What is the "gravel kit" on some Boeing 737-200s for?

A gravel kit is a modification designed to prevent damage or foreign object debris (FOD) ingestion while the aircraft operates on unpaved surfaces. The stones, dust and debris an aircraft kicks up on landing can damage vital engine parts. The kit addresses two issues: the nose gear throwing FOD toward the engine and toward the lower fuselage or wings, and each engine ingesting FOD from the ground just ahead of it. In 1969 Boeing built a gravel kit for its well-known 737, letting its commercial jets reach more remote locations. The option reduced foreign-object damage when operating from remote, unimproved or unpaved runways.

The kit includes a gravel deflector, similar in shape to a wide ski, mounted on the nose wheel, and vortex dissipators fed by compressor bleed air ahead of each engine. Deflectors are added to both the landing gear and the nose gear to keep rocks from damaging the underside of the aircraft and the flaps. The package also adds metal shields to prevent damage to brake lines and hydraulics. The vortex dissipators blow engine bleed air downward, below the intake. The work also includes reinforcing the flaps, applying Teflon paint to the underside of the wings and fuselage, and installing an extra light.

Alaska Airlines historically used this modification. It remains in use at Air Inuit, Canadian North, Air North and Nolinor Aviation, which operate in northern Canada.