This is what the underside of a Boeing 737 looks like during flight:
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 model. For instance, the Embraer E-jet family also has a similar design:
The same goes for the ATR 42/72 turboprops.
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:
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.
With the exception of the 737 Max, every 737 fitted with high-bypass turbofan engines will have engine inlets that are slightly ovoid in order to provide sufficient clearance between the engines and the ground.
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.
▲ Belt loader
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, organise 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, besides safety considerations.
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 cover that encloses the hub (center) of the airplane’s wheel.
▲ Foreground: Wheel fitted with hubcap | Background: Wheel sans hubcap
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 and form a fairly aerodynamic surface, which reduces 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.
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.
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 protect the 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:
This means the wheel wells are vulnerable to damage from foreign debris during taxiing, take-off 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 I would assume this isn’t 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.
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 the original founding design of the 737.
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.
Thank you for reading.
Author – Isaac
Appendix
(Some stuff I wrote in the draft that I’ve omitted from my answer.)
Over the years, the 737 was altered to suit the changing needs of airline industry and the evolving demands of air travel. It’s design was adapted to be compatible with new technologies, and to stay competitive in the commercial airliner industry.
In the 1980s, high-bypass jet engines became more widespread for their higher fuel efficiency and lower noise. The 737’s low wings weren’t designed for these types of wider, larger engines, but Boeing engineers worked with engine suppliers to develop an unconventional strategy for overcoming the problem—by squashing the engine nacelles, and moving several engine accessories, like the fuel pumps, from the bottom of the engine pod to the sides, resulting in an unorthodox, flat-bottomed shape.
I’ve mentioned in my answer above that the 737 Max’s engine inlets and nacelles aren’t elliptical, compared to all other high-bypass turbofan 737 models.
That’s because the Boeing 737 Max’s engines are far too large to be stuffed under the wings with the short landing gear, so two things were done:
Firstly, the nose gear strut was lengthened.
Secondly, the engines were mounted in front of the wings, rather than underneath them, creating more space allowance for larger engines. This permitted a much more rounded engine nacelle design.
Despite extending the nose gear, and freeing up additional space underneath the aircraft fuselage, the main landing gear bay doors weren’t added. I surmise that Boeing opted not to devote more resources (i.e. money and manpower) into developing landing gear doors in order to introduce the 737 Max into the market as soon as possible, and thwart Airbus’ dominance in the new-generation single-aisle market, as demonstrated by the success of the A320neo family. Boeing also wanted to skimp on the 737 Max project anyways.
I wrote about the Embraer E-jet family not having landing gear bay doors. Interestingly enough, its successor, the E-jet E2 family, does have the wheel well doors. This is in contrast to Boeing’s approach with the 737 Max.
As with the 737 Max, the landing gear of every E-jet E2 family airplane must be made longer to accommodate the enlarged engines. In doing so, Embraer found sufficient space to implement the doors.
Image credits
1st image — Wikimedia Commons
2nd — It’s About Airplanes
3rd — Aviation International News
4th — Wikimedia Commons
5th — Wikimedia Commons
6th — Alamy
7th — Aviation Stack Exchange
8th — Wikimedia Commons
9th — Wikimedia Commons
10th — Flickriver[.]com
11th — Wikimedia Commons
12th — Airplane-Pictures[.]net
13th — (Screenshot from Mentour Pilot’s video on YouTube)
14th — The Boeing 737 Technical Site
15th — Airliner Spotter
16th — Wikimedia Commons (Top) + Carrushome[.]com
