Notar520AC
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Safe Airframe Times (when Looking To Buy An A/c)

Sat Jul 16, 2005 3:32 pm

Hi-

I was wondering if there's a general rule of thumb or fixed time as to when an airframe has pretty much finished its life- are there different times for small props, small jets, & airliners? If I was in the market for a Learjet (which I'm not), what would sort of be the cutoff point? Would flying an a/c beyond its reccomended time or cycles risk mid-air damage from airframe fatigue? Sorry, lots of questions lol.

Thanks!
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320tech
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RE: Safe Airframe Times (when Looking To Buy An A/c)

Sun Jul 17, 2005 1:42 pm

Airframes used to be designed to a fixed life - I recall CF-100 Canucks being removed from service when they hit 9,000 hours or so (and those things were extremely strong). But newer aircraft are designed for an indefinite life. The main thing becomes the cost to maintain them. I know of Airbus A320's with 60,000 hours on them, and no retirement in sight.

The Cessna 172's I normally fly have about 12,000 hours, and are not close to the end of their lives.
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Newark777
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RE: Safe Airframe Times (when Looking To Buy An A/

Sun Jul 17, 2005 1:55 pm

I know of Airbus A320's with 60,000 hours on them, and no retirement in sight.

And NW still flies DC-9's from the 1960's.

BTW Don't think I'm making another NW joke, they do in fact fly a handful from the 60's. This one has 74,000 hours according to the comments.


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CWUPilot
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RE: Safe Airframe Times (when Looking To Buy An A/c)

Sun Jul 17, 2005 2:37 pm

Planes with lots of hours and cycles (takeoffs and landings or pressurizations and depresserizations) do become more maintenance heavy and prone to problems. Sometimes, those problems are deadly. The 737-200 that blew its top in Hawaii had 86,000 hours and nearly as many cycles. All of that pressurizing and depressurizing led to fatigue cracks which weakened the a/c. I have flown small unpressurized planes with more that 12,000 hours. They weren't perfect, but they were safe and flew well.

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HAWK21M
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RE: Safe Airframe Times (when Looking To Buy An A/c)

Sun Jul 17, 2005 5:10 pm

Quoting CWUPilot (Reply 3):
The 737-200 that blew its top in Hawaii had 86,000 hours and nearly as many cycles. All of that pressurizing and depressurizing led to fatigue cracks which weakened the a/c. I have flown small unpressurized planes with more that 12,000 hours. They weren't perfect, but they were safe and flew well.

Good Mx plays a big role out here.
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Starlionblue
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RE: Safe Airframe Times (when Looking To Buy An A/c)

Sun Jul 17, 2005 9:36 pm

Quoting CWUPilot (Reply 3):
The 737-200 that blew its top in Hawaii had 86,000 hours and nearly as many cycles. All of that pressurizing and depressurizing led to fatigue cracks which weakened the a/c.

Note that this was in combination with environmental factors particular to operating in Hawaii, like salt spray in the air (or something like that). So the normal Mx regime was fine in any other place...
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Woodreau
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RE: Safe Airframe Times (when Looking To Buy An A/c)

Sun Jul 17, 2005 10:14 pm

One place to start is the airplane's type certificate. The type certificate will spell out the basis on which the aircraft was certified and any limitations or exemptions which were granted. In addition you may find certain specific airplane serials which are not eligible for an airworthiness certificate in the United States (which means if you were looking at a particular airplane and that serial number is on the list as not airworthy in the US, you'd never be able to register it in the US)

For example the PA44 Seminole - the wing is life limited to 14663 hours. There are no exemptions. So if you are looking at an aircraft which has say 14000 hours on it, you can fly it 663 more hours. Then you must replace the wing in order to keep flying the airplane because there is no limitation on the airframe or any other part of the aircraft. If you wanted to keep flying that Seminole, how much are you willing to spend to replace the wing?

The type certificate for the Boeing 737-300/-400/-500 establishes a life limit of 75,000 cycles for the landing gear (referring you to Boeing Service Letter 737-SL-32-21 for the gory details) after which components of the landing gear need to be retired (or basically replace the landing gear) in order to keep flying the airplane. There is another note referring you to Boeing 737 Supplemental Structural Inspection Document D6-37089 (AD 84-21-06, Amendment 39-4933) which probably lists the inspections and repairs required at certain intervals for the airframe to be considered airworthy.

If you were looking at the Lear 25 say (it's pretty old), then the type certificate for the Lear 25 refers you to the Model 25 Learjet Service Manuals/Maintenance Manuals and Learjet Reports 25-S47 which lists the life limitations and inspection intervals for airframe, parts of the aircraft in order for it to continue to be airworthy. Then you'd need to go into the airplane's logbooks to the look at the aircraft's history and see if those items were done.

Same thing for the DC-9. You have to look in Report MDC-J0005 for structural items that need replacement at specified intervals for the DC-9/MD-80 and in Report MDC-92K9145 for life-limited non-structural components.

Oddly enough, there is nothing in the Cessna 172 and Piper 28 type certificates which mention anything about required inspections or life limited structures for continued airworthiness certification. There might be something in the maintenance manuals but nothing that is required by regulation.
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flykal
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RE: Safe Airframe Times (when Looking To Buy An A/c)

Sun Jul 17, 2005 10:36 pm

Many of our early 744's, delivered in the 90's have 50,000+ hours on the logs. These aircraft are still in good condition - that's what maintenance is there for.

If I recall correctly, I remember reading that some of NW's DC-10's have 100,000 + hours. I'm sure some of our 747 classics would have similar time, but since I don't fly them, I couldn't tell you.
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Starlionblue
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RE: Safe Airframe Times (when Looking To Buy An A/c)

Mon Jul 18, 2005 2:43 am

Quoting Woodreau (Reply 6):
Oddly enough, there is nothing in the Cessna 172 and Piper 28 type certificates which mention anything about required inspections or life limited structures for continued airworthiness certification. There might be something in the maintenance manuals but nothing that is required by regulation.

I would imagine that the structural loads on these planes are just a mite lower than on a commercial airliner, even relative to the lighter structure.
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320tech
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RE: Safe Airframe Times (when Looking To Buy An A/c)

Mon Jul 18, 2005 5:52 am

I would imagine that the structural loads on these planes are just a mite lower than on a commercial airliner, even relative to the lighter structure.

Except for all those smooth student landings.  Wink
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jetstar
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RE: Safe Airframe Times (when Looking To Buy An A/c)

Mon Jul 18, 2005 9:51 am

The Aloha Airlines B-737 that blew it top had about 86,000 cycles, not hours, it had only about 35,000 hours on the airframe. Aloha averages about one third of an hour per cycle on their aircraft because of their short up and down flight segments. On the opposite end, the TWA 747, flight 800 that exploded had almost 100,000 flight hours, but only about 19,000 cycles because of their long over water segments that the 747’s usually fly.

One of the contributing circumstances about this Aloha 737 and its sister aircraft at AQ was in the early days of the 737 production, Boeing used a cold glue method to reinforce the lap joints in the fuselage. This method trapped moisture in the glue and this moisture combined with the salt air environment these aircraft lived it caused the skin lap joints to corrode heavily. Boeing realized this problem before this incident and changed over to a hot glue method, which boiled out all the moisture from the glue before it was applied. If I remember correctly the cold glue method was used in about the first 175 Boeing 737’s, after that the hot glue method was used. This AQ 737 and 2 sisters were delivered new to AQ from Boeing and spent their entire lives flying inter island. Before this incident, Boeing factory reps on an inspection stated that these 737’s were the most heavily corroded aircraft they had ever seen. After the cause of this accident was determined, AQ removed the sister airplanes from service after an inspection reveled they to were severely corroded and were parted out.

FYI, the 737 that became a convertible was scrapped on Maui but did live on in a way after scrapping, the entire nose section was shipped to the mainland and was converted to a B-737 flight simulator.

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