GLOSTER AIR PARTS

 

 

Nosewheel Thoughts

 

Contents

Introduction

Old & New Nosewheel Forks

What are the significant factors?

So what should you do?

FAQs

Rethreading nose gear legs

New Nosewheel Forks Group Buy to save shipping costs

Standards of fork and nose wheel spacers

References & Links

Other Developments

 

 

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 England. I believe this has persuaded LAA Engineering that a Mandatory Permit Directive is not appropriate – why mandate something that has been shown not to solve the problem? I would still strongly recommend that any new-build aeroplane does incorporate a new style nose wheel fork (or Bill Fisher’s fork for a 5” nosewheel, or an RV-10 fork).

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 US has fitted an RV-10 nosewheel fork to an RV-7A (with a shortened nose leg), to allow a 5” nosewheel to be fitted. This has required some machining of the fork, and the fitting of a collar as the RV-10 noseleg is a larger diameter – interestingly less machining would have been required on an old “long” lose leg.

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 CAA that a Mandatory Permit Directive should be issued to cover the Van’s SB.

Briggs Brothers Engineers in Redditch also offer nose leg re-threading service, see foot of article.

 

 

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 “Casper”.

 

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 07-11-09 available on Van’s website (links to all relevant documentation at the end). The new design of nosewheel fork has been shipped with kits delivered since February 2005.

 

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 draw, PFA Type Certification Data Sheets, AAIB reports, Van’s Aircraft documentation and a report published by the NTSB in the USA looking into accidents to nosewheel RVs. So that everyone knows what we are talking about the photos below (courtesy Andy Karmy) show the old and new designs of nosewheel fork. Note the large nut that attaches the nosewheel yoke. All of the metal work is usually covered by the nosewheel spat and the undercarriage leg is covered with a streamlined fairing.

 

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 landing). The US has seen more accidents where the aircraft has tipped right over (in the UK only two aircraft have actually flipped although one or two others have come quite close). The report finds that low tyre pressure and a heavy load on the nose wheel (so all up weight and cg position) reduce the static clearance between the nut and the ground and so increase the chances of contact.

 

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.

 

Pilot 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 UK accidents occurred on hard runways), this factor may limit the crosswinds in which some pilots feel comfortable.

 

Nosewheel spat. Flight without the nosewheel spat fitted is specifically discouraged by Van’s and the PFA. Both organisations also encourage generous clearance between the fairing and the nosewheel.

 

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.

 

Compliance with the SB. Clearly fitting a new nose wheel fork, with a shortened nose gear leg will increase the static ground clearance by 25%. As the tyre compresses the percentage increase in ground clearance will be that much greater.

 

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.

 

Nosewheel 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 Roger Hopkinson, who has more experience than most in nosewheel RVs, recommends a pressure at the lower end of this range.

 

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).

 

So what should you do? A Mandatory Permit Directive to make compliance with the Van’s SB compulsory has not been issued (1 Feb 09, I don’t believe one will now be issued), so the decision is up to you. The new design of nosewheel fork and undercarriage leg provides 25% greater ground clearance for the fork and is clearly better than the old design. However, there is no guarantee that the new design on its own will prevent landing accidents with nosewheel RVs. The Van’s letter of 9 Nov 07 and service bulletin 07-11-9 provides some excellent advice for accident prevention. If you have only a few hours on type, are relatively in-experienced or have a new build aeroplane and operate from a soft or bumpy grass strip it would seem like a very good idea for you make the change. If you have not yet flown your aeroplane it would seem like an extremely prudent move to incorporate the new design before first flight. Other owners should make their own decision based on their own circumstances after reviewing the information above. My opinion is that all new RV nosewheel pilots should receive conversion training from a type experienced pilot who should emphasize stabilized approaches at the appropriate speed and well held off landings.

 

You might like to also consider the following:

 

  • Make yourself a reminder to check the nosewheel tyre pressure every month.
  • Ensure that the nosewheel has a spacer between the two bearings.
  • Make sure you have sufficient clearance between the nosewheel spat and the tyre.
  • Don’t go flying without the nosewheel spat fitted.
  • Take some refresher training on your landings from an experienced RV A model instructor.
  • Try to avoid heavy braking when at forward c of gs, when the ground is soft or very bumpy.
  • Read all the reference documents at the foot of this article.

 

 

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/CAA have not issued an MPD and you are free to carry out the SB whenever you want – it’s your aeroplane! You can do it right away, over the next few months, at the next annual, at a time that suits you or never – you don’t have do it right away (or ever). The SB calls for compliance at the next annual, and has been made available only via Van’s website. If Van’s believed this were a flight safety problem perhaps they would be writing to all of the owners concerned directly and calling for compliance within a much shorter time scale? In the certified world there are plenty of examples of parts suppliers and airframe companies issuing ‘mandatory’ service bulletins that airworthiness authorities (ie the CAA, EASA or the FAA) have decided need not be followed up by an airworthiness directive. If LAA Engineering and the CAA believe this issue is a flight safety problem they will issue a Mandatory Permit Directive requiring compliance with this SB. Until an MPD is issued you are free to decide when, or if, to comply.

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 Oregon are they the only place that can do the work?

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.

 

 

New Nosewheel Forks. If you decide to comply with the SB you will need the following parts as a minimum:

 

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 Roger Hopkinson, strongly recommend fitting such a spacer as a significant improvement on the original design. These spacers are now a standard mod, number 12265, for any RV-6A, 7A, 8A or 9A.

 

 

To answer some of the further questions that have been asked,

 

Re-threading and shortening of nose-gear legs.

 

Thanks to Pete Greenslade, DV Godden Engineering in Kent are able to modify the nose-gear legs. The approximate cost will be £65 + VAT, as long as there is a batch of 3 to 5. The work will take a day (say if delivered by 8.30am leg will be ready at around 4 pm), shipping is extra. Contact Godden’s on Tel : +44 (0) 1732 844072, Fax: +44 (0) 1732 845184, email: dvgoddeneng@tiscali.co.uk, ask for Barry.

 

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 US. Les is in Redditch so may be more convenient for those in the Northern half of the country. Les runs the company with his son Richard and can be contacted on 01527 66779. Both Les and Richard are fully familiar with needs of RV owners. The cost of the modification will be £55, including a spacer to fit inside the nose wheel, but not including shipping. Les needs at least 3 nose legs to modify to make it worth his while setting up one of his milling machines for the nose leg task, so is proposing to modify nose legs on a Friday afternoon when he has at least 3 on hand. If you are interested in having Les modify your nose leg please contact him or Richard directly. You will have to ship your nose leg to Redditch in packaging that can be re-used for the return journey. Shipping costs are likely to be £15 to £20 each way.

 

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.

 

Standards of fork and nose wheel spacers.

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), all UK examples should have been changed out in compliance with an earlier SB.

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 4-16-90,

 

 

 

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.

 

 

 

References

 

 

1. Van’s Aircraft Service Letter dated November 9, 2007

http://www.vansaircraft.com/pdf/Nose_gear_service_letter.pdf

2. Van’s Aircraft Service Letter dated March 10, 2005

http:// www.vansaircraft.com /pdf/letters/nosegear.pdf

3. Van’s Aircraft Service Bulletin 07-11-09 dated November 9, 2007

http://www.vansaircraft.com/pdf/sb07-11-9.pdf

4. Nose gear service bulletin FAQs

http://www.vansaircraft.com/pdf/Nosegear_sb_faq.pdf

5. NTSB Structures Study, Case No.: ANC05LA123

http://www.ntsb.gov/publictn/2006/RV_Study.pdf

6. NTSB Structures Study, Case No.: ANC05LA123 – photos and data table

http://www.ntsb.gov/publictn/2006/RV_Photos.pdf

7. Further NTSB comment

http://www.ntsb.gov/ntsb/brief2.asp?ev_id=20051006X01588&ntsbno=ANC05LA123&akey=1

 

 

Other Developments

 

RV-6A builder Bill Knott from Inverness has taken a different route to providing increased ground clearance for his nose gear, he has designed and built a nose gear to accept a 5.00x5 wheel. I believe that LAA Eng have approved this design and that Bill is now shipping units. Details (and the photos below) from Ian Corse’s website http://www.rvuk.co.uk/

 

RV-10 nose fork & wheel on 2-seat A models

In the US a couple of builders have fitted RV-10 nosewheel forks (the -10 uses a 5” nosewheel), here is what one says (http://www.vansairforce.com/community/showthread.php?t=23361),

“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.”

http://lloydsplane.50megs.com/index.html

 

Bill Knott’s nosewheel fork