Home | I seem to have some play in my horizontal tab, on the back of the tab. Should I worry about it? What can I do to fix it?

I seem to have some play in my horizontal tab, on the back of the tab. Should I worry about it? What can I do to fix it?

I seem to have some play in my horizontal tab, on the back of the tab. Should I worry about it? What can I do to fix it?

In many cases, the play you are talking about is actually play in the Anti-Servo Tab linkage. This surface across the aft end of the Stabilator is not really just a Trim Tab, as most people think. The AST is there to make the Stabilator harder to move as it departs from level trim. Otherwise, since the leading edge moves up and down, as soon as it came out of the neutral trail position, it would slam instantly against the up or down stop due to the air loads on it (just like your hand sticking out a car window at high speed, when you rotate your palm up or down).

These tabs are critical when it comes to flutter prevention. While they have performance advantages with respect to expanding the CG range, all Stabilator designs are known to be more critical regarding free play (to prevent flutter). There should be almost no play in them, neither in the outboard hinges nor in the linkage. We have had reports of “airframe vibration” or “control buzz” that was actually incipient flutter. In most cases true flutter will lead to surface destruction within seconds. There is a recurring AD for inspection of the Stabilator hinges on each side of the fuselage, as some were found to be cracked or to have loose rivets. My strong personal suspicion is that those damaged planes were the direct result of excessive play in the AST, which in turn created “control buzz” in the Stabilator.

The surest way to prevent flutter is to keep the play out of the linkages, and to keep cable tensions properly set. In many cases, simple fixes can remove the vast majority of linkage and hinge play. It is non-intuitive, but the steel fasteners and steel hinge pins often wear the fastest. This is because grit becomes embedded in the softer material of linkages and hinges, and acts like a fine grinding stone on the steel.

The first thing to try is replacing the bolts or clevis screws in the linkage, and replacing the AST hinge wire. If that doesn’t take out almost all the play, you have to move on to more expensive repairs. If you can wiggle the AST up and down in the center (where the linkage is fastened) more than 1/16″ or so, you probably have too much total play. The same goes for the outboard ends of the AST itself; they should not move up and down much more than 1/32″ (like a snug hinge would feel). I don’t have the books here with me, so I can’t look up the precise numbers. To give you some idea of relativity, some similar specs on the Beech Bonanza are .010″ (ten thousandths of an inch). It is a faster plane, so free play in a control surface is even more critical.

If everything was completely new in the hinge (new NAS40 outboard hinge sections, and new NAS40-compatible hinge wire), there would be almost no detectible free play in the outboard ends of the AST. A very limited supply of the NAS40 hinge can still be found (though it is relatively expensive). Do not buy the Beech replacement hinges by 169- part number, if you can help it, as all the replacements they now ship are rolled hinge stock. The original hinge was the much stronger (and longer lasting) extruded hinge stock. Beech has been unable to provide me with an engineering report that demonstrates that the rolled stock is a satisfactory replacement for the extruded stock (with which the plane was certified). Being the original manufacturer, it is their business what they choose to do with replacement parts. In my case, I choose to stick with the originally-certified hardware type.

Unfortunately, there are no more of the original larger-diameter AST hinge pins available. MS20253 hinge pin stock is supplied as a replacement (including by Beech-RAPID). The combination results in some slight and unavoidable play, as the pin hole in NAS40 stock is .093″, while MS20253 hinge pins have a diameter of .090″.

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