Helicopter Trim Tab Adjustments Can Be Tricky

Using a trim tab gauge

As a provider of dynamic prop balancer, aircraft vibration survey, and helicopter track-and-balance systems to the aviation maintenance industry, we meet a lot of great people who are interested in high-quality maintenance work while minimizing maintenance costs. Our experience has revealed a maintenance procedure where cutting corners can be costly: making helicopter trim tab adjustments. Here are some tips about this procedure that may prevent wasted time and unnecessary expense.

Tip 1: Use High-Quality Trim Tab Gauges
Helicopter mechanics need high quality trim tab gauges and, in some cases, the gauge is specified by the manufacturer for a particular blade set. The best trim tab gauge designs for most rotorcraft blades utilize dial indicators on a frame that attaches to the blade in a very repeatable way. These types of trim tab measurement instruments support accurate and reproducible measurements of the blade tab position.

Trim tab gauges

This trim tab measurement reproducibility is critical when it comes to tracking blades. Before making any adjustments, the mechanic should verify that the same measurement can be made repeatedly. The mechanic should be able to put the trim tab gauge on the blade, make a measurement, remove the gauge, and repeat this sequence two more times, obtaining the same position result every time. If the measurements aren’t repeatable, the mounting technique should be reviewed and the tool should be checked to verify that it is functioning properly, and that it is the correct one specified for this blade set. Verifying that the gauge readings are reproducible before making the first tab adjustment can save many hours later.
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In addition to reproducibility, trim tab measurement accuracy is also important because incredibly small adjustments can affect forward flight vibration. For a Robinson R22, a 0.015” tab adjustment can produce almost a quarter of an inch change in blade track. This is more than enough to cause a noticeable increase (or decrease) in vibration.

​Tip 2: Use Recommended Tools for Tab Bending
Use the recommended tab adjustment tool when bending blade trim tabs. A lot of improper tools are used to make adjustments, from vice grips to sledge hammers (really). It is extremely important that the manufacturer’s recommended tools are used to adjust trim tabs and keep in mind that a tab bending tool appropriate for one set of blades may be the wrong tool for a different set of blades.

Trim tab on a blade

Robinson trim tabs, for example, should not be adjusted by simply applying torque to the tab about the tab’s root. Robinson trim tab adjustments are produced by more of a curling action resulting from three precisely applied forces that are generated by Robinson’s tab bending tool as shown below. 

A specialized tool for adjusting a Robinson trim tab

If tabs are bent improperly, the result can be costly. For instance, one mechanic thought that the trim tab on a Robinson was adjusted just like many other blade tabs, simply by bending the trim tab down. After a couple of flexing adjustments using the wrong tool, the rotor blade responded with a loud pop and an expensive replacement bill. There is a reason that Robinson sells a custom device for adjusting their trim tabs.

More Information
An underlying question for both of these tips is: how can you know the proper adjustments to make to helicopter trim tabs? The answer is given by the blade offsets measured by the DynaTrack, the vibration measured by the DynaVibe accelerometers, and the procedures in the helicopter manufacturer’s maintenance manual. However, if the mechanic doesn’t have the correct tab bending tools and trim tab gauges to make these adjustments safely, precisely, and accurately then the process will take longer at best and could be a costly mistake if the tabs are bent improperly.

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