But experienced pilot knew what was constantly happening inside a jet turbine engine. In order to produce forward thrust, air enters the fan in the front and becomes compressed by the layers of fan blades inside the turbine. Towards the rear section, fuel is added and ignited. This keeps the rotation in motion. The fuel extension also provides the pressurized breathing air for cabin air. This air is bled off from the compressor stages within the turbine. It is therefore called bleed air. The fan blades must rotate constantly and smoothly around the central shaft, which requires a special lubricant, the synthetic engine oil. The oil also has to perform well with the enormous heat that can reach up to 450 degrees Celsius or even higher. It is contained by special seals made from carbon. But whenever thrust is increased or reduced, small amounts of oil may leak through the seals and enter the air stream. These particles get burned or pyrolyzed by the hot engine and travel their way via the air ducts to gather with the compressed breathing air to the cabin. Even today, there are no filters installed within the system. Nor are there any sensors measuring the air quality on board, or which could alarm the pilots in case toxic chemicals enter the air conditioning system and become inhaled and flight. So the only sensor is the sense of smell of the pilots. If the seals are aging or fail completely, then visible smoke may enter the cockpit and the cabin.