The pure jet engines you find in airliners rarely ever face such issues because they operate into well maintained aerodromes which are regularly inspected and cleaned to keep debris away from aircraft engines and other critical components. Runways and taxiways are regularly inspected to ensure that they are debris free.
Those aircraft, that are allowed to operate into poorly built aerodromes are usually fitted with what we call ‘gravel kits.’ It does not only prevent engine damage but it also protects the wheels and some critical flight controls. The Boeing 737-200 for example, can be retrofitted with such a kit. It comes with a vortex dissipator located right below the engine intake. This vortex dissipator uses engine bleed air to generate a stream of air to force away any foreign objects that can be engulfed by the engine.
Boeing 737-200 vortex dissipator.
Turboprops also need extra protection because most operators fly them into smaller aerodromes where foreign object damage is likely. In Dash 8 we have an engine intake bypass door. The engine intake of the aircraft is built in such a way that it creates a swirl. This swirl reverses the flow of air in the intake and as it tries to go forwards, the compressor pulls the air into the engine. The whole point of the process is to remove any heavy solid particles from the air. Because of the inertia most of them are dropped onto the aft area of the intake where there is the bypass door which creates an exhaust path for them to get out. When I used to fly the Dash 8s we used to operate into very small airports and our SOPs urged us to open the bypass doors for take offs and landings. It is also used in icing conditions, which allows the bypass to catch any solid ice that tries to enter the compressor stages. And the opening of the bypass door allows the operation of engine intake lip heating when temperature goes below a certain level.
The bypass door also has a safety mechanism. If the pilots forget to open it and if something heavy enters the intake, the door can shear off removing the object from the intake.
Dash 8 Q200/300 bypass door operation
Author – Anas Maaz (Airline Pilot)
