Wikipedia
Gravel kit is a term for a modification which is used on some aircraft to avoid foreign objects/debris (FOD) damage and/or ingestion while operating on unpaved surfaces. Modifications generally include methods of preventing the nose gear spraying FOD into the engine and onto the underside of the fuselage or wings, and methods of preventing each engine from ingesting FOD from the ground directly in front of it.
Examples of planes that can be fitted with this upgrade include the Boeing 737-200, which includes a gravel deflector (similar in appearance to a wide ski) mounted to the nose wheel, and vortex dissipators, using compressor bleed air, in front of each engine. This modification was used by Alaska Airlines for many years, and is still employed by Air Inuit, Canadian North, Air North, Nolinor Aviation and First Air in northern Canada. Some Boeing 727s were also fitted with specific gravel kits on the nose wheel, while the rest were fitted with devices in the form of smaller deflectors than those used on the 737, used on all wheels, to deflect FOD away so as to prevent the rear-mounted engines from ingesting FOD from a runway. Another rear-engined aircraft--the McDonnell Douglas DC-9/ MD-80/ MD-90 and Boeing 717 series--uses deflectors similar to those of the conventionally-equipped 727 on all their aircraft's landing gear to prevent FOD structural damage and ingestion.
Image:737-200_Gravelkit.jpg|Canadian North 737-200 engine gravelkit vortex dissipator. Image:737-200_Gravelkit_profile_view.jpg|Canadian North 737-200 engine gravelkit vortex dissipator in profile view. Image:C-GANV landing in Vancouver.jpg|Gravelkit on a landing Air North 737-200