The optional Upaved Strip Kit was made available for the 737-100/200 from Feb 1969. It allowed aircraft to operate from gravel, dirt or grass strips. At its peak of operation, 737s were making over 2000 movements a year from unpaved runways.
Whatever surface was to be used, certain guidelines had to be observed. The surface had to be smooth with no bumps higher than 3 inches in 100ft; good drainage with no standing water or ruts; and the surface material had to be at least 6 inches thick with no areas of deep loose gravel. Boeing offered a survey service to assess the suitability of potential strips. If a surface was not particularly hard it could still be used by reducing tyre pressure down to a minimum 40psi in accordance with a chart.
Prevent vortices forming at the engine intakes which could cause gravel to be ingested by engine. These consist of a small forward projecting tube which blows pressure regulated (55psi) engine bleed air down and aft from 3 nozzles at the tip to break up the vortices.
The dissipators are operated by a solenoid held switch on the overhead panel which switches off on a squat switch so that climb performance is not affected. (Similar to the wing anti ice switch on 3-900 series).
The Gravel Protect switch must be on or at the Anti-iceétest position at any time the engines are running on the ground. Take-offs and landings must be made with the engine bleeds off to ensure that there is sufficient bleed air for their effective operation. Air conditioning may be used with APU bleed air if required.
The nose gear gravel deflector is made of corrosion-resistant steel and has a sheet metal leading edge which acts as an aerofoil to give it aerodynamic stability.
When the gear retracts, the deflector is hydraulically rotated around the underneath of the nose wheel before seating into the faring at the front of the nose wheel well. The rotation is programmed to maintain the deflector in a nose-up attitude during transit. No extra crew action is required to use the deflector and in the event of a manual gear extension, springs and rollers will position it correctly.
The maximum speed for gear operation (V LO) is reduced considerably to 180kts and the max speed with the gear extended (V LE) is only 200kts.
Note that the ground clearance of this nose-gear unit is only 3.5 inches this is enough to allow for flat tyre clearance but care must be taken when crossing runway arrestor cables, particularly try to avoid taxying over the "doughnuts" that support any cables Operational Procedures
Gravity always wins!