Crashing a Belite Airplane

Crashing a Belite Airplane

(c) 2014 by James Wiebe

Chapter 1:  THE EVENT

An Observer saw it happen, and commented as follows:  “The plane had entered a departure stall, then spun into the ground.  When the Belite hit the ground, it sounded like a grenade went off.”

The Observer had past military experience, and brought his remembrance of ugly scenes into his thoughts:  “I ran up to the point of impact, expecting to see a bloody body.  Instead, the pilot was walking around the crashed Belite, looking at the wrecked airplane.”

The Belite UltraCub had just had its first major accident.  I’d received the call from the Observer (who happened to be the owner of this particular Belite, and our customer), and the very next day I was having lunch with him, along with the pilot who’d survived the stall / spin / smash accident.

I’d never heard of someone surviving a stall / spin, much less surviving without hardly any damage to their own body.

We’ll get to the pilot’s story in a few paragraphs.  In the meantime, the pictures of the wreckage tells a fascinating tale of impact energy, dissipated by a cabin constructed of aluminum, with impact loads absorbed by the Belite Ultracub’s lightweight and strong boxed aluminum structure.

Here’s a view of the accident site, exactly as it was:

A wrecked Belite, after a stall / spin into terra firma. The pilot survived with a scratch and two bruises.

Chapter 2:  THE AIRCRAFT’S STORY

According to the Observer, the aircraft hit the ground at an impact angle of around 45 degrees.  The impact caused the following damage:

• The carbon fiber composite propeller was destroyed.  Well, of course…
• The single cylinder Hirth F33 engine was severely damaged.  As the engine was mounted cylinder down, the muffler and cylinder head sustained damage.  The redrive was not damaged.
• The motor mount was destroyed, bent downwards by the impact to the engine lower side.
• The front cabin angle aluminums were severely bent.  These angle parts are supported by gussets.  OK, a lot of stuff was severely bent!
• The wings appeared largely undamaged.  (One wing which had hit a wingtip had subtle but substantial internal damage – the sail / anti-sail tubes were either bent in compression or snapped in tension; the main spar and false rib spar had a subtle bend.  The other wing had one sail / anti-sail mount fitting sheared off from the spar.  You can see the subtle bend in the front wing spar in the above photo.)
• The impact of the wings caused their AN-5 attachment bolts to bend – a shear stress load which was many, many thousands of pounds.  The top cabin cross connection tubes had deformation damage around the attachment bolts.  These cross connection tubes are constructed from 6061T6 1” square aluminum, with a wall thickness of .063.  Their partial deformation indicates that the shear load at the bolts (and the consequent deformation of the aluminum) was approximately 9000 pounds.
• The landing gear had bent backwards and upwards, folding into the underside of the cabin and tucking upwards towards the pilot’s bottom.  (But the landing gear and wheels never reached the pilot’s lower torso).
• The rear fuselage had a couple of bends in the aluminum longerons, immediately below the tail feathers.  These compressive bends were caused by forward impact G forces of the tail feathers, transferring into the rear fuselage.  The G force required to bend the 2024 aluminum alloy extrusions is difficult for to calculate, (because of Euler’s buckling principles), but I’ll take a stab at it – a force of around 1000 pounds was required to bend these longerons.  So perhaps the momentary G forces on this area of the structure were around 50 G’s, based on a tail feather weight of 20 pounds.  Truly this is guess-timation.

Here’s a look through the top of the cabin, taken directly downward at the pilot’s seat:

The wrecked cabin of a Belite UltraCub aircraft.

The Belite UltraCub’s cabin is composed of two aluminum side boxes and four cross boxes.  The side boxes run along the sides of the cabin, and they absorbed crash loads from the front of the cabin heading backwards; the four cross boxes under the pilot absorb loads coming up from the ground and protect the bottom part of the pilot’s torso.  Three of these undercabin cross boxes are partially visible in the above photo.  The front cross box is clearly visible, just above the control stick, with two lightening round holes of about 3” diameter.  These boxes are constructed from square tubes with very thin walls and from a front side and a backside continuous aluminum frame/gusset, which spans the entire length of the tube.

Note that the pilot’s seat is bent but intact.  Not visible below the pilot’s seat is what I call a “butt plate”, which is an aluminum plate with angle reinforcements below and extra gussets.  It did its job very, very well.  It connects to two of the cross boxes, and the pilot seat sits on top of the butt plate.

The four point safety harness did its job.  It did not break.  In the event that it had failed, the pilot’s head and torso are likely to have impacted forward, with severe consequences.

Our cheap $12 fuel tank from the home improvement store held together without any problems.

Fuel tank in a wrecked Belite ultralight airplane

Chapter 3:  THE PILOT’S STORY

Now, back to the pilot’s side of the story: he had a small round scrape (about the size of a large coin) and a couple of bruises.  He was fine, but in some sort of reasonable shock as to what had just happened.  The Belite UltraCub cabin had crushed, like an accordion, collapsing around the pilot.  No broken body parts, no lacerations.

The pilot’s story:

“I’m a multi-thousand hour commercial pilot.  I have a lot of flying time in general aviation aircraft, and I fly for a living.  I’m embarrassed by what happened, so I’ve chosen to remain Anonymous.  I’ve also chosen to share this story with others, so that the pilot community can learn from what happened.

“I’d never flown an ultralight aircraft before, much less one with just 28HP, but I was eager to try it.  Although I’d been warned not to over pitch the initial climb, that is, in fact, what I did.  I climbed through ground effect, then it felt like I lost control of the aircraft, but what was really happening was a departure stall, which quickly turned into a spin.  When I realized what was happening, I slammed the stick forward and recovered just as the aircraft impacted the ground.  Of course it was too late.  A few seconds later, I was out of the aircraft, walking around the wreck, and pondering what had just happened.

“In hind sight, it’s so obvious:  allow the aircraft to gain speed before climbing out of ground effect.  Listen to what I was briefed on.  Perhaps spend more time acquainting myself with the aircraft in ground effect hops.

“The next time I fly this aircraft–and I will fly it again after it is repaired and rebuilt–I will note the indicated airspeed at lift off and keep a healthy margin of additional speed until I flare to land!

“I am amazed that I came through this essentially unharmed.   What a great job Belite has done making this aircraft safe.  One might say the Belite people protected me from my own mistakes.

Chapter 4:  EPILOGUE

The aircraft is currently being rebuilt.  As stated, the rear fuselage had a minor bend in the lower longerons, which we easily repaired.  One wing is being replaced, the other wing is being repaired and the cabin is being replaced.  One tail feather had a minor bend in the Chromalloy steel, also easily repaired.  The engine has been repaired, and is awaiting installation.  A new Ivoprop composite propeller will be on the rebuilt aircraft.  As of this writing, the first flight of the rebuilt aircraft is scheduled for late April.

I can’t express strongly enough how pleased I am with the crash capability of the cabin.  Everything in the cabin structure ‘gave up’ under mostly compressive forces in a way which unwound energy from the impact, without causing any appreciable damage to the occupant.

If I wanted to add more strength to the cabin, I would add it on the side boxes, but this would be at the expense of weight, and FAR Part 103 is always a balancing act between the mandate for low aircraft weight and safety.  According to the law, it’s more important that the aircraft be light than it be strong.  Such is the oddity of Part 103.  Part 103 is not a safety standard, it is a weight standard.  Sigh.

Another concern is lacerations caused by sheet metal.  A possible solution is the use of rubber slip-on edging, especially around the interior pieces of the cockpit.

I’m aware of a lot of aircraft accidents.  I’ve lost a Belite customer to an aircraft accident – but not in a Belite.  He had a departure stall in a Rotax powered aircraft of another experimental design.  We’ll never know the exact details, but things went very poorly for him.  So I know how these kinds of takeoff accidents often end – poorly.  I’ve also heard of a new owner who crashed his Kitfox Lite on his first departure.  He did not survive.  That was with an engine significantly larger than our F33 28HP Hirth, but also at a higher field elevation.  The original KFL looks a lot the same, but uses a different wing, (smaller), different flaperons (smaller), different fuselage (steel instead of our aluminum), and other differences as well.

I have developed very strong personal biases towards my aircraft design.  We’ve lost sales of Belite Aircraft over perceived differences in design philosophies vs. competition, and this maddens me.  I’m aware of what happens when a non-enclosed light or ultralight aircraft impacts the ground, with the pilot’s feet literally hitting the ground before the aircraft structure.  It rarely ends well for the pilot.

I’m also confident that some percentage of the readers of this article will think I’m wired a little oddly for publishing this article.  Yes, I am.  Ultralight aircraft accidents are never investigated by the FAA, and pilots rarely ‘fess up to how that stall-spin accident actually occurred.  This is a unique opportunity to see what happened to the airplane, and hear what we can learn from the pilot’s story.  Let’s do that.

It takes a little bit of bravery to publish this story.

I haven’t hid the fact that this event happened:  We’ve had prospective customers come to our facility and see the wreck, and then order our assembled aircraft or kit.

If you have any questions about this, please send them to info AT beliteaircraft.com

Here are several more photos of the wreck:

Wrecked engine, mount and prop on a Belite UltraCub

Cabin view inside a wrecked Belite ultralight airplane

Gear folded up under Belite UltraCub ultralight wreck

Bent rear fuselage aluminum angle longerons

Cross tube on top of cabin showing deformation

Wing attachment bolt, AN5, showing bend. 9000 pounds of shear?