Flight Training - August 2012 - 52

CAREER PILOT»

TECH TALK

Do you have the power?
» By Jared Dirkmaat

STOP!

AIRPLANES ARE PRETTY good at going fast. Even a Cessna 172 can fly at 110 knots, which may not sound like much, but when was the last time you drove your car at 120 mph? Scale that speed up to larger aircraft, and now you’re cruising in the 400- to 500-knot range. That speed means you can get from point A to point B quickly, but what does it mean when it’s time to stop? In the same way that aircraft are designed for speed with aerodynamic designs and high-performance engines, they’re also designed for stopping power. Utilizing a combination of thrust reversers, ground spoilers, and anti-skid braking systems, large and heavy aircraft can be slowed to a stop in much shorter distances than without the combined technology. Thrust reversers work by redirecting the exhaust gases from the aft of the engine by channeling them forward. This creates an equal-and-opposite reaction from the force directed forward, and decelerates the aircraft. Generally these systems incorporate hydraulic actuation with electrical operation; by utilizing multiple systems to operate and actuate the reversers an inadvertent deployment can be avoided. Some aircraft are designed to be capable of in-flight deployment of the reversers; however, most aircraft today require the aircraft to be on the ground. Ground spoilers often are combined with a flight spoiler system that can be used during all phases of flight. Some aircraft even utilize the flight spoiler system

to assist in roll control during low speed flight. Most aircraft are equipped with technology that knows when the aircraft is on the ground and the wheel speed of the main landing gear. While on the ground and above a specified wheel speed, the ground spoilers will automatically deploy if they’re properly armed. This spoils the airflow over the wing, creating drag and also applying downforce to the main landing gear, improving braking effectiveness. The final element to creating all of this stopping power are the brakes. Hydraulic actuation of the brakes reduces pedal forces required by the pilot to create a desired amount of braking. This is accomplished through the assistance of a Brake Control Unit which receives inputs from the brake pedals through a position transducer that interprets the braking commands. These commands are then carried out by applying or relieving the required hydraulic pressure for either differential brake. Anti-skid is accomplished by monitoring of wheel speed and hydraulic pressure sensors by the Brake Control Unit. Should the wheel speed on one main wheel differ from the other, hydraulic pressure is automatically relieved. This helps prevent un-commanded asymmetrical braking, but it also prevents skids by not allowing the wheel to lock up. Combine these technologies and the aircraft becomes capable of shorter, safer, more effective deceleration when it most counts: landings and rejected takeoffs.

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Flight Training - August 2012

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