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Gravel Kits: Boeing 737 Dirt Runway Operation

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While most of us may be used to flying in and out of a paved runway, gravel and dirt runways are the norm for those living in isolated communities towards the Arctic Circle or other remote locales.

Boeing once provided a gravel kit for the aircraft variants 737-100 and -200 from February 1969. This kit included a selection of modifications that could enable the aircraft to arrive and depart on unpaved runways. While no longer available for new jets, this article will explain what the kit looked like.

an airplane on a runway

Gravel runways are built in remote areas where the budget would not suffice a paved runway, and these communities may only see scheduled services once or twice a week. The gravel runway is also very common for airports experiencing sub-zero temperatures, which may have ice runways in the winter or rely on critical cargo freight.

Easily matching the reliability and strength of a paved runway, gravel is a practical solution for low-volume airports. Simultaneously, they require more maintenance than a more common sealed runway; they are expected to last around ten years before complete replacement.

Unpacking an unpaved strip kit

Since not all aircraft can land safely on an unpaved runway, the kit was designed to mitigate potential damage to the engines, gear structure, and fuselage. While historically limited to smaller aircraft, Boeing designed a solution for the early 737 variant, which included the -100 and -200 versions. These packs included:

  • Deflectors are added to the nose gear to limit gravel damage from the underside of the aircraft—this required modification to ensure alignment of the deflector when the pilots would lower the load. Additionally, smaller deflectors could be added to the main landing gear to protect flaps.
  • Reinforcement on the underside of flaps.
  • Metal shields were to be placed over brake cables and hydraulic tubing on the landing gear.
  • Vortex dissipators were fitted below the engines. These prevent vortices from forming, which could cause gravel to be ingested by the aircraft engines. These worked by blowing bleed air from the engines to break up vortices. The system should be operated both during take-off and landing.
  • Teflon-based paint was applied to the wing and the underside of the fuselage.
  • An additional retractable light was added.

While each runway could differ slightly, the packs had several guidelines that outlined suitable surfaces for safe operation:

  • 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

These modifications to the aircraft introduced new operations procedures for the pilots and crew, such as limiting the maximum speed for take-off and landing and limiting the use of the reverse thrust. Extra guidelines requested a reduction in tire pressure, depending on the quality of the runway.

A small market

Having initially been used extensively by Alaska Airlines since its inception, the number of carriers using the kits has dwindled, and Boeing decided it was no longer financially feasible to continue manufacturing the product. And given more airports were moving to paved runways, the demand for gravel kits decreased. While they remain in use in Northern Canada and other isolated locations, other aircraft have also adopted similar products to aid in fly-in-fly-out operations, such as in the outback of Australia, like Cobham Aviation with its modified BAe 146 jets.

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737-200 Flies on for the next 25-30 Years!

Boeing's decision to not release these for newer types of aircraft has seen a steady decline in the number of operators using the unpaved strip kits, with Canadian North, operating out of Iqaluit, Nunavut, and Yellowknife, Northwest Territories, retiring its last capable 737-200 earlier this year in May, noting that the ability to source replacement parts for both the aircraft and the gravel kit modifications becoming increasingly difficult.

The 'Spirit of Yellowknife' aircraft operated its final flight some 43 years after rolling off the Boeing factory floor and included a 20-year tenure with the carrier. The carrier has now moved to use the ATR turboprop in these isolated locations, which don't require as many modifications to make the services possible.

While this was the last for Canadian North, it doesn't leave Northern Canada without an operator. Nolinor Aviation, a charter carrier based in Mirabel, Quebec, operates passenger and cargo services within Canada and the United States. Starting from humble beginnings in 1992, the airline purchased its first aircraft, a Convair 580, in 2001. This eventually grew, including acquiring two ex-Royal Air Maroc 737-200 combis in 2007.

I can confidently state that, due to their low flight hours and cycles, we anticipate operating these "birds" for an additional 25 to 30 years. Their longevity is a testament to our meticulous maintenance and operational efficiency. Moreover, we have a robust partnership with Boeing, who has committed to supporting this crucial segment of our fleet, particularly in our operations up north. It’s a point of pride for us at Nolinor Aviation to possess the world's largest fleet of 737-200s.

Marco Prud'Homme, President of Nolinor Aviation

While the 737 is one of the most famous aircraft variants on the planet, other rear-engined aircraft also once operated similar gravel kit modifications, including the McDonnell Douglas DC-9, MD-80, and MD-90, and Boeing 717 and 727.

Have you ever flown on an aircraft equipped with an unpaved strip kit? Let us know in the comments.

This post was brought to you by Simple Flying. Written By Chris Loh, and Aaron Bailey.