Nose Gear Steering Horn Maintenance
December 2001
Over the years, maintaining the general aviation fleet has evolved. It used to be that when you saw a 2,000 hour airframe in for an annual it was considered old, but in today’s reality of new airplane costs and low manufacturing numbers, it’s gotten to be a refreshing experience to see only 2,000 hours on an airframe. Within this reality, the Mooney fleet is also aging. Aircraft systems and components that normally would not be inspected regularly for wear are becoming worn out items. This is the case with the nose gear steering horn assembly. This month, ShopTalk will explain in detail how this steering horn is installed, shimmed and how it wears out.
The bulk of the Mooney fleet from 1968 on is equipped with a steering horn assembly, part # 720095-001. The horn assembly must be purchased with the shaft assembly. The shaft assembly, part # 720085-017 may be purchased separately. Both assemblies are now priced over $1,000.00 and the last shaft I purchased was over $250.00.
Because of the loads placed on the nose steering horn and shaft during taxi, take off and landing, this assembly tends to wear oblong on the inside bore of the horn and on a corresponding side on the shaft. This is a normal wear pattern. There is almost no wear on the horn and shaft when the shaft is rotated to the gear up position. Because this is a direct linkage, if you have a rudder input during nose wheel touch down, the tire will be at an angle and the airplane will veer in that direction. In a bungee system like Piper or Cessna, the bungee limits the forces to the steering linkage; in Mooneys, the forces are not limited until the nose wheel skids.
Refer to the picture accompanying this article to understand just how this system functions. The upper part of the horn fits into a welded stud on the airframe tubular structure. The other end of this horn fits into the rod ends that attach the horn directly to the rudder pedal torque tube. The shaft fits into the bottom of the horn and has a collar with a bolt at the end to prevent it from moving in or out of the horn bore. The other end of the shaft fits into the top of your upper nose gear truss – that is the one that is dented when the tug towing the plane exceeds the turn angle limit. There is a bushing and a ¼” bolt holding the shaft in place on top of the nose gear truss.
Now here is the tricky part. Because every airplane is hand build and no two planes are the same, Mooney uses different size shims to properly fit the shaft to the horn when it is installed into the airframe. If the upper nose gear truss is removed or the horn replaced, the assemblies will have to be reshimed. These stainless steel shims come in various sizes (.016, .025, .032, .040 and .050”) and are meant to be mixed and matched on either side on the horn where the shaft fits into the horn. If you have a gap on one side or the other, the movement of the shaft within the steering horn pumps the grease through the gaps. The shaft horn will prematurely wear out and require replacement (see cost above).
The next time your airplane is on jacks with the nose wheel free, take a careful look at the steering horn assembly while moving the nose wheel from left to right by hand. Pay close attention to just how much free play is in the horn. Using a feeler gap, measure the gap between the horn and shaft. Would re-shimming it make it tighter? Once gaps develop and if the horn is not greased, there will be metal-to-metal contact and the horn will wear out prematurely. Keeping the horn greased (at 100 hour and annual inspections) and properly shimmed will extend its life past 2,500 hrs depending on the length of your average flight. The more taxiing, take offs and landings, the more wear and tear on the steering horn.
Often the bolt that attaches the horn shaft to the upper truss is worn and needs to be replaced. In some cases, installing a close tolerance bolt will take up the slop in the system. By having a tight steering horn your plane will be less likely to shimmy at high speeds. Some looseness is inherent in this design but if there are over 1,500 hours on the steering horn, it must be inspected closely at every annual. Make sure it gets greased every annual or sooner. Insure there are no gaps on ether side of the horn where the shaft goes through. Re-shim and eliminate any gaps that have developed.
As always, if you have a question about this article, you may contact me at my aircraft repair shop, 307-789-6866 or via e-mail. Until the next ShopTalk, enjoy flying your Mooney.
