The Honeywell TPE331 engines containing a “U” in the engine model designation such as the TPE331-10UA and TPE331-12UHR are configured for an engine inlet up installation. These engines are primarily found on the Metro/Merlin and Jetstream 31/32. This inlet up configuration warrants several operational considerations that are different then the inlet down configuration found on most other aircraft.
To start, while parking these aircraft outside, moisture from rain and condensation will run down the inlet and pool in the gear case, possibly causing corrosion to the magnesium gearbox. There is a case drain at the bottom of the engine, however we have seen them get clogged by a piece of debris or dirt that has entered the engine. This can be cleared by sticking a piece of safety wire up the drain or by blowing shop air through the drain. We recommend that this is done on a regular basis.
Additionally, during winter operations this moisture that has run down into the gearcase may freeze and cause the impeller to be stuck. Pulling the propeller through by hand or by use of the starter and forcing the engine to rotate even though ice is preventing the rotating group to rotate freely may cause the first impeller to be damaged. We highly recommend checking to make sure that the engine rotates freely without using any force before each start. If there is greater than normal resistance, please make sure the ice inside the engine is melted prior to pulling the prop through or starting the engine.
Both the risk of ice damage as well as the gearcase corrosion can be reduced or even prevented by installing inlet covers while the aircraft is parked outside. Engine blankets type covers that cover the complete front of the engine are preferred, but inlet plugs work as well.
If you would like to discuss this article of if you have additional questions regarding the operation of your TPE331, feel free to contact us at (615) 624-4380.