Pressurisation check gone wrong...
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What sort of differential pressure are we talking about here to do this?
I've always wondered because the diff pressure gauges go red somewhere around 9.00 psi, which is not much as far as I can guess.
And no, I don't have a shady character friend who has a scuba tank and a plot.
I've always wondered because the diff pressure gauges go red somewhere around 9.00 psi, which is not much as far as I can guess.
And no, I don't have a shady character friend who has a scuba tank and a plot.
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9psi's nothing to sniff at.
The average bag of sugar is 2lbs/1kg, and has a "footprint" of, oh, maybe 3" by 4" (I don't have one handy, so could be off a bit there). So a single bag of sugar is 2lbs/12sq.in.
To achieve 9 psi would require a total of 108lbs over that 12sqin footprint. 54 bags of sugar.
If you were to pile bags of sugar over an entire surface in your house, and pile them 54 layers high, I wouldn't put long odds on that surface lasting too long. There's at least be some creaking going on.
Alternatively, typical feet sizes of a male are probably about 4" wide by 12" long. With two feet, that's a rough footprint of 2*3*12 = 72 sq inches If you weigh just over 200lbs - which is getting a bit chubby in most cases - that's still only 3psi. How many people could you stand (with two others sat on their shoulders) before an airplane structure might start to be a bit upset. Especially if our hypothetical 3-person load were repeated as close as you could get them to stand?
I'm actually quite impressed those thin sheets of metal can take 9psi.
The average bag of sugar is 2lbs/1kg, and has a "footprint" of, oh, maybe 3" by 4" (I don't have one handy, so could be off a bit there). So a single bag of sugar is 2lbs/12sq.in.
To achieve 9 psi would require a total of 108lbs over that 12sqin footprint. 54 bags of sugar.
If you were to pile bags of sugar over an entire surface in your house, and pile them 54 layers high, I wouldn't put long odds on that surface lasting too long. There's at least be some creaking going on.
Alternatively, typical feet sizes of a male are probably about 4" wide by 12" long. With two feet, that's a rough footprint of 2*3*12 = 72 sq inches If you weigh just over 200lbs - which is getting a bit chubby in most cases - that's still only 3psi. How many people could you stand (with two others sat on their shoulders) before an airplane structure might start to be a bit upset. Especially if our hypothetical 3-person load were repeated as close as you could get them to stand?
I'm actually quite impressed those thin sheets of metal can take 9psi.
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I wonder what they were doing when it happened?
After initial manufacture, or again after completing major repairs to a fuselage structure, it is the practice to carry out an overblow check. The fuselage is pressurised to at least 150% of the rated Maximum Differential Pressure. For passenger transport aircraft the max diff is in the order of 8.6. p.s.i so the overblow would be to 12.9 p.s.i. The purpose of this operation isn't to see if the fuselage can withstand the pressure, it is done to work harden the structure and improve the fatigue life of the whole pressurised fuselage.
In the case of this KC135, either there was a pre-existing major structural fault or the pressure limit was exceeded by a much wider margin than 150%.
After initial manufacture, or again after completing major repairs to a fuselage structure, it is the practice to carry out an overblow check. The fuselage is pressurised to at least 150% of the rated Maximum Differential Pressure. For passenger transport aircraft the max diff is in the order of 8.6. p.s.i so the overblow would be to 12.9 p.s.i. The purpose of this operation isn't to see if the fuselage can withstand the pressure, it is done to work harden the structure and improve the fatigue life of the whole pressurised fuselage.
In the case of this KC135, either there was a pre-existing major structural fault or the pressure limit was exceeded by a much wider margin than 150%.
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@ Blacksheep
At what kind of A/C are you working???
Here a Quote of the Page i have linked above:
A KC-135 Aircraft was being pressurized at ground level. The outflow valves which are used to regulate the pressure of the aircraft were capped off during a 5 year overhaul and never opened back up. The post-investigation revealed: that a civilian depot technician who, "had always done it that way," was using a homemade gauge, and no procedure.
Ohhh, this technician ist absolutly
Ingo
After initial manufacture, or again after completing major repairs to a fuselage structure, it is the practice to carry out an overblow check. The fuselage is pressurised to at least 150% of the rated Maximum Differential Pressure.
Here a Quote of the Page i have linked above:
A KC-135 Aircraft was being pressurized at ground level. The outflow valves which are used to regulate the pressure of the aircraft were capped off during a 5 year overhaul and never opened back up. The post-investigation revealed: that a civilian depot technician who, "had always done it that way," was using a homemade gauge, and no procedure.
Ohhh, this technician ist absolutly
Ingo
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Hmmm. And the safety valves didn't open either... curiouser and curiouser...
What kind of A/C? Go to the Boeing Commercial Airplanes website and enter the expression 'high blow' into the website search engine. You may wish to check FARs on the subject of testing fuselage structures too...
What kind of A/C? Go to the Boeing Commercial Airplanes website and enter the expression 'high blow' into the website search engine. You may wish to check FARs on the subject of testing fuselage structures too...
Last edited by Blacksheep; 1st Feb 2005 at 13:44.
Cunning Artificer
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I do hope that you wouldn't really refer to the AMM for a major structural repair, old chap. Do try looking in the SRM first, then when you've discovered that its beyond the SRM limit, you can contact your FAR21 design office or the manufacturer for an approved repair scheme. If its a Boeing, they will, no doubt, lend you the high blow test equipment and trained operators to do the test for you.
I am simply pointing out that an aircraft fuselage can withstand very much more pressure than the typical Max Diff of around 8.6 psi and it is a fairly routine matter to take it up to 150% of that just to bed it in after initial build or a major repair. For a KC135 fuselage to burst in the fashion shown, it must either have been taken to an almost unbelievable overload value, or else there was existing undetected damage to the structure.
Who do you work for, by the way. I like to be sure of these things when I buy my tickets.
Take care.
I am simply pointing out that an aircraft fuselage can withstand very much more pressure than the typical Max Diff of around 8.6 psi and it is a fairly routine matter to take it up to 150% of that just to bed it in after initial build or a major repair. For a KC135 fuselage to burst in the fashion shown, it must either have been taken to an almost unbelievable overload value, or else there was existing undetected damage to the structure.
Who do you work for, by the way. I like to be sure of these things when I buy my tickets.
Take care.
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I don't know what this guy/girl was doing/smoking but every AMM procedure I've ever followed for a pressurization test tells you to use the aircraft's own gauge while doing this. One thing to keep in mind is that this was a military aircraft and the AMM being used here would be USAF's manual and they have their own rules and standards which are below civilian standards at least most of the time.
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When testing the pressurisation functions of an aircraft, or checking for door seal leaks etc., it is OK to sit in the flight deck and monitor using the aircraft gauges. However, after manufacture or a major repair, the aircraft is more often pressurised from outside the aircraft and the internal pressure monitored using an external gauge. This is not an AMM procedure, it is part of an approved repair scheme.
The information provided above says that the aircraft had just undergone a Major Check and it is quite likely that the aircraft was being pressurised and monitored externally. Many years ago while I was still in the military, a colleague of mine was checking a repair for leaks and as he approached the aircraft, the repaired structure failed and he was killed. He had not been involved in the repair either...
RIP, Steve old friend. Another hard lesson in engineering that I've never forgotten.
The information provided above says that the aircraft had just undergone a Major Check and it is quite likely that the aircraft was being pressurised and monitored externally. Many years ago while I was still in the military, a colleague of mine was checking a repair for leaks and as he approached the aircraft, the repaired structure failed and he was killed. He had not been involved in the repair either...
RIP, Steve old friend. Another hard lesson in engineering that I've never forgotten.
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I recall a similar, although not so destructive, episode on an early L-1329 JetStar. Deformation, not disassembly.
The ship was deemed ferryable, whereupon it was returned to Marietta where a new fuselage was installed.
Jack up the nameplate...
The ship was deemed ferryable, whereupon it was returned to Marietta where a new fuselage was installed.
Jack up the nameplate...
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Overpressuring a fuselage after manufacturing or major repair is one lesson learned from Comet.
If (due to bad design or manufacturing ...) some stress concentration points do exist on the pressurized fuselage, they will be stressed beyond the yield limit during the overpressure test, leaving residual compression stress after the test. Therefore these critical points will not encounter critical tension stress (leading to fatigue cracks) during service.
Problem for comet was that the fuselage section that was fatigue tested for certification was used for the overpressure test before, increasing it´s fatigue life. The serial built fuselages were not operpressured before delivery, so they did not have enough fatigue resistance.
If (due to bad design or manufacturing ...) some stress concentration points do exist on the pressurized fuselage, they will be stressed beyond the yield limit during the overpressure test, leaving residual compression stress after the test. Therefore these critical points will not encounter critical tension stress (leading to fatigue cracks) during service.
Problem for comet was that the fuselage section that was fatigue tested for certification was used for the overpressure test before, increasing it´s fatigue life. The serial built fuselages were not operpressured before delivery, so they did not have enough fatigue resistance.
