How Things Work: Stopping the A380
Hint: Plan ahead.
- By Mark Huber
- Air & Space magazine, August 2011
With the Airbus A380 weighing in fully loaded at 1,265,000 pounds, you might think stopping it within a reasonable distance after landing would require a Phalanx of Heavy-duty thrust reversers.
Truth be told, in the megaliner’s braking system, thrust reversers are the least critical components. Airliners are not required to have thrust reversers, and only the two inboard engines on the A380 are equipped with them. The decision not to install reversers on the A380’s two outboard engines saved weight and lowered the chances that those engines, which sometimes hang over runway edges, would be damaged by ingesting foreign objects.
The two reversers do help slow the A380—but not by much. In fact, unlike the thrust reversers on most airliners, including the Boeing 747 jumbo, they do not stop the aircraft in a shorter distance than brakes and spoilers alone. They do, however, take some of the strain off the brakes and are useful if water or snow makes the runway slippery.
Most modern airliners use reversers that redirect engine thrust forward. On many turbofan engines, the airflow bypassing the engine core is blocked from exiting (though the combustion exhaust is not) and is channeled through an assembly of vanes, called a cascade, exposed when an outer sleeve on the engine nacelle slides aft.
Some aircraft engines have thrust reversers designed to help speed a descent: The U.S. Air Force C-17, for example, uses reverse thrust to get on the ground faster in combat zones. NASA’s Shuttle Training Aircraft, a highly modified Grumman Gulfstream II, used inflight thrust reversers to help simulate the space shuttle’s steep landing profile.
On the A380, a pilot can deploy the thrust reversers only on the ground, and can select a range of thrust reversal from idle to maximum reverse, until the aircraft has slowed to below 70 knots, or 80.5 mph (1 knot equals 1.15 mph). At that point, the thrust reversers must be set at idle reverse.
All airliner engines now have safeguards built in to keep the thrust reversers from accidentally deploying during flight. In 1991, a Boeing 767 crashed 15 minutes after taking off from Indonesia, killing all 313 on board, because the thrust reverser on one of its engines deployed at 24,000 feet, sending the aircraft into a high-speed descent. The Federal Aviation Administration responded by requiring redundant locks on the equipment. In case of future accidental deployments in spite of the locks, the agency required new training procedures for cockpit crews to prevent deployment from causing a crash. In 1998, a thrust reverser on a Korean Air Airbus A300 deployed for a few seconds in flight, but the crew was able to disable the reverser and land safely.
During certification testing of the A380, Airbus loaded the airplane to its maximum takeoff weight, equipped it with brakes that had been machined to a 90 percent level of wear, and blasted it down the runway until it reached 170 knots, the “decision” speed at which a pilot would either continue a takeoff or abort. Then the test pilots retarded the throttles to idle and stood on the brakes, an action that would be taken only in an emergency. The use of thrust reversers was not permitted in the test. In 6,070 feet, the giant aircraft screamed to a stop. The Bridgestone tires—about the size of those on a military Hummer—did deflate several minutes after the aircraft exited the runway, as expected.
To stop the A380, enormous composite Honeywell brakes on 16 of the 20 main landing gear wheels do most of the work. As on most new airliners, the A380’s brakes are anti-skid. They work like the anti-skid brakes in your car, responding to extreme pressure by automatically pulsing to prevent brake lockup and skidding. Almost as important is the aerodynamic braking of 16 giant wingtop spoilers swinging skyward to create drag and reduce lift. Reducing lift improves mechanical braking by putting more weight on the wheels.
Of course, it’s the overall design of an airliner that allows it to slow from a transonic cruise at 500 knots to a crawl in a matter of minutes. Although enormous, the A380 lands just like any other Airbus of the A320 or A330 family, says Airbus executive Larry Rockliff, who has flown the aircraft for 120 hours. Letdown starts at cruise altitude at a speed near .85 Mach. Pilots enter data such as runway winds into the redundant flight management systems and compare data during descent to ensure accuracy. Below 10,000 feet the aircraft must be slowed to about 250 knots, and it generally enters the landing pattern at 180 knots. Pilots can manually control the rate of descent and speed by using knobs on the autopilot control panel, or can let the flight management system operate according to the optimum profile.
The design of the A380’s wings, with their large area, comparatively gentle sweep (33.5 degrees), and massive flaps, give the Airbus a landing speed that is 20 knots slower than that of a 747. An A380 crosses the landing threshold at a docile 140 knots and touches down, depending on its landing weight, at a speed as slow as 130 knots, about the same touchdown speed of some corporate jets that weigh 1/50th as much as the world’s biggest airliner.
Frequent contributor Mark Huber has been fascinated with aircraft stopping systems ever since he botched a landing as a rookie pilot, frying the brakes and bald-spotting the tires on a Cessna 172.