An extra landing gear, but for what?

The DC-10, MD-11, Airbus A340 and Ilyushin Il-86 and -96 make a special appearance in 2022. A common feature of these machines is the central main landing gear.

Each aircraft type has its own landing gear. The vast majority of short-haul aircraft are equipped with a three-point undercarriage. The Boeing 777, 787, Airbus A330, A350 and Tupolev Tu-154 also have such a configuration, although unlike the rest, the 777, A350-1000 and Tu-154 have six wheels instead of four. wheel per left and per right main frame. Other large aircraft such as the Boeing 747 and Airbus A380 have five points of contact with the ground. But the DC-10-30/-40, MD-11, A340, Il-86 and 96 are an exception in the whole, they are equipped with a so-called center leg.

burden sharing

Douglas was the first to bring it to the DC-10-30. This American aircraft manufacturer’s first design of the DC-10 was the DC-10-10 with two main landing gear and one nose landing gear. That was more than enough for this machine’s starting weight. This was a device for the US market: transporting many people over (relatively) short distances. Due to interest from KLM, Lufthansa and Northwest Airlines, among others, a long-haul version was introduced, the DC-10-30. This variant had to carry more fuel and cargo and was therefore heavier. To maximum starting weight (MTOW), the structure must be adjusted somewhere so as not to overload the wing load and the wing-fuselage structure. By equipping an aircraft with a central main landing gear, a better load distribution is achieved in the fuselage. Also, this adds more stopping power because the middle leg also has brakes. The middle leg is not equipped with steering cylinders.

Similar story

The MD-11, which McDonnell Douglas later brought to market, is a further development of the DC-10-30 (KLM variant with CF6-50 engines) and DC-10-40 (Northwest variant with P&W engines). It made sense to keep the center leg in light of the ultimately higher MTOW achievable with the MD-11 design.

A similar story is involved with the twin-engine Airbus A330 and the four-engine Airbus A340. The European aircraft manufacturer built the A330 for the shorter distance, the A340 for the longer. The latter was used for the same reason as DC-10-30/-40 and the later MD-11 with center leg. While the A330-300, unlike the A330-200, is not equipped with a center tank, this machine did not need to be equipped with a center leg because the aircraft manufacturer used the same MTOW for both variants.

The Russians also supplied machines equipped with a center leg: Ilyushin Il-86 and -96. As the MD-11 is a modern version of the DC-10, the Il-96 is that of the Il-86.

power distribution

It was not just a technical choice for the aircraft manufacturers in question to provide the machines in question with a center leg. The influence of the weight of the aircraft, the track load, also played a role. With only four wheels on both the left and right main landing gear, the forces from the huge planes on the tarmac would not be optimally distributed. By adding two extra wheels in the middle, and even four on both types of Ilyushin, the forces were distributed over a larger area, limiting the impact on runways. It is also a fact that undoubtedly when the aircraft manufacturers introduce new types of aircraft that are very different from the previous ones, they will talk to the airport authorities about matters such as the runway system, the buildings and the logistics.

Countries without a middle leg

In an emergency, landing can be carried out without the center head undercarriage being unfolded. In that case, because the weight is not distributed over sufficient contact points, the impact on the fuselage may exceed the certified limits. There is a chance that the forces in the structure have become too great, as with a landing above the surface maximum landing weight (MLW). In that case, they write the manufacturer an inspection, usually divided into several phases. First, an inspection to look at specific points in the structure, regardless of whether deformation or cracks are found. If this is the case, further and more detailed inspections must be carried out. It is rare that further inspections are required after stage 1. These requirements apply not only to an ‘overweight landing’ but also to a hard landing (if it is detected in the aircraft that a certain g-value has been exceeded, or if a rate of descent is too high at the time of landing ). Prominent detail: an Airbus does not have to undergo an overweight landing inspection if it has occurred within a certain g-value. All other aircraft must also undergo an overweight landing inspection.

Leave a Comment