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FAA Partial Backflip On Fuel Tank Safety Regs  
User currently offlineZeke From Hong Kong, joined Dec 2006, 9229 posts, RR: 76
Posted (5 years 10 months 2 weeks 23 hours ago) and read 6212 times:

from http://seattletimes.nwsource.com/htm...ospace/2008719843_lightning08.html

Quote:
The FAA proposes to relax the safeguards for preventing sparks inside the fuel tank during a lightning strike, standards the agency now calls "impractical" and Boeing says its soon-to-fly 787 Dreamliner cannot meet.

Instead of requiring three independent protection measures for any feature that could cause sparking, the revised policy would allow some parts to have just one safeguard.

Seems Boeing has brought pressure upon the FAA to change the certification requirements for fuel tanks, another case of affordable safety. The NSTB and many engineers at the FAA seem unhappy/disappointed with the change.

Previous (now closed) threads relating to this giving some history on the matter :
"FAA To Order Fuel Safety Systems On Jets"
http://www.airliners.net/aviation-fo...eneral_aviation/read.main/1407560/

"Fuel-Tank Safety System Comes Standard On 787"
http://www.airliners.net/aviation-fo...eneral_aviation/read.main/3304514/

"Airbus Plans To Comply With FAA Rules On Fuel Tank"
http://www.airliners.net/aviation-fo...eneral_aviation/read.main/3172085/

"New System To Prevent Fuel Tank Explosions"
http://www.airliners.net/aviation-fo...general_aviation/read.main/984113/

"FAA To Mandate Fuel Tank Inerting Systems"
http://www.airliners.net/aviation-fo...eneral_aviation/read.main/1435203/

"New Fuel Tank Fortification To Become Mandatory"
http://www.airliners.net/aviation-fo...eneral_aviation/read.main/1407186/


We are addicted to our thoughts. We cannot change anything if we cannot change our thinking – Santosh Kalwar
34 replies: All unread, showing first 25:
 
User currently offlineCARST From Germany, joined Jul 2006, 836 posts, RR: 2
Reply 1, posted (5 years 10 months 2 weeks 22 hours ago) and read 6179 times:

I am no expert, but i read the full story now and it seems to me like it sounds worse as it is...

Some quotes...

Quoting Boeing (true?):
And Boeing experts insist the 787 will be safer in a lightning storm than any jet flying today.

Now the more interesting ones...

Quote:
The rules the FAA is now reinterpreting have been in place since 2001 after the investigation into the TWA 800 fuel-tank explosion that killed all 230 people on board the 747 jumbo jet.
While investigators concluded that the likely cause of the spark that triggered that explosion was faulty wiring, they set up standards to prevent fuel-tank ignition from any source, including a lightning storm.

The reason wasn't lightning, so why the rule?

Quote:
"To this day, we have not had one manufacturer that has been able to demonstrate compliance with that rule," said Ali Bahrami, head of the FAA's Seattle office dealing with commercial-airplane certification. "We decided it's time to re-evaluate our approach."



Quote:
Airbus applied for certification of its newest plane, the A380, before the regulation, so it did not have to comply.



Quote:
The FAA granted exemptions in 2006 and 2007 to plane makers Dassault Aviation, of France, and Hawker Beechcraft, of Wichita, Kan., allowing them to certify their Falcon 7X and Hawker 4000 business jets with only two independent layers of protection on the wing-skin fasteners.

So why should Boeing follow a rule from 2001 which Airbus, Dassault and Hawker and even the newest commercial airplane on the markert today, the A380 don't follow?

Might sound childish if they would argue with saying "they didn't why should we?", but this decision has a large economical factor and if others don't have to follow a rule that was perhaps misplaced due to the panic after TWA800 i think this decision is acceptable...


User currently offlineKhobar From United States of America, joined Mar 2006, 2379 posts, RR: 4
Reply 2, posted (5 years 10 months 2 weeks 18 hours ago) and read 6026 times:

Quoting CARST (Reply 1):
The reason wasn't lightning, so why the rule?

Quite simply because if you preclude the possibility of fuel tank ignition then it doesn't matter what ignition sources might be present. IOW, TWA800 was concluded to be faulty wiring. However, if the fuel tank had been inerted, then it wouldn't have mattered whether the source of ignition was faulty wiring, a lightening strike, a meteorite, a big goose, or an explosive device, etc. - none of them would have been able to produce the result we got.

The 787 has an inerting system, as already mentioned in the article.

[Edited 2009-02-08 08:07:04]

User currently offlineComeAndGo From United States of America, joined Mar 2005, 1041 posts, RR: 0
Reply 3, posted (5 years 10 months 2 weeks 17 hours ago) and read 5983 times:



Quoting Khobar (Reply 2):
The 787 has an inerting system, as already mentioned in the article.

... which will prevent any tank from blowing up no matter how many sparks fly.

Boeing has a particular problem with fuel tank explosions, because it is the only manufacturer to place bare wires in the fuel tank. Other manufacturers have wires running around the tanks or duct wires through pipes that prevent any sparks from propagating into the tank.


User currently offlineFlashmeister From United States of America, joined Apr 2000, 2903 posts, RR: 6
Reply 4, posted (5 years 10 months 2 weeks 17 hours ago) and read 5943 times:
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Quoting ComeAndGo (Reply 3):
which will prevent any tank from blowing up no matter how many sparks fly.

As long as the inerting system isn't MELed, which it can be for 10 days.


User currently offlineTdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 5, posted (5 years 10 months 2 weeks 15 hours ago) and read 5884 times:



Quoting ComeAndGo (Reply 3):
Boeing has a particular problem with fuel tank explosions, because it is the only manufacturer to place bare wires in the fuel tank.

All current generation aircraft except the 777 have bare wires in the fuel tank. Capacitance FQIS probes are just huge unshielded wires.

Tom.


User currently offlineKhobar From United States of America, joined Mar 2006, 2379 posts, RR: 4
Reply 6, posted (5 years 10 months 2 weeks 12 hours ago) and read 5771 times:



Quoting ComeAndGo (Reply 3):
Boeing has a particular problem with fuel tank explosions, because it is the only manufacturer to place bare wires in the fuel tank.

You are correct in that Boeings are logically more prone to these kinds of issues, but for the wrong reason.

In TWA800, the fuel tank was combustible because it was heated via air packs. In two other Boeing CWT explosions, the CWT was heated by the air packs (though the actual cause of ignition is suspected to be something different from TWA 800). Heated fuel tanks is the key. Since Airbus generally puts their air packs in a different location, they are less prone to the same type of heating.

That does not mean, however, that they don't have a risk: http://www8.landings.com/cgi-bin/get...ass=12345&ADS/2007/2007-21-14.html

And: http://rgl.faa.gov/Regulatory_and_Gu...b63d!OpenDocument&ExpandSection=-3

Here's an Australian one for the A330: http://www.casa.gov.au/airworth/airwd/ADfiles/OVER/A330/A330-063.pdf

Bottom line - if the fuel tanks are not combustible, then they will not be a problem no matter what kind of ignition source may be present.


User currently offlineZeke From Hong Kong, joined Dec 2006, 9229 posts, RR: 76
Reply 7, posted (5 years 10 months 2 weeks 9 hours ago) and read 5668 times:



Quoting Khobar (Reply 6):
Bottom line - if the fuel tanks are not combustible, then they will not be a problem no matter what kind of ignition source may be present.

Did you READ and UNDERSTAND what they were asking in the links provided ? Those Airbus aircraft were certified before the new rules came in, and the ADs simply brings their maintenance practices inline with current regulations. It included aircraft like the A330-300 which does not even have a center fuel tank.

None of those ADs from what I read had corrective action to the aircraft, they were maintenance documentation related. The documentation change asked for additional inspections and documentation changes in line with EASA D 2005/CPRO http://www.easa.europa.eu/ws_prod/c/...ment_fuel_tank_system_ignition.pdf

I am not aware of a single Airbus aircraft having a fuel tank related ignition problem, but I am aware of 707/KC-135, 727, 737, and 747 aircraft with fuel tank explosions as being the probable cause.

I am of the understanding that this is due to Airbus aircraft being designed from the outset as being fault tolerant, whereas Boeing aircraft before the 777 were designed only as fail safe (777 is fault tolerant), in my view it is the different design philosophies which lead to the different systems, and that saved lives.



We are addicted to our thoughts. We cannot change anything if we cannot change our thinking – Santosh Kalwar
User currently offlineTdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 8, posted (5 years 10 months 2 weeks 8 hours ago) and read 5630 times:



Quoting Zeke (Reply 7):
I am of the understanding that this is due to Airbus aircraft being designed from the outset as being fault tolerant, whereas Boeing aircraft before the 777 were designed only as fail safe (777 is fault tolerant)

Boeing hasn't designed fail safe since the 727.

Tom.


User currently offlineZeke From Hong Kong, joined Dec 2006, 9229 posts, RR: 76
Reply 9, posted (5 years 10 months 2 weeks 7 hours ago) and read 5580 times:



Quoting Tdscanuck (Reply 8):
Boeing hasn't designed fail safe since the 727

Not true Tom, the 747-8 will be fail safe, so is the 737, other 747s, 757, and 767. The 767 for example has fail safe fuselage joins. The 777 and 787 will be the only totally fault tolerant aircraft Boeing has.



We are addicted to our thoughts. We cannot change anything if we cannot change our thinking – Santosh Kalwar
User currently offlineTdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 10, posted (5 years 10 months 2 weeks 4 hours ago) and read 5509 times:



Quoting Zeke (Reply 9):

Not true Tom, the 747-8 will be fail safe, so is the 737, other 747s, 757, and 767. The 767 for example has fail safe fuselage joins. The 777 and 787 will be the only totally fault tolerant aircraft Boeing has.

I think we must be talking about different definitions. When I hear "fail safe" I think about a structural design philosophy where you protect against structural failures by having redundant load paths. The last aircraft done this (at Boeing, anyway) was the 727. Everything since then has been damage tolerant.

Since a fuselage joint has only one load path, what you mean by a "fail safe fuselage joint" must be different than what I'm thinking of.

So I retract everything I wrote before because I think we're talking about two different things, but I want to understand this. How are you defining "fail safe" and how is that different from "fault tolerant?"

Tom.


User currently offlinePygmalion From United States of America, joined Jun 2006, 969 posts, RR: 38
Reply 11, posted (5 years 10 months 2 weeks 3 hours ago) and read 5478 times:

especially when Fail safe design is actually more stringent/robust than damage tolerant.

Fail safe sssumes that the part will fail and surrounding structure will carry all loads around that failed part. Damage tolerant design assumes that the part will crack but the inspection plan will catch the crack before it grows past a critical length. (not full failure)

All modern civil aircraft are designed damage tolerant, Airbus and Boeing. It makes for lighter weight structure but the fatigue analysis is more complex and detailed.


User currently offlineBoeingVista From Australia, joined Jan 2009, 1584 posts, RR: 3
Reply 12, posted (5 years 10 months 2 weeks 3 hours ago) and read 5478 times:



Quoting CARST (Reply 1):

So why should Boeing follow a rule from 2001 which Airbus, Dassault and Hawker and even the newest commercial airplane on the markert today, the A380 don't follow?

The A380-800 doesnt have a centre fuel tank.



BV
User currently offlineZeke From Hong Kong, joined Dec 2006, 9229 posts, RR: 76
Reply 13, posted (5 years 10 months 1 week 6 days 23 hours ago) and read 5408 times:



Quoting Tdscanuck (Reply 10):
I think we must be talking about different definitions. When I hear "fail safe" I think about a structural design philosophy where you protect against structural failures by having redundant load paths. The last aircraft done this (at Boeing, anyway) was the 727. Everything since then has been damage tolerant.

That is a common misunderstanding, fail safe design dos not mean that at all, e.g. look at the Dan Air 707 crash demonstrated that a fail safe design concept does not by itself constitute a fail safe design. In that case the investigation traced the accident back to fatigue failure in the upper chord of the rear spar of the right-hand horizontal stabilizer. Fatigue cracking began at a fastener hole owing to higher loads than those anticipated in the design. The fatigue spread into the upper chord, with overall crack growth being accelerated by large intermittent tensile crack jumps. Fatigue crack growth finally gave way to overload fracture down through the entire rear spar, and this resulted in the stabilizer separating from the aircraft.

Damage tolerant designs also allow for fail safe design e.g. the DHL Airbus 300 missile attack at Baghdad Airport, but it also assumes that structures will fail (or even has imperfections at the time of manufacture) and requires the development of structural inspection procedures to monitor continuing airworthiness and detailed structural repair procedures and documents.

I should add, both Airbus and Boeing still use safe life design methods with ground loaded, single load path, structures such as landing gear, this is not relevant to the fuel tank issue, but I raise it now to also counter the false "Everything since then has been damage tolerant" line you keep pushing.

Other issues would include the rear pressure bulkhead on the 737, 757, 747, 767 etc are fail safe, not damage tolerant. And we saw from the China Airlines 747 crash how a fail safe design repair of that structure can fail. The 747 also does not meet current requirements for rapid decompression, sustained engine imbalance, uncontained engine failure, redundant control paths etc. Another example of fail safe vs damage tolerant design since the 727 would be the Aloha 737 zipper top.

But the biggest issue of damage tolerant design which is relevant to fuel tank safety would be the possible failure of fuel tank pumps, or placing wiring in those areas which is not inspectable.

Quoting Tdscanuck (Reply 10):
Since a fuselage joint has only one load path, what you mean by a "fail safe fuselage joint" must be different than what I'm thinking of.

That document I sent you was a fail safe joint with a slow flaw growth, it is not damage tolerant as one cannot inspect it, nor are imperfections in the join considered at design time. When it reaches ts design service life it needs to be replaced, which is what was being shown in that document.

Quoting Pygmalion (Reply 11):
All modern civil aircraft are designed damage tolerant, Airbus and Boeing. It makes for lighter weight structure but the fatigue analysis is more complex and detailed.

Disagree with that, I have read that the 747-8 is at least 5% lighter than what it would have been if it had been certified under current rules. The grandfathering of the structural design allows for a lighter structure and systems for a number of reasons. While Boeing may update changes on follow on models as being damage tolerant, they are not redesigning the whole airframe as such.

Damage tolerant design encompasses all of the fail safe design philosophy but also have some differences like the possibility of cracks or flaws in a new structure must be considered and the structure should be inspection to monitor any damage during its service life.

The EASA document I linked above includes the damage tolerant inspection requirements for fuel tanks.



We are addicted to our thoughts. We cannot change anything if we cannot change our thinking – Santosh Kalwar
User currently offlineMoo From Falkland Islands, joined May 2007, 4087 posts, RR: 4
Reply 14, posted (5 years 10 months 1 week 6 days 22 hours ago) and read 5383 times:



Quoting CARST (Reply 1):
So why should Boeing follow a rule from 2001 which Airbus, Dassault and Hawker and even the newest commercial airplane on the markert today, the A380 don't follow?

You yourself answered the question with regard to the A380...

Quoting CARST (Reply 1):
Airbus applied for certification of its newest plane, the A380, before the regulation, so it did not have to comply.

If Boeing had got their application for certification in before the cutoff date, they would also be exempt.

Its as simple as that.


User currently offlineOldAeroGuy From United States of America, joined Dec 2004, 3597 posts, RR: 66
Reply 15, posted (5 years 10 months 1 week 6 days 17 hours ago) and read 5254 times:

Quoting Zeke (Reply 13):
The 747 also does not meet current requirements for rapid decompression, sustained engine imbalance, uncontained engine failure, redundant control paths

This a pretty general statement and it depends on what version of the 747 you are talking about.

It's no surprise that a 747-100 won't meet the current version of these regs.

However, many more recently certified airplanes will not meet them either. With regard to rapid decompression, I don't know of any airplane, either Airbus or Boeing, that hasn't been granted an Exemption from FAR 25.841(a)(2)(i),(ii), and (3), per Admin 87, including the A380.

http://rgl.faa.gov/Regulatory_and_Gu...A58625754C0070D4DB/$FILE/A58NM.pdf

In this case, the FAA wrote an Amendment that airframers could not comply with in a manner that was economically viable. As a result, the FAA has allowed airframers Exemptions relative to these requirements although they have been left on the books as an inspiration to future designs.

The FAA would seem to be going through a similar process relative to fuel tank flammability.

With regard to the 748F/8i and sustained engine imbalance, uncontained engine failures and redundant control paths, I think you'll find that this version of the 747 will directly comply with the latest version of the regs or will have been granted an Equivalent Safety Finding.

[Edited 2009-02-09 09:08:50]

[Edited 2009-02-09 09:42:32]


Airplane design is easy, the difficulty is getting them to fly - Barnes Wallis
User currently offlineKhobar From United States of America, joined Mar 2006, 2379 posts, RR: 4
Reply 16, posted (5 years 10 months 1 week 6 days 13 hours ago) and read 5114 times:



Quoting Zeke (Reply 7):
Did you READ and UNDERSTAND what they were asking in the links provided ? Those Airbus aircraft were certified before the new rules came in, and the ADs simply brings their maintenance practices inline with current regulations. It included aircraft like the A330-300 which does not even have a center fuel tank.

Yes I did. If you are unhappy about the quality of the link provided, there are plenty more. For example: http://www.avantext.com/media/files/ADs/20082502.pdf

"Consequently, the following measures [are]
rendered mandatory * * *:
• [inspection and] replacement [if
necessary] of the white P-clips by blue Pclips
which are more fuel resistant remove
the risks of fuel quantity indicator (FQI) and
fuel level sensor system (FLSS) harnesses
chafing against the metallic part of the P-clip,
• Modification of electrical bonding of
equipment installed in fuel tanks in order to
re-establish the conformity with the design
definition by introducing additional bonding
leads, electrical bonding points and electrical
bonding of a support bracket for a diffuser
assembly installed between Rib 1 and Rib 2
on the stringers of the Number 1 bottom skin
panel,• Modification of bonding points,
installation of additional bonding leads and
other modifications of the Additional Center
Tank (ACT),
• Modification to increase the distance
between metallic parts on the THS Trim
Tank,
• Installation of a bonding lead between
the bonding tags on the Jettison valve
actuator and drive assembly.

Doesn't sound like it's just paperwork, but that's beside the point that ignition sources do exist inside the fuel tanks of various Airbus types.

And FYI, CWT's are NOT the only fuel tanks that are at potential risk which is consistent with the fact that fuel tank explosions have not been restricted to CWT's. Go figure.


User currently offlineKhobar From United States of America, joined Mar 2006, 2379 posts, RR: 4
Reply 17, posted (5 years 10 months 1 week 6 days 13 hours ago) and read 5087 times:



Quoting Zeke (Reply 7):
I am not aware of a single Airbus aircraft having a fuel tank related ignition problem, but I am aware of 707/KC-135, 727, 737, and 747 aircraft with fuel tank explosions as being the probable cause.

Can you certify, with absolute certainty, that Airbus fuel tanks are NEVER vulnerable? Just because it hasn't happened - yet, doesn't mean it can't, or won't.

Fuel tank explosions are rare to begin with, and I would remind you to check the fuel types and all factors involved when assigning blame in such events.


User currently offlineTdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 18, posted (5 years 10 months 1 week 6 days 4 hours ago) and read 4959 times:



Quoting Zeke (Reply 13):
That is a common misunderstanding, fail safe design dos not mean that at all, e.g. look at the Dan Air 707 crash demonstrated that a fail safe design concept does not by itself constitute a fail safe design.

Dan Air 707 was indeed a fail safe *design*...that crash was one of a major drivers towards damage tolerance (as well as the disintegrating F-111). The whole principal of that particular hunk of structure was that there were three chords, and any two could carry the full load. By design, one of the chords was unloaded. However, they didn't take into account interaction across the chords. It was a faulty fail safe design, but it was intended to be fail safe design. The design concept was definitely fail safe (just badly executed).

Quoting Zeke (Reply 13):
I should add, both Airbus and Boeing still use safe life design methods with ground loaded, single load path, structures such as landing gear, this is not relevant to the fuel tank issue, but I raise it now to also counter the false "Everything since then has been damage tolerant" line you keep pushing.

This is absolutely true. I was unacceptably broad before.

Quoting Zeke (Reply 13):
Other issues would include the rear pressure bulkhead on the 737, 757, 747, 767 etc are fail safe, not damage tolerant.

How are they fail safe? They're inspected for cracks at regular intervals (the hallmark of damage tolerant design) and they don't have redundant load paths...when the rear pressure bulkhead lets go, it's gone.

Quoting Zeke (Reply 13):
And we saw from the China Airlines 747 crash how a fail safe design repair of that structure can fail.

The repair design on that one was supposed to be damage tolerant, I think. The repair, as accomplished, was neither damage tolerant nor fail safe (it only had a single row of rivets in bearing).

Quoting Zeke (Reply 13):
Another example of fail safe vs damage tolerant design since the 727 would be the Aloha 737 zipper top.

The 737 fuselage skin was designed as damage tolerant. The problem with the Aloha 737 was that they only looked at fatigue cracks from each hole in isolation, not what would happen if multiple cracks linked up. Aloha kicked off the entire multi-site damage aspect of damage tolerant design.

Quoting Zeke (Reply 13):
But the biggest issue of damage tolerant design which is relevant to fuel tank safety would be the possible failure of fuel tank pumps, or placing wiring in those areas which is not inspectable.

I'm not sure about Airbus, but Boeing doesn't have any non-inspectable wiring in the fuel tank. There are inspect tasks on all the in-tank wiring. In the context of fuel pump failure, what's the difference between damage tolerant and fail safe?

Quoting Zeke (Reply 13):
That document I sent you was a fail safe joint with a slow flaw growth, it is not damage tolerant as one cannot inspect it, nor are imperfections in the join considered at design time. When it reaches ts design service life it needs to be replaced, which is what was being shown in that document.

Do you have more data on this? That would mean the 767 fuselage itself is a life limited part, which I find pretty surprising.

Tom.


User currently offlineZeke From Hong Kong, joined Dec 2006, 9229 posts, RR: 76
Reply 19, posted (5 years 10 months 1 week 5 days 14 hours ago) and read 4835 times:



Quoting OldAeroGuy (Reply 15):

This a pretty general statement and it depends on what version of the 747 you are talking about.

Not really, all 747s built today or in the future are inferior to the design standard used in the 777 or 787. It is an old aircraft design, with grandfather rights to old design requirements, it pre dates amendment 25-72, e.g. the 747-400 Damage Tolerance and Fatigue Evaluation of Structure is at 25-9 levels according to the TCDS, which is back around mid 1966 amendment, a lot has changed since then. I have no reason to believe the 747-8 will have any improvement over that.

Quoting Khobar (Reply 16):
Doesn't sound like it's just paperwork, but that's beside the point that ignition sources do exist inside the fuel tanks of various Airbus types.

That one, like your previous ones is yet again in response to SFAR 88, it is not in response to problem with the aircraft.

Quoting Khobar (Reply 16):
And FYI, CWT's are NOT the only fuel tanks that are at potential risk which is consistent with the fact that fuel tank explosions have not been restricted to CWT's. Go figure.

True, but the risk on wing/horizontal stab tanks is about 10 time lower than the designs used by Boeing for fuselage tanks (except for the 787 due the lower thermal conductivity of the wing)

Quoting Khobar (Reply 17):
Can you certify, with absolute certainty, that Airbus fuel tanks are NEVER vulnerable? Just because it hasn't happened - yet, doesn't mean it can't, or won't.

Of course not, we have seen from the A300 in Baghdad that the A300 wing fuel tank is vulnerable to a missile attack, but then again, what aircraft is not vulnerable to such an attack ?

The main difference between Boeing and Airbus center fuel tanks is that Airbus forces ram air to cool the packs on the ground, and Boeing uses the fuel, or the air in the fuel tank for pack cooling.

Quoting Khobar (Reply 17):
Fuel tank explosions are rare to begin with, and I would remind you to check the fuel types and all factors involved when assigning blame in such events.

Meaning ? we have only been talking about jet turbine aircraft here, and not military aircraft.

Quoting Tdscanuck (Reply 18):
It was a faulty fail safe design, but it was intended to be fail safe design. The design concept was definitely fail safe (just badly executed).

It was not designed for the correct loads, but it did hold together for a long time, and 7% of the rest of the fleet was found to have the same damage, so I do not agree with the "faulty fail safe design" comment

from http://www.aaib.gov.uk/publications/formal_reports/9_1978_g_bebp.cfm

"The total number of flights between the initiation of the fatigue crack and the final failure of the upper chord is estimated at 7,200 flights, with 3,500 of the flights being the duration to grow the crack across the exposed surface of the top chord."

Quoting Tdscanuck (Reply 18):
when the rear pressure bulkhead lets go, it's gone

Not true, I know PIA had a rear pressure bulkhead failure in an A300, emergency descent and landed, the aircraft was not lost, as the failure was contained.

Quoting Tdscanuck (Reply 18):
The repair design on that one was supposed to be damage tolerant

I don't know, the aircraft is not designed to be damage tolerant, so I do not understand how a repair could be if the underlying structure is not damage tolerant.

Quoting Tdscanuck (Reply 18):
The 737 fuselage skin was designed as damage tolerant.

Doubt that, it was designed well before amendment 25-72.

Quoting Tdscanuck (Reply 18):
Boeing doesn't have any non-inspectable wiring in the fuel tank.

Doubt that as well, seem to recall tanks being removed from BBJs for that reason, and seem to recall similar issues on the TWA 800 report. Boeing also put higher voltage into its tank from what I recall as well.

Quoting Tdscanuck (Reply 18):
That would mean the 767 fuselage itself is a life limited part, which I find pretty surprising.

I don't, I cannot think of an aircraft flying that does not have life limited parts on them. And they were not looking at replacing the fuselage, just a join which would be a stress concentration.



We are addicted to our thoughts. We cannot change anything if we cannot change our thinking – Santosh Kalwar
User currently offlineOldAeroGuy From United States of America, joined Dec 2004, 3597 posts, RR: 66
Reply 20, posted (5 years 10 months 1 week 5 days 10 hours ago) and read 4781 times:



Quoting Zeke (Reply 19):
Quoting OldAeroGuy (Reply 15):

This a pretty general statement and it depends on what version of the 747 you are talking about.

Not really, all 747s built today or in the future are inferior to the design standard used in the 777 or 787. It is an old aircraft design, with grandfather rights to old design requirements, it pre dates amendment 25-72, e.g. the 747-400 Damage Tolerance and Fatigue Evaluation of Structure is at 25-9 levels according to the TCDS, which is back around mid 1966 amendment, a lot has changed since then. I have no reason to believe the 747-8 will have any improvement over that.

Ah, but you're changing the area of regulatory coverage. You specifically referred to the areas mentioned below.

Quoting Zeke (Reply 13):
The 747 also does not meet current requirements for rapid decompression, sustained engine imbalance, uncontained engine failure, redundant control paths etc

That's why I said this:

Quoting OldAeroGuy (Reply 15):
It's no surprise that a 747-100 won't meet the current version of these regs.

However, many more recently certified airplanes will not meet them either. With regard to rapid decompression, I don't know of any airplane, either Airbus or Boeing, that hasn't been granted an Exemption from FAR 25.841(a)(2)(i),(ii), and (3), per Admin 87, including the A380.

http://rgl.faa.gov/Regulatory_and_Gu...A58625754C0070D4DB/$FILE/A58NM.pdf

In this case, the FAA wrote an Amendment that airframers could not comply with in a manner that was economically viable. As a result, the FAA has allowed airframers Exemptions relative to these requirements although they have been left on the books as an inspiration to future designs.

and this:

Quoting OldAeroGuy (Reply 15):
With regard to the 748F/8i and sustained engine imbalance, uncontained engine failures and redundant control paths, I think you'll find that this version of the 747 will directly comply with the latest version of the regs or will have been granted an Equivalent Safety Finding.

There is nothing wrong with an Equivalent Safety Finding (ESF), the A380 TCDS referenced above shows that the A380 was granted 20 of them.

An ESF acknowledges that the airplane may not meet the exact compliance method of the latest version of the regulations but it does meet the intent of the regulations and is therefore Equivalently Safe.

The 747-8F/8i should be just fine for sustained engine imbalance, uncontained engine failure and redundant control paths relative to the current regulation Amendment level.

You might read AC 21.101-1 and consider the effect this Advisory Circular has had on "grandfathering" in general. Certifying under "Changed Product Rule" is significantly different after 2001 compared to what it was like in 1988.



Airplane design is easy, the difficulty is getting them to fly - Barnes Wallis
User currently offlineStressedout From United States of America, joined Dec 2008, 78 posts, RR: 0
Reply 21, posted (5 years 10 months 1 week 5 days 5 hours ago) and read 4710 times:

The 757, 767 and 777 airframes were all designed with a damage tolerance design criteria. Damage tolerance was established for the 747 via an AD and the associated Boeing document called "Supplementary Structural Inspection Document for the 747" or the SSID. Structural repairs on fatigue critical baseline structure for 747 airplanes were required to be shown to be damage tolerant from Jan 2008 forward. By no means could one say that the repairs designed before that date were not damage tolerant, however, because they typically are.

Those who are interested in damage tolerance might like reading the following articles:

Damage Tolerance, Fact and Fiction by U. G. Goranson

Damage Tolerance in pressurized fuselages by Tom Swift.

Both men are absolute experts in the field. Tom Swift has written many other articles or chapters in books on the subject and I highly recommend reading any and all of them.

Quoting Zeke (Reply 13):
Other issues would include the rear pressure bulkhead on the 737, 757, 747, 767 etc are fail safe, not damage tolerant. And we saw from the China Airlines 747 crash how a fail safe design repair of that structure can fail.

The bit about rear pressure bulkheads on 757's and 767's is categorically not true. I haven't done much stress analysis on 737's and haven't studied them so I can't say for sure there. With regard to the 747, DT is ensured by an AD and SSID document.

With regard to the China airlines repair, how do you know it was designed as a fail safe repair Zeke? Have you seen the backup notes. I doubt it. From what I recall the long scratch from the tail strike wasn't blended out correctly causing the failure. I doubt that it had much to do with whether or not the the repair was designed with damage tolerant criteria, which it probably was!

Quoting Zeke (Reply 13):
Damage tolerant design encompasses all of the fail safe design philosophy

I have heard (in person) individuals with years of large airframe stress analysis experience argue this point both ways. I wonder if anyone on this forum can definitively make this statement?

Quoting Zeke (Reply 13):
Another example of fail safe vs damage tolerant design since the 727 would be the Aloha 737 zipper top.

Actually this isn't an example at all. This accident was the beginning of the study of MSD or Multiple Site Damage. Not very many people had thought about what would happen if many, many fatigue cracks all linked up at the same time along a detail that provided an opportunity for this to happen such as a lap joint. Besides the airline had been warned about their maintenance practices with regard to corrosion in this area.

Quoting Zeke (Reply 19):
I don't, I cannot think of an aircraft flying that does not have life limited parts on them. And they were not looking at replacing the fuselage, just a join which would be a stress concentration.

What do you mean by life limited? Do you mean safe life parts? Landing gears are safe life parts and there could be others although I can't think of any off of the top of my head. The 767 was designed as a damage tolerant airplane so I highly doubt that fuselage joints would be designed with a safe life methodology. I haven't ever heard this either. Maybe because of a design error or economics a part or several parts were designed this way. Please provide documentation.


User currently offlineTdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 22, posted (5 years 10 months 1 week 5 days 4 hours ago) and read 4698 times:



Quoting Zeke (Reply 19):
The main difference between Boeing and Airbus center fuel tanks is that Airbus forces ram air to cool the packs on the ground, and Boeing uses the fuel, or the air in the fuel tank for pack cooling.

Boeing does no such thing. The packs are under the fuel tank, so the fuel tank gets some of the heat rejected from the packs into the pack bay, but the packs are not cooled by fuel or air from the fuel tank. Airbus ventilates the pack bay and usually has the packs up in the fairing cheeks, where they're not directly below the wing box.

Quoting Zeke (Reply 19):
Quoting Tdscanuck (Reply 18):
when the rear pressure bulkhead lets go, it's gone

Not true, I know PIA had a rear pressure bulkhead failure in an A300, emergency descent and landed, the aircraft was not lost, as the failure was contained.

When I say "let go" I mean no longer carried their design load. Or did the failed PIA pressure bulkhead continue to hold pressure? If so, very impressive, but I doubt that's what happened. A fail safe structure, by definition, continues to carry full design load after a failure.

Quoting Zeke (Reply 19):
Quoting Tdscanuck (Reply 18):
Boeing doesn't have any non-inspectable wiring in the fuel tank.

Doubt that as well, seem to recall tanks being removed from BBJs for that reason, and seem to recall similar issues on the TWA 800 report. Boeing also put higher voltage into its tank from what I recall as well.

I believe BBJ tanks are an STC (true, a factory installed STC, but an STC none the less). In the TWA800 report there were quite a few examples of non-inspectable wiring, but not in the wing. The issue was shorts in wire bundles *outside* the wing, which is an issue for all airplanes that use bundles (i.e. all of them). As for voltage, I'm not sure, since I don't know what Airbus runs their fuel pumps on...I strongly suspect 115VAC though, since doing a fuel pump on 28VDC would require huge wires.

Quoting Zeke (Reply 19):

Quoting Tdscanuck (Reply 18):
That would mean the 767 fuselage itself is a life limited part, which I find pretty surprising.

I don't, I cannot think of an aircraft flying that does not have life limited parts on them. And they were not looking at replacing the fuselage, just a join which would be a stress concentration.

I can't think of an aircraft flying that does not have life limited parts either. But I can't think of any modern commercial jet that has a life limited fuselage (or fuselage primary structure), which is what you're saying the 767 has.

Tom.


User currently offlineZeke From Hong Kong, joined Dec 2006, 9229 posts, RR: 76
Reply 23, posted (5 years 10 months 1 week 4 days 15 hours ago) and read 4624 times:

Quoting OldAeroGuy (Reply 20):
Ah, but you're changing the area of regulatory coverage.

No I am not, you are just trying to limit what I can discuss. I will repeat again, the 747-8 will be inferior in design to that of the 777 and 787 becuase the design basis of that airframe is based upon 1960s regulations. Where Being has made major changes over earlier models, e.g. the 747-8 wing, I fully expect that to meet the agreed FAA regulation level (most prob something in 2005), just like the 737 did with its new wing.

Quoting Stressedout (Reply 21):
The 757, 767 and 777 airframes were all designed with a damage tolerance design criteria.

That is not true, damage tolerant design as we know it today only came out after the report into the 1977 Dan Air 707 crash which was issued in 1979, after that crash amendment 25-72 was issued. The 757 and 767 are not designed to that level, if you do not believe me check their TCDS you will see it is at 25-45 for FAR 25.571.

Quoting Stressedout (Reply 21):
With regard to the 747, DT is ensured by an AD and SSID document.

It is not damage tolerant as per amendment 25-72.

Quoting Stressedout (Reply 21):
With regard to the China airlines repair, how do you know it was designed as a fail safe repair Zeke?

Suggest you reread my reply 19 again.

Quoting Stressedout (Reply 21):
This accident was the beginning of the study of MSD or Multiple Site Damage.

Current damage tolerant design assumes multiple site damage at the time of manufacture.

Quoting Tdscanuck (Reply 22):
Boeing does no such thing. The packs are under the fuel tank, so the fuel tank gets some of the heat rejected from the packs into the pack bay, but the packs are not cooled by fuel or air from the fuel tank.

If heat it transferred from the pack bay to the fuel, and the fuel is adsorbing heat from the packs, that process is called cooling. If you can get you hands on a Air Safety Week article "Heating of B747 Center Wing Tank Documented in 1980 Study" it provides some numbers of ho much heating of the fuel has been measured in the 747 tanks.

Also from from http://seattlepi.nwsource.com/business/18352_boeing12.shtml

"The air conditioning units are located under the center fuel tank on Boeing-made planes, and the heat given off during ground operations can be get into the tank, raising the temperature of fuel vapors."

"The safety board last year criticized the design of Boeing jets because of the placement of the air conditioning units and the lack of a ventilation cooling system."

Quoting Tdscanuck (Reply 22):
A fail safe structure, by definition, continues to carry full design load after a failure.

Incorrect. From the preamble to amendment 25-72, "Fail-safe generally means a design such that the airplane can survive the failure of an element of a system or, in some instances one or more entire systems, without catastrophic consequences. Fail-safe, as applied to structures prior to Amendment 25-45, meant complete element failure or obvious partial failure of large panels. It was assumed that a complete element failure or partial failure would be obvious during a general area inspection and would be corrected within a very short time. The probability of detecting damage during routine inspections before it could progress to catastrophic limits was very high. Damage-tolerance, on the other hand, does not require consideration of complete element failures or obvious partial failures, although fail-safe features may be included in structure that is designed to damage-tolerance requirements."

The ability of that aircraft to land after the failure of the primary structure without catastrophic consequences means it was fail safe.

Edit : Should add that aircraft was repaired after that 2001 incident and then was retired in 2005.

Quoting Tdscanuck (Reply 22):
I believe BBJ tanks are an STC (true, a factory installed STC, but an STC none the less). In the TWA800 report there were quite a few examples of non-inspectable wiring, but not in the wing

I might have that wrong then, I was of the understand that Boeing had sent out new fuel tank wiring in response to AD for a few models and have changed the wiring on new models.

Also I now understand that all the manufacturers of Boeing FQIS have changed their wiring to nickel-plated wire with gold plated ring connectors after the previous silver plated and silver soldered terminal connections were found to interact with the sulfur in Jet A fuel to form conductive silver sulfate deposits. The accumulation of these conductive deposits on fuel tank FQIS terminal blocks means that there is a breakdown in that terminal blocks insulation and then a very small voltage can cause an arc across that FQIS terminal block. Boeing as found that this can happen with as little as 750 hours in service, see Boeing Analytical Engineering Report 2-5323-WP-91-97.

Quoting Tdscanuck (Reply 22):
But I can't think of any modern commercial jet that has a life limited fuselage (or fuselage primary structure), which is what you're saying the 767 has.

No the document I sent you was for one area of the fuselage, not the whole fuselage, and you can hardly deny it when airlines are doing the joint replacement. Any idea what the Design Service Objective for the 767 is ? and what was critical ?



[Edited 2009-02-11 11:22:24]


We are addicted to our thoughts. We cannot change anything if we cannot change our thinking – Santosh Kalwar
User currently offlineR2rho From Germany, joined Feb 2007, 2771 posts, RR: 1
Reply 24, posted (5 years 10 months 1 week 3 days 13 hours ago) and read 4470 times:

I though this was a thread about fuel-tank safety?  Confused How about starting a separate thread on structural design philosophiy for this debate? Let's get on with the topic!

25 Post contains links Khobar : Something is messed up with the reply. Not sure what got deleted, but I'll try to resend: Quoting Zeke (Reply 19): Meaning ? we have only been talkin
26 Post contains links Stitch : Since we've wandered off the 787 fuel tank to other systems on other Boeing airplanes, I figure this is now relevant to the discussion at hand: S. Ahm
27 ArabAirX : There is a very detailed podcast on the IAG site - Dominic Gates goes into detail about the 787 spark issue and how the fasteners (again?!) play a rol
28 Post contains links Lumberton : Boeing responded to this via an e-mail to Leeham. See comment #4. Here's a link. http://leehamnews.wordpress.com/2009/02/08/787-lightning-protection/
29 ArabAirX : Wouldnt it be better then that the 747-8 perhaps adhere to a new type certificate rather than then orginal and updating that?
30 Stitch : Probably, but then it wouldn't be a 747 because you can't build an airplane like it if you follow all the current guidelines (no passengers forward o
31 Tdscanuck : If they had to do a new type certificate, there likely wouldn't be a 747-8. The effort involved in certifying even a derivative is pretty horrifying.
32 ArabAirX : Didnt Airbus not so long ago advocate that the 747-8 be certified as a new airplane? How then would that have worked if as Stitch notes no pax forward
33 Stitch : I do not recall if it was Airbus specifically who advocated the 747-8 Intercontinental be certified as a new airplane. Since a passenger plane with t
34 ArabAirX : Found this link from Flight global, and Leahy too says the same thing you do regarding Door 1.
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