Jet fuel is a fairly common smell in the passenger cabin when a plane is preparing to taxi. Far less so is the aroma of dirty socks, rancid cheese, or a wet dog— unpleasant signs that engine oil vapors have seeped in, too.

These smells are usually found in jet engine “bleed air” -- outside air that’s been shunted from the engines into an air conditioning system and then to the cabin. In various systems—cabin pressurization, water-tank storage pressure, even heating to dissipate wing ice—this air is need for the plane’s operation.