GLOSTER AIR PARTS
Nosewheel
Thoughts
Contents
What are the significant factors?
New Nosewheel Forks Group Buy to
save shipping costs
Standards of fork and nose wheel
spacers
Latest news 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
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
draw, PFA Type Certification Data Sheets, AAIB reports, Van’s Aircraft
documentation and a report published by the NTSB in the
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
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
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? To date
PFA Engineering and the CAA have not issued a Mandatory Permit Directive to
make compliance with the Van’s SB compulsory, 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
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. Until the PFA and CAA issue 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. 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 PFA 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
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 PFA 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 PFA 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 ’ave 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 sa’s 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 shipping of the nosewheel fork will be expensive. Gloster Air Parts would like to co-ordinate the group buy of
Nosewheel Forks. I would propose that we make available a kit of parts that
includes:
1 x Nosewheel Fork,
WD-630-1
2 x Brackets (L
& R), U-713C L/R
1 x Nosewheel
bearing spacer
At an approximate
cost of £145, based on a group buy of 10, final cost will depend on order size.
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. By the time these spacers are
available they will be approved as a standard mod.
I estimate that
individual nosewheel forks, with brackets, shipped direct from Van’s, plus a
spacer from Gloster Air Parts will cost £170.
If you would like
to take advantage of the group buy please send a deposit cheque of £100,
payable to Gloster Air Parts, at
Church Crookham
Fleet
Hants
GU52 6PQ
We will order the nosewheel forks and brackets when we
have at least 6 orders, but please bear in mind that the lead time from Van’s
(from order to shipping) is running at about 6 weeks. An article is being
prepared with LAA Engineering for the February issue of LA describing the
nosewheel SB. If you have questions please ring Gloster Air Parts on 01252
617484 or email on orders@glosterairparts.co.uk.
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
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 can put owners into batches if we cannot do that via
the RV Sqn, but don’t really want to be involved with
this aspect.
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, I’m not sure there will be a huge saving if there is only a
small group buy. If you are interested and would like me to investigate the
savings please email orders@glosterairparts.co.uk.
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
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. Gloster Air
Parts have organized a group buy of forks and brackets to save on shipping
costs. Shipping of an individual fork would be approximately $50, plus Vat and
customs clearance charges. 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
http://www.vansaircraft.com/pdf/Nose_gear_service_letter.pdf
2. Van’s Aircraft Service Letter dated
http://
www.vansaircraft.com /pdf/letters/nosegear.pdf
3. Van’s Aircraft Service Bulletin
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
RV-6A builder Bill Knott from
