Test Criteria for Large Civil Aircraft
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Test Criteria for Large Civil Aircraft
General query to Genghis or any of you other knowledgeable chaps (EX FTE).
What levels are large commercial aircraft flight tested to? ie is it the norm to test a double (unrelated) failure for example? Is there any legislation or guidelines in this area?
The examples I'm thinking of are a hydraulic system failure followed by a severe electrical generation failure.
Answers on a postcard please.
What levels are large commercial aircraft flight tested to? ie is it the norm to test a double (unrelated) failure for example? Is there any legislation or guidelines in this area?
The examples I'm thinking of are a hydraulic system failure followed by a severe electrical generation failure.
Answers on a postcard please.
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Biggest thing I've ever tested only had about 50 seats, so I'm no authority on these big beasties (particularly this week when I've been using JAR-22 which has nothing to do with that end of the spectrum).
My opinion (and it's only that) is that the airworthiness department would carry out a probability analysis of the various double and treble failure cases. If any of these cases showed a probability significantly below the acceptable limit (probably 1 per 10E5 flying hours), it would need evaluation.
Thinking about FAR/JAR-25 which would be the relevant standards; the obvious double failure case is VMCL-2, but I seem also to recall a few mentions of required minimum handling qualities in the event of certain failure combinations in the flight control systems.
Any Airbus or Boeing FTE's lurking around here with something to say?
G
N.B. Jetnoise, if there are regulations they're almost certainly in JAR 25 or FAR-25. Both are available online, the former at http://www.jaa.nl/jar/jar.html and the latter at http://www.amerflyers.com/htm/rfars.htm
My opinion (and it's only that) is that the airworthiness department would carry out a probability analysis of the various double and treble failure cases. If any of these cases showed a probability significantly below the acceptable limit (probably 1 per 10E5 flying hours), it would need evaluation.
Thinking about FAR/JAR-25 which would be the relevant standards; the obvious double failure case is VMCL-2, but I seem also to recall a few mentions of required minimum handling qualities in the event of certain failure combinations in the flight control systems.
Any Airbus or Boeing FTE's lurking around here with something to say?
G
N.B. Jetnoise, if there are regulations they're almost certainly in JAR 25 or FAR-25. Both are available online, the former at http://www.jaa.nl/jar/jar.html and the latter at http://www.amerflyers.com/htm/rfars.htm
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To: Jet Noise
Under normal conditions,unrelated failures are not considered, with certain exceptions. If you were performing a Safety Hazards Analysis on the hydraulic system you could factor in a failure of a supporting system such as the electrical system. Since the electrical system is very important to the entire aircraft its’ failure would be factored as a single input to an AND gate within the hydraulic system diagram and the frequency of all failures that went into that particular AND gate would be calculated. In any case the electrical system and the hydraulic system would be the subject of their own Hazards Analysis and in most cases total system loss could not occur more frequently than 1 10-9 for the entire fleet of that particular aircraft.
The fallacy about this type of analysis is that the systems are treated separately and not at the aircraft level. To do so would prove that at the aircraft level the probability of a single point failure causing loss of the aircraft would occur at a frequency well below 1 10-9 flight hours.
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The Cat
Under normal conditions,unrelated failures are not considered, with certain exceptions. If you were performing a Safety Hazards Analysis on the hydraulic system you could factor in a failure of a supporting system such as the electrical system. Since the electrical system is very important to the entire aircraft its’ failure would be factored as a single input to an AND gate within the hydraulic system diagram and the frequency of all failures that went into that particular AND gate would be calculated. In any case the electrical system and the hydraulic system would be the subject of their own Hazards Analysis and in most cases total system loss could not occur more frequently than 1 10-9 for the entire fleet of that particular aircraft.
The fallacy about this type of analysis is that the systems are treated separately and not at the aircraft level. To do so would prove that at the aircraft level the probability of a single point failure causing loss of the aircraft would occur at a frequency well below 1 10-9 flight hours.
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The Cat
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Endorse what Genghis says; JAR Part 25 is the place to look for requirements.
Last programme I was on we didnt consider multiple unrelated failures on the flight test aircraft but consequential failures and redundancy certainly do come up.
For example we did a test for a failure of the AP to disengage and consequently had to demonstrate an AP engaged landing (NB Not an autoland!!) For that to occur all the modes of AP disengagement must fail (ICO, Mode Selectors, Trim etc). They are related in the sense that they are AP disengage methods but unrelated in the sense that one is a control panel fault, one is a stick switch fault and the other is a trim wheel fault!!!
Transport Canada and the FAA publish advisory circulars (AC25) which takes the Part 25 regs and suggest how to demonstrate compliance. Not been able to find on line copies yet but I believe there is a JAA AC available (AC25J?)
Not sure what the boys on the iron birds do.
Looks like I will be joining the boys in Seattle shortly so I will have a look see at what they have.
Finally, as Genghis said, talk with the Airworthiness spods. It is most certainly their job to compile the compliance plan (albeit from engineering input). I'll add caution to that as they will tend to over demonstrate and you will end up doing days and weeks of tests rather than have someone sit down and work out the risks & probablities.
On the subject of probabilities, I have seen at least 3 "10 to the minus 9 events" on some of the equipment that you guys are going to be getting Jet Noise!! Course its fixed now.....
Last programme I was on we didnt consider multiple unrelated failures on the flight test aircraft but consequential failures and redundancy certainly do come up.
For example we did a test for a failure of the AP to disengage and consequently had to demonstrate an AP engaged landing (NB Not an autoland!!) For that to occur all the modes of AP disengagement must fail (ICO, Mode Selectors, Trim etc). They are related in the sense that they are AP disengage methods but unrelated in the sense that one is a control panel fault, one is a stick switch fault and the other is a trim wheel fault!!!
Transport Canada and the FAA publish advisory circulars (AC25) which takes the Part 25 regs and suggest how to demonstrate compliance. Not been able to find on line copies yet but I believe there is a JAA AC available (AC25J?)
Not sure what the boys on the iron birds do.
Looks like I will be joining the boys in Seattle shortly so I will have a look see at what they have.
Finally, as Genghis said, talk with the Airworthiness spods. It is most certainly their job to compile the compliance plan (albeit from engineering input). I'll add caution to that as they will tend to over demonstrate and you will end up doing days and weeks of tests rather than have someone sit down and work out the risks & probablities.
On the subject of probabilities, I have seen at least 3 "10 to the minus 9 events" on some of the equipment that you guys are going to be getting Jet Noise!! Course its fixed now.....
