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Free Play in the Mooney Tail

 

March 2001

 

This month’s ShopTalk will answer another common query about Mooney Aircraft: “How much free play is okay in my Mooney tail assembly?” Mechanics not familiar with Mooney Aircraft and owners alike ask this question frequently.

 

The Mooney elevator trim system begins with a manual trim wheel located between the two front seats. Attached to this is a sprocket wheel which drives a bicycle-type chain down to another sprocket wheel located directly below, under the floor. These wheels are similar aircraft to aircraft, but are not always interchangeable. Different model aircraft will have different tooth counts on their particular trim wheel and therefore different gear ratios. When the pilot rotates the aircraft elevator trim wheel the lower wheel turns a worm gear assembly. This is directly attached to a multi-piece shaft assembly which extends to the aft bulkhead in the aircraft tail cone. There are some universal joints along the shaft to allow for alignment concerns. This shaft is hooked up to a jack screw (Mooney refers to this as a trim screw) assembly that pushes or pulls on the lower portion of the vertical fin. The entire tail moves, trimming the aircraft nose up or down, respectively.

If the aircraft has an electric trim system, about midway along the shaft assembly, a sprocket wheel is attached to the shaft. Another bicycle chain runs to the sprocket wheel which is part of the integral electric clutch/trim motor assembly.

 

Moving the entire tail plane is more efficient than hanging a trim tab in the breeze and causing more drag on the aircraft. The entire horizontal stabilizer is utilized and when in trim, the elevator is very nearly faired. In the Mooney system the entire tail plane (horizontal and vertical fin, rudder and elevators) are hinged to the aft tail cone bulkhead with two ¼" close tolerance (not standard AN) bolts, each with a self-locking nut. This hinge point is located near the top of the aft bulkhead. Below that is the jack screw assembly and below that, at the bottom of this bulkhead is a large multiple hinge assembly. This hinge assembly along with the ¼” bolts provide torsional stability between the tail section and the aft bulkhead.

 

It’s not uncommon to lift up or push down on the base of the rudder and feel a noticeable clunk or movement. There are two sources for this free play in the tail assembly. The first, at the ¼” bolt assembly, is addressed in your Mooney Service Manual. In summary, the horizontal stabilizer tip is moved horizontally and vertically and the amount of movement is measured. These tolerances are very tight and typically are not more than .15". If you apply too much force, due to structural flexing, you can easily get a deflection of more than that. Don’t force it, just measure the free play.

 

The second source of movement is in the jack screw assembly and is addressed in Mooney Service Bulletin M20-62, dated April, 1960. Yes, a 1960 bulletin covers your 2000 model Ovation and TLS/BRAVO! This is amazing when you consider today’s M20M TLS is at least 70 knots faster than a 1960 M20A or B. This is possible because of the extensive and rigorous flight tests carried out by Bill Wheat. He was the Mooney factory test pilot during the time frame that service bulletin was written. If Bill ever writes a book on his experiences at Mooney, it would document why you need not worry about a little free play in the tail plane. But that is another story.

 

Service bulletin M20-62 starts by immobilizing the tail of the aircraft at the rear tie down ring. No vertical movement must occur. Little or no wind should be present. The measurement taken is the relative distance from the ground (or other reference plane) to the root (bottom) rib at the trailing edge of the rudder. Using a step ladder, apply to the top of the vertical fin, by hand, about 40 pounds of force rearward. Slowly release the force and place a mark on the measuring stick at the root rib. Using the same amount of force pull the vertical fin forward, slowly release and place another mark on the measuring stick at the root rib. It is best to have two people accomplish this procedure. With one to apply the forces and one to do the measuring, it will only take a few minutes. If the distance between the marks is not more than 5/16", the aircraft is within tolerance.

We can all appreciate Mooney’s straightforward engineering, circa 1960. A common area for excessive free play to occur is in the bushings and brackets that attach the aft jack screw to the tail plane. Additionally, the jack screw internally can become worn and sloppy. Mooney manufactured a number of different styles of jack screws, so I won’t go in to the details of their disassembly and repair in this article.

 

As temperatures begin to warm (not here in Wyoming), go out with a friend and see how much free play is in your Mooney tail. Save this article so you’ll know how to check it at every annual.

 

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.

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