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Monday, March 24, 2014

Horizontal Stabilizer Talk

It is said that necessity is the mother of invention. The truth of that saying is seen in the modifications MMS Aviation mechanics make to airplanes used in missionary service. In fact, the Federal Aviation Administration provides a way changes can be made to production aircraft that are approved by the FAA and issued a Supplemental Type Certificate. As in any acronym-ridden system, we simply refer to this approval as an STC.

One STC we have installed several times was developed by a company in Alaska to improve the durability of the horizontal stabilizer on Cessna 206 and 207 airplanes. The stabilizer skins are removed, new skins are installed in a slightly different configuration, and the leading edge skin is held on with screws instead of rivets. This is valuable because, when a plane is operated on unpaved airstrips, stones kicked up by the wheels and propeller blast can beat up the horizontal stabilizer leading edge quite badly.

Phil works on leading edge attachment. The yellow frame in the picture is a fixture to keep the stabilizer straight while giving good access to it for repair.
After the original aluminum sheet skins are removed the horizontal stabilizer structure is cleaned, inspected, and repaired as necessary. Additional pieces are added to hold the "plate nuts" for the leading edge screws. The leading edge material is thicker than the original and is more resistant to damage.

Josh, a LeTourneau University student on spring break, drives rivets while Phil (standing) bucks the rivets. This swells the aluminum rivet to hold the layers of sheet metal together. 
Skins aft of the leading edge skin are riveted in place as they don't need changed as often as the leading edge. With this STC installed, a damaged stabilizer leading edge can be changed with a screwdriver instead of drills, a rivet gun, and bucking bars. Considerable time is saved as well.

Bob bucks a rivet while Phil drives it with the rivet gun. The more skins in place reduces easy access for the bucking bar (a piece of smooth steel). Added concentration is often required to achieve desired results.
Riveting 101: The repeated hammering of the rivet gun on the rivet head (outside the structure) against the bucking bar (inside) swells the rivet and holds the pieces together. The FAA publishes standards the driven rivet must meet.

An FAA Advisory Circular gives guidance.