December 2004/January 2005 
Volume 03, Issue 8 
Tech Talk

Striking out tailstrikes


Boeing 777-300ER during its flight test programBoeing has developed an industry-leading system to strike out tailstrikes.

The Airplane Tailstrike Protection System is designed for longer commercial airplanes, such as the 777-300ER (Extended Range), to reduce tailstrikes. A tailstrike occurs when a plane's rear fuselage hits the runway during takeoffs or landings. The electronic tailstrike protection system, invented by five Boeing colleagues, is a significant technical advancement because it addresses all factors needed to effectively curb tailstrikes.

Currently, the tailstrike protection system is installed in the flight-control system on the 777-300ER and will also be included in the configuration design of the 7E7's stretch-body version.

Previously, Boeing helped counter inadvertent tailstrikes on longer jets, as in the case of the 767-300, by having tailskids-plates with shock absorbers- fitted to airplanes' aft underbodies. While the 777-300ER supplements the plate-version tailskid with the electronic protection system, the next application even will eliminate the physical tailskid, which will save weight, maintenance and cost.

When is a tailstrike protection system needed? If a pilot follows normal takeoff procedure and rotates the airplane nose upward at the correct speed and rate of rotation, it's unlikely for a tailstrike to happen. But if the pilot begins to rotate too soon or too fast, there's more risk of making runway contact.

"For a stretch-body airplane equipped with the physical tailskid, [damage from] a light tailstrike is minor, but moderate to severe strike can be costly to an airline," said Mithra Sankrithi, manager, Boeing Commercial Airplanes Product Development, Configuration and Engineering Analysis, and a coinventor who originated the idea for the system. "It can put an airplane out of service to replace shock absorbers, inspect the body structure for damage or, if necessary, undergo repair." He added that a tailstrike is a maintenance rather than a flight-safety issue, as the pilots still have full control of the airplane.

The electronic protection system also optimizes stretch-body airplanes' aerodynamic performance on takeoff and landing, enabling greater payloads.

"With this tailstrike protection system, customers have a reliable plane that will help reduce their operating expenses and help increase performance," Sankrithi said. "Cost-savings include fewer schedule interruptions and less maintenance and repairs. Also, increased payloads help airlines increase revenues."

How it works

Visualize a takeoff. After accelerating, the pilot commands a rotation of the aircraft to a nose-high angle of fuselage relative to the runway. Passengers experience this rotation, or "pitching," of the airplane on takeoff as a leaning back in their seats. The optimal nose-up pitch angle is a complex function of airplane aerodynamic, weight and loading parameters. For stretch-body airplanes, the allowable nose-up pitch angle has specific limits, depending on these parameters, that lower the risk of tailstrikes.

As the jet's wings generate lift and the aircraft begins to climb away from the runway, the motion of the aft body is affected by the plane's rate of climb and nose-up rotation relative to the runway. During slower-speed takeoffs, this nose-up rotation is greater, which puts the tail closer to the ground.

The tailstrike protection system's technology, embedded in the airplane's fly-by-wire flight-control computer, continuously surveys key aircraft performance data and sensor input. The technology computes the location of the airplane's aft fuselage relative to the runway, as well as the closure rate at which the fuselage approaches the runway during takeoff. If the system senses an impending tailstrike, it automatically commands the aircraft's elevators-moveable, hydraulically powered control surfaces located on the tail's horizontal surface that cause an airplane's body to rotate about its pitch axis-to generate nose-down pitching movements as needed to avoid a tailstrike.

The Aircraft Tailstrike Protection System was invented by Mithra Sankrithi, Stephen Wells, Wendi DeWitt, Monte Evans and David Eggold.




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