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Safety Of Planes Beyond Useful Service Life  
User currently offlineThrust From United States of America, joined Sep 2003, 2688 posts, RR: 10
Posted (6 years 8 months 2 weeks 22 hours ago) and read 3839 times:

I felt compelled to ask this question because of some of the TWA 800 lawsuits that I read which I feel were not necessarily true about the planes at the time. While TWA was the operator of the plane that crashed as TWA 800, a lot of people claimed that the 747, which was originally designed for a 60,000 hour service life and had 92,000 hours of service as of July 17th, 1996, was dangerous because of this very reason. My understanding was that Boeing had never recognized that N93119 had an equal chance of explosion as any other aircraft, age or not....747 wiring and placement of the air conditioning packs below the fuel tank and hot temperatures and the type of fuel used caused the accident to my knowledge. Plus, I've seen many airlines operate aircraft much older than N93119 was at the time it exploded...NW is operating 747s that are 30 years old, and DC-9s in excess of 40 years old. i guess my question is, how much more dangerous does an aircraft become when it is beyond its useful service life? Was age a contributing factor to this accident? Let me know. Thanks.


Fly one thing; Fly it well
25 replies: All unread, showing first 25:
 
User currently offlineMQTmxguy From United States of America, joined Nov 2007, 197 posts, RR: 0
Reply 1, posted (6 years 8 months 2 weeks 20 hours ago) and read 3788 times:



Quoting Thrust (Thread starter):
I felt compelled to ask this question because of some of the TWA 800 lawsuits that I read which I feel were not necessarily true about the planes at the time. While TWA was the operator of the plane that crashed as TWA 800, a lot of people claimed that the 747, which was originally designed for a 60,000 hour service life and had 92,000 hours of service as of July 17th, 1996, was dangerous because of this very reason. My understanding was that Boeing had never recognized that N93119 had an equal chance of explosion as any other aircraft, age or not....747 wiring and placement of the air conditioning packs below the fuel tank and hot temperatures and the type of fuel used caused the accident to my knowledge. Plus, I've seen many airlines operate aircraft much older than N93119 was at the time it exploded...NW is operating 747s that are 30 years old, and DC-9s in excess of 40 years old. i guess my question is, how much more dangerous does an aircraft become when it is beyond its useful service life? Was age a contributing factor to this accident? Let me know. Thanks.

nmhkuf


Firstly, large airframe usage is measured in cycles, not hours. An airframe cycle is normally measured from when the aircraft is pressurized before takeoff to when the aircraft is depressurized after landing. How much time elapses in between is largely irrelevant.
The manufacture designs the airframe for a certain number of cycles, however after the aircraft has been in service a long time, the manufacture may re-certify the frame for more cycles along with ADs to fix any aging issues (the DC-9 has done this several times.) Rest assured, the manufactures have no desire to let one of their aircraft get so old as to be dangerous. It has everything to do with maintenance practices and compliance with ADs and manufacturer's instructions.

TWA800 was a case of design flaw, not age.



Well at least we can all take comfort in the fact that NW will never retire their DC-9s
User currently offlineStarlionblue From Greenland, joined Feb 2004, 17015 posts, RR: 67
Reply 2, posted (6 years 8 months 2 weeks 15 hours ago) and read 3722 times:

Quoting Thrust (Thread starter):
how much more dangerous does an aircraft become when it is beyond its useful service life? Was age a contributing factor to this accident? Let me know. Thanks.


It's a question of maintenance. In theory, you can keep an aircraft flying indefinitely and safely as long as it is maintained correctly.

Maintenance will become more and more complex and expensive as more and more parts need to be inspected and eventually replaced. Once you start getting into primary structural members the owners typically say "screw this". That's why a lot of airliners are scrapped instead of going through a D check. It's cheaper to get a new one in some cases.

[Edited 2007-12-10 15:55:47]


"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineJetstar From United States of America, joined May 2003, 1644 posts, RR: 10
Reply 3, posted (6 years 8 months 2 weeks 14 hours ago) and read 3693 times:
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Quoting MQTmxguy (Reply 1):
Firstly, large airframe usage is measured in cycles, not hours. An airframe cycle is normally measured from when the aircraft is pressurized before takeoff to when the aircraft is depressurized after landing. How much time elapses in between is largely irrelevant.

I don’t agree with your statement about hours do not mean anything. I have never seen, although it might be in some cases major airframe inspections based solely on cycles. While airframe hours are usually the main limit on inspections, there are 3 time limits, hours, cycles and calendar.

I know on the JetStar, a Part 25 certified airplane, the engines were on an hour based time between overhaul, the compressor and turbine discs were cycle limited and the engine had a 12 year limit between overhauls, so all three limits applied. You could legally overhaul the engine and put back in the discs if they were still under the cycle limit, but then when the discs reached their cycle limit, the engine had to be removed and disassembled and the cycle limited discs replaced, and not all the discs had the same cycle limit.

Case in point, the Aloha 737 that lost its roof had about 33,000 hours but over 85,000 cycles while the TWA flight 800 747 had just over 19,000 cycles with about 97,000 hours on the airframe. While each cycle does put a strain on the airframe, you can’t compare an airplane that averages a little over 1 cycle per hour like the average 737 against a long haul airplane that averages over 12 hours per cycle like a 747-400. While in flight the long haul airplane is subject to much more flexing that its short haul cousin, just look at the wings and engine pylons flex in turbulence on a 747 which all leads to more metal fatigue than a 737. What you don’t see in flight is the empennage and fuselage also flexing at the same time, so the 747 airframe is flexing a lot more in one cycle than a 737 flexes in one cycle. I believe the 747 is certified to 20,000 cycles, but that is an airframe limit, and the airframe can go well over 100,000 hours.

When Boeing designed the 707, the airframe was certified for 30,000 hours because Boeing did not think the airlines would put any more time on their airplanes than they did with their old piston engine airliners. When the airlines realized that the 707 did not need anywhere near the maintenance of their piston engine airplanes, they started flying them more and the 707 reached the 30,000 hour limit less than halfway in time that Boeing anticipated they would. Boeing had to come up with life extension programs to extend the operating hours which included the crown replacement, the entire fuselage skins above the center wing box had to be replaced because of fatigue. Boeing kept upping the hours with these extension programs until some of the fleet reached 100,000 hours. Even the 747, designed as a long haul airplane, had to undergo a life extension improvement in reinforcing section 41, the section just aft of the cockpit because of metal fatigue.

When Boeing fatigue test an airframe and wing, they build in to their test program the amount of flexing that would occur over 100,000 hours of flight, they can simulate the takeoff, cruise and landing stresses and compress them into about one tenth the time to see how many hours the airframe can be certified to, which is usually 100,000 for a long haul airplane.

When Douglas designed the DC-9 in the early 1960’s computer assisted design and analysis programs didn’t exist, the airplane was designed in the slide rule era so the engineers at Douglas to side with caution designed it much stronger than required and this is evident today with the longer life with less fatigue problems in the DC-9 as compared to the 737.

Some DC-9’s, a short haul airplane have exceeded 100,000 airframe hours while I doubt many if any 737’s have reached 100,000 hours and as far as I know the DC-9 does not have an airframe hour limit.

I am sure that in some airline operations, like Aloha and Hawaiian because of their extremely short flights might be on a cycle based maintenance program, other operators of the same type airplane would be on a hour based program.


User currently offlineTdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 4, posted (6 years 8 months 2 weeks 13 hours ago) and read 3675 times:



Quoting Thrust (Thread starter):
While TWA was the operator of the plane that crashed as TWA 800, a lot of people claimed that the 747, which was originally designed for a 60,000 hour service life and had 92,000 hours of service as of July 17th, 1996, was dangerous because of this very reason.

TWA800 had nothing to do with service life. Service life is almost entirely a structural phenomenon and TWA800 wasn't a structural failure.

Quoting Thrust (Thread starter):
My understanding was that Boeing had never recognized that N93119 had an equal chance of explosion as any other aircraft, age or not....747 wiring and placement of the air conditioning packs below the fuel tank and hot temperatures and the type of fuel used caused the accident to my knowledge.

That's pretty much true. All the OEM's do everything assuming that their maintenance program (or an equivalent) is complied with. Under that theory, TWA800 was no better or worse off than any other Boeing airplane. In reality, maintenance isn't done in accordance with the maintenance program and that eventually catches up with you.

Quoting Thrust (Thread starter):
how much more dangerous does an aircraft become when it is beyond its useful service life?

It really depends on who's maintaining it. Done right, the life is effectively infinite (although cost and complexity will continue to increase). Done wrong, the older the airplane gets the more maintenance errors accumulate and the more likely that one of them will get you.

Quoting Thrust (Thread starter):
Was age a contributing factor to this accident?

Yes. Not airframe hours/cycles but decades of iffy work on and near the wiring.

Quoting MQTmxguy (Reply 1):
TWA800 was a case of design flaw, not age.

It wasn't a design flaw, it was wiring damaged by maintenance.

Quoting Jetstar (Reply 3):
I believe the 747 is certified to 20,000 cycles, but that is an airframe limit, and the airframe can go well over 100,000 hours.

As far as I know, there is no certified limit for any Boeing aircraft. There is a requirement that your fatigue test frame has to keep ahead of your high time in-service airplane, but there's nothing that says you lose your certification if you go over x cycles.

Tom.


User currently offlineSpeedbird128 From Pitcairn Islands, joined Oct 2003, 1648 posts, RR: 2
Reply 5, posted (6 years 8 months 2 weeks 13 hours ago) and read 3673 times:



Quoting Tdscanuck (Reply 4):
It really depends on who's maintaining it. Done right

Or, if not, in CE's case at Cape Town a couple weeks back, an engine can fall off the aircraft :S Thankfully that was all that went wrong...



A306, A313, A319, A320, A321, A332, A343, A345, A346 A388, AC90, B06, B722, B732, B733, B735, B738, B744, B762, B772, B7
User currently offlineJetstar From United States of America, joined May 2003, 1644 posts, RR: 10
Reply 6, posted (6 years 8 months 2 weeks 12 hours ago) and read 3650 times:
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Quoting Tdscanuck (Reply 4):
As far as I know, there is no certified limit for any Boeing aircraft. There is a requirement that your fatigue test frame has to keep ahead of your high time in-service airplane, but there's nothing that says you lose your certification if you go over x cycles.

I agree that even though Boeing certified the 747 to 20,000 cycles, it is not the maximum limit on the airframe. The only way a manufacturer can place an absolute limit on an airframe is to incorporate the maximum time into the type certificate. I believe the Piper Malibu wings are limited to around 10,000 hours, then the wings have to replaced, as per the aircraft’s type certificate.

On the JetStar, there is only one life limited component on the entire airframe, the main fuel boost pumps, they are limited to 2000 hours, it is listed on the type certificate and because of this there are no extensions or on condition allowed, it is mandatory they must be replaced. I never was able to find out why Lockheed mandated only the fuel boost pumps had a time limit, especially since they were not a safety of flight item because of the fuel cross feed system, probably was to keep the FAA happy that something was life limited on the JetStar..

I doubt that Boeing has kept the –100/200 fatigue test airframe in service for over 20 years. When I took the tour of Boeings 747 plant years ago, there was a former high time JAL 747-100 that Boeing had bought back from JAL, the fuselage was sitting outside and Boeing had partially disassembled it to check for fatigue and was using this airframe as a basis for life extension programs.

From what I have seen and read, the manufacturers keep a watch on the high time airframes and use the inspection reports and other data as a basis for extending or requiring repairs or modifications to extend the airframe life of the rest of the fleet.
The Air Force, many years ago in conjunction with Lockheed completely disassembled a high time C-5A cargo airplane to see how much hidden fatigue and corrosion there was in the airframe and to establish a base line for the rest of the fleet.


User currently offlineMQTmxguy From United States of America, joined Nov 2007, 197 posts, RR: 0
Reply 7, posted (6 years 8 months 2 weeks 11 hours ago) and read 3650 times:

Quoting Jetstar (Reply 3):
Case in point, the Aloha 737 that lost its roof had about 33,000 hours but over 85,000 cycles while the TWA flight 800 747 had just over 19,000 cycles with about 97,000 hours on the airframe. While each cycle does put a strain on the airframe, you can’t compare an airplane that averages a little over 1 cycle per hour like the average 737 against a long haul airplane that averages over 12 hours per cycle like a 747-400. While in flight the long haul airplane is subject to much more flexing that its short haul cousin,

You somewhat proved my point there. The TWA800 incident was not an issue of hours or cycles, however the Aloha 243 incident (a much higher cycle airframe) was a failure due to fatigue and corrosion.

I should have been a little more clear, I am fully aware that hours and calender time are factors in maintenance, but they are not nearly as important in large transport category aircraft as cycles. How many times that aircraft has been pressurized, launched, cooled at altitude, warmed up on desent, depressurized, and hit the Earth again is far more important than how many hours it actually spent in the air. Engines, even though they do also work on cycles, are much more total time intensive than airframes.

edited to add "and corrosion"

[Edited 2007-12-10 19:50:33]


Well at least we can all take comfort in the fact that NW will never retire their DC-9s
User currently offline474218 From United States of America, joined Oct 2005, 6340 posts, RR: 9
Reply 8, posted (6 years 8 months 2 weeks 10 hours ago) and read 3617 times:



Quoting Jetstar (Reply 3):
I don't agree with your statement about hours do not mean anything. I have never seen, although it might be in some cases major airframe inspections based solely on cycles. While airframe hours are usually the main limit on inspections, there are 3 time limits, hours, cycles and calendar.

As part of the FAA mandated Aging Aircraft Program all airframe manufactures were required to establish a Design Life Goal for the airframes they produced. This Design Life Goal is quoted only in flight cycles. Additional maintenance and or inspections maybe required to operated an aircraft beyond the established Design Life Goal. However, you are correct that maintenance requirements are still based on airframe time and calender time in addition to flight cycles.


User currently offlineSEPilot From United States of America, joined Dec 2006, 6875 posts, RR: 46
Reply 9, posted (6 years 8 months 2 weeks 4 hours ago) and read 3566 times:



Quoting Tdscanuck (Reply 4):
It wasn't a design flaw, it was wiring damaged by maintenance.

I was under the impression that no ignition source was ever identified; do you have any more info on this?



The problem with making things foolproof is that fools are so doggone ingenious...Dan Keebler
User currently offlineMQTmxguy From United States of America, joined Nov 2007, 197 posts, RR: 0
Reply 10, posted (6 years 8 months 2 weeks 1 hour ago) and read 3535 times:



Quoting SEPilot (Reply 9):



Quoting NTSB:
Abstract: On July 17, 1996, about 2031 eastern daylight time, Trans World Airlines, Inc. (TWA) flight 800, a Boeing 747-131, N93119, crashed in the Atlantic Ocean near East Moriches, New York. TWA flight 800 was operating under the provisions of 14 Code of Federal Regulations Part 121 as a scheduled international passenger flight from John F. Kennedy International Airport (JFK), New York, New York, to Charles DeGaulle International Airport, Paris, France. The flight departed JFK about 2019, with 2 pilots, 2 flight engineers, 14 flight attendants, and 212 passengers on board. All 230 people on board were killed, and the airplane was destroyed. Visual meteorological conditions prevailed for the flight, which operated on an instrument flight rules flight plan.

The National Transportation Safety Board determines that the probable cause of the TWA flight 800 accident was an explosion of the center wing fuel tank (CWT), resulting from ignition of the flammable fuel/air mixture in the tank. The source of ignition energy for the explosion could not be determined with certainty, but, of the sources evaluated by the investigation, the most likely was a short circuit outside of the CWT that allowed excessive voltage to enter it through electrical wiring associated with the fuel quantity indication system.

Contributing factors to the accident were the design and certification concept that fuel tank explosions could be prevented solely by precluding all ignition sources and the design and certification of the Boeing 747 with heat sources located beneath the CWT with no means to reduce the heat transferred into the CWT or to render the fuel vapor in the tank nonflammable.

The safety issues in this report focus on fuel tank flammability, fuel tank ignition sources, design and certification standards, and the maintenance and aging of aircraft systems. Safety recommendations concerning these issues are addressed to the Federal Aviation Administration

http://www.ntsb.gov/Publictn/2000/aar0003.htm



Well at least we can all take comfort in the fact that NW will never retire their DC-9s
User currently offlineHAWK21M From India, joined Jan 2001, 31667 posts, RR: 56
Reply 11, posted (6 years 8 months 2 weeks 1 hour ago) and read 3530 times:

Design Economic Life.....Aircraft can be well maintained thereafter,its only when Mgmt feels the costs of Mods is outweighing the Income when these Airframes are Rested/sold off.

regds
MEL



Think of the brighter side!
User currently offlineThrust From United States of America, joined Sep 2003, 2688 posts, RR: 10
Reply 12, posted (6 years 8 months 1 week 6 days 22 hours ago) and read 3501 times:

To SEPilot, the proof that wiring may have been the cause of the explosion lies actually in the cockpit voice recorder and black boxes themselves. Seconds before contact was lost, the recording skipped something like twice...this indicated as I recall some type of shortage or electrical fault in the wiring. While no ignition source was ever identified, it was not a bomb or a missile either...the PETN is solidly explained way by training exercises in which dogs sniffed out bomb residue, which were conducted in June 1996 on N93119 when it was parked at STL, and there was no evidence of a missile either...if you look at the number of eyewitnesses who claimed it was a missile, if I recall, the number who said they saw a streak towards the plane originating from the ground or horizon was maybe about a quarter of those who gave testimony. Plus, there was no evidence of pitting or gas-washing on the metal, and the mood inside the cockpit of Flight 800 was calm all the way up to the very end. I am almost 100% convinced that Flight 800 was some kind of electrical or mechanical failure.


Fly one thing; Fly it well
User currently offlineBAe146QT From United Kingdom, joined Sep 2006, 996 posts, RR: 0
Reply 13, posted (6 years 8 months 1 week 6 days 21 hours ago) and read 3467 times:



Quoting Thrust (Reply 12):
if you look at the number of eyewitnesses who claimed it was a missile, if I recall, the number who said they saw a streak towards the plane originating from the ground or horizon was maybe about a quarter of those who gave testimony.

The trouble was that at least two of those witnesses were National Guardsmen - helo pilots - who were in the air at the time. In part it was their testimony which lent undue credence to the shoot-down theory.

I have no idea what happened that evening since I wasn't there, and I'm not a professional accident investigator, so my opinion on the subject is neither here nor there. Suffice to say that I treat the conspiracy theory with same contempt that I would any other.

To be a little closer to the subject;

Quoting Jetstar (Reply 6):
On the JetStar, there is only one life limited component on the entire airframe, the main fuel boost pumps, they are limited to 2000 hours, it is listed on the type certificate and because of this there are no extensions or on condition allowed, it is mandatory they must be replaced.

I don't doubt that you are correct, but could you please indulge me? It has always been my understanding that the ultimate lifespan of any aircraft is determined by the main spar, be it by cyles or hours. Am I thinking of propliner-era and light aircraft? Or does that ultimately apply to all planes?



Todos mis dominós son totalmente pegajosos
User currently offlineMQTmxguy From United States of America, joined Nov 2007, 197 posts, RR: 0
Reply 14, posted (6 years 8 months 1 week 6 days 20 hours ago) and read 3443 times:



Quoting BAe146QT (Reply 13):
I don't doubt that you are correct, but could you please indulge me? It has always been my understanding that the ultimate lifespan of any aircraft is determined by the main spar, be it by cyles or hours. Am I thinking of propliner-era and light aircraft? Or does that ultimately apply to all planes?

The main wing spar is a big one, probably the biggest, but once you start talking about replacing any large main structural members on a large aircraft, keeping it flying begins to get prohibitively expensive. As has been said before, you can pretty much keep an aircraft flying as long as you want to keep throwing money at it (just ask the USAF.)



Well at least we can all take comfort in the fact that NW will never retire their DC-9s
User currently offlineBAe146QT From United Kingdom, joined Sep 2006, 996 posts, RR: 0
Reply 15, posted (6 years 8 months 1 week 6 days 19 hours ago) and read 3420 times:

Quoting MQTmxguy (Reply 14):
you want to keep throwing money at it (just ask the USAF.)

I'm thinking BUFF here. And probably a load of kit like the F-14, F4, the A-7and the A-6 variants?

In our auto club we have an expression, (almost certainly not original) which goes; "Power is just a question of money. How fast can you afford to go?". Change that to, "Lifespan is just a question of money..."

Maybe the same is true of aircraft. A Piper Cub can last until the last trump as long as you have the money to pay for replacement parts. The trick is knowing when those parts need replacing. Maybe TWA800 was a case of pushing it too far, either unknowingly or without the investment.

Have we found the OP's answer for him? How about;

"It doesn't matter how old an aircraft is or what its design life was. As long as safety-critical parts are replaced to their own schedule, you can fly that bird as long as you want as long as you can afford it.".

After all, there's BUFFs, Lancs, Yorks, V-Bombers, 747s, 737s and all manner of kit buzzing around like they own the skies. Hell, I saw a JU52 flying a few weeks ago, and by rights it should have been dust.

[Edited 2007-12-11 12:17:19]


Todos mis dominós son totalmente pegajosos
User currently offlineMQTmxguy From United States of America, joined Nov 2007, 197 posts, RR: 0
Reply 16, posted (6 years 8 months 1 week 6 days 17 hours ago) and read 3368 times:



Quoting BAe146QT (Reply 15):
I'm thinking BUFF here.

Yep, and don't forget the ol' KC-135!

Quoting BAe146QT (Reply 15):
Have we found the OP's answer for him? How about;

"It doesn't matter how old an aircraft is or what its design life was. As long as safety-critical parts are replaced to their own schedule, you can fly that bird as long as you want as long as you can afford it.".

I like that one.



Well at least we can all take comfort in the fact that NW will never retire their DC-9s
User currently offlineThrust From United States of America, joined Sep 2003, 2688 posts, RR: 10
Reply 17, posted (6 years 8 months 1 week 6 days 16 hours ago) and read 3354 times:

I've actually heard that even after Flight 800 occured, while the wiring inside the 747 was changed, Boeing continues to place the air conditioning packs in all types of its aircraft underneath the fuel tank....that is something IMO that needs to be fixed. Any type of source that could heat the fuel tank needs to be kept away from it IMO. Something else that I think could clear TWA of a lot of blame for the accident was that N93119 was in compliance with every maintenance directive issued by Boeing. So IMO I'm not sure if anybody could have seen anything wrong with it ahead of time.

[Edited 2007-12-11 14:59:08]


Fly one thing; Fly it well
User currently offlineCoolGuy From United States of America, joined Jan 2005, 414 posts, RR: 0
Reply 18, posted (6 years 8 months 1 week 6 days 16 hours ago) and read 3344 times:



Quoting Jetstar (Reply 3):
Case in point, the Aloha 737 that lost its roof had about 33,000 hours but over 85,000 cycles

How is that possible? Ten a day for ten years is appx. 36500 only. Even 19,000, that's pretty impressive.


User currently offline474218 From United States of America, joined Oct 2005, 6340 posts, RR: 9
Reply 19, posted (6 years 8 months 1 week 6 days 16 hours ago) and read 3333 times:

Quoting CoolGuy (Reply 18):
How is that possible? Ten a day for ten years is appx. 36500 only. Even 19,000, that's pretty impressive.

The actual numbers were 89,680 cycles and 35.496 hours. The Aloha 737 first flew in 1969, so it averaged approximately 13 flights per day over its 19 year life, with each flight averaging about 20 minutes.

[Edited 2007-12-11 15:44:22]

User currently offlineStarlionblue From Greenland, joined Feb 2004, 17015 posts, RR: 67
Reply 20, posted (6 years 8 months 1 week 6 days 14 hours ago) and read 3293 times:



Quoting Thrust (Reply 17):
I've actually heard that even after Flight 800 occured, while the wiring inside the 747 was changed, Boeing continues to place the air conditioning packs in all types of its aircraft underneath the fuel tank....that is something IMO that needs to be fixed. Any type of source that could heat the fuel tank needs to be kept away from it IMO.

This is actually a feature, not a bug. The packs are there in part to heat the fuel. Fuel can get awfully cold on long haul, creating problems. If you didn't have the packs there you'd simply have to heat it some other way.

Jetfuel is pretty hard to ignite. Heat was not really the problem with TWA800. Gaseous suspension, (a specific) pressure and sparks were.



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineJetstar From United States of America, joined May 2003, 1644 posts, RR: 10
Reply 21, posted (6 years 8 months 1 week 6 days 13 hours ago) and read 3266 times:
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Quoting 474218 (Reply 19):
The actual numbers were 89,680 cycles and 35.496 hours. The Aloha 737 first flew in 1969, so it averaged approximately 13 flights per day over its 19 year life, with each flight averaging about 20 minutes.

Thanks 424718 for the actual numbers, I was going from memory on the cycles and hours.

I hear at Hawaiian the morning flight crew shift normally works 4 turns, thats 8 landings, and the afternoon crew works about the same, so they can get 16 landings a day.

I am sure HAL or any other Hawaiian or Aloha a.netter could give us the normal landings a day for their inter island flights


User currently offlineTdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 22, posted (6 years 8 months 1 week 6 days 8 hours ago) and read 3219 times:



Quoting SEPilot (Reply 9):
Quoting Tdscanuck (Reply 4):
It wasn't a design flaw, it was wiring damaged by maintenance.

I was under the impression that no ignition source was ever identified; do you have any more info on this?

MQTmxguy has it right in Reply 10...they never conclusively identified the source of the ignition but the most likely cause, based on available evidence, was a hot short on the FQIS wiring. It was absolutely positively a CWT explosion and an electrical ignition from aircraft is the only possible cause for which there is any evidence at all.

Quoting Thrust (Reply 12):
To SEPilot, the proof that wiring may have been the cause of the explosion lies actually in the cockpit voice recorder and black boxes themselves. Seconds before contact was lost, the recording skipped something like twice...this indicated as I recall some type of shortage or electrical fault in the wiring.

In addition, the flight crew commented that the center fuel quantity gauge was going screwy just before the explosion, which is exactly what you'd expect with a short on the FQIS wiring.

Quoting BAe146QT (Reply 13):
I don't doubt that you are correct, but could you please indulge me? It has always been my understanding that the ultimate lifespan of any aircraft is determined by the main spar, be it by cyles or hours. Am I thinking of propliner-era and light aircraft? Or does that ultimately apply to all planes?

The main spar is a major fatigue critical component, but it's not the only one. Fuselage skins are another biggie. The 737-200's will be grounded for skins before they're grounded for spars.

Quoting Thrust (Reply 17):
I've actually heard that even after Flight 800 occured, while the wiring inside the 747 was changed, Boeing continues to place the air conditioning packs in all types of its aircraft underneath the fuel tank....that is something IMO that needs to be fixed.

You have to heat the fuel somehow or you can't use it. Even if you did want to move the packs elsewhere, on the current production models there's nowhere to move them to without a massive (and economically impractical) redesign. If you take ignition source prevention (standard in aviation for 60+ years) and add inerting on top of that, heating is pretty much moot.

Quoting Thrust (Reply 17):
Any type of source that could heat the fuel tank needs to be kept away from it IMO.

In which case you've just grossly limited the range of most aircraft due to fuel freezing.

Quoting Thrust (Reply 17):
Something else that I think could clear TWA of a lot of blame for the accident was that N93119 was in compliance with every maintenance directive issued by Boeing.

No, it wasn't. The wiring of TWA800 was not maintained per Boeing's instructions. That is not at all uncommon, for any manufacturer, but it's still true.

Tom.


User currently offlineThrust From United States of America, joined Sep 2003, 2688 posts, RR: 10
Reply 23, posted (6 years 8 months 1 week 5 days 8 hours ago) and read 3129 times:

I see. I understand then. Thanks Tom. Was unsure if the wiring was or not myself, but found a source that claimed it was in every maintenance directive from Boeing. I guess TWA was trying to minimize their role in the accident. I guess that explains that. Somebody should've taken better care of the wiring.

[Edited 2007-12-12 23:18:37]

[Edited 2007-12-12 23:18:58]


Fly one thing; Fly it well
User currently offlineStarlionblue From Greenland, joined Feb 2004, 17015 posts, RR: 67
Reply 24, posted (6 years 8 months 1 week 5 days 4 hours ago) and read 3108 times:



Quoting Thrust (Reply 23):
Somebody should've taken better care of the wiring.

Well yes. But hindsight is 20/20. If there had been an awareness of the possible consequences from any of parties, no doubt the wiring would have been better maintained. Profit motive or non, if there's a chance something will ignite outside the engines, the most "frugal" airline will take care of it.



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineTdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 25, posted (6 years 8 months 1 week 4 days 10 hours ago) and read 3031 times:



Quoting Thrust (Reply 23):
I see. I understand then. Thanks Tom. Was unsure if the wiring was or not myself, but found a source that claimed it was in every maintenance directive from Boeing. I guess TWA was trying to minimize their role in the accident. I guess that explains that. Somebody should've taken better care of the wiring.

It really depends on what you mean by "maintenance directive." I have no doubt that TWA complied with all Airworthiness Directives (from the FAA, not Boeing) and with the relevant Boeing Service Bulletins. And I'm almost positive that a review of TWA's paperwork would show that everything was maintained per their operating certificate (which is either the Boeing maintenance manuals or TWA's equivalent).

However...when you look at the actual wiring on real airplanes, especially old ones, you realize that maintaining per the manual isn't nearly as easy as it sounds. Manuals will say things like "ensure that all metal chips are captured" while drilling a hole, but that doesn't mean all the chips really do get captured. If you miss one, how would you know? What happened on TWA800 was what happens on all old airplanes...you build up junk and dirt and chips and all manner of stuff that works its way into the wire bundles and, eventually, starts chaffing wires.

Airplanes are extremely fault tolerant so this is usually an annoyance, not a catastrophe, but if you short exactly the right wire to exactly the right other wire, you can get a boom. That's why the post TWA800 rules are hypersensitive to fuel system wiring maintenance and proximity to all other wires.

Tom.


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