Update Feb 2010 – Nosewheel spacer mod number is 12265, standard mod for any RV-6A, 7A, 8A or 9A
Latest news 01 Feb
09 – A few months ago a nosewheel RV that had been modified with a new
nosewheel fork came to grief in a landing tip-over accident in
Bill Knott is now shipping nosewheel forks that allow a 5” wheel to be fitted, details at the foot of this article and a link to contact information. Bill was injured in a flying accident last summer but is well on the road to recovery, best wishes to Bill.
A builder in the
As a consequence Gloster Air Parts will not be coordinating any further group buys of nose wheel forks.
23 Jan 08 – It
appears that LAA Engineering are about to recommend to the
Briggs Brothers Engineers in
Introduction. Many people may have seen the shocking
video on the internet of an RV-7A flipping over at a fly-in this summer. Some
of you may also have heard that this is just the latest in a series of such
accidents that call into question the RV nosewheel design. This article is an
attempt to present all of the data about nosewheel RVs and RV nosewheels that I
have been able to find over the last few months. I do have a vested interest in
the subject, I have owned an RV-6A for 5 years and now have about 300 hours
experience in “
Accidents are always traumatic affairs. Even if there are no physical injuries, repairs are often costly and egos get a severe bruising. In this article I have tried to avoid pointing any fingers at any particular individual, and would ask that anyone who has been involved to date takes this in the spirit of increased awareness for all and not in any way of passing judgment on the actions of any particular person.
In early November Van’s Aircraft issued a Service
Bulletin, which was labeled as “Mandatory”, calling for all nosewheel aircraft
to be fitted with an updated design of nosewheel attachment yoke, with an
attendant modification to the nose undercarriage leg, at or before the next
annual inspection. See SB
There is a weight of opinion that says further operation with the original design nosewheel set up is dangerous. One the other hand, some owners have operated for hundreds of hours without incident using the original design. For the average owner it seems difficult to determine what the real situation is, I have tried to present the available information to allow individual owners to decide for themselves what is the best way forward.
There are four sources of reliable data on which to
Figure 1 Old and new nose wheel forks
As of August this year the AAIB’s database listed 8 reportable accidents involving nosewheel RVs in this country that occurred during landing or take-off. Add the accident in the video to make 9. There have also been a few more incidents that have not required to be reported. Of these accidents 3 happened on a hard runway, in 6 the pilot either have less than 4 hours on type or had less than 6 hours in the last 90 days. In some the ground was soft, in others it was rough. There is little data on the nosewheel configuration (which mod standard was fitted, was a spat fitted), or the pilot’s technique, the cg position (and hence the load on the nosewheel), wheel fairing to tyre clearance or the nose wheel tyre pressure. There may be other relevant factors that have yet to come to light.
The NTSB report concludes that as soon as the large
nut (that holds the nose wheel fork to the nose gear leg) contacts the ground
the pilot is almost certain to lose control as the nose down moment from the
nut acting as a plough is greater than the nose up pitching moment from the
elevator – as that is low because of the slow speed (and usually reducing on
The new Van’s design is clearly an improvement on the old as it provides another inch of static clearance (an increase of 25%) between the nut and the ground. Van’s data shows that there have been no accidents with the new design of leg & fork, but the number of hours flown with the new design may be small compared to the original set up. But perhaps the basic unanswered question remains, is the old design an accident waiting to happen?
What are the significant factors? Reviewing all of available data, it appears that there are several factors that influence the outcome of each landing in an RV-xA. It is certainly not true to say that the old nosewheel fork is “an accident waiting to happen”, the situation is far more complex than that.
proficiency and technique. AAIB data shows that in the majority
of accidents the pilot was inexperienced on type, had not flown much in the
previous 90 days or was relatively inexperienced (in half the reported accidents
less than 400 hours total). Van’s letter emphasizes that the nose gear does not
react well to techniques that might be acceptable in the Piper or Cessna that
you learnt in. In particular nose wheel first arrivals are unlikely to have a
happy ending. Hard braking on soft or bumpy surfaces, especially with a forward
c of g, can significantly increase the likelihood of nose undercarriage
failure. Typical crosswind techniques of allowing the nosewheel down onto the
runway at speeds approaching touch down speed may also not be helpful (two
Nosewheel spat. Flight
without the nosewheel spat fitted is specifically discouraged by Van’s and the
Nosewheel bearing configuration. Van’s have offered two different nosewheel axle and bearing support arrangements. The first was offered with the original RV-6A in 1990, and the second version is now supplied with any RV-6A, 7A, 8A or 9A. Figures 2 & 3 below, copies of Van’s drawings, show the differences. Some owners strongly recommend installing a spacer in the later configuration (see figure 4) to control the tension in the bearings and to prevent seizure.
Nose leg damper. In the early RV-6A drawings Van’s shows a wooden damper glassed to the back of the nose leg, that also doubled as a fairing. In the late 1990s a glassfiber fairing was introduced (see Van’s drawing C1 issued in 1999), and the damper was deleted. It has been reported on some on-line newsgroups that Van’s is now advising against the installation of a damper as it changes the bending and resonance characteristics of the nose leg.
The distance of the large nut securing the fork to the nose leg from the ground. As pointed out by the NTSB, when the metal work contacts the ground the aeroplane is very likely to stop abruptly. That may result in damage to the nose leg and the aeroplane tipping over. Any thing that can increase the distance between the ground and the large nut will decrease the likelihood of contact.
Load on the nose leg. As the aircraft becomes heavier, and the cg moves forward (not that those two things happen together), the load on the nose wheel increases. Van’s have published a series of graphs (Reference 1) that give a maximum acceptable load on the nose wheel.
Level of braking. Heavy braking will increase the load on the nosewheel and could reduce the ground clearance. If making a landing where several of the other factors mentioned here are present, such as a forward cg and soft runway, pilots should avoid the use of heavy braking if at all possible. It may be preferable to go-around rather than continue with a landing that requires heavy braking to stop in the runway available.
tyre pressure. Low nosewheel tyre pressure
significantly decreases the ground clearance, however high tyre pressures
increase the likelihood of nosewheel shimmy. Van’s
recommend a pressure of between 25 and 35 psi, RV-6A pilot
Runway surface. Most accidents have occurred on soft and/or bumpy surfaces, but that should not be news to most people! Clearly it is up to the pilot to satisfy himself that the surface he is about to land on is suitable. Long period larger bumps that change the load on the nosewheel appreciably can be as troublesome as smaller ridges. No matter how far the large nut is off the ground, if you drive into a large enough hole or ridge there will be contact between the two.
RV-6A, 7A, 8A or 9A? Are nose wheel versions of each of the Van’s models equally affected? Probably not, its difficult to be certain as the number of A versions in a fleet is not known, but from the NTSB data RV-6s seem least effected while 7s & 9s are more at risk. It may be that RV-8As are most at risk, but as there are no 8As flying in the UK yet that might not be very relevant (the new design of nose wheel fork has been included in all finish kits shipped for the last 2 ½ years so any UK -8As should be fitted with the new set up).
should you do? A
Mandatory Permit Directive to make compl
You might like to also consider the following:
Below are some questions that might be asked along with the best answer that can be given at the moment.
Q. Van’s have made this a mandatory service bulletin, shouldn’t I do this right away?
A. As the LAA/
Q. As this is a Van's SB do I need to get my’inspector to sign it off?
A. Yes, all work that is not on the list of approved owner maintenance items must be signed off by your inspector.
Q. As this SB calls for the gear legs to be modified
at Langair in
A. No, as long as the nose geaI leg is modified as shown on the appropriate drawing (the thread is cut rather than rolled) the work may be carried out anywhere that is acceptable to the inspector signing off the installation.
Q. Do I need to apply for a mod to install the new components called out by the SB?
A. Definitely not! This is a modification specified by the designer (Van’s Aircraft) and as such may be embodied by the owner, and signed off by his inspector, without reference to LAA Engineering.
Q. When I built my aeroplane I modified the nose gear leg/fork/spat slightly and applied for a mod, can I modify the new gear leg/fork/spat to the same mod?
A. Probably no, but talk to your inspector. As the new gear leg & fork have different part numbers to the old items, your original mod submission may have to be updated to call up the new parts. There is no guarantee that the updated mod will be passed by LAA Engineering.
Q. Can I convert my nose wheel RV to a tailwheel?
A. Yes you can, but is it really necessary? The conversion is not cheap as a new engine mount and undercarriage legs are required. The A model fleet have flown hundreds of thousands of hours with only a very few problems. Heed the advice in Van's letter and you should have many more hours of enjoyable flying.
Q. One of our group members consistently lands all our RV-9A with all 3 wheels together, he says that is the way he was taught to fly it. I feel the aircraft should be landed on the main wheels only with the nosewheel held off. Who is right?
A. It is always difficult to arbitrate on a matter of flying style from a written description. Van's letter specifically says that the nose gear is not designed to withstand landing loads and implies that the nose wheel should only be lowered on to the ground after touchdown and initial deceleration. It appears that your group member is not heeding this advice and that the way he was taught was incorrect. I would suspect he would not find many people to endorse his technique on any aircraft type.
1 x Nosewheel Fork, WD-630-1
2 x Brackets (L & R), U-713C L/R
1 x Nosewheel bearing spacer (not available from Van’s)
Gloster Air Parts is no longer coordinating group buys of nosewheel parts.
The spacer is to be
placed in the centre of the nosewheel to maintain proper torque in the
nosewheel bearings. Those “in the know”, such as
To answer some of the further questions that have been asked,
Thanks to Pete Greenslade, DV Godden Engineering in
RV-9A builder Les Clark, who runs Briggs Brothers
Engineers, has built a fixture to machine the additional thread using the same
thread milling techniques as Harmon Lange in the
I suggest owners use the RV Sqn to coordinate batches.
Supply of new nose-gear legs (U-603-3X).
Please deal direct with Van’s for new gear legs. The cost is $194.00, plus shipping, etc. If you would like to ship your old leg to them Van’s will match drill your new leg for a further $58. I’m sure there are engineering companies in this country that could carry out the match drilling, but I have not yet heard of them. Harmon Lange’s website has a useful set of instructions for match drilling legs (http://www.langair.com/matchdrilling1.html). Because the legs are that much heavier than the forks, making the shipping that much more costly.
From the information I have to hand this is the situation as I can determine it.
Early RV-6As were supplied with a thin nose gear leg
(1” at the top as it exits the socket on the engine mount),
Until the mid/late 90s a “thin” nose wheel spat was supplied, after that the “pressure-recovery” spats became standard. I suspect the thin spat will not be able to use the new U-713C attachment brackets.
RV-6As, until the late 90s were supplied with a thick
axle, see drawing below, RV-6/A drawing number 62 dated
These aeroplanes will not be a candidate for using the spacer I described above. Note also that the drawing shows a wooden damper glassed to be back of the gear leg – in lieu of a fairing that was supplied with later kits.
Nose wheel finish kits supplied after February 1999 used a different nose wheel bearing set-up as shown in the drawing below, RV-6A, 7A, 8A, 9A dwg C1 R2 dated 10/10/01 (initial issue 2/16/99).
This drawing shows the axle adapters, U-623-1, or ‘mushrooms’ that support the wheel bearing. The spacer bears on the inner face of the bearing, not on the inner face of the mushroom. Note also that no damper is shown on the nose gear leg. The other thing to note from this drawing is the position of the wheel spat attachment brackets.
The drawing below is taken from the FAQ sheet made available with the SB.
The costs of embodying the change (at the time of writing) are $154 for the new fork, $15x 2 for new brackets and $75 for shortening the nose leg and cutting a new thread (at Langair in OR). The two British companies above can modify nose legs also. All prices are exclusive of shipping and VAT, Langair requires noselegs to be packed in a sturdy wooden box.
Some have asked if they need to submit a mod to make the change. The answer is definitely no! This mod has been designed by the factory and as such can be signed off by your inspector after you comply with the provisions of the SB.
1. Van’s Aircraft Service Letter dated
Van’s Aircraft Service Letter dated
Van’s Aircraft Service Bulletin
4. Nose gear service bulletin FAQs
5. NTSB Structures Study, Case No.: ANC05LA123
6. NTSB Structures Study, Case No.: ANC05LA123 – photos and data table
7. Further NTSB comment
RV-6A builder Bill Knott from
RV-10 nose fork & wheel on 2-seat A models
“I have had several requests that I should post the
details of my nose gear modification to use a RV-10 nose gear, so here goes.
The reason I did the conversion was due to the lack of clearance between the
ground and the nose fork nut as well as the wheel pants. This is not a factor
when flying off of hard surfaces. I got about 1 1/4 inches more clearance after
the modification, I was hoping for more, but it's
better than it was. The gear leg is unchanged. I purchased a new RV 10 fork. It
is exactly one inch taller than the 9a fork (at the main part of the fork where
the gear leg goes through). I took this new fork to a local machine shop and
had them remove one inch from the bottom portion of the fork. Removing one inch
from the bottom rather than the top gives the most ground clearance. The hole
in the fork where the gear leg goes through is larger in diameter so I had the
guys at the machine shop cut me some new oilite
bushings to make things fit. You will also need the part that mounts to the
gear leg just above the fork, the part that the bearing rides against that has
the little ears on it for travel stops, this needs to be bushed to fit the gear
leg as well (you can use one of the original oilite
bushings that came with your -9A fork, it fits perfectly, just trim off the shoulder).
The nut and cup washers are the stock -9a parts. It uses the same size tire as
the main tires used on all of Van's two seat models, 500x5. No change to the
wheels. The stock -9a wheel pant will not fit.
“The mains really weren't necessary. I found a tire that was one inch larger in diameter that would still fit on the stock wheels. In the end I only got 1/2 inch more clearance at the mains. My stock main wheel pants still fit nicely with these larger tires, I only had to trim the opening a little where the tire sticks out of the bottom of the pants.
“The whole reason I did this was that on my very first taxi test with standard size tires and beautiful wheel pants, I hit a soft spot in the grass. Fortunately I was going very slowly, but it damaged my front wheel pant and upon closer inspection I could see that the nut on the bottom of the gear leg had contacted the ground. This could be catastrophic if done at take off or landing speeds. Since I have done the conversion I have not had any clearance problems. And, to be honest, this is the reason I am selling the plane. I have decided to find a tail dragger, something more suited for rough fields. I just hate the thought of hitting a really large soft spot, even with the larger gear, at higher speeds and what it will do to my brand new airplane. I hope this helps answer your questions. Again, in the end, I did not get as much extra clearance as I had hoped for but if it saves me from a collapsed nose gear and all of the damage an incident like that would cause, then it was well worth it.
“I want to make it clear that this modification is untested by Van's aircraft, so, any builder wanting to make this modification is on his own. Any body concerned with tricycle geared airplanes on soft airstrips should consider a taildragger instead. I talked to Van's Aircraft about this today. They made it clear that they do not want builders trying to contact their venders to build a part for them that has not been designed and tested by Van's Aircraft.”
Bill Knott’s nosewheel fork