Not all runways are smooth; some are rough surfaces such as gravel while others use grass or sand surfaces. This has implications for which aircraft can use them. A modification kit was offered to help the Boeing 737 with gravel runway landings, and in this article, we explore the details.
Boeing’s Unpaved Strip Kit
Not all aircraft can safely land on unpaved surfaces, with the potential to damage the engines, gear structures, or fuselage. This is usually limited to smaller aircraft, rather than larger jets. One of the limited solutions made available, though, was for early Boeing 737 aircraft. From 1969, Boeing made an optional Unpaved Strip Kit available for their 737-100 and 737-200 series aircraft.
These kits made several modifications to the aircraft to enable them to operate more safely from gravel surfaces. There were also several guidelines for the type of surface suitable. These included:
- Generally smooth, with no bumps raised more than three inches.
- Good drainage and no standing water
- Surface material of at least six inches thick, with no deep loose gravel
Modifying the 737-200
So what modifications were made to the aircraft? The kits offered several changes and additions to the 737, designed to protect the fuselage and engines from gravel damage, and also to reduce wear and maintenance expense. These details for the kits are based on coverage at The 737 Information Site.
- Deflectors added to the nose gear to keep gravel from the underside of the aircraft. This required modification to automatically align the deflector when the gear was lowered. Additionally, smaller deflectors added to the main landing gear to protect the flaps.
- Metal shields placed over brake cables and hydraulic tubing on the landing gear.
- Reinforcement on the underside of flaps.
- Teflon based paint applied to wing and the underside of the fuselage.
- An additional retractable light added.
- Vortex dissipators were fitted below the engines. These prevent vortices forming, which could cause gravel to be ingested by the engine. These worked by blowing bleed air from the engines to break up vortices. The system should be operated both during take-off and landing.
These structural modifications, of course, brought changes to the operating procedures and limits for the aircraft. For example, the maximum speed for gear operation was reduced, and when landing limits were placed on the use of reverse thrust. There were also guidelines for reducing tire pressure, depending on how soft the surface was.
Mostly used in Canada and Alaska
According to discussion on b737.org.uk, at peak level, there were over 2,000 Boeing 737 operations a year from unpaved runways. These were mostly runways in Alaska and Canada, many of them serving remote locations. Airlines that have made use of the kits include Alaska Airlines, Canadian North, First Air, Air Inuit, Nolinor, and Air North. They were mostly fitted to the 737-200C (Combi version – allowing easy conversion between passenger and freight use).
As of early 2020, a few of these airlines are still flying 737s with Unpaved Strip Kits. Nolinor Aviation in Canada operates up to nine 737-200C aircraft, Air Inuit operates four, and Canadian North operates two (according to fleet data on Wikipedia). Chrono Aviation in Canada uses three 737-200 that are equipped with gravel landing gear also.
Alaska Airlines retired its eight 737-200C aircraft by 2007. These were replaced with 737-400 Combi aircraft, without any conversion for gravel landings.
Unpaved Strip Kits have not been made available for newer 737s, and are a rare sight now. Over time, many runways have been upgraded (this has certainly helped Alaska Airlines, for example), and the northern Canadian airlines do have other aircraft in their fleets that can land on rough surfaces.
Do let us know in the comments if you know of any other airlines or locations where these aircraft may still be operating.