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Fri 19 Sep 2008 04:00 AM

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Ask the expert: John Hamilton

Question: How do steel and carbon brakes compare?

Question: How do steel and carbon brakes compare?

Expert:John Hamilton, Chief engineer on the 737 Program

At Boeing we offer steel brakes as standard with the introduction of the next generation 737 family. But we also offer carbon brakes, which are the latest in a series of product enhancements that Boeing commercial aeroplanes have introduced for its 737 family and its BBJ business jets.

Each customer will determine which brakes are more suited to its needs. Steel brakes have thermodynamic capabilities which are more capable for higher frequency, quicker turn, shorter runway and higher landing weight operations.

Carbon brakes are considered more environmentally friendly. Reduced weight contributes to a reduction in associated fuel burn and CO2 emissions.

Carbon brakes are lighter and work well for average frequency, turn time, runway and landing weight operations. Boeing can perform a thermal analysis tailored to an airline's actual operations to determine which brake is best.

When Boeing entered the first Next-Generation 737 into service in 1997, only steel brakes could support the higher frequency; quicker turn profile of airlines purchasing those jets.

Steel brakes cooled quickly enough to allow operators to service their aeroplanes and be off on the next flight. As carbon technology evolved, Boeing determined it was time to reconsider carbon brakes. The weight savings of carbon brakes was an attractive incentive to launch a study. We chose a product produced by aeronautical brake manufacturing company, Messier-Bugatti, part of Safran group.

The brakes produced were very successful and the company is now targeting a potential market of several thousand aircraft, either new or retrofits on aircraft already in service, which will be worth up to US$3 billion for Safran. In addition carbon brakes are considered more environmentally friendly.

Reduced weight contributes to a reduction in associated fuel burn and CO2 emission depending on airline operations. Airline Operators and owners can expect 320 kg weight savings compared to high capacity steel brakes for 737-700, 737-800, 737-900ER and 250kg weight savings on standard capacity steel brakes for 737-600/-700.

Testing of the brakes demonstrated equivalent performance between steel and carbon brakes - this was one of the most important goals of the certification testing.

The testing took place over 36 days; 35 were spent testing at Roswell, New Mexico and one day at Glasgow, Montana. All of the brakes were changed 25 times and wheels were changed 58 times.

The things we were looking for included, Performance take-offs and landings; Refused/rejected takeoffs at different airplane speeds; Anti-skid testing on a combination of wet and dry pavement; 100% maximum brake energy stops with worn brakes and landing with and without auto brake at different flap settings. The U.S. Federal Aviation Administration certified the brakes July 24, 2008.

Shortly after certification, the first set of brakes was delivered and entered service on a Delta Air Lines Next-Generation 737-700. It was Delta's first 737-700. The airline will receive nine more of that model over the next several years.

The brakes are mounted on the main landing gear wheels. The steel brakes that are on the 737 airplane provide superior stopping performance and because they cool quickly, they allow for fast turn times to support better utilisation of the airplane.

During flight testing we determined that the carbon brakes offer comparable stopping performance to the steel brakes and provide nearly equivalent cooling times. Thus an airline in most cases can maintain their existing turn times they use with steel brakes and save approximately 700 pounds on every takeoff resulting in fuel burn savings and a reduction in CO2.

Under some conditions it makes sense to stay with the steel brake, the main difference between the two types of brakes is the weight. Airlines can negotiate with the supplier on overhaul costs and acquisition costs.

We continue to study and introduce product enhancements for the Next-Generation 737. We're always studying innovations to help airlines save fuel, lower emissions and reduce operating costs.

The enhancements we study fall into the categories of improved performance, passenger comfort and navigation precision.

We will share the details of future enhancements when we have decided to implement them. The CFM Tech Insertion engines and Blended Winglets are two other examples of enhancements that save fuel and lower emissions.

The CFM Tech Insertion lowers airline costs by reducing engine maintenance; increasing time-on-wing for fewer shop visits; fewer spare engines are required; reduces NOx emissions; complies with 2008 CAEP 6 requirements.

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