ok, thanks alot. now when i think about dirt and old plane engines it reminds me of my trip to Paris this summer, I went there for Le Bourget of course.
the showed an old Lockheed Constellation, i had never heard of it before and at the time i didnt apreciate it as much as the new state of the art fighters.
anyways, when they started it in front of the audience there was a huge cloud of thick black smoke and then flames of fire coming out of the engines. So you can understand if an old WWII (dont know for sure but i think the Constellation flew some time around WWII) time plane gets some exhaust stains over time
really old stuff i guess. or maybe it was standard procedure in the old days with fire in the engines?