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Convoluted NTSB Report on Roll Overs

S

skikrazi

Member
The report goes on and on ad nauseam in great technical detail about what may cause forks to dig in and flip over nose-wheel RV's like 6 through 9A's. There are so many pages in great technical detail, yata, yata, yata, that it caused my eyes to glaze over. I still have an imprint of the keyboard on my forehead from dozing off.

It's simple: the flat face of the fork with a big nut sticking below it does not make a good plow. Pictures of furrows created by the fork prior to the plane flipping over prove what causes the accident. I know good pilots who have had this kind of accident. Any gap between the fiberglass front cup on the nosewheel and the fork will allow even a small bounce and impact to crush the fiberglass and thus expose the bottom nut and allow it and then the fork to dig in. You won't plow too far before the strut bends back and you are suddenly on your back. Even if the fork is presently rounded and sets an inch higher, as of February, 2005, it still does not make a good plow. The nut is still exposed and will dig in until the fork catches and also digs in. A rounded, metal skid plate behind the fiberglass cup of the front wheel pant does offer protection--especially if that cup has 3 layers of fiberglass on the inside leading edge of it. This fiberglass layer is IMPORTANT! The 3-layers of fiberglass is rock hard and fits snug against the skid plate and does not allow a gap between the skid plate and nose cap to permit breaking the fiberglass cup. The cup and metal skid become essentially one unit for sliding/bouncing over uneven "stuff". If the nose cap is put on while the 3-layers of added fiberglass is still wet, it forms snug to the skid plate. The new one-inch higher fork may or may not solve the problem. Size does matter, but I'm not sure one more inch will make a difference. The concept is too new to provide statistics for proving one way or the other. I have 485 hours on my RV-7A with over half of that using the skid plate. Don't get me wrong, I plan on doing the "Mandatory" Service Bulletin for insurance and resale reasons; but I'll be also pulling off the "jock strap" and moving it to the new fork for extra insurance because I'm convinced it has saved my RV-7A and my bacon more than once. I enjoy fly-ins to out-of-the-way places like Reklaw where you land on uneven dirt strips. I intend to keep flying into places like that. My fiberglass nose cap was once damaged bad on the front bottom when I hit a 4" concrete lip I did not see while taxiing through high grass. The nose wheel jumped the lip (sounded like a sledge hammer hit) and landed on the concrete apron but the skid saved the day. I have witnesses and a picture to show how lucky I was. I also have pictures of the skid mounted on the fork. If you are interested, email me at [email protected] and I'll send pictures & more info.
 
I'm doing the same thing, but with my all-fiberglass design.

The new fork will have the nut protected with a skidplate. I will also stake my bearings to lower rolling resistance. I've already modified my rudder pedals to reduce brake dragging on landing/taxi.

At least 35 lbs of pressure in the tire as well. Also, flying with 20 lbs of ballast in the baggage compartment (my flight bag and O2 tank works well for this).

Check with me in about 10 years, I'll let you know if it works.

V
 
What Causes the nut to hit the ground?

skikrazi said:
The report goes on and on ad nauseam in great technical detail about what may cause forks to dig in and flip over nose-wheel RV's like 6 through 9A's. There are so many pages in great technical detail, yata, yata, yata, that it caused my eyes to glaze over. I still have an imprint of the keyboard on my forehead from dozing off.

It's simple: the flat face of the fork with a big nut sticking below it does not make a good plow. Pictures of furrows created by the fork prior to the plane flipping over prove what causes the accident.
Click to expand...

The part of the report that you found so boring is what caused the nut to hit the ground in the first place. First things first. Trying to keep the nut from hitting the ground is the first priority. Keep the pressure in the tire up, try to stay out of a rute, stay off the brakes, keep the weight on the nose gear light, raise the nut higher, keep the nose gear in the air as long as possible are all good things for that.

If that proves unsuccesfull then the skid plate may or may not solve the problem. You seem to have the first documented evidence that it does work.
 
plehrke said:
The part of the report that you found so boring is what caused the nut to hit the ground in the first place. First things first. Trying to keep the nut from hitting the ground is the first priority. Keep the pressure in the tire up, try to stay out of a rute, stay off the brakes, keep the weight on the nose gear light, raise the nut higher, keep the nose gear in the air as long as possible are all good things for that.

If that proves unsuccesfull then the skid plate may or may not solve the problem. You seem to have the first documented evidence that it does work.
Click to expand...

"The nose gear tire pressure is not known for any of the accidents or incidents cited in this study" (quoted from the report). My pressure is kept at28 to 30 psi.

I know it is probably necessary in a NTSB report, but it was all the technical detail that bored me--stuff like this:

"The weight on the nose gear at empty weight is about 370 pounds. At 30 pounds per square inch (psi) tire pressure, a tire footprint of about 12.3 square inches is required. The tire is about 3.7 inches wide at the tread, thus the flat spot on the tire would be about 3.3 inches when viewed from the side. At a static condition, the strut would now be about 3.7 inches from the ground. At twice the static load (740 pounds), the flat spot would be about 6.7 inches when viewed from the side. Because of the tire radius, the compression is significantly greater to obtain twice the tire footprint. In
this case, the strut would be 2.9 inches from the ground. At three times the load (1,110 pounds), the ground clearance would be 1.4 inches. At 20 psi, each footprint would be 50% greater for each load factor. Starting at a load
factor of one, the strut to ground clearance is 3.4 inches. At twice the load factor the ground clearance is reduced to only 0.8 inches, by calculation.
The nose gear tire pressure is not known for any of the accidents or incidents cited in this study. In addition, the bottom of the nose wheel is below the bottom edge of the strut and fork and would provide some resistance to ground penetration."


Nuts hitting the ground were caused by many things, e.g., landing on the nose wheel, not enough clearance between nut and ground, rough runways/taxiways, etc., and some were not pilot error. I personally am not convinced, until more time goes by, that one more inch of clearance will avoid another nut strike against the fiberglass cone. A strike will let the flimsy cone punch inward and allow the nut to dig in and then cause the fork to dig in--another accident--possible even with the newer rounded fork! Stick back pressures may still be inadequate for TO & landings. Hard landings will still be made. Many Rv's will still land on less than smooth runways. So, after seeing my friend's RV upside down at the 2005 Reklaw flyin, because he is such a good pilot, I began looking for something that would protect the nut on the fork as well as the nut behind the control stick (me). Van's solution was to raise the nut up from the ground by one inch. I plan on doing that, but because of my experience when I was taxiing and did not see a 4" concrete lip due to overgrown grass--and then jumped the lip with my skid behind a fiberglassed-reinforced nosecone, I'm betting on the skid and hardened nosecone to save me again. Time will tell because I still plan to make many more landings on rough runways at Texas flyins and at dirt/shell airstrips in Baja, Mexico.

Cheers,
Chuck
 
I found the report to take leap in logic

It went A, B , C and conclusion was Z. The conclusion was not supported by more than opinion. It was not a neutral conclusion but Pro or a positive one, which I don't think can be drawn.

The engineering and factual data I felt was fair, which did point to somewhat significant limitations in the design, for example if tire pressure is not proper or ground is uneven the gear can dig in. How can they conclude its OK. Yea its an OK design, with some real limitations. Besides, its experimental and what is the criteria? For a fast homebuilt it's acceptable. For a bush plane or factory plane, built to Part 23, may be not.

Nothing wrong with limitations or sacrificing iron work gear for speed, right. As Clint Eastwood said in the movies: "A mans got to know his limitations."

A talented home builder could make a more stout nose gear/larger tire design even less susceptible to digging in. Of course the no free lunch rule is always in effect; it will be heavier, more draggy and not as attractive.
 
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skikrazi said:
I personally am not convinced, until more time goes by, that one more inch of clearance will avoid another nut strike against the fiberglass cone. A strike will let the flimsy cone punch inward and allow the nut to dig in and then cause the fork to dig in--another accident--possible even with the newer rounded fork! ....... Van's solution was to raise the nut up from the ground by one inch. I plan on doing that, but because of my experience when I was taxiing and did not see a 4" concrete lip due to overgrown grass--and then jumped the lip with my skid behind a fiberglassed-reinforced nosecone, I'm betting on the skid and hardened nosecone to save me again. Time will tell because I still plan to make many more landings on rough runways at Texas flyins and at dirt/shell airstrips in Baja, Mexico.
Click to expand...


I agree only time will tell if the 1" additional clearance actually has an effect. I also believe that we need to prevent the the nut from hitting any way we can and the skid may be the best answer overall.

Sorry, must be the engineer in me, but all data that could be relevant is useful and I found that the data in the NTSB report useful. More usefull than just my speculations. Conclusions are usually supported by data otherwise they are only opinions. The more data the better the conclusion.
 
plehrke said:
I agree only time will tell if the 1" additional clearance actually has an effect. I also believe that we need to prevent the the nut from hitting any way we can and the skid may be the best answer overall.

Sorry, must be the engineer in me, but all data that could be relevant is useful and I found that the data in the NTSB report useful. More usefull than just my speculations. Conclusions are usually supported by data otherwise they are only opinions. The more data the better the conclusion.
Click to expand...

All data is indeed relevant, and should be examined closely especially when it comes to safety. A nice part about building an experimental is that you are free to agree or disagree with the data and conclusions presented in a NTSB report. Van's Aircraft calls the service bulletin "mandatory" but it is up to each builder to comply with it...or not. I'll comply, mostly because of insurance and re-sell concerns, but I'll bet a Crispy Cream donut there are many who will not. I'll also transfer my skid and reinforced nose cone to the new fork.
 
It is really Krispy Kreme

I found this at an airport in the southeast last week. Apparently the landing was not ideal and the nose gear itself bent a lot. Note that the wheel is raised off the ground.

6ANoseGearSmall.jpg


Reference the flip at some airfield mentioned, did anyone look at the ground at the flip point to see if the nose gear would have dropped enough to allow the nut to catch on the ground?
 
When I first saw this picture and saw the new style wheel pant bracket, I thought it was the new fork. I would have been somewhat upset if it was, having just received my new fork and sent my old leg in to Van's for drilling of a new one.

I think the new fork will help, but I am certain someone will eventually fold that up too. I am also convinced that flying without the nosewheel pant on is a bad idea. And the reinforcing of the pant and/or skid is certainly helpful.

JMHO

Roberta
 
Picture analysis

I don't have the details on the incident but my GUESS is that there was a huge downward force on the nose wheel which caused the gear to bend. The fork was not a factor and the runway was paved.
 
NTSB Reports

Having retired from the FAA, I have read NTSB reports for probably 20 years and they are all long, complex and mind numbing BUT there is good data there if you can dig it out of all the superflous detail. While I do not have an "A" model I did find the report interesting as it did point to a design problem which may or may not be solved by the new modification. I did find it strange that this is the first NTSB report I know of based upon an experimental design. I don't know what this means to the experimental owner in the future because we have always operated below NTSB radar in the past. To dismiss this report due to the mind numbing size of the data would be a mistake. Read what you can and use the data to improve your airplane if you feel the data applies to you. We are "the" manufacturer of our aircraft...true, Van's supplied the basic design and materials kit, but according to law we are the manufacturer so we have to act accordingly by accepting or rejecting details such as this report. Choose wisely as your project or your neck is on the line here.

Dick DeCramer
N500DD RV6
Northfield, MN
 
It is Really Krispy Kreme

Ignoring whether the pictured aircraft had trouble on grass or pavement (there's a lot of mud and grass shown), the photo illustrates a common misunderstanding. The nut or threaded tube are unlikely to dig in. By the time things are going horribly wrong, they are pointed rearward and should simply skip over the ground. The two large Belleville washers above the nut, however, are positioned to form an effective plow, dig in, and trip the airplane. Since the washers are an essential part of the design, Van's move to raise them seems logical. In a normal static condition, the added percentage clearance seems minor. Under accident conditions (high dynamic load, extreme gear deflection, possibly low tire pressure) one inch could make a huge difference. For those still worried, a "jock strap" could add a smooth, non-digging front surface. The foregoing is just an old farm boy's observation intended to provoke more thought--now back to plowing the back forty.
 
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