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RV-8 Pilot Seat Failure

Ironflight

VAF Moderator / Line Boy
Mentor
I am posting this for a friend who isn’t on VAF, but agrees that the community should know about this. He was on take-off, full throttle, when the lower part of his aftermarket seat back buckled, allowing the top to slip UNDER the cross-bar, and putting him halfway into the back seat, with his head below longeron level. He told me he was doing about 50 knots at the time, ended up going off the runway, and damaging a wheel pant - fortunately, that was it (as well as the seat back, which is trashed). Imagine if this had happened a few seconds later!

He bought this -8, and is not a builder, and is not sure who did the seat backs - but they appear to be aftermarket, and when he pulled the upholstery off, it is obvious that this is not a stock seat back design. I’m posting the pictures of the seat back with the upholstery as well as the picture of the failed frame for reference, in case it looks familiar. This, to me, is somethign that anyone with a similar seat frame should look at carefully - there appears to be a weak point which could get bent with age.

In aircraft after incident:
472D63F9-70D4-4501-A3B3-523AADF195C3.jpeg

With upholstery removed, bottom of seat back to left:
952269F7-F464-489C-9008-87334D1CB7CC.jpeg

If you have something other than a stock seat back - take a look, down in the lower six inches to see if you have any sign of bending or buckling!

Paul
 
Mine failed same location years ago. Stock seats. Loop entry with 3 wingmen. Could have ended badly.
Banged the back of my neck pretty good. Helmet may have played a positive role for no injury. An attention getter for sure.
Seat back quickly ended up in the back seat. No rear stick in. I never put or leave the rear stick in.
 
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Holy smokes! That could have been a much worse outcome for the pilot. I'm glad he's okay, and thanks for posting these pics Paul.
 
What after market?

I have the Classic Aero Aviator seats....should I be worried?

Inspection tomorrow.

Thanks


DP
 
I have the Classic Aero Aviator seats....should I be worried?

Inspection tomorrow.

Thanks


DP

Yeah - I wish we could definitively ID the seat manufacturer to narrow it down, but I don't have access to anything but these pictures, so I’d take a look. I already looked at my stock seat just to make sure!

Paul
 
Looks like Classic Aero to me... yikes! Wonder if I should open mine up for a look?
 
Yeah - I wish we could definitively ID the seat manufacturer to narrow it down, but I don't have access to anything but these pictures, so I’d take a look. I already looked at my stock seat just to make sure!

Paul

Looks like Classic Aero Aviator Seats. (Would be good to confirm from Classic Aero / Builder).

(http://www.classic.aero/web/public/...=72&ProductCategory=RV-8&ProductCategoryID=21)

The front backrest has an integral frame on the Aviator seats. (The other option, Sportsman seat, uses the frame that comes with the RV-8 kit from Vans).

Inspecting my seat tomorrow as well - glad that the pilot is safe.
 
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Yes, I had the same failure with mine...pulling out of the bottom of a loop, perhaps 3.5 to 4 Gs. Called Classic Aero and they sent me some beefed up angle to rivet into the seat frame. Clearly the frame was not designed for aerobics. That’s been a decade ago, no issues since.
 
Has anyone contacted Classic Aero about this? Do we have a year of manufacture (in case there were design changes since that time)?

Do we know anything about the hours on the plane and type of flying the owner usually did (aerobatics)?

That's...disconcerting.
 
Seat failure

It might be interesting to know which bottom hinge line was used. I'm certainly not an engineer but it would seem to me that a more reclined seat (using the more forward bottom hinge line) would direct more force towards the seat back than one which is more vertical (more aft bottom hinge line). I also noticed when changing our stock seat back to the most rearward bottom hinge position that the vertical side seat supports moved higher where they rest on the center crossover support. This could provide a little more margin before the seat back slipped under the crossover. Lastly, I'm going to inspect our seat today and look into some sort of safety strap connecting the seat back and the center crossover support as a fail-safe. Should be easy to do.
 
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Is the cross bar the tall man option Vans sells? Hard to tell in the photo but if it is , the top of the bar is moved to the rear and leaves very little of the seat top resting against the bar. The seat back needs to be extended if using the tall man cross bar.
 
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seat back

The stock seat seems pretty beefy to me, but perhaps there is a weakness in the design that's not obvious.

The F-832C angle along each side is 1" or 1.25" by 0.125" if I'm not mistaken.

RV-8 Front Seat Back - Dwg 76.png
 
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Lap Belt type for those who had failures

Hi,
What lap belt was in use for those who had back seat failures? The point of failure on the picture is close (if not at) the pass through for the lap belt. If the lap belt was a ratchet type belt, there will be additional bending moment applied at that point in the seat 'beams' before any G is applied.

Just curious if that is a factor.
 
Seat back failure

Just looked at mine. No obvious issue but it looks like a cushioned adel clamp around the middle of the crossover that is attached to the seat back top would keep the seat from failing to the point of dumping you into the back seat. Anyone see a problem with that? I have an ex A-10 driver son that loves aerobatics. Sure don’t want that happening to him!
 
Going from memory since my failure happened 10+ years ago, but my aircraft and seats were new (less than 50 hours). As I finished a loop at under perhaps 3.5 to 4 Gs, the seat failed. One of the vertical angle supports on the side had buckled. It wasn’t a violent failure, and I did not lose control of the aircraft. When I contacted the company, they said there was a retrofit beefed up frame they would send me to repair the seat. I told them that since the RV-8 was an aerobatic aircraft, the seat really needed to be designed to withstand at least 6 Gs (not just look nice for cross country flying). They sent the fix which I installed fairly easily, and I have had no issues since. I fly aerobatics regularly. For the record I’m 6’ 1” and weigh 168 lbs; well within an average range.
 
Hi all, that was my airplane and near-incident. I just received a stock Vans seat kit and comparing the one that was in the airplane (which I had assumed was just stock and custom upholstered!) points to a pretty critical design flaw in the installed seat. The Vans seat back design uses 1/8" aluminum angle along both sides of the entire seat. The installed seat side walls are 1/16". While they are deeper and include an extra bend over most of the length (thus raising the section modulus), this is not true of the failure area. There the stiffener profile is nearly identical to stock, but with half the material thickness. That means that back-of-the-envelope buckling strength of each side of the seat rail is about 1/4 the stock design. More importantly, it sets the seats up for increased local stress that in this case appear to have resulted in work hardening and then brittle failure of the left-side seat rail in the high-stress region. That precipitated further bilateral buckling of the structure that left me half way in the back with the controls out of reach. I am about 200lbs, and I have done basic aerobatics at up to 4G. On the morning of the accident, there was basically no warning. The seat did not slowly deform and then slip under the crossbar. It was a brittle failure followed by plastic buckling in the the high stress region created by the shape of the seat back. If you have custom seats, I'd recommend checking them and making sure that they are not designed to fail.
 
To answer (or not!) some questions: I don't know off the top of my head who made the seats, but I do have a receipt in the paperwork that came with the airframe for the upholstery. I just can't fly there right now to retrieve the data, but I will once I've built a new seat. The belts were not ratcheting style, I don't think that they contributed significantly to the bending moment during normal use. The guy that I bought the airplane from was tall, so it may be a tall crossbar, but there was ample overlap between the bar and the seat back (on the order of a few inches). The only reason it was able slide under the bar is the bend that shows up in the pictures...that seat back used to be straight! Btw, I have put about 450 hours on the plane since July 2017 when I purchased it, but the seat was consistently riding in the same place (per the marks on the back of the upholstery) until last week. I'm not sure about the hinge line positioning (I'll check it when I'm next with the plane), but the seat was normally fairly upright in flight...in fact if it had been any more upright I would have been leaning forward during straight and level flight.
 
Check stock seats, too!

I know of two RV-8s with stock Van's seat backs which have shown significant deformation of the seat back. I one instance I was noticing that I couldn't apply sufficient forward stick pressure to maintain inverted flight and subsequently found that a large bend in the seat back angle brackets was causing me to be positioned several inches back from the normal position. The owner replaced the seat back with one fashioned with much larger angle braces. The second bent seat back was discovered by a friend who regularly flies aerobatics.

I never saw a deformation of the seat back in my own RV-8 despite years of competition aerobatics. I'm six feet tall, 190 pounds and regularly pulled just under six Gs. I did have a ratcheting seat belt but fashioned a second attach point for that belt directly under the top of my thighs so that would not cause extra stress on the seat back.

Bottom line: If your seat back is a stock Van's part or an aftermarket part, take a look at the seat angle braces and make sure they are not bending. If they are, replace with new angles, preferably with larger, stronger stock.
 
wow

Thank you Paul for posting, very interesting and safety issue.
Inspection needed !
ps : Could be interesting to know the pilot weight and the math into the project seat frame.
 
i just completed making carbon fiber seat baks for my RV-6A. I evaluated the potential pilot loads in a 6 G pull and a 1G vertical maneuver (plus 1.5 safety factor) given the slope of the seat back, and for pilot reaction loads from the flight control inputs. I tested to 180 lbs at about 10” up from the bottom with the upper part supported at the same support point as in the fuselage. Passed fine. I can’t think of anything that would present higher bending loads to the seat back, other than a pilot weight over 220 lbs.

But that does say that in aerobatics the seat back loads can be important.
 
I think what may be missed here is the failure mechanism...it's cumulative but doesn't necessarily have any outward indication (short of a flux measurement). It's not that the seat doesn't support an accelerated load with a given safety factor. Rather, metal fatigue plays a role, particularly in light of the fact that aluminum (unlike steel) does not have a stress vs #-cycles-to-failure curve that saturates. That means that for any repetitive stress, no matter how small, aluminum will eventually fail. That said, properly sized components can have a design life sufficiently long that it doesn't matter...but that's only guaranteed if the design is intentional. To that end, it is not sufficient to measure/calculate the yield strength and/or section modulus, multiply by some factor (over assumed maximum load) and call it done. That's because brittle fracture mechanics may eventually come for you (as they did me). Again the seat that was installed in my aircraft was inherently weaker than the stock Van's seat (I didn't know it wasn't stock, being a mere buyer), and that lead to an 800-ish hour lifetime under my "normal use" conditions. Also the fact that seat failure seems relatively uncommon (though significant if it happens) suggests that it's not an epidemic problem at the age of a typical airplane in the fleet. However, if you really want to know whether you should expect your seat to not fail (or rather, to fail), fatigue which is a cumulative effect must be taken into account. That's not necessarily easy and perhaps a good proxy is just to overbuild the seat in a reasoned way (does it really matter if you add a pound to the seat back in the quest for robustness...my water bottle weighs more than that), but at any rate because your seat is made from aluminum, it definitely has a fatigue life. Knowing how long your seat has to live (or at least that it has a very long life) is important...actually, having experienced a failure on takeoff roll and subsequent left-turn departure from the runway, headed at 50kts+ toward a ramp full of planes and some people (with the stick pinned full back meaning I was moments from leaving the ground thanks to that awesome RV performance)...I'd say it's critical.
 
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Another data point

I have 450 nonaerobatic hours on Classic Aero Aviator seats with the integral pilot seat back frame. The seats were purchased in 2012. I have just completed an inspection and find no evidence of deformity. Since the seat is removed annually during the condition inspection it is a simple matter to unzip the upholstery and check the frame, a step I will now add to my check list.

I also built the standard Van’s frame intending to use it with a back pack chute for aerobatics. I can confirm differences in design and material used. The Van’s frame uses 0.125” x 1” aluminum angle for the side beams while the Classic Aero design uses “Z” shaped beams of 0.063” aluminum with a 1.25” web and 0.75” flanges. The beams seem, by palpation, to extend unaltered the full length of the frame in both designs and both use 0.025” corrugated material for the backs, however, the corrugations face back on the Van’s frame and forward on the Classic Aero design. Both include penetrations for the lap belts.
 
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To answer (or not!) some questions: I don't know off the top of my head who made the seats, but I do have a receipt in the paperwork that came with the airframe for the upholstery. I just can't fly there right now to retrieve the data, but I will once I've built a new seat. The belts were not ratcheting style, I don't think that they contributed significantly to the bending moment during normal use. The guy that I bought the airplane from was tall, so it may be a tall crossbar, but there was ample overlap between the bar and the seat back (on the order of a few inches). The only reason it was able slide under the bar is the bend that shows up in the pictures...that seat back used to be straight! Btw, I have put about 450 hours on the plane since July 2017 when I purchased it, but the seat was consistently riding in the same place (per the marks on the back of the upholstery) until last week. I'm not sure about the hinge line positioning (I'll check it when I'm next with the plane), but the seat was normally fairly upright in flight...in fact if it had been any more upright I would have been leaning forward during straight and level flight.

What year was the airplane built?
 
I have 450 nonaerobatic hours on Classic Aero Aviator seats with the integral pilot seat back frame. The seats were purchased in 2012. I have just completed an inspection and find no evidence of deformity. Since the seat is removed annually during the condition inspection it is a simple matter to unzip the upholstery and check the frame, a step I will now add to my check list.

I have Classic Aero Aviator seats (with headrests), but don't see any way to "unzip" the upholstery...?
 
I also built the standard Van’s frame intending to use it with a back pack chute for aerobatics. I can confirm differences in design and material used. The Van’s frame uses 0.125” x 1” aluminum angle for the side beams while the Classic Aero design uses “Z” shaped beams of 0.063” aluminum with a 1.25” web and 0.75” flanges. The beams seem, by palpation, to extend unaltered the full length of the frame in both designs and both use 0.025” corrugated material for the backs

In the seat that was in my plane (maybe an older version of a Classic Aero design?), the extra bend that forms the Z-shaped beams end about 1" above where the failure occurred (call it 6"-8" from the bottom of the seat back, but I don't have it in front of me right now). For the last portion it's just an L-bend, comprising 1/16" aluminum.
 
Also, visual inspection for deformation isn't necessarily diagnostic of the seat's integrity. Indeed, fatigue effects result from dislocations created in the microstructure of the aluminum at the site of repetitive high stress. The location/existence of high-stress site is going to be a function of the overall structure and any stress raisers (whether internal to the material or from the construction) which may arise as dislocations induced in the microstructure and so evolve over time. As the material work-hardens with continued stress it becomes subject to brittle fracture...which I'm fairly sure is what happened here (there is a clear crack extending across the in-plane cross section of the left side beam).
 
I just heard from Classic Aero. They confirmed it was one of their seats. They also said that mine was one of the first few of that design and that they very quickly changed the design to extend the Z-bend structure all the way to the floor (as someone up-thread found). He said that for all the seats that were shipped with the truncated Z-bend design, they shipped a modification kit to reinforce those parts--it was not installed on my seat, however. In this case, since the seat back was destroyed, they offered to replace it at no charge, which is impressive since I wasn't the builder.

I'm still going to take a look at an FEM model (assuming I can get my hands on something like Ansys or Comsol again) just to see if I can get a read on what the expected lifetime would be under typical repetitive stress loads. Honestly, I spent a lot of time worrying about the engine, controls, electrical, landing gear, brakes, etc...never even stopped to think that the seat might get me into trouble (I am familiar with the issues in the C172, but the RV seats are fixed in place....)
 
I know of two RV-8s with stock Van's seat backs which have shown significant deformation of the seat back. I one instance I was noticing that I couldn't apply sufficient forward stick pressure to maintain inverted flight and subsequently found that a large bend in the seat back angle brackets was causing me to be positioned several inches back from the normal position. The owner replaced the seat back with one fashioned with much larger angle braces. The second bent seat back was discovered by a friend who regularly flies aerobatics.

I never saw a deformation of the seat back in my own RV-8 despite years of competition aerobatics. I'm six feet tall, 190 pounds and regularly pulled just under six Gs. I did have a ratcheting seat belt but fashioned a second attach point for that belt directly under the top of my thighs so that would not cause extra stress on the seat back.

Bottom line: If your seat back is a stock Van's part or an aftermarket part, take a look at the seat angle braces and make sure they are not bending. If they are, replace with new angles, preferably with larger, stronger stock.

I’m one of the friends that has experienced significant seat back deformation. I’m actually in the midst of putting the finishing touches on my new and improved (read very heavy) seat back. The new one has 1.5 x 1.5 x 0.167 vertical angles, two layers of corrugated seat back and an additional flat 0.032 sheet on top of the two corrugated pieces. The aerobatics and my chute really did a number on it. I think it was more due to the chute rather than the aerobatics. The flat sheet and the two corrugated pieces are all being riveted together. The purpose of the flat sheet is to give the chute something better to bear against.

Ultimately I resolved the issue by purchasing another airplane. But that’s a topic for another thread. I know, it’s a little extreme! :)
 
This incident reminds me of a near accident I had many decades ago in a C-150 Texas Taildragger conversion. Back then, Cessna’s were experiencing seat track failures and I became the victim of such a failure.

I was landing on a grass strip and in the flare I pulled the throttle toward idle and had a beautiful touchdown. The bumps in the grass was enough for the seat track to fail and the seat rotated 90 degrees with my feet up in the air and my head in the baggage compartment! Luckily I had my hand on the throttle when I fell back and was able to fully close it. Unbelievably, the airplane tracked straight ahead and came to a complete stop on the grass runway with the engine idling. It wasn’t so funny then, but looking back it seems so ridiculous!
 
A seatback failure in a Citabria on departure killed a mentor and friend back in the 80's, one of the older guys who was always good to us new kids. Pushed the rear stick full aft, and they stalled/spun.

Maybe that was why I was so pissed when Legend Cub had a rash of broken seats in the early serial numbers, and didn't send service bulletins. This one broke under me while sitting still on the ground.
.
 

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I wonder if the headrest was installed or not, and if it would have helped in this case.

Sounds like great customer service from Classic Aero to offer to replace it at no charge though. One would expect that the previous owner received the repair kit and elected not to install it for some reason.
 
Seat back

...Honestly, I spent a lot of time worrying about the engine, controls, electrical, landing gear, brakes, etc...never even stopped to think that the seat might get me into trouble (I am familiar with the issues in the C172, but the RV seats are fixed in place....)
This thread opened my eyes to a new set of things to keep in mind and check. Like many people, I've had the cessna seat slide back on takeoff, but never considered this kind of issue in my RV-8. Another point for VAF helping to keep the fleet safer.
 
I've been away from the shop the last few days, so I missed this thread when it got started. I just talked to Jeremiah in our shop. He received Tony's email yesterday.

We learned about this problem not long after we changed our RV-8 seats from the earlier chromoly frame design to the aluminum design. This was around 2008. When we learned about it, we contacted all customers who had the affected seats to let them know. Shortly after that, we developed a retrofit kit to reinforce the affected area, and sent it out to customers. Those who were not comfortable performing the retrofit sent the seat back to us, and we installed it for them. It's possible that some customers didn't install the kit, but I thought that we had confirmed this with everyone.

For those who may have purchase a used RV-8, and are wondering if your seat might be affected, let me describe how to inspect your seat. Remove the front seat back from the plane. If you look at the bottom of the seat frame where the piano hinge is visible, you will be able to see the outer edges of the frame. The outer edge is the frame rail. This is the area in question. You don't need to remove the upholstery.

What you are looking for is the cross section shape of the frame rail at the bottom of the frame. It will be either an "L" shape (single 90 degree bend), or a "Z" bend (90 degree flange at the top, and one at the bottom). There will be some foam around the outer edge that you may need to pull back a little bit to get a clear view of the cross section. If you have the retrofit kit installed, there will be an additional piece riveted to the original "L" cross section to form a "Z" cross section.

Also, keep in mind that if your seats were shipped after about mid 2008, they would not be affected. This accounts for the vast majority of our RV-8 Aviator seats.

We did quite a bit of testing on the frames with the retrofit installed. We also tested a stock Vans seat frame for comparison. The testing involved supporting the frame at both ends (simply supported). We tested the Vans frame to failure. We were not able to achieve a precise failure on the retrofit frame using weight though. We balanced something like 1200 lb on it before we ran out of weights that would fit.

One other note is that our RV-7/9/14 seat frames are a completely different design. They use a mandrel bent tube frame that is much stronger than stock, so if you have one of those frames, the are definitely not affected.

I have reached out to Tony. If you think your seat frame may be affected, please let us know. This has been kind of a weird week for my schedule. I will be out today, but Jeremiah or myself will be back in on Monday.

Best regards,
 
Luke,

For those of us who still have the chromoly frame design (2006), were there any retrofits needed? Also, any graphics/drawings/photos of the retrofits would certainly be helpful here. Thanks!

Hi Scott,

No retrofits needed for the chromoly frame. I'll work on getting some graphics or pictures next week.
 
Yes sir--pictures as soon as you can sir. Mine is a 2008 RV 8, and I didn't build it. Pictures worth a thousand words.
 
Luke,
thanks for the great info. I feel better knowing that my Aviator seats were purchased in early 2009.
 
I am the original builder of the RV-8 with the failed seatback.

Tony,
I have the same seats in my current RV-8, Classic Aero Designs purchased in 2018. I can bring a replacement seatback to you today. Are you with your airplane?
Call me 858 229-6389.
 
Yes sir--pictures as soon as you can sir. Mine is a 2008 RV 8, and I didn't build it. Pictures worth a thousand words.

Hi Don,

Here are some screen shots from solidworks. These parts were actually pre-solid modeling days (I drew them up in 2D way back), so I had to kind of make up some mockup 3d parts from old dxf files.

The first picture shows the original design. The second shows the reinforced design, and the last picture shows the updated design.

Note: the reinforcement angle includes rivets in real life, but I didn't know the location off hand, so they are not shown here.
 

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