01 February 2010

One item of historical interest attached to British Airways' Boeing 777s relates back to the flight testing phase when one of the 777 prototypes was at Edwards AFB conducting RTO (Rejected/Refused Take-Off) testing- an RTO test is the most demanding test of an aircraft's braking systems. The aircraft is loaded with ballast to represent a typical fuel/pax/cargo payload, taken up to takeoff speed, and then the engines are throttled back and the brakes slammed to stop the aircraft before it reaches the end of the runway. No thrust reversers/spoilers are used in the RTO certification tests. These are some of the most dangerous tests in flight testing as the brakes absorb tremendous amounts of energy, glowing red hot and often the tires will also explode. In an RTO test, the fire trucks stand by and do NOTHING for five minutes to simulate the time needed for them to reach an aircraft on the runway.

When the 777 was in flight testing, the Boeing and FAA engineers were preparing for the maximum RTO test called "The Big One"- this was the worst case scenario- full payload, maximum brakes, and brakes purposely worn down to remove any additional safety margin. As test preparations were under way, British Airways notified Boeing that they planned to use the 777 on the London Gatwick-Dallas route and this would require an increase in the payload weight and takeoff speeds (and thus maximum braking forces) than what the engineers were originally prepared to perform as "The Big One". It was BA's worst case scenario- an aborted takeoff out of DFW on a hot summer day and it was worse than what the Boeing engineers had considered the worst case scenario in the RTO tests.

Long story short, after a lot of hand-wringing, they went ahead and conducted "The Big One" using BA's Dallas parameters. The 777 was loaded to a takeoff weight of 632000 pounds, accelerated to 183 knots, then 100% braking applied as thrust went to idle. The 777 stopped in 4000 feet, the brake temperatures went off the scale past 3000 degrees Celsius, and as the 777 turned off the runway, smoke could be seen from the brakes as they glowed white hot.

With the fire trucks standing by for the required five minutes, the first tire blew (in a controlled manner using what's called a fuse plug in such emergencies- the plug melts and lets air out without a blowout) before the plane stopped. All 12 tires' fuse plugs blew and steam billowed out from the wheels as the fire trucks began to pump water on the undercarriage. The 777 passed "The Big One".

It was estimated that about 97 million foot-pounds of energy were absorbed by the brakes in this test. By comparison:

-A general aviation aircraft weighing about 2000 pounds uses 200,000 foot pounds of brake energy.
-A typical jetliner lands and uses about 3-4 million foot-pounds of energy.
-The Space Shuttle uses 30-36 million foot-pounds of braking energy when it lands at the Cape.
-A carrier landing can require as much as 74 million foot-pounds of stopping power.

Source: Twenty-First Century Jet: The Making and Marketing of the Boeing 777 by Karl Sabbagh. Scribner Press, 1996, p291-305.

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