Why does the PA-38 Tomahawk have a wing life of 11,000 hours?
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Why does the PA-38 Tomahawk have a wing life of 11,000 hours?
I've got a fair bit of time in Tomahawks, and love them. They're a great trainer, and a reasonable touring aircraft.
I've never had a proper answer from anyone as to why the wings are lifed at 11,000 hours, when I can't find another aircraft anywhere that has a similar restriction. Does anyone know why Piper decided on this restriction?
I've never had a proper answer from anyone as to why the wings are lifed at 11,000 hours, when I can't find another aircraft anywhere that has a similar restriction. Does anyone know why Piper decided on this restriction?
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Can't find another aircraft with a similar restriction? Cirrus aircraft are Airframe life limited at 12000 hours. The PA-44 wing is life limited at 14, 663 hours. There are many other examples. 
Many aeroplanes have big bits of structure lifed like that. The Twin Otter mainplane is another such example, so are many of the wing tubes in the Pegasus flexwings.
In a nutshell, the design and manufacture ensure a maximum size of flaw in the wing structure. It's then worked out the number of hours at a representative operating regime it would take for any of these flaws to turn into a critical and endangering crack.
This number of hours is then divided by a safety factor - typically around 3, to allow for errors in calculation or manufacture.
And this is then the maximum hours that the component is lifed to.
It makes a lot of sense in terms of long term economics, just frustrating if you find yourself owner of a late-in-life aeroplane.
G
In a nutshell, the design and manufacture ensure a maximum size of flaw in the wing structure. It's then worked out the number of hours at a representative operating regime it would take for any of these flaws to turn into a critical and endangering crack.
This number of hours is then divided by a safety factor - typically around 3, to allow for errors in calculation or manufacture.
And this is then the maximum hours that the component is lifed to.
It makes a lot of sense in terms of long term economics, just frustrating if you find yourself owner of a late-in-life aeroplane.
G
The rules under which aircraft are certified evolve over time. The simple Cessna singles are all certified under the old CAR3 rules dating back to the 1950's, which did not require establishing life limits and grandfathered onto todays Part 23 regulations. The Traumahawk was certified as a new design under the rules that has been updated in the mid 1970's. One of the updates was requiring airframe life limits. I think Piper simply did not want to spend the money to do the required engineering work and testing to go past 11,000 hours.
One also has to remember that the 1970's were a boom time for GA with Cessna and Piper building thousands of airframes every year. Schools would routinely use a trainer for a couple of years and then replace it with a new one, so 11,000 hours probably seemed like plenty. I don't think that even in their wildest dreams, did the light plane designers of that era think that you would routinely see trainers still on the line that were 35 + years old with flight times in over 10 K hours. The highest time C 152 I know of has 24 K + total time, all as an ab inito trainer, and is still working every day
One also has to remember that the 1970's were a boom time for GA with Cessna and Piper building thousands of airframes every year. Schools would routinely use a trainer for a couple of years and then replace it with a new one, so 11,000 hours probably seemed like plenty. I don't think that even in their wildest dreams, did the light plane designers of that era think that you would routinely see trainers still on the line that were 35 + years old with flight times in over 10 K hours. The highest time C 152 I know of has 24 K + total time, all as an ab inito trainer, and is still working every day
Last edited by Big Pistons Forever; 11th Dec 2011 at 22:48.
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It is a conservative calculation for the purposes of meeting the certification criteria in lieu of actual testing. By the way, there is an FAA STC available for wing life service extension.
Even gliders have airframe lives.
A Discus is initially 6,000 hrs, an inspection at that point raises it to 9,000. Further inspections at 10,000 and 11,000 hrs and then buy a new one at 12,000 hrs. That's a plastic airframe - mainly carbon.
Most Blaniks are now grounded (metal airframe) if over something like 3000 hours due fatigue problems in main spar. A mod exists to extend life - but costs more than the gliders are worth.
A Discus is initially 6,000 hrs, an inspection at that point raises it to 9,000. Further inspections at 10,000 and 11,000 hrs and then buy a new one at 12,000 hrs. That's a plastic airframe - mainly carbon.
Most Blaniks are now grounded (metal airframe) if over something like 3000 hours due fatigue problems in main spar. A mod exists to extend life - but costs more than the gliders are worth.
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A longer life
There is a life extension program for the PA38 that involves some wing modification, I think that this would be an option if Piper supported the aircraft in terms of parts. Five or so years back I was looking at buying two PA38's as they were cheap and then doing the life extension work. What did for the deal was the spake parts situation not the modification work.
The Buldog is another aircraft with an airframe life issue hanging over its head with the RAF getting withdrawing the aircraft from service because of the cost of doing the life extension work that BAe had proposed. As always BAe had gold plated the contract and pitched it at just under the cost of buying new aircraft. The RAF found a cheaper way of financing new aircraft and ditched the Buldog.
Buldog owners are now facing the choice of a very expensive modification or scrapping the aircraft if they get the work done individually, the last time I looked at the issue the problem was the cost of setting up the manufacture of the spar bolts, if all the Buldog owners got together they could drive the unit costs of the bolts down to something reasonable and each of them have a set of bolts ready for the day when the aircraft needs the work doing.
Things on the Buldog front may well have moved on in the last few weeks with the change in status of DH support and the chance to put the Buldog on a permit to fly, however I doubt if the cracks radiating from the spar bolts understand the difference certification management and are likely to grow at the same rate making the modification mandatory who ever provides the paperwork.
The Buldog is another aircraft with an airframe life issue hanging over its head with the RAF getting withdrawing the aircraft from service because of the cost of doing the life extension work that BAe had proposed. As always BAe had gold plated the contract and pitched it at just under the cost of buying new aircraft. The RAF found a cheaper way of financing new aircraft and ditched the Buldog.
Buldog owners are now facing the choice of a very expensive modification or scrapping the aircraft if they get the work done individually, the last time I looked at the issue the problem was the cost of setting up the manufacture of the spar bolts, if all the Buldog owners got together they could drive the unit costs of the bolts down to something reasonable and each of them have a set of bolts ready for the day when the aircraft needs the work doing.
Things on the Buldog front may well have moved on in the last few weeks with the change in status of DH support and the chance to put the Buldog on a permit to fly, however I doubt if the cracks radiating from the spar bolts understand the difference certification management and are likely to grow at the same rate making the modification mandatory who ever provides the paperwork.
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Haven't Piper recently sold their spares operation? I vaguely recall reading something about that.
Can't be much left of Piper, without the parts business. Just the odd few Meridians, Malibus, Matrixes, miniscule numbers of PA28s...
Can't be much left of Piper, without the parts business. Just the odd few Meridians, Malibus, Matrixes, miniscule numbers of PA28s...
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Last time I flew a PA38, I was not impressed at how flimsy it all was... Like it was made out of the thinnest Alu they could get away with? Maybe the wings just fall apart after a few thousand hours..
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I'm not quick to second guess experienced structural designers as to their choice in aluminum thickness. Indeed, ad good resign can be made bad by going one gauge thicker. There are many cases where the gauge of aluminum chosen for a skin, is more based in it's need to be stiff for ground handling loads (people leaning, and bumping things against the fuselage), rather that flight loads.
Consider the venerable, and arguably heavily built Cessna 206. Some of the fuselage skins are only .016" thick. You don't hear people saying that 206's are flimsy....
Similarly, those "well built" Rockwells, have have aileron skins, which are surprisingly thin, but it's what the plane needs to do the job, and be balancable. Just watch out for hangar rash!
The Twin Otter is one of many mid sized aircraft which have life limits imposed. deHavilland in the day, really was not expecting those aircraft to make it through to the limits, but imposed them, and the aircraft was largely intended as a military design. Most common is a 33,000 hour and 66,000 [landing] cycle service limit, though certain conditions could increase or decrease these values per aircraft. I believe these numbers were reached on analysis early in the designing, and then supplemented by testing, once the aircraft were in service. I have personally seen a wing strut strut/spar attaching fitting in which one of the two laminated aluminum plates was seriously cracked, and a definite safety concern. Happily, the wing had been removed from service already. I was aware of deHavilland engineers having informally said in the 1990's that had they to do it again, they would not have made the Twin Otter wing life as great as 33,000 hours. This supported the DH position of not endorsing extending it. There were other STC efforts to extend and relife the wings. "Extend" was a stop gap, which sort of helped, but the relife STC became the much more viable action, for a long term safe fix. In the late 1990's, the high time Twin Otter had more than 109,000 hours in service. This is much more than DH ever expected.
It is the continued use of aircraft, well beyond the manufacture's vision for the aircraft, whic his introduction thee concerns. The buzz word is "aging aircraft". It's not finding it's way down to our little GA types yet, but it will in the long term. Events like the Aloha and Southwest B737 upper fuselage skin failures are really a big motivator for this new policy. In those cases, like the Comet, it's cylced pressurization loads, but flight loads can be every bit as harmful. Piper PA-28's did have a wing attach AD decades ago, but it was eventually recinded, for lack of actually finding a problem, in any other than the one aircraft which started it all.
The ideal, is to assure that the design of the aircraft is such that detail inspection, or obvious external distress will indicate an impending structural failure, before it becomes a safety hazard. There after, the aircraft is a good repairable design if the repair or replacement of these vulnerable parts is easily accomplished.
It is this easy repair, which makes the much maligned Cessna spam cans an excellent choice for low cost longevity. The Cessna structural repair manuals are comprehensive, and simple. The majority of most light Cessna airframes are folded sheet metal (as opposed to extrusions (more often found in Piper low wings), homeycomb (Grummans), or non metallics of some of most recent GA aircraft.
When a maximum time in service for an airframe is expressed as a "limitation", take it seriously. This is akin to time/cycle limitations on dynamic components (mostly found in helicopters). These "limitations" are much more serious, that the "recommended" intervals for overhaul which manufacturer's state for piston engines
It has been said about structures: "Them that bends, don't break". Perhaps over simplified, poor grammar, but there it a worthy theme in there...
Consider the venerable, and arguably heavily built Cessna 206. Some of the fuselage skins are only .016" thick. You don't hear people saying that 206's are flimsy....
Similarly, those "well built" Rockwells, have have aileron skins, which are surprisingly thin, but it's what the plane needs to do the job, and be balancable. Just watch out for hangar rash!
The Twin Otter is one of many mid sized aircraft which have life limits imposed. deHavilland in the day, really was not expecting those aircraft to make it through to the limits, but imposed them, and the aircraft was largely intended as a military design. Most common is a 33,000 hour and 66,000 [landing] cycle service limit, though certain conditions could increase or decrease these values per aircraft. I believe these numbers were reached on analysis early in the designing, and then supplemented by testing, once the aircraft were in service. I have personally seen a wing strut strut/spar attaching fitting in which one of the two laminated aluminum plates was seriously cracked, and a definite safety concern. Happily, the wing had been removed from service already. I was aware of deHavilland engineers having informally said in the 1990's that had they to do it again, they would not have made the Twin Otter wing life as great as 33,000 hours. This supported the DH position of not endorsing extending it. There were other STC efforts to extend and relife the wings. "Extend" was a stop gap, which sort of helped, but the relife STC became the much more viable action, for a long term safe fix. In the late 1990's, the high time Twin Otter had more than 109,000 hours in service. This is much more than DH ever expected.
It is the continued use of aircraft, well beyond the manufacture's vision for the aircraft, whic his introduction thee concerns. The buzz word is "aging aircraft". It's not finding it's way down to our little GA types yet, but it will in the long term. Events like the Aloha and Southwest B737 upper fuselage skin failures are really a big motivator for this new policy. In those cases, like the Comet, it's cylced pressurization loads, but flight loads can be every bit as harmful. Piper PA-28's did have a wing attach AD decades ago, but it was eventually recinded, for lack of actually finding a problem, in any other than the one aircraft which started it all.
The ideal, is to assure that the design of the aircraft is such that detail inspection, or obvious external distress will indicate an impending structural failure, before it becomes a safety hazard. There after, the aircraft is a good repairable design if the repair or replacement of these vulnerable parts is easily accomplished.
It is this easy repair, which makes the much maligned Cessna spam cans an excellent choice for low cost longevity. The Cessna structural repair manuals are comprehensive, and simple. The majority of most light Cessna airframes are folded sheet metal (as opposed to extrusions (more often found in Piper low wings), homeycomb (Grummans), or non metallics of some of most recent GA aircraft.
When a maximum time in service for an airframe is expressed as a "limitation", take it seriously. This is akin to time/cycle limitations on dynamic components (mostly found in helicopters). These "limitations" are much more serious, that the "recommended" intervals for overhaul which manufacturer's state for piston engines
It has been said about structures: "Them that bends, don't break". Perhaps over simplified, poor grammar, but there it a worthy theme in there...
Maths does beat pilot opinion, sometimes.
G
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I hadn't realised that wing limits were so much the norm. I had heard some old wives tale about the piper legal team demanding a life limit on the tomahawk for fear of getting sued years down the line, and the number had simply been plucked from the air so was just wondering what the real story would be.
It is kind of crazy that tomahawks are still flying now, 30+ years after being built, I'm sure surpassing all expectations of those who made them.
It is kind of crazy that tomahawks are still flying now, 30+ years after being built, I'm sure surpassing all expectations of those who made them.
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Flying school gossip - by schools that never ran them.
OK I'm a sample size of one but I've spin tomahawks dozens of times totally without incidence and I'm no ace of the base.
Personnel I think the Tommy is a lovely training aircraft and its a shame there isn't a few more of them about these days.
OK I'm a sample size of one but I've spin tomahawks dozens of times totally without incidence and I'm no ace of the base.
Personnel I think the Tommy is a lovely training aircraft and its a shame there isn't a few more of them about these days.
Piper Tomahawks were intended to stall properly and spin aggressively. As a student PPL on the Tomahawk I can confirm that the stall can be moderately alarming. Certainly far more so than on the PA28.
The controversy is due to whether some Tomahawks don't exit the spin as readily as they should. There's a story (that I don't know quite whether to believe) that production Tomahawks differed from the prototype and have less rigid wings. Some apparently spin more predictably than others.
I've never spun a Tomahawk, but both my instructors have. One says 'never again' whilst I think the other would rather enjoy the experience. Unfortunately, Tommies aren't certified for intentional spins any more.
The controversy is due to whether some Tomahawks don't exit the spin as readily as they should. There's a story (that I don't know quite whether to believe) that production Tomahawks differed from the prototype and have less rigid wings. Some apparently spin more predictably than others.
I've never spun a Tomahawk, but both my instructors have. One says 'never again' whilst I think the other would rather enjoy the experience. Unfortunately, Tommies aren't certified for intentional spins any more.