While no-one would deny that landing gear is a necessary and vital component of any aircraft, how much attention do we pay to it when flying? We know when it’s been retracted by the "whoosh" we hear in the cabin and how much quieter it suddenly seems. Conversely, we know when it’s been extended by the loud "thud" which occurs prior to landing. Because a moving aircraft causes friction and turbulence, it triggers sound waves. Generally, the faster the aircraft is flying, the more turbulence and friction will occur. So, when the aircraft's landing gear has been extended, more noise is made because more resistance has been created.
As aviation has advanced and technology has improved, landing gear has seen constant re-engineering and improvements. Writing about the importance of the landing gear in a white paper document, the consulting firm Infosys described this importance and improvement in engineering as follows, “The need to design landing gear with minimum weight, minimum volume, reduced life cycle cost and short development cycle time poses many challenges to landing gear designers. These challenges have been met by employing advanced technologies, materials, analysis methods processes and production methods.” Most landing gear are made by third-party suppliers, rather than by aircraft manufacturers themselves. Safran Landing Systems is one of the largest such companies, making landing gears for most Airbus aircraft, as well as the Boeing 787.
The primary function of the landing gear is to absorb the force of landing and, of course, to prevent the fuselage from striking the ground. This force absorption happens in several ways. Firstly, the main landing gear strut has a shock absorption system which uses compressible fluids. Secondly, the landing force is spread over a number of wheels. Several smaller and medium-sized aircraft have two-wheel-long landing gears (four wheels on each side in a 2-2 configuration). The larger Boeing 777 is notable for adding a third wheel to this. And most heavier aircraft, including the Airbus A340 and A380, and the Boeing 747 add an additional landing gear in the center of the fuselage.
If you’ve ever watched an aircraft take off, you’ve noticed that the gear is raised soon after leaving the runway. The landing gear structure is a significant source of drag, so raising it quickly is important when the aircraft needs to gain speed. Pilots will do this as soon as a 'positive rate of climb' is reached. Before this, the gear remains extended in case the aircraft descends back to the ground. On take-off, once there is an indicated positive rate of climb, the flying pilot will call "positive rate - gear up" and the monitoring pilot will select the landing gear handle to the up position. The landing gear is then raised into a compartment in the fuselage. Some aircraft have doors that close over the gear, which protects the gear structure and also ensures the aerodynamics of the aircraft. Others, like the Boeing 737, retract into a cavity in the aircraft's belly. Although things differ from aircraft to aircraft, Virgin Atlantic outlined how things work on its Airbus A330. In this case, the gear retraction (and extention for landing) is computer-controlled by Landing Gear Control and Interface Units. All aircraft have backup gear systems as well. The A330 has two independent control systems, alternated in use each time the gear is activated. The Boeing 747 has four separate hydraulic systems. As the gear is raised, it will often shorten to take up less space in the fuselage compartment. On the A330, that's achieved by retracting the shock absorbers in the landing gear strut.
The gear is lowered for landing once the airspeed reaches a designated level (this is 280 knots on the A330, for example). Doing so earlier could damage the gear, and it's usually prevented as lowering of the gear is controlled by the same dual computer system. There is a further backup system in case the hydraulic systems fail. On the A330, this is gravity-assisted, using electromagnets. Older aircraft may even feature a manual handle to lower the gear. Ensuring that the gear is lowered correctly and locked before landing has always been vital. (I recall, during training for my Commercial Pilot-Single Engine License, verbally calling out “three green and locked,” whenever all cockpit lights indicated the landing gear had been extended.) These days for commercial airline pilots, it's indicated by the computer unit using lights in the cockpit. There is usually a secondary, independent system indicating this as well. It was not that long ago that it was also possible to check this visually. Many older Boeing 737 aircraft, as described on the Boeing 737 Technical Site, have a viewing periscope in the fuselage above the main landing gear. This was discontinued from the 737 Next Generation Series when an independent indicator backup system was introduced.
Despite improvements in technology and numerous backup systems, emergency incidents involving the landing gear do happen. With so many moving parts and hydraulic systems in play, risk remains. Fortunately, there are cases of aircraft landing successfully despite problems. These so-called 'belly landings' are, of course, very dangerous though, and even if successful, will cause extensive damage to the aircraft. Simple Flying regularly reports aircraft incidents involving landing gear. In March 2020, for example, a Virgin Atlantic Airbus A330 was forced to return to London Heathrow after its landing gear failed to retract. And just a month earlier, the main landing gear collapsed on an Icelandair Boeing 757 at Keflavik International Airport. More recently, a fractured landing gear component prevented the pilots of a Delta Air Lines Boeing 717 from lowering its nose gear during a flight on June 28 of this year that ended with a gear-up landing. That is according to a preliminary report released by the National Transportation Safety Board (NTSB), which identified the defective part as an “upper lock link.”
Some other newsworthy and significant emergencies include a JetBlue Airbus A320 in 2005, which landed safely at the Los Angeles International Airport after circling the region for three hours with its nose wheels turned sideways, unable to be retracted into the plane. And in 2011, a LOT Polish Airlines Boeing 767 landed at Warsaw’s Chopin Airport with complete landing gear failure, after the hydraulic system failed. There have been additional notable examples of landing gear issues in recent years. For instance, an SAS ATR-72 had nose gear trouble when landing in Hamburg last year. The gear rotated 90 degrees to the surprise of the crew. In May of 2022, an AZUL Embraer returned to Recife, Brazil, following a landing gear problem.
But, these are exceptions to the rule, which is why unfortunate incidents like this are so newsworthy. Landing gear on commercial airliners are amazing feats of engineering brilliance. They are huge, complicated structures supporting enormous weight and extreme landing forces, all the while protecting precious cargo. Think about that during an upcoming landing.
Until next time…safe travels.
Great insider information. I will feel confident when flying next waiting for and hearing the “ whoosh” and the “ thud”.