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).]