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Metal Fatigue

by Had Robinson

After 100's of hours of flying, pilots should thoroughly check their paramotor for fatigue cracks and breaks.

Breakage of the ABM sidebars, their attachment points to the frame, and the side bar pins (bolts) would be catastrophic.  These areas must be carefully checked at least yearly, especially on motors with many hours.  Careful attention must be especially given to the bolts that attach the arms to the frame on ABM models and similar configurations.  These should be replaced at the slightest sign of wear.

On the Top 80, breakage of the fins on the cooling box can also be expected on models that lack the silicone bond to the cylinder.  Cooling efficiency will be less but if a CHT (cylinder head temperature) gauge is used, pilots can easily monitor any increase in running temperatures.  For the last few years (as of 2018), Miniplane has put spots of silicone cement between the cooling box fins and the cylinder to stop them from vibrating -- and eventually breaking off.  When removing the cylinder, pilots must be certain to re-apply silicone in the same manner as the original.

Another area of failure noticed for the Miniplane is the tubular frame near the lower cross member and the lower attachment points for the harness.  A competent automobile body shop or, preferably, an aircraft repair facility can TIG weld a break or crack.  DO NOT LET A MUFFLER SHOP OR ANY NON-SPECIALIST WELD THIS FRAME!  It is very thin aluminum tubing and must ONLY be repaired by an aluminum TIG welding expert.

Alternatively, it may be better to not weld the frame but to rivet a piece of aluminum angle to both halves of the frame (see photos below).  The advantage is that a rivet repair allows some movement of the the two pieces which would basically stop it from occurring in the same place again.  Miniplane needs to use slightly better quality and thicker tubing in this area of the frame.

Metal fatigue on a Miniplane frame

Metal fatigue on a Miniplane frame

The photo below is of a repaired Miniplane frame that had upwards of 400 hours.  The crack was at the black arrows (on both photos).  The cracks were barely visible and were only noticed on close inspection.  The frame, otherwise, was in perfect condition with no bends or other damage.  The green arrows point to additional welding that was done to strengthen where the cross member is joined to the vertical frame members.  Had this part of the frame completely separated or failed it would not have posed any danger, just inconvenience.  The welding was done by a firm that manufacturers aluminum gas tanks for racing cars.  They are experts at working with aluminum.  The repair was successful.  Nonetheless, pilots have to examine their equipment carefully and regularly.

Miniplane repaired frame fatigue cracks

Miniplane repaired frame fatigue cracks

Below is a photo of a throttle and kill switch failure caused by metal fatigue of the support bracket in a Top 80 paramotor.  When the bracket broke (white arrows), the ground circuit for the kill switch was also lost.  This is why pilots must always have a functioning choke to stop the engine if the kill switch circuit fails.  Also visible is an improved throttle return spring setup.  It is smoother and longer lasting than the OEM design.

throttle failure from metal fatigue in a Top 80 paramotor

The support bracket was easily repaired with a piece of aluminum and pop rivets.  Steel could also be used to make the repair but it weighs more but is less subject to fatigue.  The arrow shows the original point of failure.  All paramotor engines engines generate severe vibration which is the cause of metal fatigue.  Modifying the throttle by adding a spring (prominent in the photo below) decreases stress on carburetor parts and the throttle shaft, in particular.  Check the link above for how to do this.

repair of the Miniplane Top 80 throttle support bracket

It is also convenient to cut a notch in the throttle bracket so that the throttle cable can be easily removed and replaced.  This can be done with a fine hacksaw or a Dremel cut-off wheel.  All bicycle Bowden cables have a notch so the cable can be easily removed.

Top 80 modification to the throttle bracket for easy removal to throttle cable

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