Broken Exhaust Threatens Wing!
October 1995
This happened to be a Cozy MKIV, but the wing attach system, exhaust system, and engine cowling area are essentially the same as the Long-EZ and Defiant. RAF is publishing the story here in the hope that this knowledge may prevent a similar incident in one of our airplanes. While flying at 10,000 feet over the Gulf of Mexico near Pensacola at night, the exhaust pipe on cylinder number 4 broke off. Fortunately it remained, in the cowling and did not go through the prop. However, hot exhaust gases traveled between the wing and the center-section spar, heating the epoxy in the wing near the wing attach hard points. The epoxy softened enough for both wings to move upward at the wing tips, 1/8 inch on the left wing, and 3/8 inch on the right wing.
The spar caps were not damaged, but the shear web on the right wing actually fractured near the outboard wing-attach point, allowing the wing to move to a new dihedral angle. Unfortunately, the pilot was unable to land when he first heard the exhaust let go, but had to fly for nearly an hour to the nearest suitable airport. It is possible that an immediate landing would have prevented the damage and resulting enormous repair job. The pilot reported that the engine sound made an abrupt change. Performance was not affected, but the noise level was obviously higher, and led him to suspect a broken exhaust system.
He throttled back to 1800 RPM and continued on. He noticed that cylinder head temperatures on 2 and 3 settled down to around 300 degrees F, but cylinder 4 remained up around 400 degrees F. He landed safely, and had the exhaust stack repaired. He did not notice the wing problem until the next day. There was considerable foam shrinkage (due to heat) all around the hard points. He found a small hole in the inboard glass rib, near the aileron torque tube bearing, and the heat had gotten into the wing through this hole. The only visible damage anywhere in the cowling was a small blister on the cowl itself Fortunately all of his fuel lines were fire-sleeved, and his wing ribs were protected with 1/8 inch fiberfrax glued on with high-temp silicone. None of the glass on the firewall or in the wing roots were damaged.
What can be learned from this incident? First of all, exhaust systems are subject to vibration and high temperatures and are vulnerable to cracking, even in an type-certificated aircraft. Inspect your exhaust stacks often and carefully, using a strong flashlight. All visible glass in the cowling area, firewall, center section spar aft face, win roots, etc., should be protected using fiberfrax.
The 1/8 inch-thick material is best, and it should be cut to fit perfectly, and then glued onto the glass using red (high temp) silicon, available at any auto @ store. Seal all possible paths for hot air, such as the gap between the center section spar and the wing, and any holes you may have made in the wing root ribs.- All of the air, hot or cold, should have to exit the cowl around the spinner in front of the prop, except the air that flows through your oil cooler. If you ever hear an abrupt, unusual increase in the noise level from your engine compartment, make a precautionary landing at the nearest suitable airport and remove the cowling for a thorough inspection.