Engine Exhaust Tube Cracks

Cracks in the exhaust system that can occur on some engines. These cracks can occur on any of the four exhaust tubes but most commonly on #2 but I’ve discovered cracks in #1 also. These cracks occur on the first bend of the tube as it exits the engine.DSC_0372
On your existing exhaust tube this area is modified from the standard Rotax exhaust tube configuration to accommodate the installation of the engine in the S10 engine compartment.  There are several welds in this area along with an EGT Service Port and on the opposite side a welded up EGT Port. Cracks will start and emirate from this area around the bend, EGT Service Port, and welded up EGT port.
DSC_0375Stemme has recognized this and provided Service Bulletin SB A31-10-075 having to do with exhaust cracks. Stemme has also informed the FAA and EASA and they have published FAA AD2006-19-08 and EASA EAD2006-0217. All these SB/AD’s indicate the exhaust tubes should be inspected at the next 100 inspection/annual but I highly recommend this be a continuous part of your routine maintenance program and even your preflight examination if you are likely to pull the side cowls (particularly the left) for your checks. The SB/AD’s indicates it’s a one time check but I have discovered that this can happen at any time. I have replaced or repaired eleven exhaust tubes and have discovered several things. Somewhere starting around 150 engine hours and up to and around 250 engine hours you may get a crack.

During your inspection you may need a 10X magnifier to see the early stages of a small crack all the way up to the very obvious and visible crack or cracks. During your examination you will have to remove about six inches of the exhaust wrap that extends up to the flange. It’s brittle, so be careful unwrapping it or expect to replace it.

In some cases it will be easy to see even without removing the exhaust wrap. If the crack is big enough, it will blow a hole in the exhaust wrap and deposit a large amount of exhaust soot into the engine compartment.

On exhaust tube #1 generally cracks are on the inboard side of the bend and deposits exhaust soot all over the oil filter, the frame and forward heatshield. The #2 exhaust tubes generally cracks on the outboard side around the EGT Boss and coats the forward heatshield and side cowl. I have not seen bit cracks on the #1 like I have on the #2 and have only repaired two #1 tubes. Half of the #2 tubes were repaired (small cracks) and half were replaced (big or multiple cracks).

The new replacement tubes are now one piece stingless steel tubes formed into the proper configuration with only couplings welded to the ends for mating to the engine. This should eliminate cracks developing in the future.

new exh tube 4I have found the new tubes don’t fit as closely as I’d like and needed some manual stimulation to get them to mate properly. Obviously, new exhaust wrap is required when fitting the new tube.

SB A31-10-075 has a comment sheet that needs to be filled out and emailed back to Stemme so they can track the cracks and repairs or replacement of the tube.

As always, please feel free to contact me if you have any questions or need parts.