1 year 11 months 1 week 6 days 19 hours ago) and read 5823 times:

I was lucky enough to ride one of Pan Am's first 747SP's out of SFO. An "acceptance flight" with about 6 pax (and their test gear) in the cabin, three crew and two jumpseat riders (me & my dad - Pan Am Captain). Brake release to FL400 was just a touch over 5 minutes (the chief pilot was a bit annoyed at himself for not getting it done in less than 5 minutes. No problems with the pressurization system. Heck, nobody noticed it at all. Probably too busy admiring the 35+ nose attitude... and accelerating.   


*NO CARRIER* -- A Naval Aviator's worst nightmare!
User currently offlineMax Q From United States of America, joined May 2001, 4435 posts, RR: 19
Reply 11, posted (1 year 11 months 1 week 5 days 20 hours ago) and read 5675 times:

Quoting AAR90 (Reply 10):


I was lucky enough to ride one of Pan Am's first 747SP's out of SFO. An "acceptance flight" with about 6 pax (and their test gear) in the cabin, three crew and two jumpseat riders (me & my dad - Pan Am Captain). Brake release to FL400 was just a touch over 5 minutes (the chief pilot was a bit annoyed at himself for not getting it done in less than 5 minutes. No problems with the pressurization system. Heck, nobody noticed it at all. Probably too busy admiring the 35+ nose attitude... and accelerating.

Now that is something I would love to have seen.


Truly awesome !



The best contribution to safety is a competent Pilot.
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