Fatigue life on light aircraft
Thread Starter
Fatigue life on light aircraft
If this is in the wrong forum, mods please move to the right one.
OK I fly a mixture of light A/C (C152, C172, PA28) that are standard flying school fare. What I wanted to know is will the standard 100hour insp (or any visual) show up any airframe abuse that could lead to failure.
To expand - these a/c will have been subjected to some rough/poor handling from students and/or may have been spun/aeroed/oversped without being reported. Is there any obvious sign that would show?
Thanks
OK I fly a mixture of light A/C (C152, C172, PA28) that are standard flying school fare. What I wanted to know is will the standard 100hour insp (or any visual) show up any airframe abuse that could lead to failure.
To expand - these a/c will have been subjected to some rough/poor handling from students and/or may have been spun/aeroed/oversped without being reported. Is there any obvious sign that would show?
Thanks
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Perhaps depends on your engineer. I used to own a PA38 that had a broken engine mounting frame spotted at a 50hr (analysis showed failed in overload with no pre-existing defect so presumably a very hard and unreported landing). Also had strange creaking noise that was traced to undercarriage leg mounting bolts but as part of that investigation found wing spar web corrosion (required a re-wing!)
Neither obviously part of the LAMS so I put it down to having a bl&*dy good engineering organisation.
And incase you ask - Tayside Aviation at Dundee.
Neither obviously part of the LAMS so I put it down to having a bl&*dy good engineering organisation.
And incase you ask - Tayside Aviation at Dundee.
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It is reasonable to expect that a maintenance facility who are familiar with the aircraft type involved, would have the experience to detect any troublesome airframe condition before it becomes dangerous. Each type has it's unique things to look for. For example, Cessna 150/150 are known to be weak in the inboard forward ribs of the H stab, by which the H stab is bolted to the fuselage. These tend to crack, and the tail assembly can become loose, and certainly unsafe. A keen walkaround will turn this up.
Generally these types are so well experienced that most weak areas have long since been found, and publicised for maintainers. One of the plusses of an older type!
These aircraft will generally show abuse by outwardly visible wrinkled skins and smoking rivets before any unsafe internal condition exsists. Engine mounts and landing gear mounts are an exception to this rule...
Be reassured that the aircraft design includes a capacity to exceed stated limitations by a required margin with some safety, if cautiously done. This does not consitute permission to do this, but simply a reassurance. I am required to fly to 1.1 of Vne during some flight testing. Smooth air, and gentle control inputs are vital at these speeds!
Treat the planes you fly as though you own them, and your family will fly in them one day. Both you, and the plane will be better off!
Pilot DAR
Generally these types are so well experienced that most weak areas have long since been found, and publicised for maintainers. One of the plusses of an older type!
These aircraft will generally show abuse by outwardly visible wrinkled skins and smoking rivets before any unsafe internal condition exsists. Engine mounts and landing gear mounts are an exception to this rule...
Be reassured that the aircraft design includes a capacity to exceed stated limitations by a required margin with some safety, if cautiously done. This does not consitute permission to do this, but simply a reassurance. I am required to fly to 1.1 of Vne during some flight testing. Smooth air, and gentle control inputs are vital at these speeds!
Treat the planes you fly as though you own them, and your family will fly in them one day. Both you, and the plane will be better off!
Pilot DAR
Thread Starter
Thanks for the replies, this is one section of the forum where quality answers can be found.
As a bit of thread creep, would it be possible to fly on just the elevator trim or rudder trim if the primary cables / rods failed? The PA28 has a paticularly large trim tab. I know i'd certainly give it a go if it happened.
As a bit of thread creep, would it be possible to fly on just the elevator trim or rudder trim if the primary cables / rods failed? The PA28 has a paticularly large trim tab. I know i'd certainly give it a go if it happened.
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If an airplane was overstressed and it resulted in permanent deformation, then it is possible that the damage is visible. Unfortunately fatigue damage is not visible until it results in cracks. In high stressed components like the wing spar the damage may progress from a small crack to a failed wing rather suddenly. This kind of damage is usually impossible to detect without a very thourough inspection of the component in the right stage (small crack developed but not completely failed). The time from visible crack to failed wing can be only a few cycles, depending on the load factors. This has happened on some metal (aluminum) airplanes which unexpectedly lost a wing. They were usually subjected to higher than normal load cycles (lots of time flying low level through turbulence or aerobatics) for much of their life. Composite or wood airplanes are much less affected by fatigue. The only way to determine fatigue damage is by monitoring the load cycles, which as far as I know only the military does. Very few small airplanes have been fatigue tested and have a known life.
If you fly a certified airplane, it should be able to land without elevator control, just the elevator trim. In case the aileron control fails, you should be able to fly with rudder, but be carefull with full flaps, in most cases lateral stability is rather marginal there. Better to land with flaps up for better control.
If you fly a certified airplane, it should be able to land without elevator control, just the elevator trim. In case the aileron control fails, you should be able to fly with rudder, but be carefull with full flaps, in most cases lateral stability is rather marginal there. Better to land with flaps up for better control.
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If you want to fly using trim alone, there is a caveat. Depending on what *exactly* happened, and the *exact* layout of your trim tabs vs. the control surfaces, the trim *might* work in reverse.
If the control cables have broken, leaving the elevator to free-float in the wind, trim will work conventionally: If you move the trim wheel down, the trim tab moves up, pushing the trailing edge of the elevator down. This will deflect the air over the tail downwards, leading to a pitch down.
But if the elevator on for instance a PA-28 somehow got jammed and won't move anymore, then the trim tab becomes the only elevator you have left and it works in the reverse sense, e.g. trim down to go up.
And of course, in the latter scenario, your trim tab is only a very, very small elevator. At low speeds you might not have enough authority to prevent pitch up or pitch down. It might be a good idea to do a low-speed handing test at altitude before committing to landing, and use the time to rearrange the CoG in the aircraft if necessary.
If the control cables have broken, leaving the elevator to free-float in the wind, trim will work conventionally: If you move the trim wheel down, the trim tab moves up, pushing the trailing edge of the elevator down. This will deflect the air over the tail downwards, leading to a pitch down.
But if the elevator on for instance a PA-28 somehow got jammed and won't move anymore, then the trim tab becomes the only elevator you have left and it works in the reverse sense, e.g. trim down to go up.
And of course, in the latter scenario, your trim tab is only a very, very small elevator. At low speeds you might not have enough authority to prevent pitch up or pitch down. It might be a good idea to do a low-speed handing test at altitude before committing to landing, and use the time to rearrange the CoG in the aircraft if necessary.
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Interesting about fatigue.
My personal aeroplane is subject to an Airworthiness Directive whereby a spar strap is required to be fitted at 5500 hours airframe (fortunately, quite a long time off as I purchased a very low time example), and indeed a similar type from the same manufacturer had a 'nearly' failed forward wing spar, the source of which was a manufacturing defect.
So, even though your airplane is looked after properly, problems can be found...fortunately most times early enough to prevent smokin' holes.
With regard to composite structures, this remains to be seen.
Aluminum aeroplanes have been around a very long time, composite ones much less so.
My personal aeroplane is subject to an Airworthiness Directive whereby a spar strap is required to be fitted at 5500 hours airframe (fortunately, quite a long time off as I purchased a very low time example), and indeed a similar type from the same manufacturer had a 'nearly' failed forward wing spar, the source of which was a manufacturing defect.
So, even though your airplane is looked after properly, problems can be found...fortunately most times early enough to prevent smokin' holes.
Composite or wood airplanes are much less affected by fatigue.
Aluminum aeroplanes have been around a very long time, composite ones much less so.
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But if the elevator on for instance a PA-28 somehow got jammed and won't move anymore, then the trim tab becomes the only elevator you have left and it works in the reverse sense, e.g. trim down to go up.
In a PA28, you will not find an elevator, only a stabilator (the so called 'all flying tail.')
And you won't find a trim tab either, but you will find an anti balance tab, which works in the reverse sense to an elevator trim tab.
Pedantic reply? Possibly, but we need to understand how our aircraft systems work and using the correct terminology is the 1st step.
In a PA28, you will not find an elevator, only a stabilator (the so called 'all flying tail.')
And you won't find a trim tab either, but you will find an anti balance tab, which works in the reverse sense to an elevator trim tab.
Pedantic reply? Possibly, but we need to understand how our aircraft systems work and using the correct terminology is the 1st step.
Cessna 150/150 are known to be weak in the inboard forward ribs of the H stab, by which the H stab is bolted to the fuselage.
Cracked stabiliser leading edge ribs are pretty common on Cessna 206 aircraft used for bush flying (usually on the left side). Reason - same as ZeBedie suggests: pilot sit on it to take the weight off the NLG and 'paddle' the aircraft until it's facing where they want.
As a bit of thread creep, would it be possible to fly on just the elevator trim or rudder trim if the primary cables / rods failed? The PA28 has a paticularly large trim tab. I know i'd certainly give it a go if it happened.
. Takes a few seconds for it to sink in, but when it does it's quite a simple fix, albeit a temporary one.Whichever side goes u/s just wind on FULL TRIM in that direction (ie: left goes, full left trim, right goes, full right trim) AND, this is most important, LEAVE IT THERE. You can gain enough control back with the opposite pedal alone to get you on the ground safely.
Same can be done with a broken aileron cable, although perhaps not so effectively if you don't have aileron trim, and with the elevator (ie: UP elevator breaks, wind on nose up trim and balance with forward pressure on the control column. Next time you go flying, try it with the rudder (safely of course!) and see how it feels. It takes a bit of getting used to but it could save your life.
Stay safe
AS
Thread Starter
Slight creep here but how about control linkages? any obvious sign of failure that you could pick up on from just a walk around - does the metal change colour when about to fail like plastic does?
I'd rather learn from other people's mistakes before i make them!!
I'd rather learn from other people's mistakes before i make them!!
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You shouldn't get many problems on linkages or indeed cables, fatigued metal will tend to go greyer slightly lighter and have a granular surface appearance in some cases if that makes sense, but you should never get that type of forces on it.
I myself change any cables that show signs of polishing at all on the Annual Inspection, this is for 2 reasons, one and the primary one is safety and the other is downtime, if it needs replacing later on in the year........ there are acceptable limits but i just replace them.
There is a manual issued by Cessna that covers higher houred 100 series aircraft and what addition things to look at based on their experiances and feedback........ Nothing really in it I do not do already and nothing your Engineers would not normally be looking at...
To be honest one of our worst enemies to fatigue is the pilot, In this I mean when one has a hard landing etc, they would rather keep it quiet and to themselves, this can have a detrimantal effect as checks we would normally carry out in these instances do not get done...
I had a person who came to me sheepishly of late and told me when moving the aircraft on the steering arm he had clipped the tip light....... He expected me to be upset and was suprised that I thanked him for his honesty, but his honesty allowed me to check not just the light but the wing structure itself... people do not realise the forces that can be exerted further down the wing from a slight impact on the wing tip.
I remember the report on a Robin pilot in the UK that clipped a haybale taxying, no visual damage to the wing or very little, so he moved the bale and continued on his flight, I believe the wing separated and he was killed. That is why honesty is important..
As for the larger light stuff, Well, Cessna concerned about the amount of large twins still flying Cessna 421's etc, something not envisaged when built. So they bought a couple of high houred examples, stripped them down to the component parts, inspected them and issued some service bulletins (SID's) on what it found required to be carried out to maintain the aircraft in flyable conditions... this involved pulling the wings and inspecting the mountings, inspecting the windscreens( Pressurised) and also the fuselage etc...
I myself change any cables that show signs of polishing at all on the Annual Inspection, this is for 2 reasons, one and the primary one is safety and the other is downtime, if it needs replacing later on in the year........ there are acceptable limits but i just replace them.
There is a manual issued by Cessna that covers higher houred 100 series aircraft and what addition things to look at based on their experiances and feedback........ Nothing really in it I do not do already and nothing your Engineers would not normally be looking at...
To be honest one of our worst enemies to fatigue is the pilot, In this I mean when one has a hard landing etc, they would rather keep it quiet and to themselves, this can have a detrimantal effect as checks we would normally carry out in these instances do not get done...
I had a person who came to me sheepishly of late and told me when moving the aircraft on the steering arm he had clipped the tip light....... He expected me to be upset and was suprised that I thanked him for his honesty, but his honesty allowed me to check not just the light but the wing structure itself... people do not realise the forces that can be exerted further down the wing from a slight impact on the wing tip.
I remember the report on a Robin pilot in the UK that clipped a haybale taxying, no visual damage to the wing or very little, so he moved the bale and continued on his flight, I believe the wing separated and he was killed. That is why honesty is important..
As for the larger light stuff, Well, Cessna concerned about the amount of large twins still flying Cessna 421's etc, something not envisaged when built. So they bought a couple of high houred examples, stripped them down to the component parts, inspected them and issued some service bulletins (SID's) on what it found required to be carried out to maintain the aircraft in flyable conditions... this involved pulling the wings and inspecting the mountings, inspecting the windscreens( Pressurised) and also the fuselage etc...
Last edited by NutLoose; 30th Jul 2007 at 02:49.
