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Home > Flying on the Boeing 777 > What cause the periodic resonance in the cabin of the Boeing 777?
What cause the periodic resonance in the cabin of the Boeing 777?
Flying - Flying on the Boeing 777
Wednesday, 01 November 2006 20:49

Hi Capt Lim,

I'm a former U.S.A.F. pilot and can help answer Jochen Buur's question about engine resonance.

He's referring to the harmonic resonance that almost all twin-engine aircraft have. I dealt with it quite a bit in the T-38 and in the Beech King Air.

When the engines are not quite synchronized (same RPM) they tend to have a period of resonance that is felt as vibration in the cabin. As the engines get closer to the same rpm, the resonance period becomes longer until it fades away. I often wonder if it is felt or discussed by the pilots in a commercial airliner cockpit, because often times it is left unchanged and the passengers (at least me) suffer. It's sometimes quite a pain to listen to. All it takes is a simple tweaking of one engine's speed setting to get the harmonic to disappear.

Gary Rolf
Anchorage
Alaska.

Hi Gary,

Thank you for your additional information. I attach Jochen Buur's question below for the benefit of the other readers:

Hello Capt Lim,

First off all, thanks for the excellent website. It really has helped me to understand flying more and so reduce my fear of flying a bit!

Anyway, I have this question about resonance. I fly frequently between Australia and Europe and these flights are serviced by the Boeing 777s and 747s. I seem to notice a slow periodic resonance that can be more felt than heard, particularly when I am using earplugs to shut out cabin noise. This resonance seems more pronounced in the 777 than in the 747 but also differs in intensity between individual planes and/or flights.

I wonder if this resonance, which makes sleeping sometimes uncomfortable, is just a by-product of the mechanical nature of a plane and/or has something to do with load, weather etc? Surely resonance is something you would like to minimize from a maintenance point of view?

Your thoughts are highly appreciated! Kind regards,

Jochen Buur,
Brisbane, Australia

PS. I feel that the Boeing 747 is more comfortable while flying in turbulent weather. It seems a bit more "flexible" and therefore able to cushion the bumps a bit better ... Would you agree (probably not!)?

Hi Jochen,

Your question is rather technical for me to give a precise answer. I have consulted the Boeing Company, but was not able to get an official response from them. However, I have met one of their safety pilots and pose this similar question to him. His initial response was that the noise and vibration within the cabin could be rather subjective and it depends on each passenger*s perception. Boeing would need more specific data or measurements to compare the situation to their baseline measurements. However, he pointed out that the Boeing 777 meets or exceeds all government certification requirements for cabin noise and comfort.

Regarding the question on whether the Boeing 747 can take turbulence better, I have not had any official report about it from Boeing. Perhaps the consumers are the better parties to give a more objective feedback.

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Harmonic Resonance - lack of fan sync
I've noticed that the 777 is particularly bad for this fan-sync (or lack of) induced vibration, which I guess is down to the enormous fan size on the 777.

Having flown on lots of different 777s, and usually sat in roughly the same area of the aircraft, I found that Pratt engined 777s (e.g. UA) are the worst for this resonant vibration caused by the fans being out of sync, whilst Rolls engined aircraft (NZ, SQ, BA) hardly seem to suffer it at all.

I guess that quads such as the 747 don't show this as much because there's more "randomness" across 4 engines, giving a cancellation effect.

Mike
Mike Hughes , 19 Jan, 2008

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