Gravel kit

While I was a student pilot I had a friend and mentor who was an instructor at a big school in California. I gave him an outdated Canada Flight Supplement, the fat then-green [now blue] book that lists all Canadian airports along with their frequencies, runways, nav aids, services, and other information for pilots. He had already given me an equivalent book for California, the A/FD, and I wanted to show him how the information was presented differently in the Canadian equivalent. I thought ours was better, as it includes pictures, and fewer cryptic abbreviations for information required in flight. The FAA seems to agree with me about the pictures, because they've since added them to the American publication.

I've told you this bit and some other parts before, but it's a good introduction to the rest of the post. My friend flipped open the CFS and looked at the first entry that met his eye. "Huh? Why does it say they have Jet-A fuel for sale at an airport with a gravel runway? Why would someone land a jet on a gravel runway?"

Even then I knew enough about my country's geography to have the answer. "Because that's their destination, and it's the only runway for hundreds of kilometres, and it's too far from anywhere to serve easily with piston aircraft." Paving is difficult in remote locations. Gravel is easily relevelled when the ground heaves with the freeze and thaw. It's not as slippery as pavement when covered with ice, and once the snow is packed deep, it doesn't matter what kind of surface is underneath. Pavement is nice to land on, but it won't keep you from sliding off the end on a snowy day.

You need particular techniques and equipment to cope with northern conditions. A lot of it is simply being diligent in applying the techniques you already know. Land under control, at the lowest speed that is safe to maintain on approach, at the beginning of the runway, and with no sideways momentum. Sturdy landing gear and runway friction will forgive a multitude of sins, but if you're going even a little bit sideways at touchdown and there's limited friction, you'll keep going sideways until you meet some resistance, maybe a snowbank at the side of the runway.

Gravel that is not covered in snow actually allows for some very soft landings, maybe because it crunches slightly out of your way, spreading the deceleration from any remaining vertical speed over a longer time than wheels hitting pavement. I flew on gravel for so long I can remember the "whoa, lines!" feeling when I went down south and landed on a paved runway with paint markings. At startup, taxi, and takeoff on gravel you have to be careful not to damage the airplane with flying gravel. Many gravel airports have little paved pads you can park between, positioning your engines over them so that when you start up or apply taxi power, the propellers won't kick up gravel. With jets, the danger is the nosewheel launching gravel that subsequently gets ingested into the engines. 

Here, and this is the whole point of this post, I have some pictures of the B737-200 gravel kit, a modification that physically deflects gravel from being kicked up by the nosewheel and then pneumatically protects the engine intakes.

Gravel kit-fitted B737-200

The optional Unpaved 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.

Components included:

• Nose-gear gravel deflector to keep gravel off the underbelly.
• Smaller deflectors on the oversized main gear to prevent damage to the flaps.
• Protective metal shields over hydraulic tubing and brake cables on the main gear strut.
• Protective metal shields over speed brake cables.
• Glass fibre reinforced underside of the inboard flaps.
• Metal edge band on elephant ear faring.
• Abrasion resistant Teflon based paint on wing and fuselage undersurfaces.
• Strengthened under-fuselage aerials.
• Retractable anti-collision light.
• Vortex dissipators fitted to the engine nacelles.
• Screens in the wheel well to protect components against damage.

The protective equipment listed above means that extra maintenance costs due to tire wear, paint loss, etc is only about $15 per landing (1975 rates).

Vortex Dissipators

Vortex dissipators that shoot out compressed air in front
of the engines

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-ICE/TEST 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.

It looks as if it is resting on the ground, but there is actually
a nine centimetre gap between the plate and the ground

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

• Antiskid must be ON for takeoff and landing. (AFM)
• Vortex dissipators must be ON for takeoff and landing. (AFM)
• Maximum taxi EPR on gravel: 1.4. (AFM)
• Gravel Protect switch: ANTI-ICE position when using engine inlet anti-ice. (AFM)
• Use of rudder pedal steering rather than tiller is recommended to make all turns as large as possible to prevent nose gear from digging in.
• Thrust to be kept to a minimum to sustain a slow taxy speed.
• If runway is dusty try to manouvre so that your jet blast does not pick up loose debris that may be blown back over the runway in a crosswind. Dust should be allowed to settle before starting takeoff roll.
• Notwithstanding the above, use a rolling takeoff wherever possible to avoid debris ingestion when takeoff thrust is set. EPR should be limited to 1.4 or less before brake release.
• For landings, use of autobrake is recommended.
• When landing on gravel, use approximately idle reverse, not to exceed 1.8 EPR. Stow reversers by approx 60kts. (AFM)

 
Source:
http://airplanepilot.blogspot.hu/2011/02/gravel-kit.html
http://www.b737.org.uk/unpavedstripkit.htm