The staggering chicken test

In response to a previous column in which I expounded on the problem of exhaust valve recession with unleaded fuels, I received a note from Ron Newberg, which reminded me of work done by the oil companies back in the early 1970s. I actually ran some of these tests, in which we demonstrated that Tri-Cresyl Phosphorous (TCP) added to an unleaded fuel reduces the amount of exhaust valve recession. It worked well. Since TCP is approved for almost all aircraft piston engines, it is an immediate approved solution for the exhaust valve recession problem. But, alas, nothing is that simple. TCP will work, but it has some health concerns.

Back then we ran staggering chicken tests. In these tests, we shaved the necks of disease-free chickens and coated the area with the test material. After a short period the chickens were released and if they did not walk straight, you failed.

Most of you are probably thinking that I am stretching the details slightly, but I am not. This is an approved standardized test. Interestingly, the tests can also be run in rats, but the nervous system of a chicken is closer to that of a human. Disease-free rats are a dime a dozen, whereas disease-free chickens are very difficult and expensive to find. Remember this the next time you go out for a chicken dinner.

Back to my main point, TCP failed the stagger test, so is considered a neurotoxin. That is why most applications are changing over to butylated tri-phenyl phosphorus (BTPP). The point of all of this rattling on is that it may be a good idea for engine manufacturers to start looking at approving BTPP for use in current engines as a fuel additive. It is presently approved as a lubricant additive, so it should not be too difficult. The engine manufacturers may also want to test the additive for exhaust valve recession performance to document the performance.


I received a note from George Bracke who also had a problem with ethanol gas in his tractor. To test how ethanol fuel works, he filled a small olive jar half full of 10% ethanol auto gas. He added some water to the jar, put the lid on and shook it up. He then let the jar sit and watched what happened. Shortly, the water settled to the bottom and the gasoline went to the top with a yellow substance in the middle. This was the ethanol water mixture that would not go back into solution. He then let the jar sit with the lid off and the gasoline evaporated over time. What was left was the water on the bottom and a corrosive yellow cooking oil type substance on top. This is the stuff that can really do a job on your carb or fuel system. This is why you do not want any ethanol-containing fuel near your aircraft.

And now for the person who sent me a note in which he complained that in my last column I did not stress that pilots should always check their mogas for ethanol. A quote from that column: “I CANNOT STRESS ENOUGH THE IMPORTANCE FOR PILOTS TO ALWAYS CHECK EVERY FUEL PURCHASE FOR ETHANOL.” I am not sure how I can make that statement any stronger.


Ben Visser is an aviation fuels and lubricants expert who spent 33 years with Shell Oil. He has been a private pilot since 1985. You can contact him at


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