Engine exhaust creates unsafe condition for Colt owner

My Piper Colt has a strong odor of engine exhaust in the cockpit. The mechanic says he is sure the muffler is good, and turning off the heater does not change the amount of odor.

Looking at the floor board, directly below the seats, I can see a lot of daylight. There’s an upholstery cover from the seat to the floorboard that snaps down and covers the area.

Lying on the floor and looking up at the bottom of the fuselage at the area of the landing gear bungee springs, it’s obvious this is where the daylight is coming through. Problem is, the exhaust pipe is directly in front of the right landing gear strut, about 24 inches forward. It’s easy to see exhaust residue all over this area and I’m fairly convinced this is where the exhaust odor is coming from.

Also worth mentioning is the tail pipe has been cut and only protrudes about one inch outside the cowl.

How far should the tailpipe extend below the fuse, and is it straight down or bent? Should the area under the fuselage be covered with a shroud or otherwise sealed to prevent exhaust gases entering?


Jacksonville, Fla.

I was a bit shocked and very concerned for your safety when I read your question and thought I should respond as soon as possible. There is no doubt you’ve got a potentially unsafe condition with your Piper Colt. It should be addressed as soon as possible.

Since my expertise is focused primarily on Lycoming engines, I felt I should contact someone who has a great deal of knowledge regarding the Piper product line, especially the fabric models. I contacted my old friend Clyde Smith, Jr. who is a former Piper employee and recognized worldwide as an expert on the entire product line of Piper fabric aircraft.

The first thing Clyde mentioned was to be certain you have your maintenance facility comply with the FAA AD Note 68-05-01, which is a muffler inspection. After we discussed the details and I described your observations, Clyde was very concerned about the tailpipe. It appears somewhere in the past it may have been modified and the current arrangement does not appear to be correct. You didn’t mention anything about a scarf cut on the tailpipe, which is very important for the exhaust system to function normally. It’s important that the scarf is towards the rear. It would appear the present configuration, allowing the pipe to extend only about one inch below the cowl, is too short and may be causing some of the problem. While Clyde didn’t mention any specific distance the pipe should extend below the cowl, he did feel yours was too short. He mentioned that there are aftermarket suppliers who can furnish the correct tailpipe for your aircraft, which is something you may want to consider.

Due to the design and normal operation of the aircraft landing gear, it’s impossible to seal the entire area off, but our feeling is with the proper exhaust/tailpipe configuration you shouldn’t have any problems with exhaust odor in the cabin.

I’d like to thank Clyde Smith, Jr. for his expertise and comments on this question and, Arthur, we both hope you’ll correct this potentially unsafe condition as quickly as possible.

I have a Lycoming O-290-D2 and notice there is an AD note that requires the timing to be set at 18° because of possible detonation under certain conditions. Why is that only for the 290 and not the rest of the Lycoming family?


Acworth, Ga.

Regarding your question why the O-290-D2 has an FAA AD Note to require the timing to be set at 18° vs. 25°, which is more common on the other O-290 models, as well as most other Lycoming engines, here are my thoughts: Since the compression ratio of the D2 is 7.50:1 vs. 6.50:1 and 7.00:1 on the other O-290 models, this would be my guess for the different timing on the D2. I can’t think of any other factor that would be a reason for the timing to change.

This is about the best I can do on this one Steve, and this change would impact the potential for detonation where higher compression is used.

Paul McBride, recognized worldwide as an expert on engines, retired after almost 40 years with Lycoming. Send your questions to: AskPaul@GeneralAviationNews.com.

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