Pneumatics question

[mriya225]

Okay guys,
I'm probably going to get blasted for this, but I'd rather be made fun of by a bunch of strangers than the guys at school, so here it goes...
I know pneumatics are a tremendous weight savings and I'm sure that for the most part pneumatic systems are quite sufficient. Even so - I can't bring myself to have as much faith in it as I do hydraulics.
I'm probably clinging to illusions here, but it seems to me that once you blow that air charge--you're scr*w*d--and at least you have some residual pressure with hydraulic systems.
So, am I out of line here? Are pneumatic systems as reliable as hydraulic systems? 'Cause in my gut--I don't buy it.

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Un diva très doué.

I should've made it clear I'm refering to situations where the integrity of the system has been compromised - a failure of some kind.

[This message has been edited by mriya225 (edited 14 November 2000).]

[Blacksheep]

Just as hydraulic systems have pumps, most pneumatic systems have a compressor of some sort. In a turbine engined aircraft its usually the engine itself that provides engine bleed air.

A hydraulic system needs a reservoir of fluid, if you get a leak and all the hydraulic fluid is gone, its gone. End of story. With pneumatics there is usually an inexhaustable supply of 'fluid.' As long as any leak is within reasonable limits the pneumatic system still works. In fact most operate on a "total loss" principle anyway, used air is exhausted.

Both systems have their uses and most large aircraft have both. (Although such a thing probably exists somewhere, I've yet to see an aircraft with a hydraulic engine start system!)

By the way, check out where the residual pressure in hydraulic systems comes from. (As a clue - you're breathing some right now)

Cheers m'dear ;)

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Through difficulties to the cinema

[This message has been edited by Blacksheep (edited 14 November 2000).]

[Lu Zuckerman]

First to answer Blacksheep. The Apache uses hydraulic start. I don't know if the UH-60 uses the same type of system but it uses the same engine type. T-700

To: mriya225

There are many systems on large commercial aircraft that utilize pneumatics as a power source. The pneumatic pressure is supplied as bleed air from the engines. Most large aircraft have cross feed valves that can maintain the pressure supply in the event of an engine failure. It is true that the system elements can fail and you could lose the air supply but the rate of failure of those elements is no greater than similar elements in a hydraulic system. The 747 and the 767-400 have pneumatically powered hydraulic pumps that are utilized when there is a high demand on the system. The failure of these types of turbine driven hydraulic pumping systems is mainly due to control or electrical malfunctions and not the loss of supply. Loss of pressure would be caused by a compromise in the tubing in the delivery system but the same would hold true for hydraulics. Another major system on commercial aircraft that is powered by pneumatics is the air conditioning system and in some aircraft the windshields are cleaned not with windshield wipers but by hot bleed air. Assuming the loss of all engines resulting in no bleed air then the air conditioning system shuts down and everybody starts to sweat. Of course there is the APU but that does not help in maintaining altitude.

The system you described is a pressurized bottle such as that used to blow the landing gear down in the event of the loss of the hydraulic system. Other than a pressure gage on the bottle you would have to assume that it will always work.

In another thread I spoke about a second order failure. That is a failure of a backup system or any system that is assumed to be available when the primary system fails. In the performance of the FMECA you can only address the primary failure and state that there is a back-up system that will perform the function. As far as the back-up system is concerned you can only address its' failure as contributing to system maintenance. On the safety hazards analysis the back-up and primary system are considered as both passing through an And gate which means that both systems must fail to lose the function.

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The Cat

[This message has been edited by Lu Zuckerman (edited 14 November 2000).]

[mriya225]

Gentlemen,

I'll give it a while to sink into my insticts, but I understand pneumatics a little better now and I certianly trust the judgement of each of you.

Thanks for taking the time!

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Un diva très doué.

[SchmiteGoBust]

Mriya,
There is one major advantage of hydraulics and this is why they are used mostly on flying controls. Due to hydraulic fluid not being compressible(for all intents and purposes) it is much more controllable for precise movement. Pneumatics on the other hand are much better at extremely rapid movement..

[mriya225]

Hi SchmiteGoBust,
Haven't seen you around in a while, I hope all is well with you.
The qualities you mentioned of air and liquid charges... That makes sense, so even on European aircraft where they use more pneumatics than we generally use - they're using hydraulics for the flight controls?
If memory serves, those qualities are put to good use in the struts right? Standard shock absorbtion handled primarily by the fluid passing through an orifice and the landing shock handled by air charge.
God, I hope that's right - not getting much sleep lately... feeling a bit weenie-eyed and foggy of mind. http://www.pprune.org/ubb/NonCGI/tongue.gif



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Un diva très doué.

[SchmiteGoBust]

Hi Mriya,
How's it going? Keep you morale up, you sound as though you know what your talking about to me.Quit worrying if possible!!!!!