Where the rubber meets the runway
Admire most any jet aircraft sitting on a ramp, and the landing gear probably isn't high on your list of the airplane's most eye-catching attributes. Nevertheless, this pedestrian system has some added twists in jets, in deference to their higher takeoff and landing speeds, large gross weights, and the sheer physical size of some aircraft.
For starters, the tires need to be able to hold together on the runway at jet takeoff and landing speeds, and thus they have higher pressure and speed ratings than most light-aircraft tires. The clouds of bluish smoke that accompany wheel touchdown are testament to the abuse that jet aircraft tires regularly experience, going from zero to 140 knots or more in an instant. Ratings of 195 kt are common for commercial jet aircraft, and those of some military aircraft tires are even higher.
While corporate and commercial jets operate at significantly lower runway speeds most of the time, it's worth noting that high-gross weight operations at high-altitude airports can result in speeds that meet and even exceed tire limits. This is because true airspeed (and thus runway groundspeed) increases as altitude increases for the same indicated airspeed. During a heavyweight takeoff from Bogota, Colombia's 8,300-foot-high airport last year, an outer ply of one of the main tires on our Boeing 757 separated at rotation. The groundspeed readout showed we were traveling at the 195-kt tire-limit speed as the wheels lifted off the runway.
While this incident resulted in only minor damage to the underside of the wing from shed rubber pieces, the destructive potential of tire debris flung at high speeds is considerable. Such events have caused engine failures or damage to important components such as hydraulic lines and fuel tanks — and worse. After the Concorde experienced its first-ever fatal accident in 2000 in part because of damage from tire debris, tire manufacturer Michelin created a new kind of radial tire that was a critical component of returning the aircraft to service. The new design is lighter and more resistant to damage at high speeds than those it replaced. If it does fail, tests have shown it will disintegrate into much smaller pieces than traditional aircraft tires do, which should help limit aircraft damage.
Heat can create big problems for jet aircraft tires and is a prime cause of tire failures. Heat is generated through braking action as well as by tire flexing during runway operation. Just like the tires on your car, improper inflation can result in excessive flexing and therefore greater heat buildup. Carried to an extreme, heat can even lead to tire fires, and tires are pressurized with inert nitrogen gas to minimize this fire potential. Nevertheless, there have been cases of wheel fires in jets following excessive taxiing, although this more commonly happens after a rejected takeoff (RTO). Some jet aircraft are equipped with brake temperature gauges with which the crew can monitor individual wheel temperatures from the cockpit. If concerned about overly warm tires after takeoff, one helpful technique is to leave the landing gear down for several minutes of air cooling. Heat also negatively affects brakes, which become less effective with increasing temperature. When sufficiently hot, they may be incapable of providing enough braking energy to stop an aircraft during an RTO or other high-demand situation. A series of short, closely spaced flights can have a cumulative effect on brake heat buildup. Crews can reference so-called quick-turnaround charts to better judge the likely effects of heat buildup on wheels and brakes during a series of flights or following an RTO. These charts account for aircraft weight, field elevation, and other factors, and they provide a handy reference for required ground cooling times between flights. Absent quick-turnaround charts, pilots can request that a mechanic physically measure the temperature of the brakes for a more accurate assessment of the situation.
A high-speed RTO in a jet can transfer a tremendous amount of heat energy to wheels and tires in a short time. To prevent subsequent tire explosions as brake heat radiates through the wheels and tires, tires are equipped with fusible plugs. These have a lower melting temperature than the main tire rubber itself, and will allow pressure to release before the tire can explode. Fusible plugs will not prevent a tire explosion caused by overinflation, however.
Unlike light aircraft, most jets require more than one main tire per side because of their heavier weights. The Boeing 777, for example, has three axles and six tires per side, arranged in what is called a main gear truck. But the Soviet Antonov 225 cargo airplane appears to win the prize for the greatest number of tires on a single jet aircraft. Capable of gross takeoff weights as high as 1.3 million pounds, this six-engine transport has no less than 32 main and nose gear tires.
Getting around on the ground is generally accomplished using hydraulically powered nosewheel steering in jets, although even the largest transports retain some lesser degree of rudder pedal steering. Nosewheel steering on the Boeing B-777 also steers the aft- most pair of wheels on each main gear truck. This interconnect feature is found on only a few large jets.
The extremely long length of some jet transports creates interesting landing gear considerations for both designers and pilots. During takeoff and landing, many jet aircraft are said to be geometrically limited. This means it is not possible to rotate or flare beyond a certain angle without striking the tail on the runway. For example, when properly rotated at between 2 and 3 degrees of pitch per second, the Boeing 767-400, which is slightly more than 197 feet long, will lift off the runway when passing through an attitude of about 7 degrees. The tail will clear the pavement by just 32 inches. However, if rotated too soon or too quickly, a tail strike will occur at 9.5 degrees pitch. Rather than design extra tall landing gear to minimize the chance of tail strikes (which would create its own set of design issues), Boeing instead depends upon proper pilot technique to do the trick.
Vincent Czaplyski holds ATP and CFI certificates. He flies as a Boeing 757/767 captain for a major U.S. airline.
