8KCAB Gear Strut Failure

[zackbloom]

I bought a 1975 8KCAB Decathlon less than a week ago. To celebrate I began a camping trip up from Austin, TX through Colorado, Oklahoma, Nebraska, and eventually South Dakota, Yellowstone, Idaho and Friday Harbor. Unfortunately, the trip took a sudden stop in Nebraska.

After sleeping in the terminal building I preflighted the plane and began some landing practice. My three point landings aren't bad, being similar to what I'm used to in nose-wheel aircraft, but my wheel landings could use some work, and I had a mild bounce or two. I never really tried to rescue anything though, whenever things went sideways I powered up and continued with my practice. On my last pass, I had one of these bounces and heard a "kerrunch", as I was going around I turned to see the right brake line trailing in the wind. A drop of the wing revealed the right landing gear was lying in the runway below!

It was immediately very clear my trip would not be going as planned! I radioed down to the excellent FBO (thank you George in Sidney, Nebraska!) who called the fire department just in case. I flew around for about half an hour while they made a plan (this is a small town!) with somewhat less than usual engine power. I later discovered the departure had resulted in the two trailing inches of each propeller blade being bent 90 degrees by the concrete runway (likely the sound I heard).

After everyone was ready I made an uneventful three-point landing in the grass by the runway. Ultimately the headwind was such (thank you Nebraska!) that the right wingtip didn't make it to the ground until the plane was only going 15 MPH or so. The damage, other than the gear leg and prop/engine is confined to a bit of missing fiberglass on the wingtip.

I'm now sorting out insurance obviously, but I would LOVE to hear if anyone has heard of this happening to Decathlons of my plane's vintage before. Any guesses if this was something which could have been detected?

 

[Bartman]

holy hell! I've never heard of anything like that happening, the metal at the break doesn't look like it had a prior crack, it looks clean all of the way across the break.

sorry to hear it! it will be interesting to hear what you find out as other people chime in that might have had a similar experience. is the FAA going to send that gear leg to a lab to investigate the cause of the crack? might be interesting to trace the serial number on it to see if it was ever re-sprung. the serial number is on the lower tip of the gear leg, maybe a log book entry if it was replaced or sent out to be re-bent.

glad you're ok and kudos to you for handling a very bad situation so well!

 

[zackbloom]

I did just find one similar story: https://blog.zawodny.com/2010/02/25/citabria-n5156x-landing-gear-accident/

 

[Bob Turner]

Glad I went with aluminum. No, I have seen these things really bent up into pretzels, but not fractured like that.

Wheel landings are hard on spring steel gear - the tendency to bounce is ever-present, but the real problem is torsion. The best landing for a Decathlon is a 3-point wheel landing. But nice save - a new gear leg and prop and a little magnafluxing and you are back at it.

Looks like a very nice bird.

 

[Goodyear]

Make sure they check that wing out before you accept any insurance money. The original wing AD is believed to have related to wings impacting fixed objects while not in flight leading to stress fractures/cracks.

 

[Goodyear]

Amazing it flew that well with the condition of that propeller. How many hours on that engine? Maybe an opportunity for insurance assisted overhaul.

 

[zackbloom]

Make sure they check that wing out before you accept any insurance money. The original wing AD is believed to have related to wings impacting fixed objects while not in flight leading to stress fractures/cracks.

The airplane had the metal wing conversion in the 90s fortunately.

Amazing it flew that well with the condition of that propeller. How many hours on that engine? Maybe an opportunity for insurance assisted overhaul.

That's the fortunate part, 1570 hours on a 1600 TBO. The prop was in the 1200s, with a likely overhaul at 1600 due to the aerobatics prop departures people have experienced.

 

[BB57]

Glad I went with aluminum. .../
/...

Time will tell.

Steel has an elastic limit, a fatigue limit and a plastic limit. Steel that is deformed under load but then returns to its original shape has experienced elastic deformation, while steel that is deformed under load and does not return to its original shape has experienced plastic deformation. The fatigue limit lives in the upper end of the elastic region somewhere short of plastic deformation. Consequently, if a steel part is designed so that it operates comfortably within it's elastic limit, it won't have a fatigue life. A few excursions above the fatigue limit will however begin to create a fatigue life, and the greater the excursion the more fatigue will occur.

3AA steel and 3AL aluminum high pressure tanks, where there is mandatory requalification every 5 years, are a useful way to illustrate the differences. 3AA steel tanks are very conservatively designed and do not exceed their fatigue limit even at pressures close to their test pressure (which is 5/3rds of service pressure). Rust is what kills them and if properly protected from rust and operated under their fatigue limit, they'll last essentially forever and undergo essentially unlimited cycles with no fatigue. The oldest tank I've seen come in for requalification was made in 1911, and still qualified for a "+" rating allowing a 10% over fill.

Similarly, cave divers also routinely fill 3AA 2400 psi service pressure steel tanks to 3600 psi (and they have a 4000 psi test pressure). They've been doing that for over 25 years and they are neither failing in service nor failing requalification every five years, suggesting that 3600 psi fills are still staying under the fatigue limit, or that the fatigue life at those loads is still suitably long.

With this in mind for spring steel landing gear, damage history certainly matters. If you land hard and permanently bend a spring steel gear leg, you've clearly exceeded the plastic limit, you've massively exceeded the fatigue limit, and greatly reduced the fatigue life - and probably the strength - of the part. I've seen Citabrias with a landing gear leg bent in service (usually as trainers) have the gear leg bent back into shape and be put back into service. However, the molecular structure in that gear where the bend has occurred has been permanently altered and that gear leg is now an interesting science experiment on ultimate load and fatigue life. The gear leg is not as strong as it use to be and will potentially exceed it's fatigue limit much more frequently in normal use, potentially leading to a fatigue failure.

Depending on the design margin and whether normal loads on the gear exceed the fatigue limit, usage history in terms of number of landings, bounces, and or landings on rough terrain might matter.

----

In comparison, aluminum fatigues with every load cycle and the greater the load in a cycle the more fatigue occurs. This means aluminum tanks have a fatigue limit. 3AL aluminum tanks are very conservatively designed and one major manufacturer tests samples of their aluminum tanks from 0 psi to the 5000 psi test pressure 10,000 times to test for fatigue.

This is an area where usage patterns matter. There is substantially more load and fatigue involved in testing to a 5000 psi test pressure than a 3000 psi service pressure, so it's not clear how many cycles are available in the life of a tank at its 3000 psi service pressure. That basically means that 10,000 cycles is a very conservative fatigue life limit. However, a dive shop in a busy resort setting filling the tank twice per day, every day will reach 10,000 cycles in about 13 years, and about 18,000 cycles in 25 years. When we were in Bermuda, the rental tanks we were given had been in service for over 40 years. They'd probably seen a LOT of cycles. In comparison, the average recreational scuba diver owning 2 tanks and diving around 50 dives per year, would take about 400 years to reach 10,000 cycles.

Applying the same principles to aluminum landing gear, the fatigue life of aluminum landing gear will depend on 1) how over designed that gear is relative to the loads it encounters, 2) how many cycles it experiences, and the 3) magnitude of the loads in those cycles.

Generally speaking aluminum needs to be much thicker to carry the same loads as steel. In a highly stressed part the weight savings can be less than you'd think. In short, if you have a much lighter aluminum part than it's steel counterpart, you probably also have less margin between the service and ultimate loads than you'd have with the steel part. There's no free lunch.

Again, usage history matters. Aluminum gear used by a weekend flyer, who is a proficient pilot, doing 1 or 2 landings per hours and landing on smooth pavement will be much kinder to the gear than a student pilot in the pattern doing a landing every 5 to 6 minutes - with harder arrivals, a couple bounces, with the wheels not moving parallel to the runway on touch down. A pilot flying off a rough strip will probably fall somewhere in the middle.

The reality is that we probably won't know how well aluminum gear holds up over the long term with different types of usage, but we do know that aluminum more prone to fatigue than steel.

----

These two spring steel gear failures do give me pause, particularly with my Citabria on gear made in 1967. The breaks are occurring at a major/permanent bend in the gear and that isn't a surprise. That bend also takes the load every time the gear flexes so it's no surprise that's where the breaks are occurring.

I'm interested in how many hours each of the aircraft involved had on them, their usage history and estimated number of landings, their accident history and whether they've ever had their gear bent and then bent back into shape in the past.

If these were high time aircraft used as trainers (aerobatic or otherwise), with a high number of landings, and operated off rough strips, then it suggests a life limit issue. If these aircraft had an accident history with the gear leg previously bent in a ground loop, or bent in a hard landing, then it potentially means something different. If they are comparatively low time with no trainer history, then it means I'd be really surprised as it would essentially suggest a minor defect in individual gear legs that took 40 plus years to reach failure.

That said, looking at how ACA responded to the very small number of wood spar failures in the 7 and 8 series aircraft, I strongly suspect ACA would have a similar approach with older 7 and 8 series spring steel gear. ACA has sold a lot of metal wings at $20K-$25K per set as a result of their SB and the eventual AD, even though the spars that failed were all on aircraft with history of a tip strike or and over turn. If there is an SB and eventual AD for the gear legs, I suspect damage history will again be ignored, if it is present, and that there will be a move to replace gear at some number of hours or landings, even though losing a gear leg is much less likely to result in death than losing a wing.

 

[Bob Turner]

Interesting post. Robbie Grove makes all ACA gear legs. They are indeed of a much fatter cross section. He is one of the sharpest engineers I have met.

I have not heard of any aluminum gear leg failures. I do know that steel gear legs are routinely straightened, and that they routinely sag a bit after a few decades. I believe, like auto/truck springs, they are straightened cold, and not re- heat treated.

I will ask . . .

 

[Bruce]

Will be interesting to see how your insurance deals with this, I would suggest looking for the replacement aircraft depending on what you insured the hull for will determine, weather they total or just repair. I will assume this will require the airframe inspection for landing damage, engine dismantle and inspect, (may as well overhaul) new prop, you may be half the value of the plane. Great to hear no one hurt sad to hear about the trip interruption. But another good landing in the books and great story to tell. Please keep us posted.

 

[Bob Turner]

Probably repair. Airframe damage sounds minimal. A new prop is expensive, but an engine tear down is only a couple grand. Wear due to 1600 hours of operation is not part of the insurance coverage.

We did have a crank failure due to a prop strike. Insurance covered the new crank and bearings. Not cheap!

 

[BB57]

Interesting post. Robbie Grove makes all ACA gear legs. They are indeed of a much fatter cross section. He is one of the sharpest engineers I have met.

I have not heard of any aluminum gear leg failures. I do know that steel gear legs are routinely straightened, and that they routinely sag a bit after a few decades. I believe, like auto/truck springs, they are straightened cold, and not re- heat treated.

I will ask . . .

I haven't see the aluminum gear up close yet. It's good news to hear it is substantially thicker.

It's been a long time since I cracked an aeronautical engineering text book, but I suspect the steel used is something close to 6150 chrome vanadium steel bent while red hot, reheated and stress relieved and then heat treated to a Rockwell C hardness of 40 to 48. Less than 40 and it'll start to sag. Much over 48 and it'll be too brittle and break. The final step in the process should be to magnaflux the gear leck to ensure no cracks were formed in the manufacture process. I don't know if they did that back then.
----
The hardness issue got me to thinking today about the original heat treatment of the steel gear legs and whether they might become more brittle with age and use.

For example, a friend of mine who spent his career in ordinance was asked by the US Army to look into the high percentage M9 service pistols with breakage of fire control parts. This was occurring even in M9 service pistols that had fairly low round counts. He eventually concluded that while they had fairly low round counts the affected pistols were still old and had been carried on a regular basis, with the required function checks resulting in multiple impacts on the fire control parts. The fire control parts were face hardened for good wear resistance, while the internal metal was softer and tougher for high strength. However, repeated impacts eventually change the structure of the metal and harden the part all the way through, making it brittle and prone to failure. You'll see the same thing in older Walther PP and PPK pistols that have not been fired excessively, but are 75 plus years old.

I'm wondering if the spring steel gear on the Citabrias and Decathlons has a similar embrittlement issue over time and use.

 

[scoutdog]

Time will tell.

Steel has an elastic limit, a fatigue limit and a plastic limit. Steel that is deformed under load but then returns to its original shape has experienced elastic deformation, while steel that is deformed under load and does not return to its original shape has experienced plastic deformation. The fatigue limit lives in the upper end of the elastic region somewhere short of plastic deformation. Consequently, if a steel part is designed so that it operates comfortably within it's elastic limit, it won't have a fatigue life. A few excursions above the fatigue limit will however begin to create a fatigue life, and the greater the excursion the more fatigue will occur.

3AA steel and 3AL aluminum high pressure tanks, where there is mandatory requalification every 5 years, are a useful way to illustrate the differences. 3AA steel tanks are very conservatively designed and do not exceed their fatigue limit even at pressures close to their test pressure (which is 5/3rds of service pressure). Rust is what kills them and if properly protected from rust and operated under their fatigue limit, they'll last essentially forever and undergo essentially unlimited cycles with no fatigue. The oldest tank I've seen come in for requalification was made in 1911, and still qualified for a "+" rating allowing a 10% over fill.

Similarly, cave divers also routinely fill 3AA 2400 psi service pressure steel tanks to 3600 psi (and they have a 4000 psi test pressure). They've been doing that for over 25 years and they are neither failing in service nor failing requalification every five years, suggesting that 3600 psi fills are still staying under the fatigue limit, or that the fatigue life at those loads is still suitably long.

With this in mind for spring steel landing gear, damage history certainly matters. If you land hard and permanently bend a spring steel gear leg, you've clearly exceeded the plastic limit, you've massively exceeded the fatigue limit, and greatly reduced the fatigue life - and probably the strength - of the part. I've seen Citabrias with a landing gear leg bent in service (usually as trainers) have the gear leg bent back into shape and be put back into service. However, the molecular structure in that gear where the bend has occurred has been permanently altered and that gear leg is now an interesting science experiment on ultimate load and fatigue life. The gear leg is not as strong as it use to be and will potentially exceed it's fatigue limit much more frequently in normal use, potentially leading to a fatigue failure.

Depending on the design margin and whether normal loads on the gear exceed the fatigue limit, usage history in terms of number of landings, bounces, and or landings on rough terrain might matter.

----

In comparison, aluminum fatigues with every load cycle and the greater the load in a cycle the more fatigue occurs. This means aluminum tanks have a fatigue limit. 3AL aluminum tanks are very conservatively designed and one major manufacturer tests samples of their aluminum tanks from 0 psi to the 5000 psi test pressure 10,000 times to test for fatigue.

This is an area where usage patterns matter. There is substantially more load and fatigue involved in testing to a 5000 psi test pressure than a 3000 psi service pressure, so it's not clear how many cycles are available in the life of a tank at its 3000 psi service pressure. That basically means that 10,000 cycles is a very conservative fatigue life limit. However, a dive shop in a busy resort setting filling the tank twice per day, every day will reach 10,000 cycles in about 13 years, and about 18,000 cycles in 25 years. When we were in Bermuda, the rental tanks we were given had been in service for over 40 years. They'd probably seen a LOT of cycles. In comparison, the average recreational scuba diver owning 2 tanks and diving around 50 dives per year, would take about 400 years to reach 10,000 cycles.

Applying the same principles to aluminum landing gear, the fatigue life of aluminum landing gear will depend on 1) how over designed that gear is relative to the loads it encounters, 2) how many cycles it experiences, and the 3) magnitude of the loads in those cycles.

Generally speaking aluminum needs to be much thicker to carry the same loads as steel. In a highly stressed part the weight savings can be less than you'd think. In short, if you have a much lighter aluminum part than it's steel counterpart, you probably also have less margin between the service and ultimate loads than you'd have with the steel part. There's no free lunch.

Again, usage history matters. Aluminum gear used by a weekend flyer, who is a proficient pilot, doing 1 or 2 landings per hours and landing on smooth pavement will be much kinder to the gear than a student pilot in the pattern doing a landing every 5 to 6 minutes - with harder arrivals, a couple bounces, with the wheels not moving parallel to the runway on touch down. A pilot flying off a rough strip will probably fall somewhere in the middle.

The reality is that we probably won't know how well aluminum gear holds up over the long term with different types of usage, but we do know that aluminum more prone to fatigue than steel.

----

These two spring steel gear failures do give me pause, particularly with my Citabria on gear made in 1967. The breaks are occurring at a major/permanent bend in the gear and that isn't a surprise. That bend also takes the load every time the gear flexes so it's no surprise that's where the breaks are occurring.

I'm interested in how many hours each of the aircraft involved had on them, their usage history and estimated number of landings, their accident history and whether they've ever had their gear bent and then bent back into shape in the past.

If these were high time aircraft used as trainers (aerobatic or otherwise), with a high number of landings, and operated off rough strips, then it suggests a life limit issue. If these aircraft had an accident history with the gear leg previously bent in a ground loop, or bent in a hard landing, then it potentially means something different. If they are comparatively low time with no trainer history, then it means I'd be really surprised as it would essentially suggest a minor defect in individual gear legs that took 40 plus years to reach failure.

That said, looking at how ACA responded to the very small number of wood spar failures in the 7 and 8 series aircraft, I strongly suspect ACA would have a similar approach with older 7 and 8 series spring steel gear. ACA has sold a lot of metal wings at $20K-$25K per set as a result of their SB and the eventual AD, even though the spars that failed were all on aircraft with history of a tip strike or and over turn. If there is an SB and eventual AD for the gear legs, I suspect damage history will again be ignored, if it is present, and that there will be a move to replace gear at some number of hours or landings, even though losing a gear leg is much less likely to result in death than losing a wing.

I think BB57 is onto something: Certainly knows more about properties of metal than most of us, and a good explanation.

I had a Scout MLG break on landing, just outboard of clamp to lower longeron. It was a mildly bounced 3-point landing on grass. And of course this got one wing and the prop.

This particular 8GCBC was an early '74, >6000 hours on airframe, and ~1800 since IRAN and recover. It had spent virtually its entire life as a glider tug, the first 4,000 in CA, and some or much of that near the ocean. Extrapolate 5 or 6 landings per hour, and that's a LOT of landing cycles. NTSB took the gear leg, and determined it was due to "abusive grinding" leading to a stress riser on the inside radius of the gear leg. An awkward place to preflight, (and believe me, when back in service started looking REALLY hard there, and at attach hardware), also painted dark blue, which might have masked cracks in marginal light. Typical flying weight would be <1700#, and the original steel gear legs eyeball about the same dimensions as a C-185, which has a much higher Gross Weight. At the time this tug was getting Annual Inspections every 100 hours by a picky I.A., probably 50 hours since last Annual.

I've also "broken" many tailsprings, just by looking at them (OK, they were broken already, and I probably preflight harder than most folks). This is not just on Scouts, by the way. When Bill Duncan owned Alaska BushWheels, he came up with a much improved tailspring for the Aviat Huskies. Bill wasn't happy with the 1920's automotive recipe spring steel which was a carryover from Piper taildraggers, so specified a better steel (German source?), and improved geometry. The Supercub folks seem to like the ABW springs too, and pretty sure they're available for the 8GCBC now.

Sorry for the long-winded comments, but the radius of the MLG needs a detailed inspection, both pre-flight and post-flight, along with the tailsprings.

Thanks. Scoutdog

 

[Bartman]

@BB57 Great post, thanks for taking the time to explain the problem in so much detail!

@scoutdog good suggestion to look under the gear bend for cracks, am curious to see if increased vigilance at that spot on the plane turns up additional squawks.

 

[BB57]

One of the things that makes it hard to determine the fatigue life of a component in a civilian aircraft is the wide range in how those aircraft are used.

In contrast, military aircraft fleets are managed to maximize airframe life and determine how frequently major depot maintenance must be done. For example the US Navy will rotate aircraft do different squadrons flying different missions to balance the wear caused by certain demanding missions across the fleet.

The F-18 legacy Hornets are a good example as they were originally 6000 hour airframes due to fatigue limits. However, that has been extended to 8000 hours for the F/A-18A and C aircraft using four life-limiting criteria:
1) flight hours,
2) wing root fatigue life expended,
3) catapults and traps, and
4) landings.

By moving the aircraft around between various squadrons and missions those 4 factors can be managed to minimize fatigue and maximize airframe life.

The C-130 is an example where aircraft are managed by squadron and mission but also exchanged between the regular military, reserve, and coast Guard commands. That management is done on the basis of a modeling program that looks at:
(1) aircraft age (between .5 and 30 years, with an average of 5 years),
(2) total flying hours (between 20 and 16,000),
(3) average yearly flying hours (between 250 and 700),
(4) mission profile severity factor (0.9 to 7.68 with an average of 1.94), and
(5) operating location of the aircraft (including corrosion factors from 0 to 25).

The single most critical life limited item on the C-130 is the wing spar box and it's very expensive and labor intensive to replace. However, it's life limited not on a flight hour basis but rather on a combination of flight hours and mission severity that range from .09 to 7.68 with an average of 1.94. By moving the aircraft around between missions and even services, the life of the wing spar box can be matched to the life of other major components so that it won't need to be replaced until the aircraft is either retired or essentially rebuilt.

----

In terms of 7 and 8 series Champion/Bellanca/ACA aircraft, they do get bought and sold periodically and may get used in very different roles over their service lives. It just isn't managed in any intentional way.

The prospective PA-28 wing spar AD is a good example. There have been a couple wing spar failures, but in both cases the aircraft had over 7000 hours and a long history of intensive use as trainers, with way more than their share of not just landings but also hard landings, as well as things like 2G 60 degree steep turns.

It's been proposed that mandatory spar inspections be based on a 5000 hour limit, but it has also been proposed that the requirement be based on high intensity use as indicated by the number of 100 hour inspections the aircraft has received.

The former proposal s conservative, while the latter would have some complications. If the aircraft does not have complete airframe logs, documenting number of 100 hour inspections is not possible and it would have to default to total airframe hours. Some aircraft are also used as rentals with little or no training but still have 100 hour inspections that would still be counted toward "intensive" use. There are also aircraft that get 100 hour inspections but are flown not much more than 100 hours per year, so that use isn't significantly more intensive than an aircraft with an annual inspection.

-----

Applying that to the even more diverse uses of Citabrias, Scouts and Decathlons makes it even more likely that if there are ever enough accidents to prompt a SB (and an almost inevitable AD) it'll most likely be based on airframe time not use.

 

[aftCG]

Wow, new one for me. I wonder if there is some kind of DIY dye penetrant inspection that could be performed with the gear in place.

 

[Bob Turner]

In spite of the unfortunate failures noted here, I suspect this is not a really serious problem. Cessna has used spring gear for almost 70 years and Champs for maybe a half century. They should be failing all over the place, and yet we have only a few scattered stories. The Champ gear are being straightened cold, I am told.

I am happy, though, to tell you I will never fly a Herc, and probably will never again set foot in a PA-28R. Yuk!

 

[aftCG]

I'd give up a kidney to log C-130 time.

Regarding statistics and other types (can't use the C word in mixed company) with similar gear, that was my first thought. I didn't want to go so far as to declare "fake news" given an above zero occurrence but it's certainly rare.

Looking at the OP photos it looks like there is a small amount of that fracture which is discolored. Could that have been the initial crack that went undetected?

 

[Bartman]

kidney for an A-10 ride if anyone is taking notes.

 

[Bob Turner]

Not giving up any body parts, but the only two I would put on my bucket list are the PBY amphib and the P-51. I could afford to do both, but the Greek might leave me - and besides, I am getting lazy.
I flew the Nord 262 - kinda like a twin engine Herc. Fun.