Boeing test pilots made a cautionary landing at the Moses Lake airport in central Washington over the weekend after experiencing an "uncommanded loss of thrust in one of the engines." Boeing's Randy Tinseth posted the news on his blog, saying the problem was linked to a pressure sensing component and was quickly fixed. The first 787 Dreamliner, ZA001 returned to Seattle Sunday after the repairs were made. ZA001 is pictured above over Runway 13R at Boeing Field in Seattle during a previous flight.
The flight test department at Boeing is very busy these days with two new designs undergoing certification flights at the same time. Since the 787 first flew back in December, the company says airframes ZA001 and ZA002 have flown 49 flights totaling more than 158 hours as of Monday morning.
A third 787 is expected to make its first flight soon, possibly later today. But with the engine issues experienced over the weekend, that flight may be delayed. Boeing expects to fly more than 3,000 hours during flight testing before delivering the first customer aircraft.
For the past week the 787 flight test team has been expanding the flight envelope and pushing towards the edges as ZA001 began flutter tests on Valentine's Day. Flutter is something that is often uttered with disdain in the flight test community and can be one of the most challenging and dangerous parts of a modern flight test program.
During flutter testing, the pilots and engineers purposely induce vibration into the wing, tail or flight control surfaces to ensure that the vibrations will dampen out automatically. The danger of flutter is that if aerodynamic forces were to induce vibrations in the wing for instance, and the flutter did not dampen out, the vibrations could increase rapidly and lead to structural failure. It is similar to the structural failure a bridge can experience as vibrations increase.
Like a bridge, a wing has a natural frequency at which it vibrates. During flight, the wing is stressed while it is lifting several hundred thousand pounds in the case of an airliner. When the wing experiences some sort of aerodynamic force such as minor turbulence, the stressed wing will vibrate at a relatively low frequency. This is common and can be seen during almost any flight by anybody with a window seat and a view of the wing.
Engineers design the wing so the vibrations are dampened similar to how a shock absorber in a car dampens the vibrations of a rough road. When the vibrations are dampened, the resulting motion is kept within the design limitations of the structure of the aircraft.
If the small force continues at the natural frequency of the wing (or other flight surface), and is not dampened out, resonance can occur. Resonance results in an increase of the amplitude of the vibration and in the case of an aircraft can quickly lead to the failure of a wing or other component. There have been several airplane crashes over the years due to flutter and resonance. But perhaps the most famous failure related to resonance is the Tacoma Narrows bridge in 1940.
Boeing's Randy Tinseth says ZA001 will soon return to flutter testing. Flutter tests take place at a wide range of speeds and flight conditions. Once complete, passengers can rest assured that any vibration experienced by the flight surfaces will quickly dampen out and result in nothing more than the normal wing movement seen on any airplane during flight.
In the meantime, the company's largest airplane ever, the 747-8 freighter is expected to make its second flight today. The redesigned 747 will fly to Moses Lake in central Washington state where Boeing conducts much of its flight testing. Later, the 747-8 flight test team will move to Palmdale, California where they will complete the majority of the flight testing before delivery of the first customer aircraft towards the end of the year.
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Photo: Boeing*
