Airplanes do not have filters

First, we need to consider the effect of altitude and gravity on particles in the air. Clean air is vital to all combustion processes, however, ambient air contains multiple particles coming from sources as diverse as industrial processes and transportation to vegetation and volcanoes. One of the major factors in keeping a land based gas turbine clean is to prevent these particles from entering the turbine. The filter removal systems protect the turbines from particulates, which, if they would enter the compressor, could cause erosion, fouling or corrosion. At flying altitude, thanks to gravity, there are much less particulates in the ambient air that are smaller and lighter (volcanic ashes during eruptions would be the exception, in which case airplanes are not allowed to fly).

Secondly, let’s consider the by-pass ratio on the jet engine. We can compare the first blade of an airplane engine to a first stage or inertial separator in a filtration system. Speed and circumference of the first turbofan blade pushes heavier and larger particles away from the compressor into the bypass stream (bypass ratio¹) that goes around the engine, reducing the number and size of particles that get to the compressor.

At last, let’s consider maintenance. Both aircraft engines and gas turbines are expensive investments that require regular maintenance, but aircraft engines follow a much more extensive maintenance schedule. The turbine engines of airliners have to ingest some dust when on the ground, but they also get on average 20-50 man-hours of maintenance for every 250 flight hours. On average, depending on the use and the size (whether the unit is a frame unit or an aeroderivative unit), a turbine will run approximately 3 000 to 4 000 hours before an inspection is done. In one year, if you serviced your gas turbine as often as an airplane engine your turbine would only be available approximately 69% of the time. An airplane’s engine lifespan also averages a 20 year life cycle while gas turbines last on average 30 to 40 years, plus any life extending repairs.

In short, gas turbine air filters are built to protect the turbine against pollution in the ambient air. It is safe to say that an air inlet filtration system helps to reduce gas turbine maintenance by keeping the engine clean and also to extend the life of the equipment.

What we can conclude from he information above is:

• Particle ingestion at airplane altitudes are minimal as the air is cleaner, unless there is a volcanic eruption
• Thanks to the bypass ratio, larger particles are thrust into the bypass stream and not into the compressor.
• Gas turbine engines operate for longer intervals and are not serviced as frequently as airplane engines.

¹ Bypass ratio (BRP) is a term used to express the ratio between the quantity of air flowing though the bypass fan and around the core of a modern jet engine and that which passes through the core.