Why doesn’t the Boeing 737 have landing gear doors?

This is what the underside of a Boeing 737 looks like during flight:

Image credit: Wikimedia Commons

The aircraft’s main landing gear wheels are not covered, and they remain exposed even when fully retracted. The nose wheel, on the other hand, is covered.

Given the lack of space under the plane while it’s on the ground, Boeing engineers determined that landing gear doors were impractical and unnecessary for the main undercarriage. Let me explain.

These “holes” on the Boeing 737’s belly isn’t unique to the plane. For instance, the Embraer E-jet family also has a similar design:

Image credit: It’s About Airplanes

The same goes for the ATR 42/72 turboprops.

Image credit: Aviation International News

These planes all share similar characteristics — they’re short-range, domestic/regional airliners with short landing gears and limited space between the fuselage and the ground.

This is the earliest 737 model from the late 1960s:

Image credit: Wikimedia Commons

The modern-day 737–800 was built on the foundation of the first-generation, retaining much of the original structural design, even as the 737’s role evolved from a short-range, domestic aircraft to now a medium-, and even long-range, airliner flying transatlantic missions, for example.

Image credit: Wikimedia Commons
Image credit: Alamy

With the exception of the 737 Max, every 737 fitted with high-bypass turbofan engines will have engine inlets that are slightly ovoid to provide sufficient clearance between the engines and the ground.

Image credit: Aviation Stack Exchange
Image credit: Wikimedia Commons

The short landing gear struts on the 737 necessitate such workarounds. So why make them short?

In the 1960s, Boeing positioned the 737 as an airliner that could serve small, domestic airports. Unlike major international airports, domestic aerodrome in those days didn’t have sophisticated facilities, such as airbridges, cargo loaders and belt loaders that could reach high places.

A belt loader | Image credit: Wikimedia Commons

Therefore, Boeing decided to make the 737 as low to the ground as possible. This allowed baggage handlers to load luggage without requiring much ground support equipment, and to climb into the cargo hold to load, organize and retrieve the bags.

The plane could be refuelled and serviced using smaller domestic airport trucks, and ground staff would not need to use lifting platforms to access the valves and servicing panels on the underside of the aircraft.

Passengers will board and disembark the 737 using airstairs, rather than airbridges.

Since the plane was deliberately designed to be low, there wasn’t enough space under the aircraft to accommodate wheel well doors for the main landing gear. If implemented, the doors will likely come in contact with the runway, taxiway or apron surface should they be deployed on the ground. When the plane is in motion, they could potentially scrape the asphalt or concrete and cause damage.

You might be thinking — but the doors will never be actuated on the ground, only in the air. It’s important for Boeing to ensure absolute safety in the design of their airplanes. There had been numerous incidents where the landing gear failed to deploy, resulting in emergency belly landings.

The landing gear is susceptible to failure, and so are their doors. If the 737 were to be equipped with landing gear bay doors, and they fail to close up before landing, things wouldn’t be good when the plane touches down.

But there’s more to it.

Landing gear doors and their mechanisms take up space within the belly of the aircraft, add weight to the aircraft, increase design complexity and require additional maintenance. This led Boeing engineers to ponder if the landing gear doors were really a necessity.

Given the reduced ground clearance of the 737, which rendered the wheel well doors much less feasible than other airplanes, and complications associated with developing and implementing the doors, Boeing decided to omit them in favour of a simpler alternative solution.

This solution involves two items — a hubcap and a rubber seal.

The hubcap is a circular metal rim cover that encloses the hub (center) of the airplane’s wheel.

Foreground: Wheel fitted with hubcap | Background: Wheel sans hubcap | Image credit: Flickriver.com

The hubcaps are attached onto the sides of two main landing gear wheels that are visible after the gear is retracted, and their main purpose is to smoothen the surface of the wheels, forming a fairly aerodynamic surface and reducing drag.

Both exposed wheels on this 737 have hubcaps, one of which is very dirty

The rubber seal surrounds the opening of each wheel well, sealing off the gap between the wheel and fuselage surface once the gear is retracted. This creates a protective barrier that prevents foreign objects, rain, and other elements from entering and potentially damaging the components inside.

Image credit: Airplane-Pictures.net

This seal consists of many thick, curved pieces of rubber overlapping each other. When the wheel is fully retracted, it will push the drooping rubber pieces up, securing them in place.

Image credit: Mentour Pilot/YouTube

So, to recap, the exposed sides of the main landing gear wheels are capped with hubcaps, making them more aerodynamic. The rubber seals fitted along the border of the wheel well safeguard inner components from external conditions.

This is essentially Boeing’s workaround for the lack of landing gear bay doors. However, it cannot fully compensate for the doors’ absence, because the aircraft’s wheel well is wide open when the main landing gear is extended, as shown in this picture:

Image credit: The Boeing 737 Technical Site

This means the wheel wells are vulnerable to damage from foreign debris during taxiing, takeoff and landing, which is the main drawback of this design.

However, the 737 has been around for 50+ years, and it has gone through many generations of upgrades and modernisations. Yet, not a single wheel bay door has ever been installed, so this isn’t really a pressing issue faced by 737 operators.

The unprotected wheel wells on the 737 also makes it easier for stowaways to climb in, compared to an Airbus A320, for instance.

Image credit: Airliner Spotter

However, this is negligible, because there’s no data to suggest airliners without landing gear bay doors more commonly involved in stowaways flights than those with the doors.

In conclusion, the reason why the Boeing 737 does not have covers for the landing gear wheels is mostly because of its original founding design.

It was built to be a short-haul, regional airliner, with low ground separation, allowing ground staff working in small domestic airports to service and prepare the aircraft easily in light of the constrains and limitations of those airports in the 1960s.

With such a design, landing gear doors were not deemed feasible by Boeing, from a safety standpoint. Additionally, the complications surrounding development of the landing gear bay doors, as well as the added weight and maintenance considerations ultimately led Boeing to conceive a workaround that reduced the overall complexity of the Boeing 737 project.

Thanks for reading.

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