Inspecting Exhaust Systems

December 2000

Now that Independence Day has passed, winter has officially arrived in Wyoming. Therefore, this month’s ShopTalk will be about the proper inspection of aircraft exhaust systems. The good folks at MAPA tell me that they get three or four calls a year about people affected by carbon monoxide (CO) poisoning while using the heaters in their airplanes. This dangerous situation is easily avoidable with some simple inspections and preventative maintenance on the exhaust system.

The first photo shows a typical 200HP M20-E or -F exhaust component. This is commonly referred to as the heater muff or muffler but it is constructed quite differently from a car muffler. A sheet metal shroud is wrapped around a metal can with an air gap between the shroud and the can. In photo #2 you can see the main heater can with the shroud removed. Some systems have studs sticking out of this can, some are smooth but the most typical is the ribbed can as shown in the picture. During every annual inspection, accomplishing the following will help insure you will not be the next victim of CO poisoning.

First, remove the shroud and inspect the outside of the can for blisters, bulges or cracks. A blister or a bulge indicates a structural breakdown in the metal and will soon become a crack. Exhaust gases will then leak into the shroud allowing CO into the cockpit. Any damage in this area is reason for removal from the aircraft and replacement or overhaul of the component.

Next, remove the tail pipe and using a flashlight observe that the flame tubes inside the can are not warped, broken or missing (see photo #3). The flame tubes guide the hottest exhaust gases away from the manifold sidewall. At extreme temperatures, the exhaust gas is slightly corrosive and flame tubes will wear out, sometimes falling away completely. This will cause excessive and uneven heating of the heater can, leading to blistering and a crack. A loose flame tube can also get lodged in the tail pipe exit causing a serious loss in manifold pressure and horsepower. Always remove the tail pipe and inspect the condition of the flame tubes.

Photo #4 shows how the pipes are welded onto the can. They typically will crack at the weld as the pipe goes into the can. In this photo, you can see a crack next to the weld on the pipe closest to the camera. Photo #5 shows a typical ball socket type attachment for the header pipe to the muffler can .You will notice that a hole and a crack are visible once the attached plate and bolts have been removed. Any exhaust leak into the engine compartment is cause for grave concern. These leaks cannot only cause an in-flight fire but the CO can get into the flexible heater hose leading to the cockpit. Photos #6 and #7 show the typical telltale signs of an exhaust leak. Do not ignore any signs of exhaust stains coming out of the cowling. The only exhaust stain should be directly behind the tail pipe exit.

As the GA fleet ages, these exhaust systems are becoming unserviceable and sometimes will have to be replaced with new components. Fifteen years ago a 1969 M20F muffler can was rebuildable. Now these parts have been overhauled so many times that they just keep cracking and aren’t providing a reasonable service life. When inspecting the exhaust system always look at the condition of the bolts and the springs that hold the ball socket joints together. These bolts will erode and the springs may become slack. Always install new springs and bolts if you question their integrity. Inspect the fresh air and heater air hoses for their security, no holes or tears, and the security of the supporting coil spring. A hole in any air hose coupled with an exhaust leak will allow CO to enter the heater system

Header pipes that have slip joints may develop exhaust leaks. This may indicate a worn slip joint and allow CO into the engine compartment. On all exhaust systems, inspect where the header pipe is welded on to the cylinder attach flange. This area cracks all the time on older systems and it is not uncommon to find numerous re-welds in this area. Most non-turbo Mooneys with Lycoming engines had a split #4 header pipe with a U-bolt type clamp. When you replace any part of the exhaust system, you must first disassemble the #4 pipe and reset it after you have installed a new component. This prevents stress on the entire exhaust assembly.

Photos #8 and #9 show a typical blister on a turbo-charged Mooney exhaust system. A blister on a turbo system can become a blowtorch in flight causing all kinds of ugly things to happen (like in-flight fire). Never allow any blister to go un-repaired. Remove the heater can shroud on all heat muffs at every annual and inspect for blisters or cracks (see photo #10). Always inspect the slip joints for excessive exhaust dust. Verify that all cylinder flanges are intact and not cracked. Look carefully at the turbo collector for cracks, for loose bolts securing the turbo and for blisters or bulges. Carefully verify that all V-band clamps are secured, properly torqued and safety wired. The torque on these V-band clamps varies depending on the size of the clamp and bolt. Most are 20 to 40 inch-pounds, but look up the correct torque for your model. Over-torqueing a V-band clamp can cause the assembly to fail.

Verify that the tail pipe is tight by giving it a simple tug. Be sure it doesn’t move against the turbo flange. On all exhaust systems, verify that all the nuts are tight on the header pipe studs where the pipe is bolted to the cylinder.

An easy and thorough method of checking for cracks and loose couplings is to use a Shop-vac in the blower mode, to slightly pressurize the exhaust system and check for leaks. But first, clean the inside of the exhaust pipe of any carbon build-up and insure the air stream from the Shop-vac is clean. You don’t want to blow dirt and dust up into the exhaust system and engine. Once the blower is connected and running, use a liquid detergent solution on suspected areas and check for bubbles. Remember, slip joints are meant to leak a small amount when cold.

Another sign of a possible exhaust leak on turbocharged systems is if the manifold pressure increases when you close the cowl flaps and decreases when you open them.

If you follow these basic instructions on a regular basis (at least every annual) and keep an eye out for the telltale signs of exhaust leaks, you will not likely be a victim of carbon monoxide poisoning. It’s always a good idea to install a quality carbon monoxide detector in the cabin. Placing a detector, such as DeadStop, in the air stream of a heater vent is an excellent method of detecting an exhaust leak before CO buildup becomes dangerous. If CO is detected in flight, turn off the heat, increase fresh air ventilation, use supplemental oxygen if available and land. Don’t forget that exhaust leaks can lead to fire.

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.