While most commercial aircraft today operate with two or three main landing gears, the Boeing 747 is one of a small number of aircraft to use four. We explore precisely why the 747 was designed with four main landing gears.
The Boeing 747 landing gear system
While the Boeing 747 was being designed in the 1960s, engineers were concerned about its heavy weight which could affect its ability to land safely. To counter this, they incorporated four main landing gears into the design, along with other features including triple-slotted flaps and split-control surfaces.
The Boeing 747 has four main landing gears in its center section, as well as its single nose landing gear. Each main landing gear consists of four wheels and two axles. An early variant of the 747 - the 747SR - added more structural support to the landing gears to help improve the aircraft's longevity.
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Distributing weight during landings
The 747 was the largest and heaviest commercial aircraft ever envisioned during its design and rollout, and to this day remains one of the biggest. Another mammoth of the skies, the Airbus A380, also uses four main landing gears given its size.
More gears allow weight to be distributed across the aircraft in a more balanced way. This reduces stress on each gear and increases longevity, an important quality for planes taking off and landing several times a week. While the 747 is technically cleared to land on just two landing gears, this would not be safe practice in the long-term.
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Weight distribution is also important when landing at certain airports. By spreading the weight of the aircraft across four different points, damage to the runway is reduced. This allows the 747 to land at airports with a lower load-bearing capacity on its runways. Designers of the Airbus A340 also had this in mind when adding a middle landing gear to the plane.
Fail-safe if landing gears malfunction
Landing gear malfunctions happen every now and then, usually forcing planes to make an emergency landing. One incident in Madrid last year led to a rough landing and a shower of sparks after a 737s landing gear fell into an engine. While landing gear problems rarely lead to major accidents, they will cause damage to an aircraft and can impact an airport's runway and scheduling too.
However, with the 747s quadruple main landing gears, pilots are still able to land safely in the event of a problem. The plane is able to land and take-off with just two operational landing gears should a gear malfunction. Despite this, landing with just two gears increases stress on the aircraft's structure so it is not standard practice.
What do you think of the Boeing 747 quadruple landing gear system? Let us know in the comments.