Engine Problems In High Altitude Flights With Low OAT

On recent flights I encountered a new problem I never had before : I almost had to shut down RH engine in FL 240 !

The story :

On 9.4.98 we refuelled 475 ltr. Avgas 100LL in Nimes-Garons after a 1:45 flight from Germany. Immediately after refuelling we flew on to Biggin Hill, GB in FL 240 (-42 deg.C). Soon after reaching FL 240, being permanently in clouds, the fuel flow (= fuel pressure on the build-in instrument) gradually increased while fuel flow on the Shadin remained constant. Fuel pressure increased on both engines but was significantly higher on the RH engine. The RH tanks were filled first.

When I tried to increase fuel flow on the RH engine to compensate for an increase in TIT, fuel pressure went sky high while fuel flow dropped to an unacceptable level to keep the engine running. I had to make an emergency descent with one engine in idle, still prefireing due to a too lean mixture. In FL 150, where temperatures were warmer, everything returned to normal and we could go on with our flight.

I must emphasize that this problem has never occurred before at any temperature. And we had flown this winter in up to minus 50 deg. C.

At the end of this loop back in Germany I had the fuel and injection system as well as the fuel pumps thoroughly checked. No malfunction was found. The hangar reported as the most likely cause a fuel contamination (water ? Fuel out of specification ?) from the fuel we took on board in Nimes. The contaminated fuel must have closed off the fuel supply in the injector most likely due to freezing.

Beechcraft`s POH does not say anything about this problem, but other handbooks do. Cessna recommends the addition of ISOPROPYLALCOHOL to the fuel up to the maximum of 1 % by volume. Lycoming says in SERVICE INSTRUCTION No 1070L the following


Isopropyl alcohol in amounts not to exceed 1 % by volume may be added to the fuel to prevent ice formation in fuel lines and tanks. Although approved for use in Textron Lycoming engines, isopropyl alcohol should not be used in the aircraft fuel systems unless recommended by the aircraft manufacturer.

Since neither the DUKE`s POH nor the maintenance manual says anything about isopropyl alcohol we decided to go ahead and try it anyway. I now fly with a concentration 0,5 % by volume isopropyl alcohol and voila´: the problem is gone. No more increase in fuel pressure, constant fuel flow and everything is fine again. If you are experiencing similar problems I strongly recommend the same procedure and I certainly want to hear your ideas about the topic. One word of caution : please dig deep for the old calculating skills so that you always know the exact percentage of isopropyl alcohol in your tanks. Too much can have some serious adverse effects on your fuel system.

One other thing that is interesting when flying hot and cold : oil temperatures. On some DUKES,mine as well, oil temperatures can drop well below 70 deg. C in a high and cold environment. Sometimes it even drops below 60 deg. C. According to the Operator`s Manual Avco Lycoming TIO-541Series Aircraft Engines this is unacceptable. Two reasons : (1) all engine governing action is adversely affected by too cold and therefore too "hard" oil and (2) acid residues that form during engine operation and are a main reason for internal engine corrosion are not evaporated at that low temperatures. If your engine behaves like this, and this is actually a sign of good health of the piston rings, please close off part of the oil cooler by using the DUKE`s baffle assy (P/N 60-590020-1 still available new) or construct something similar made from wood or metal that closes off half the oil cooler. You will find out that will not have a problem with too high oil temperatures in climb but that your oil temperature in cruise will stabilize around 75-80 deg C. Leave your baffles on until the ambient air temperature exceeds 20 deg. C..

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The page was last modified 04.08.06