Trouble Shooting the Quicksilver MX
Like anything else in life, the older something gets the more prone it is to break down,
humans are no exception to this rule, and unfortunately neither are aircraft.
One of the oldest and most proven designs on the ultralight market is that of the VENERABLE QUICKSILVER MX . The MX was
probably the most highly promoted and successfully marketed ultralight aircraft
in the world, with some reports of over 8,000 having been sold.
One of the reasons for its popularity was the fact that it came as a complete ready to
bolt together package, with superior assembly manual, and with all parts neatly packaged
for easy identification.
This allowed the average ultralight enthusiast to assemble his kit, ready to fly in as
little as 50 hours, in some cases as little as a weekend.
Now no matter how well something is built, someone always finds a way to break it, and no
matter how much testing a factory does in a controlled environment, owners always find
someway of doing it just a little different and come up with a problem.
An advantage both to the factory and potential future purchasers of an ultralight is a
quick build aircraft with thousands flying. The reason is quite simple, the faster the
craft is built and flying the sooner the PROBLEMS , show up, allowing the factory to
improve their product.
You can see that if it takes a year to build a craft, and another year to put 50 hours on
it, (before problems were reported to the factory) it would be two years from delivery
before the craft was IMPROVED, on the assembly line.
In Canada the MX, and MX11 were probably the first ultralights to be used for training new
pilots. In the hay day of ultralight aviation 1981/83, some schools in Canada were putting
50 to 60 hours week on their craft in TRAINING.
When you put that kind of hours on a plane primarily designed for recreational flight, it
like an 18 year old working the street, starts to show its age, very quickly.
In an effort to help smooth out some of the wrinkles on the old MX and make it a little
more respectable the following is a run down of some of the problems and their possible
solutions, encountered in the use of the MX for training.
These problems may not show up at all on some craft because of the different kind of
demands put on it, while others might be a little closer to the training environment than
at first realized.
The problems areas are broken down into .
3. Drive train
4. Control system
We will be discussing some of the problems associated with the engines used on the MX, but
it is suggested that you read Problem solving the 430 Cayuna R, and Troubleshooting the
Rotax family of ultralight engines, which if not published by this publication will be in
the near future, or by contacting Ultralight News for a full
report on the problems encountered in the use of these engines on various ultralights.
Whether using the Cayuna or Rotax engine use Premium unleaded fuel, with a good grade of
oil. When purchasing your gas buy it from a busy station, mix 5 gallons at a time, or
whatever you will be using, and under no circumstances store premixed fuel in a bulk
container, as it quickly looses its octane rating.
Make sure that the spark plugs you are using are the kind recommended by the manufacturer,
and that they are properly gapped, prior to installation.
Plugs should be inspected periodically replaced about every 25 hours, or at the first sign
of a miss), as should the fuel filter.
The air filter supplied by the manufacturer was a sponge filter which is very prone to
water absorption, and is not a filter that I personally recommend.
A K & N airfilter model RU 0600 has proven to work very
effectively, and does not suffer from the water draw back. A word of caution to owners who
have installed weight shift Quicksilver engines, into an MX frame. The fuel supply system
on most of the weight shifts were a GRAVITY FEED SYSTEM, the MX uses a fuel pump, the
carburetor needle and seat are DIFFERENT.
Also any owner who for one reason on another replaces his carburetor should make sure that
the carburetor is set up for a fuel pump.
Pilots flying Cayuna powered MXs have reported erratic engine performance, engine
overheating and engine seizures, and or lack of performance both on the ground and in
flight. Upon closer inspection it was found that the rubber flange connecting the
carburetor to the intake manifold had partially separated, UNDERNEATH the worm drive clamp
used to hold the carburetor to the intake manifold.
The separation was not evident unless the pilot removed the carburetor completely, and
inspected the flange closely. In some cases pilots took several months to locate the
source of the problem.
The solution to the problem is to replace the carburetor mounting flange with a new
flange, and if at all possible support the back of the carburetor so that the flange is
not taking the complete weight of the carburetor and breather.
Another commonly reported problem was improper ventilation of the gas tank. In early model
MXs the gas cap had a screw in vent on its top that was used to vent the tank. If this
vent was inadvertently left closed the engine would run for a short while and them simply
Later models eliminated the screw in vent, replacing it with a small hole. This hole has
been reported to become plugged, and should be an area of periodic inspection.
Another reported problem is in the deterioration of the cork liner in the cap. Apparently
with time the cork liner rots and falls into the tank, where it has the potential to clog
gas lines, filters, or fuel pumps.
The airframe on the MX was made up of aircraft grade anodized aluminums tubing and in most
cases AN nuts and bolts. This made for a good reliable low maintenance, combination.
That is it made a good combination if the pilot made perfect landings and take offs,
directly into the wind from a smooth runway, which unfortunately was not always the case,
in a training environment.
One of the first problems encountered when abusing the MX was that of the KING POST CHANNEL BRACKETS FAILING, usually after a
rough series of landings. In all reported cases that I know of where this has happened the
King Post remained upright, held by the gas tank and sail.
The point of failure was at the bend in the channel bracket. In early 1984 if memory
serves me correct the manufacturer did update their brackets to a higher strength because
of this problem. Thus if you have an MX prior to 84, it is suggested you check this area,
and update immediately.
Another reported problem was that of customers in the field over tightening the King Post.
The proper way to tighten an MX King Post is to bring it up snug, by hand, DO NOT USE
VISE©GRIPS OR A PIPE WRENCH.
Some MX owners tried lubricating the threads on the king post, this resulted in the king
post backing itself down during flight, causing loose flying wires and an improper angle
of incidence to the tail, and in some cases a loss of proper elevator control.
Another problem encountered due to hard landings was the bending of the King Post. On some
later model MX 11's the aluminums King Post was replaced by a chromoly KING POST.
WIRE TANGS WIRE TANGS
Another problem area, located above the wing was in the tang used to connect the King Post
upper nose wire to the front of the root tube.
This tang was reported to crack at the bend. As well the bolt used to retain the upper and
lower nose wires was reported to have broken.
To help eliminate this problem it was suggested that the tang on the lower nose wire be
replaced with a double tang, with a larger bolt hole, which used two separate wires for
support, and that the upper tang be replaced with a stronger tang drilled out, to allow
the use of a 5/16" bolt rather than the original 1/4"bolt.
ROOT TUBE ROOT TUBE
The manufacturers recommendation was to replace the root tube on the MX 11 every 200
hours, and on most other models at 300 hours. (A complete maintenance schedule is
available from any Quicksilver dealer, or parts centre).
The areas of most concern involved:
1. The area where the King post channel bracket fastens to the root tube. It was reported
that the root tube was bending, or indenting immediately below the channel bracket.
This could be caused by over tightening the king post, or by numerous hard landings or a
combination of the two.
2. The area where the leading and trailing edge channel brackets attach to the root tube.
This area was reported to show signs of wear and cracking, as well as elongation of the
In several cases in Canada it was reported on the MX 11 that the root tube had SPLIT FROM
THE LEADING TO TRAILING EDGE BOLTS , on one side of the root tube. This must have happened
during flight since a thorough preflight was done prior to the planes taking off.
The main axle on the MX proved time and again to be very forgiving. In fact many pilots
simply kept flying even when the main axle was bent.
This AXLE HAS BEEN REPORTED TO BREAK , at the bolt which ‹ connects the lower tail wires to the axle, and/or
where rear down tubes connect to the channel bracket on the axle.
Several pilots have been in the process of taking off when the plane has simply collapsed
This is not something that happens overnight but is rather caused by hard landings over a
period of time. It is suggested that if your main axle is bent that you replace it.
As well check the hole in the axle where the bolt holding the down tube bracket to the
axle is, elongation of this hole has also been reported.
TAIL BRACE AND CHANNEL BRACKET WEAR
Another area of reported wear is in the tail brace tube (40440). The wear can be found in
both the channel bracket and tube, at both ends, where the tube rests in the channel
A simple solution to this is to install small plastic washers on both sides of the tube,
between the tube and channel bracket. This makes the wear part the plastic spacer rather
than the tube and bracket.
The "U" shaped tube used to support the seat, located directly below the seat,
(seat mount assembly), was reported to fail (in early model MX's,) at the point where the
bolts go through to attach the seat belts, and seat, to the "U" bracket.
Later models were strengthened in this area.
Another problem reported was the breaking of the seat, at ‹ its base. This was caused by
pilots restraining their aircraft during run up, while seated in the craft, and applying
pressure to the back of the seat to prevent the craft from moving.
TRIBAR CROSS TUBE ASSEMBLY
On the tribar cross tube are located four channel brackets used to connect the nose struts
and axle struts to the assembly. Numerous pilots have reported the bending of the flat
plate which the channel brackets and flat saddles bolt to, again caused by hard landings,
usually by the nose wheel hitting first.
The bolts used to connect these groups of brackets and saddles has also been reported to
bend and it is suggested that it be an area of regular inspection.
It was also noted during the disassembly and reassembly of a damaged MX that the TRI BAR CROSS TUBE ASSEMBLY WAS BENT. This
could only be noted by complete disassembly, and the laying of the unit on a flat surface.
It was also noted prior to disassembly that the craft required considerable pilot input to
fly straight and level, while after replacement of the assembly with a new unit it again
flew as an MX should.
REAR DOWN TUBES
The rear downtubes on the MX have been reported to show elongation of the holes where the
top of tube fastens to the root tube. This elongation is generally not visible unless the
pilot removes the down tubes and examines in behind the plastic saddle, which acts as a
stand off from the root tube.
Another wear point on the rear downtubes is on the tubes inner sides. This wear is caused
by the two tubes contacting the driveshaft coupler, or coupler bolt.
Also if the pilot has updated to the flex drive coupler it is necessary to reposition the
rear downtubes in such a way as to clear the coupler. (For more info contact a parts depot
or service centre).
The MX has two suspension struts running from the axle to the nose wheel. Elongation of
the holes in these tubes has been reported, where the bolt passes through a channel
bracket, and then into the suspension strut, immediately in front of the axle.
In training one of the first things many schools in Canada did was to sleeve these tubes
by riveting a short sleeve over the end of the tube.
As well many schools replaced the wing nuts and safety pins used here with an AN self
locking nut and bolt, and used a saddle between the channel bracket and tube. This helped
cut down on wear by eliminating some of the movement allowed by the use of wing nuts.
Another tube which has shown to wear over time is the tail skid. The most noticeable area
of wear is on the area of the skid that comes in contact with the ground. A simple
solution is to over sleeve the tail skid with a clear replaceable chunk of rubber hose. The
tail skid has also shown wear in the holes used to connect the tail skid to the horizontal
stab, and tail support.
This wear can be eliminated somewhat by the use of an AN bolt and self locking nut, rather
than the wing nut and safety ring supplied by the manufacturer.
All early model MXs came with pit pins for ease of assembly and disassembly. These proved
over the long run to be not as strong as an AN bolt, or the stainless T pins which
The original pins were known to bend, break, rust, and in some cases work there way out of
the channel brackets. It is highly recommended that if your MX has these style of pins
that they be immediately removed and replaced with AN nuts and bolts or stainless T pins
with lock rings.
NOSE STRUTS BOLTS
Attaching the tension strut to the nose strut on old style MXs are two eye bolts, and an
AN3 bolt. This eye bolt has been known to shear at the head. When this happens it allows
the tension strut to fall down. On takeoff it can result in the strut digging into the
ground, causing possible injury to the pilot, and severe damage to the aircraft.
If the bolt breaks during flight the strut simply hangs down but is generally bent beyond
repair after being landed on.
The new MX Sprint features a bracket and bolt system that eliminates this problem, and is
adaptable to the MX.
Several float plane pilots flying 503 equipped MXs with larger fuel tanks have reported
the bending of the compression struts on early models.
We were recommending the oversleeving of the compression struts to any plane which might
be subjected to heavier than normal loads.
WING WIRE E BOLTS
MX pilots have reported the BENDING AND OR FAILURE of the wing wire bolts on the MX.
During normal preflight they have been able to simply reach up and remove the strut. To my
knowledge none has ever fallen out during flight.
It is suggested that this be a regular area of inspection, by all MX pilots. Over the
years two solutions have been suggested for this problem.
The first suggested solution was published in the EAA Light Plane World, in their March
1985 issue. The author of the report was Graham Lee, of Edmonton Alberta. I am sure that
Mary Jones of EAA would be more than happy to forward this information to you.
Or for another suggested solution contact Ultralight Systems Inc. Old Airport Mines Road
Laredo TX 78041, 512©725©9092.
Pilots have reported several problems with the wheels used on the MX.
In photo A the damage on the outer rim is caused by under inflation of the tire and hard
landings. The damage to the centre of the rim is caused by excessive side loads on
The damage in photo C was caused by excessive tire pressure.
The solution to this problem is to check your wheels during preflight for any signs of
abnormal wear. Also check your tire pressure on a regular basis. To much pressure results
in the rims breaking where the bolts holding them together go. To little pressure results
in the outer rim and bearing areas being damaged.
Another problem is the bolts used to hold the two rim halves together will back off
allowing the tube to get pinched in the opening between the two rim halves, resulting in a
flat tire. Check these bolts during your routine preflight.
Another frequently reported problem is in the loss of the whole wheel assembly, sometimes
on take off other times when the craft is in flight.
On the MX a pin with a safety ring, going through a short collar was used to keep the
wheel fastened to the axle. In long grass, or muddy conditions the retaining ring can
become damaged in such a way that it no longer retains the pin and thus the collar is
allowed to fall off, followed very shortly by the wheel.
If your aircraft is equipped with a pin using a safety ring it is strongly advised that
you replace it with an AN bolt and self locking nut.
LOWER TAIL WIRE WEAR
It has been reported by several Quicksilver MX owners that during routine preflight they
have noticed abnormal wear on their lower tail boom wires, where they come in contact with
the tail boom support tubes, in some cases the cables have sawed a hole through the tube,
in other cases the lower tail wire had started to fray.
The solution to the problem has been to rivet a cable guide or stand off on to the lower
boom tube, (This still allows the cable to move yet any wear is in the plastic guide).
Another solution is to switch to the lower boom tube conversion kit which stiffens up the
tail and eliminates the lower wires. I personally prefer the second alternative, and have
found it well worth the money.
ROOT TUBE BEARING WEAR
MX pilots have reported vibration in their aircraft, upon further examination it was found
that a collar inside the root rube which the top pulley shaft goes through had begun to
wear, allowing the bearing which it retains to move causing the vibration.
It was generally found that all these aircraft had more than 100 hours on them.
A solution to the problem is to drill a hole through the root tube directly in front of
the bolt that holds the collar and install another bolt. What this does is holds the
collar in place and takes the strain off of the bearing and collar, by giving support to
the collar at two locations instead of one.
Of course if you have already started to feel the vibration then the bearing is probably
gone and must be replaced.
There is also a retrofit available which replaces the round bearing retaining collar with
a new solid aluminums insert which fits snugly into the root tube, and which has a drilled
out section for the bearing to fit into eliminating the need to drill the second hole.
Another option available is put out by Ultralight Systems, this calls for the replacing of
the shaft and pulley and bearing, with a solid shaft with the bearings moved out into the
large reduction drive, pulley for more information contact them at 512©725©9092.
Other parts can be obtained from LEAF, QUICKSILVER & GLIDER EXCHANGE, OR WINDSTAR
AVIATION, or any authorized Eipper dealer.
PILLAR BLOCK BEARING
MX pilots have reported that the pillar block bearing used to tension the belts on the
back end of the driveshaft has
1. seized up
2. worn into the driveshaft 3. become worn out.
It is suggested that this be a regular area of inspection and that at the first sign of
wear, that this pillar bearing be replaced.
DRIVE SHAFT COUPLER
Several ultralight pilots have reported loss of propulsion both on the ground and in the
air. Upon closer examination it was found that the coupler connecting the driveshaft to
the engine had broken in half.
In several other cases the bolt connecting the shaft to the coupler had severed. The bolt
severing incidence were generally after the craft had been flown for 50 or 60 hours with
an untraceable vibration, generally at idle and around 5,000 RPM.
A possible solution to this problem is to replace the coupler with a new style which is
nearly twice the diameter plus substantially longer. (these are generally found on the
newer Rotax powered aircraft).
Another solution is to replace the coupler with a flex drive coupler, which eliminates
Another problem encountered with the coupler is that of the bolt retaining the coupler to
the crankshaft coming loose or, if removed not being properly tightened, when reinstalled.
This allows the coupler to spin on the crankshaft, generally ruining the coupler. A pilot
with a loose bolt will notice his engine revving up while under load, similar to what
would happen if his drive belts were loose.
Another area of slippage is on the two seat MX on the large reduction drive pulley. On the
side of the pulley are located two bolts for tightening the pulley on to the upper
The manufacturer recommended that loctite be applied between the pulley and shaft and then
that the bolts be tightened this helps prevent the pulley from turning on the shaft. On
two seat aircraft the bolts were and AN 5 rather than the AN 4 used on the singles.
The manufacturer also changed the kind of driveshaft they were using from a hollow shaft
with an aluminums plug to a solid shaft, for more information on this you could contact any
of the above service outlets.
It has been reported by several MX pilots that the rudder frame (part no 40333) has hooked
up on the upper tail wires, in most cases the aircraft had over 150 hours on them and the
king post had been adjusted to or near the maximum allowable.
The entanglements have been reported, on the ground during a take off run, on a rough
field, and in the air, in turbulent flying conditions.
When the frame was supplied from the factory it was usually longer than is necessary, and
this could contribute to the problem.
If you are flying an MX we strongly advise you check to make sure that your rudder frame
can not come past the upper tail wires, and that your rudder frame is cut off flush with
the bottom of the tension strut on the rudder.
Also the retrofit tail boom kit has been found to help eliminate this problem.
Early model MXs used a plastic tab to deploy the spoilers, these required considerable
amounts of pressure to engage and generally just ended up stretching the deployment lines.
Later models used a horn which worked well and allowed full deflection of the spoilerons.
Another problem with early MXs was in the use of plywood to make the spoileron plates.
These ended up rotting, or buckling over time. Later models used a bevelled aluminums
Early model MX instruction manuals came with instructions which told owners to install
their battens with the plastic retaining tips so that they came out on the top of the
Pilots flying with them in this position found that the battens backed out of their
pockets and in some cases into the prop.
Later manuals had the owners turning the tips so that they exited the underside of the
wing, and were held in place by air pressure and the trailing edge.
Owners who have installed their battens the other way who didn't want to change simply
drilled holes in the tips and tie wrapped them in. This worked well although in many cases
the tips simply broke off where the holes had been drilled.
It was also recommend that MX owners make a pattern from a new batten and then on a yearly
basis check and rebend any battens that require it, to the shape of the new one. This can
be done very easily using two pieces of plywood, the first as a base, the second the shape
of the batten.
When rebending the batten simply lock it in place between two pieces of wood, at its
leading edge, and carefully rebend it around the pattern.
Several pilots flying Cayuna powered MXs with the motor mount arms shown in this picture
have reported the arms breaking where the bolts go through to retain it to the motor, and
or root tube bracket.
If you are flying on this style of mount it should be an area of regular preflight, and or
update to the mount used on the Rotax powered MXs which to date have not been reported to
have this problem.
GAS TANK GROMMETS
Several MX pilots have reported loss of fuel, both on the ground and in air. Upon closer
inspection it was found that the rubber grommets used to seal the gas tank where fittings
are placed to supply fuel, had rotted and then cracked allowing fuel to escape. In other
cases the sharp edges of the plastic tank had actually cut into the grommets again
resulting in leakage.
The solution to this problem is to
1. round the inside corners of the holes that the grommets fit in.
2. replace the grommets yearly, or at the first sign of deterioration or leakage.
NOSE WIRE BREAKAGE
It has been reported by several MX owners that the nose wire located at the front of the
root tube and running down to the triangle bar has snapped on landing. This usually causes
the A frame to collapse around the pilot.
A possible solution to this problem is to replace the single wire, and tang, with the new
style double wire and tang shown in the attached photo.
The MX used several different sizes of saddles throughout its airframe. These saddles were
generally used to help eliminate wear and tear on parts, by preventing them from rubbing
against each other.
Pilots have reported that these saddles have broken causing improper spacing, and or wear
An area in which these saddles are located, which is critical to the flight
characteristics of the MX is in the area of the horizontal stabilizer.
If the saddles break in this area it causes the horizontal stab to flutter, in flight,
which can be very disconcerting to the pilot. Also the horizontal stab is used to
determine the correct angle of incidence from the tail to the wing, and the loss of these
saddles can result in control problems.
This is an area that should be regularly inspected and if any saddle is found to be
cracked or cracking it should be immediately replaced.
Another problem recently encountered on the MX is that of the fabric on the horizontal
stabilizer becoming very loose. This generally results in the aircraft flying with a nose
high attitude. If the fabric on your craft is loose it can be tightened somewhat by using
a hot iron and passing it carefully and quickly over the fabric.
If this does not work the fabric should be replaced, or the tail section updated to the
new style which features a leading edge tube which joins back up to the trailing edge of
the horizontal stab.
The fabric used on the MX was Dacron Sailcloth. This material is very susceptible to UV
light. Even if a pilot stores his plane inside, or uses wing covers the flying surfaces
are still exposed to UV when flying.
Tests have shown that the fabric can loose its strength in as little as 6 weeks in some
regions of the country. To date I have found only two products effective in helping
prevent UV degradation.
They are Stits Aerothane, and EXTERIOR HOUSE PAINT. These coatings have extended fabric
life from 1 to 8 years on craft located from Florida (1 & 2 years) to Ontario (8
years), when the planes have been left outside most of the time.
Check out UV Protection
Either process can be applied by gun, brush, or roller, with the gun doing the best job.
The coatings are all very toxic to use and should be applied only after reading and
following the instructions very carefully.
Well its taken some 3 months, and 5 rewrites to get this all down on paper, I hope that it
is of some use to fellow MX owners and pilots.