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Imagine you're driving at the maximum speed allowed on a major highway and you have to suddenly stop the car.
It's basically what Boeing test pilot Captain Kirk Vining had to do recently with the new 747-8 Freighter during one of the most dramatic airplane certification tests. However, instead of a 4,000-pound (1815 kilograms) car, the 747-8 weighed in at close to one million pounds.
"In the emergency event that we have to make a rejected takeoff at these heavy weights, it takes a long distance for the airplane to stop," says Capt. Vining.
The Boeing 747-8 Intercontinental and 747-8 Freighter are the new, high-capacity 747s that offer the lowest operating costs and best economics of any large passenger or freighter airplane. Both are on track to make their first deliveries later this year after certification.
To simulate the ultimate rejected takeoff (RTO), crews loaded the airplane above its maximum takeoff weight of 975,000 pounds and installed a set of 100% worn-out brakes.
In April, the Boeing Test & Evaluation team brought the 747-8 Freighter to a long runway in California to simulate the ultimate rejected takeoff (RTO). Crews loaded the airplane above its maximum takeoff weight of 975,000 pounds (443,181 kilograms) and installed a set of 100% worn-out brakes.
"It's down to the studs," says Andy Hammer, the 747-8 flight test manager. "They've got them machined down so there's basically no material left."
Capt. Vining taxied the airplane out to the start of the runway and began the takeoff roll as usual by pushing all four engines to maximum thrust.
Just as the airplane was going over 200 miles per hour (320 kilometers per hour), Capt. Vining slammed on the brakes. To channel maximum energy to the carbon brakes, he was not allowed to use the thrust reversers.
"It's just brakes," says Hammer, explaining why the condition is also called the maximum brake energy test. "The whole intent is to demonstrate that under the worst conditions you can safely bring the airplane to a stop."
"The whole intent is to demonstrate that under the worst conditions you can safely bring the airplane to a stop." Andy Hammer, 747-8 flight test manager
The brakes, made by Goodrich, did exactly as they were supposed to. In fact, the 747-8 stopped earlier than the team had hoped, beating the target by more than 700 feet (200 meters).
"It went amazing," says flight test engineer Cassie Martin. "The [deceleration] was exactly what we expected. Great stopping, no major faults."
But stopping was just half the challenge.
The kinetic energy from the moving airplane was transferred to the brakes in the form of tremendous heat, estimated to be more than 2,500 degrees Fahrenheit or 1,400 degrees Celsius. As expected, the brakes glowed a bright orange as smoke poured out.
Firefighters arrived and were available to act in an emergency, but certification requirements called for the airplane to sit unassisted for five minutes.
After the airplane comes to a stop, it's estimated to be more than 2,500 degrees Fahrenheit (1,400 degrees Celsius) inside the brakes.
"And that's to simulate that if you were at an airport, you wouldn't necessarily have the fire department right there," says Hammer.
By design, special fuse plugs in the tires were activated, deflating the tires before they exploded. At the five-minute mark, firefighters moved in with plenty of water. While the tires and brakes were damaged, the rest of the airplane remained in great shape.
"This was the worst case," says Hammer. "So when the airplane is in service, you can be assured that at 975,000 pounds, with worn brakes, worst center of gravity, and worst cut speed that you actually can stop the airplane."