Prop Strike Damage Assessment
Posted: Fri Feb 09, 2024 11:10 am
by Twosblind
Hello,
I am considering buying a used Legacy Sonex that was involved in a prop strike incident after a botched landing and was looking for opinions on the extent of damage to the airframe. I fully realize this is impossible to truly assess via the internet but any guidance would be greatly appreciated.
The aircraft has an aerovee engine that is probably a total loss (I plan to convert to a Jabiru anyway) and the engine mount appears to have suffered a slight tweak which doesn’t concern me since I will be getting a new one anyway. What does concern me is where the engine mount attaches to the airframe. The top two aluminum angle iron supports appear to be bent. The bend does not appear to be serious but still concerning. Pictures of this are attached. Worst case scenario, are these easy to replace?
Secondarily, one gear collapsed during the landing enough to strike the left wing of the aircraft. There is very light damage to the sheet metal on the wing and one rib has a slight crease from the hit (pictures attached). This is something that doesn’t concern me much but I wanted to get opinions on whether a rib replacement is warranted.
Overall, the aircraft appears to have excellent build quality and the seller is very upfront about the issues that need addressing. An airworthiness certificate was issued in 2010 and the aircraft flew from then up until the accident in late 2021. Thank you in advance for your help!
Re: Prop Strike Damage Assessment
Posted: Fri Feb 09, 2024 11:29 am
by lgsievila
Hard to really tell from the pics but what I see would cause me to pass on this airframe. While I built a Onex, not a Sonex, I believe you might be able to save the wings but you probably would have to rebuild the entire front half of the fuselage. Think you would be better off trying to find an unfinished partially built kit IMHO
Re: Prop Strike Damage Assessment
Posted: Fri Feb 09, 2024 12:02 pm
by Bryan Cotton
My assessment is different from Loren’s. I don’t have my plans in front of me but I believe those angles are held in by the phillips head #10 screws on the corners. If you were replacing the motor mount anyway then it would be good to change those parts, because you could match drill them starting with a #40 bit and working your way up as opposed to match drilling a 3/16" hole. The wing damage seems minimal and the effort to replace the rib seems like it is not worth it. I am sure the Aerovee would be good for parts at least.
Edit: during the build you need to use shims between the aluminum angles and the mount. I would ask the builder if those bends were from the accident or did they try and bend the angles to match the mount better.
Re: Prop Strike Damage Assessment
Posted: Fri Feb 09, 2024 12:22 pm
by Area 51%
Looks like a bit of a stretch to call this a “prop strike”.
My money is on a forward fuselage rebuild. I have an unstarted Sonex fuselage kit for sale cheap if that helps your decision making. .
Re: Prop Strike Damage Assessment
Posted: Fri Feb 09, 2024 12:57 pm
by daleandee
Not much to add to what’s already been given. If the price is right it should be a great deal. You really won’t know how much you have to do until you get the firewall off the front. The firewall itself may be reusable. I don’t have plans in front of me either but I believe that Bryan is correct as to the attachment of the engine mount supports.
Did you check to see if the front of the fuse is still square? I also noticed that one gear leg appears to have been heated (I could be wrong) but if so was that done after the “prop strike” or was that from a previous hard landing? Can’t tell much about the wings but the injury to them seems minimal.
First thing I might do is send these over to Sonex to get their take on it …
Dale
3.0 Corvair/Tailwheel
Re: Prop Strike Damage Assessment
Posted: Fri Feb 09, 2024 2:13 pm
by kmacht
One other thing to consider since it’s an A model Sonex is if it was scratch built or kit built. You may find that if it was scratch built or an earlier kit without matched holes that you are on your own to fabricate any replacement parts. The ones you buy from the factory may not line up if the builder wasn’t extremely precise in laying out and drilling the holes. If it were me the price would have to be very very low to consider buying as you will be either reworking or replacing everything in the forward fuselage. Have you put a straight edge down the row of rivets on the wing skin to make sure the wings are straight? Inspected the spar tunnel for any sign of movement? The fact that there was some wing damage and the twisted motor mount says this was probably more than just a simple prop strike event. Like most accidents, you tend to find more damage than just the obvious once you start the repairs.
Re: Prop Strike Damage Assessment
Posted: Fri Feb 09, 2024 4:43 pm
by BRS
Buy the airframe and convert it to a -B model. Damage eliminated.
Re: Prop Strike Damage Assessment
Posted: Fri Feb 09, 2024 5:06 pm
by Kai
Oops!
If there is any doubt, there is no doubt! Either pass it up or rebuild the front to a B-model. As stated above: problem gone.
Kai
Sonex A #0525 SG TD
120HP EP915 ECI
680 hrs
Re: Prop Strike Damage Assessment
Posted: Fri Feb 09, 2024 8:50 pm
by Twosblind
Thank you all for the insight. I definitely agree it’s worth pulling the firewall and inspecting everything the load could have transferred to. I feel for the price of the airplane it’s probably worth doing. The airplane is kit built with all rivet and screw lines straight as an arrow so I’m not too concerned about replacement part fitment.
As Bryan pointed out, I also noticed when I was looking it over the angle aluminum that attaches to the motor mounts that they appear to be attached by machine screws where everything around them is riveted. This leads me to think that they may be reasonably simple to replace. I also noticed the shims and wondered if the bend was intentional. I don’t believe they are though since you can kind of see where the two parts rubbed together prior to some kind of impact. Also the upper machine screw appears to be pulled into the countersink a little bit.
I haven’t considered the B model front end conversion, that’s an interesting idea. I have also emailed Sonex directly to see what their assessment of this is. Thanks again for all the input and I’ll keep this thread updated with my decision!
Re: Prop Strike Damage Assessment
Posted: Sat Feb 10, 2024 10:00 am
by lgsievila
Bryan, Twosblind,
As an EAA Tech Rep, it is what you cannot see is what concerns me. There are a whole lot of rivets that will have to be removed and that is a big task by itself. And then re-riveting will probably require rivets bigger than 1/8 " in many cases in order to have a proper fit. If that firewall is tweaked after reconstruction and you don’t see that, you wont find out until you hang the engine/prop and your prop tracking is way out of spec.
Fitting a “B” model on the front would probably work provided the wings can be used. Before I started this project I would try to determine if your rebuild is enough to qualify you for a Repairman ticket so you could do your own annual inspections. I just see this as a project full of unknowns and frustrations. IMHO, it is a lot more fun to build than fix a wreck.
Re: Prop Strike Damage Assessment
Posted: Sat Feb 10, 2024 10:06 am
by Scott Todd
Try to be objective and look at things from an engineering perspective. First of all, aluminum airplanes are VERY fragile when loaded in directions they are not intended for. It doesn’t take much to really wrinkle one with a simple ground touch. For example, the wing rib dent could almost happen from someone bumping into it on a walkaround. I’ve seen it happen. This airplane is not showing signs of significant damage. Contrary to urban legend, it wont just fly apart in the air because something is slightly bent. Of course it should be fixed but ‘hidden’ damage is really hard to miss on aluminum airplanes.
I must not be looking at the same pictures as everyone else. I don’t see why the entire front end has to be re-built because a motor mount is bent. That those parts off and further inspect. I suspect the material around there is in good shape and a new angle bracket can be installed.
The gear leg may have been straightened. So what? Our airplanes are designed to fly, not crash. They are not tolerant of hard landings. Pound it and bend it. Get some proper training and learn how to land. My students don’t pound airplanes. But many do…The quality of current flight instruction could be a new topic for another day.
The tip wing rib is nearly a non-issue. Look at the loads out there. Theoretically they go to zero at the tip. The tip is NOT supporting the entire weight of the airplane. removing the rib and bending it all back to original shape is probably fine. Again, NO loads out there…
The upper machine screws were not pulled into the angle. The countersinks were deep. The loads required to deform the aluminum by ‘pulling’ a screw head into it are unimaginable. Unless the countersink was WAY too deep, stuff would bend and break before that deformation could be seen. Looking at the backside will tell but I suspect it wasn’t ‘pulled’ in. Put one in a press and try it.
I’ve also worked on and flown Sonex’s that were far more damaged than this one from hard landings and off-field landings. But I’ve also seen pilots that see a small dent in an airplane and deem it complete scrap and would NEVER get in it. Dale mentions to check the squareness. I think it would be pretty hard to knock one out of square without a lot more visible damage but its a good starting point.
Re: Prop Strike Damage Assessment
Posted: Sat Feb 10, 2024 10:12 am
by Scott Todd
I don’t see how a ‘tweaked’ firewall can cause prop tracking issues. Prop tracking issues come from bent engine parts and damaged propellers. Wood props are REALLY hard to get out of track without bent turning engine parts.
I don’t see where a whole lot of rivets need to be removed to replace the angle brackets. And if so, fine. Its pretty easy to drill out rivets once and put back the same size. More than once becomes an issue but this one isn’t there yet.
The FAA will probably never issue a Repairman certificate for a re-build. Who cares. There are plenty of A&P’s in your local EAA chapter that will be glad to help.
I think repairing and bringing something back to life can be just as rewarding, if not more, than putting new parts together. Lots of people restore airplanes instead of building new ones 
FWIW, I’m an EAA Tech Counselor, Flight Adviser, current CFI, and retired Mechanical/Aerospace Engineer. My Onex was my third completed airplane (Kitfox and scratch built Biplane). I also restored/flew a crashed Sonex and helped re-build another one with serious landing damage. I watched an A&P friend rebuild one that was flipped over in a storm. All three of those were WAY worse than the one in question.
Re: Prop Strike Damage Assessment
Posted: Sat Feb 10, 2024 11:12 am
by Area 51%
Scott Todd wrote:The upper machine screws were not pulled into the angle. The countersinks were deep.
I partially disagree. There is evidence the screws have been pulled into the holes rather than simply deep countersinks. As you stated, it would require a monumental amount of force to push/pull those fasteners through the angles. That would be the case if trying to drive the screw straight through the hole. However, the screw head is no longer parallel to the angle’s surface, but has been pulled on an angle causing the head to depress at the rear, leaving witness marks in it’s wake, and become “proud” in front.
Re: Prop Strike Damage Assessment
Posted: Sun Feb 11, 2024 9:52 am
by Scott Todd
That’s a good catch. I can now see how the screw has been moved or angled to one side. I didn’t study the photo as well on the first pass. I jumped the gun trying to dispel a myth. The deformation casing the two parts to move against each other and dragging the screw is totally plausible. The screw being ‘pulled’ into the countersink and deforming the aluminum is not as much.
I did a Tech Counselor visit on an RV project recently. It was acquired from a deceased friend. The new Owners inspection revealed a rivet with an deformed shop head in the middle of the vertical fin. He was told by various sources it was un-airworthy and the skin would have to be removed. He was also told he couldn’t drill out all those rivets and re-use the skin. He was asking for advice on buying a laser cut skin, making one from scratch, or possibly using a pulled rivet to replace it. He was also told a pulled rivet wouldn’t be strong enough to replace the solid one. All for one deformed rivet in the middle of the fin. I think we convinced him to leave it alone. He is worried it will catastrophically fail in flight causing his airplane to disintegrate. His words. These myths and well intended, but misguided, opinions are killing GA.
Re: Prop Strike Damage Assessment
Posted: Sun Feb 11, 2024 11:44 am
by Twosblind
Loren, thank you for that insight. Looking into the B model front end conversion a little further gave me a little bit of sticker shock. It seems like with the price of the aircraft as-is and the B model conversion I would be in the neighborhood of just buying a flying Sonex with a Jabiru already installed. I like a project but I definitely have to weigh this one.
Scott, I tend to agree that from what I see on this aircraft the damage is not extensive and likely doesn’t go beyond the L brackets that the engine mount attaches to. In a previous life I was a mechanical engineer (now just a braindead stick jockey) and thinking about the load transfer that was likely absorbed through first the gear, then the prop, then the crank, then the engine mount and finally (hopefully) the L brackets and machine screws; it’s difficult to imagine there was enough left over to deform the airframe itself. I could be proven completely wrong on this upon further inspection but I think it’s worth the look.
I also agree that a well intentioned but overabundance of caution is hurting the hobby of GA overall. Some of the repairs made to supersonic jets with 15k hours on the airframe would draw the ire of many GA pilots at your local grass strip and yet those jets keep on just fine. Definitely good to be careful but we aren’t talking about the space shuttle here either.
Thanks again for everyone’s responses on both side of this debate. Still waiting to hear back from Sonex on this. I’m not sure what they’ll be able to tell me from a liability standpoint but I’m definitely interested and will keep this thread updated.
Re: Prop Strike Damage Assessment
Posted: Mon Feb 12, 2024 10:29 am
by lgsievila
Scott Todd, you are correct. What I meant to say was thrust centerline could be affected by a tweaked firewall which may or may not be a big deal. I still wonder how the wing shows damage from a prop strike and how much heat was applied to the landing gear. Really hard to be sure about anything by only seeing pictures.
Re: Prop Strike Damage Assessment
Posted: Tue Feb 13, 2024 11:19 am
by Twosblind
So the wing was damaged when the left gear struck it as the left strut collapsed. The tire impacted the bottom of the wing. The strut was removed from the airplane and a torch was used to straighten the strut tube before it was reinstalled to allow for moving the airplane around post crash. Obviously the strut tubes are writeoffs but the heat was at least applied while the strut was removed.
Re: Prop Strike Damage Assessment
Posted: Tue Feb 13, 2024 12:10 pm
by Bryan Cotton
I am not sure I like the Sonex gear alignment procedure. If I was replacing gear legs I would probably figure out a way to set the alignment of the axle mounts with weight on the gear. You have a golden opportunity here.
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