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Minimax airframe strength  This thread currently has 677 views. Print
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bob.hood
August 7, 2021, 2:41pm Report to Moderator

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Hello all,

I sadly have to report that my 'max has suffered a fairly terminal event. It's a UK built 88 model, which is like a cross between the US 1030 and 1100 models, but with some additions that were mandated by the LAA (Liight Aircraft Association, formerly known as the PFA or Popular Flying Association). As such the plane has a redesigned anchor point for the shoulder straps, heavier teleflex cables all round, and twin elevator teleflexes. It also has the light tail and light wings from the 1030 and the short narrow fuselage (20") as opposed to the later UK 91 model which uses the heavy wing and tail, and has the 23" wide longer fuselage. Because of these changes the 88 model has a VNE of 90mph instead of the 100mph of the later 91 model.

The plane is powered by a Rotax 447 engine that has a single CDI ignition system, and a single coil that fires both spark plugs using the 'wasted spark' idea.

I've attached some recent photos showing the plane as it was prior to Wednesday August 4th. You may notice that it has a sliding canopy, which is both a blessing and a curse in operation, and has some bearing on what happened when I crashed the plane on Wednesday evening.

I was out for a flight around our local area, and had flown around 5 miles away from the field, then turned and was returning to overfly our microlight club field, then I intended to fly around the local city, which is quite small in size. However, as I was travelling along at around 1200 AGL and 55mph, the engine suddenly stopped firing. I didn't know if it was a fuel problem, so started the electric pump (the plane has both vacuum and electric pumps) and hit the starter button repeatedly (it also has an M5 electric starter on it) but the engine didn't fire once. So I headed for our field to try and get back to our runway if I could.

I set the glide speed at around 55mph and tried to glide the mile or so to the runway, as all the fields around our area are currently under a 2ft high wheat crop, including our field (apart from the runway that is). Despite my best efforts I realised I wasn't going to make it over a line of trees to our field, so elected to land in the crop in the next field, and as close as I could get it to the gate in the corner of the field. At around 100ft I turned everything off, then concentrated on keeping the wings level as the aircraft lost height and speed, and tried to flare at around 35 - 40mph when less than 10ft above the crop. The aircraft flared slightly, but then the axle hit the top of the crop and dragged the plane down. The nose hit the ground and flipped the plane upside down, and the tail fin took most of the impact.

I was left hanging in the seat belt and shoulder straps, but soon undid those and lowered myself so that I was lying on my back inside the sliding canopy roof. I then discovered that the builder had made the side windows of the canopy almost crash proof because I couldn't force the panels away from the metal frame. The panels were rivetted to the frame, and I found I didn't have enough room to turn sideways in the cockpit and push out the window. However, my luck was in because I landed around 100ft from a road next to some horse stables and paddocks, and one of the horse owners, a lady called Marcia, came over to see if she could help. When I told her of my predicament she went away and came back with a claw hammer. By this time I'd managed to break the fabric on one side of the canopy, that covered in the side area behind each of the side windows, so she passed me the hammer and I was able to use the claw part to break the rivets and open the rear seam of the window and crawl out.

Apart from some abrasions on both arms and legs, and some incipient bruises, I seem to be alright, although I'm taking things steady for the next few days, till all the bruises come out.

I took some photos on my phone camera, then left the plane there till the next day, when some of our club members helped me remove the wings and tail feathers, then lift the fuselage onto the top of the grass cutter on the back of our club tractor. With this we managed to slowly get all the pieces back to my hangar, and the plane is now sitting there till I decide what to do with it. Both wings suffered fractures of the rear spar, and all four struts were bent. So was the axle, and both suspension legs broke from the impact.

I took more photos as the plane was dismantled, and in one of them you can see what I had to do to the canopy in order to get out. Surprisingly the canopy wasn't badly damaged, and the front screen and frame seem to have been completely untouched, but this may have something to do with the built in roll over bar the builder built into it when he built it back in 1992.

Strong planes Minimaxes, as I can attest to, and even though the front of the plane looks to be a mess, I think that once I've taken off the front fibreglass fairing and removed the broken Ivoprop and spinner, the underlying damage won't be as severe as it first appears. The undercarriage legs will need total replacement, as will the axle and struts, but from what I've seen of the fin and tailplane the structure appears solid. I'll strip all covering off the whole airframe to check, but the main part of the fuselage seems quite solid. The wings though will definitely need either major surgery or replacement before the plane flies again.

As to the engine and what caused it to stop? I strongly suspect that it was my doing. I spent some time on Wednesday afternoon with the dashboard out while I changed one of the gauges, and I strongly suspect that somehow when I put it back in, another wire must have been routed too closely to the back of the mag switch. Flying around for 15 minutes or so allowed the wire to fall against the back of the switch and short out the mag, thus killing the engine. I'll find out for sure when I take the dashboard out again and check all the electrical wiring behind it, but that's my initial suspicion.

As the engine wasn't turning and the prop was stationary when I hit the crop, the crankshaft won't have suffered any shock loading, although I may remove the gearbox and get that checked out. However, once I've given it a good going over I'll see if I can start it again, and I feel pretty certain it'll either show some sparks at the plugs, or be totally dead electrically. Either way I won't be flying behind that engine again. I'm going to re-engine the plane with a water cooled Rotax 462 that has points, because these have 2 coils that can be earthed separately. So if one coil is accidentally earthed the other one will still fire and keep the plane flying a little longer. If I'd had a 462 on the plane when it stopped, it would have just dropped to 1 cylinder and I'd have made it back to our runway without problems, but as it was I now have a very bent aircraft to mend.

Ho Hum, such is life!

Because of the limitations on attachments I'll add some more photos to separate posts below this one.




























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bob.hood
August 7, 2021, 2:46pm Report to Moderator

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Next photos



Attachment: canopy_s_6368.jpg
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Attachment: fuselage_3_4944.jpg
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bob.hood
August 7, 2021, 2:49pm Report to Moderator

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And more photos



Attachment: tailplane_1_s_8529.jpg
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Attachment: recovery_2_s_6084.jpg
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bob.hood
August 7, 2021, 2:53pm Report to Moderator

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And more photos



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Attachment: undercarriage_1a_s_3857.jpg
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Attachment: undercarriage_2a_s_9056.jpg
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Max SSDR
August 7, 2021, 7:59pm Report to Moderator
If it flies, floats or fornicates.... rent it!
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Hell Bob! Glad you are ok! The business of getting out of an inverted 'Max has occupied my mind a lot but without coming to any satisfactory conclusion.
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beragoobruce
August 8, 2021, 12:36am Report to Moderator
Built an Eros - now I'm flying it!
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Hey Bob

So sorry to read of your misfortune. You obviously did everything you could to get her down, but the Law of Sod is hard to beat. Such a shame for your flying machine,but so glad you're ok.

Inverted post arrival was something that always nagged at me. Best solution I came up with was to buy a lightweight emergency windscreen breaking hammer from Fleabay (sold for car escapes). It has a spike at one end, and a storage pouch. Don't know how useful it would be if used on polycarbonate though.

Good luck with reviving your plane. I'm sure she'll fly again.

Bruce
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ITman496
August 8, 2021, 12:44am Report to Moderator

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This makes me realize that having something strong enough to take the plane's weight in front is important.  The windscreen with aluminum hoop I suppose is strong enough, but........


I'm glad you are okay!  I hope you can get her in the air once more!
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Greg Doe
August 8, 2021, 4:30am Report to Moderator
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Good to know you're OK. I did exactly the same "arrival" two years ago, and I think there are pictures on this site somewhere. I spent one night in the hospital for a very slight concussion. My damage was a little less then yours, and I repaired it, and have flown it since. There are many possibilities of what could have caused your engine to stop. Don't fixate on any one thing. Just about every time I've had a engine issue, it has turned out to be something other then my first suspicion. Good luck.
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aeronut
August 8, 2021, 4:17pm Report to Moderator

blue sky and tail winds to everyone
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Glade you are doing fine. The Max did a good job of absorbing the impact. I hop that the rebuild goes well for you.


never surrender; never give-up
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bob.hood
August 8, 2021, 4:27pm Report to Moderator

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Hello all,

I see several of you have also had inverted landings, and I must admit the thought of how to get out if that happened had occupied my mind before it happened to me. However, what I found was that although the polycarb side windows are made of quite thin material, they wouldn't break! Nor would they separate from the pop rivets that attached them to the lightweight but strong tubular frame of the sliding canopy fitted on my plane. I was grateful for the roll over bar built into my plane, but it doesn't seem to have been touched, but then again, it was inside the sliding canopy, so would not have been abraded on impact. I was also very glad of the seat belt and shoulder straps in the plane, as these undoubtedly saved me from serious injury. As it is all I suffered were abrasions to both elbows and shins, and bruises to elbows and shoulders.

What I found when hanging upside down in the straps was that there's so little room to move around inside the cockpit that even after I'd undone the straps and lowered myself down to lie on the roof of the canopy, I couldn't turn sideways and use my legs or feet to pop out the side window, and carrying a glass breaking hammer wouldn't have made any difference for me. It was the use of a claw hammer that allowed me to lever the plastic off the rivets and crawl out from the side of the cockpit. So if you're going to carry anything at all then a small tyre iron or something similar would be much more useful than something designed to shatter glass.

As you can see from the photos, the tail fin took most of the impact when the plane flipped, along with the rear end of the fuselage, and it's obvious from looking at the way the fabric has buckled so much that the paint has split off, that the rear fuselage has suffered some shock damage. I'm going to strip all the fabric off and have a very close look at the whole fuselage, then replace any snapped wood, reglue any separated joints, and add extra gusseting where I think the most strain has been applied.

Oddly, when I got the tail feathers off, I couldn't find anything wrong with them, but I'll be taking the fabric off for a forensic examination all the same.

As for the front of the fuselage, that looks rather sorry for itself, but the engine appears to be still securely bolted in place, so I'll see if I can get it out to assess any damage to the wooden engine mounting plate and/or the Barry mounts that attach it to the fuselage, as well as the firewall area around it.

Regarding the suspension legs. They'll need a complete rebuild from scratch as well as a new axle, and I think all four struts will need to be replaced as well, even though one of the front struts appears to be more or less straight. Sadly all the work I put in to making up the aerodynamic sleeves for the struts seems to have been wasted, but who knows, if I'm careful about removing the bent struts I may find that the sleeves will spring back to more or less straight? Then a bit of careful pulling and tapping might get them back to useable condition, and with some lightweight filler and a fresh coat of paint they could pass for new. Luckily they are not at all structural, but are simply sleeves bolted on at each end of the strut, so they can be unbolted and removed easily.

Anyway, that's the state of play at the moment. I've been in touch with some people here in the UK to see if anyone has a serviceable pair of wings, and I'm hoping that by this time next year the plane will be flying again and looking as good, if not better than it did before the accident.

Cheers,

Bob




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flydog
August 8, 2021, 9:05pm Report to Moderator
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Good job landing it! Kept your cool and did it all right, or maybe you were not too cool at the time? But you did it.

I still just can not picture anyone but a dwarf, or Houdini, being able to fit into the canopy like you say.

Do we still call them dwarfs? Or is the PC term "vertically challenged"?
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bob.hood
August 8, 2021, 9:51pm Report to Moderator

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Flydog,

I'm 5' 6" tall, so I am a bit 'vertically challenged' anyway.

As for getting hot under the collar, I knew that I couldn't avoid landing in the crop, and I knew it would mean the possible end of the plane, so it was just a case of accepting the inevitable and getting on with it.

Many years ago I worked as a motorcycle courier in London for ten long years. During that time I lost count of the number of times car and van drivers did unexpected things and tried to take me off my bike. I quickly learned that panicking gets you nowhere, so you overcome the fear and instead react as quickly as you can to try to mitigate any possible negative outcome. That ten year's worth of experience on motorbikes has stood me in good stead ever since, and now when I realise I'm in a situation I can't avoid, I just do what I can to minimise damage and injury.  

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bob.hood
April 18, 2022, 12:05am Report to Moderator

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Hello all,

Just to let you know that I've given the airframe to someone who's building a 'max. He's already got one airframe, or most of one, so my airframe will give him more options for how to proceed with the build. As such I won't be rebuilding HH, but have now passed it on to a new owner who assured me that parts of it will fly again. Let's hope he's a man of his word?  
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