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777 Engine Startup - Why Such A Low Sound?

Posted: Mon Jan 18, 2010 8:43 pm
by edoca
I couldn't find an answer to this on the forum... apologies if it has been answered before.

I was travelling on a 777-300 for the first time last week (Jet Airways), and I can't remember even hearing such a low and vibrating sound at engine startup - for a moment, it even hurt my ears. Anyway, I think the best ever aircraft sound is when this beast starts its takeoff roll...

I read the basics about how a turbojet engine starts up, but what is it exactly that causes this extremely low sound?

RE: 777 Engine Startup - Why Such A Low Sound?

Posted: Tue Jan 19, 2010 12:14 am
by EMBQA
1... it's a BIG engine.....
2... every engine start up sounds different depending on where you sit in the cabin

RE: 777 Engine Startup - Why Such A Low Sound?

Posted: Tue Jan 19, 2010 1:24 am
by 411A


Quoting Edoca (Thread starter):
I read the basics about how a turbojet engine starts up, but what is it exactly that causes this extremely low sound?

If said 777 was equipped the RollsRoyce engines, the initial start sound is caused by flame propagation in the combustion section...very similar on all three-shaft RollsRoyce engines.
Quite a unique sound with this series of turbine engines.
IE: very distinctive.

RE: 777 Engine Startup - Why Such A Low Sound?

Posted: Wed Jan 20, 2010 2:54 am
by comorin
is this the famous mooing sound?

RE: 777 Engine Startup - Why Such A Low Sound?

Posted: Wed Jan 20, 2010 4:01 am
by maddogjt8d
This is very much dependent on which engine you mean - GE90 or RR Trent. I've noticed that the RR Trent doesn't make that much noise when starting up until it is just about at idle when it starts to whine. The mooing occurs with the GE90 throughout the startup sequence. I'm not sure by what you mean by "low" sound, it could apply to either the tone of the engine (GE90) or the volume (RR).

Either way I think the above post got it right - they are BIG engines.

RE: 777 Engine Startup - Why Such A Low Sound?

Posted: Wed Jan 20, 2010 9:23 am
by AA737-823
JET, the airline in question, uses GE-90-115B engines on their 777-3ER aircraft, as this is the only type of engine certificated for the airframe. Thus, discussion of Rolls Royce engines is irrelevant.

Large GE engines make some unique startup noises. I am particularly attached to the CF6-80 on the 747-400 aircraft... a bit of gear whine at first, followed by a small pop at lightoff, and then some mooing/whooshing as air is routed through bleed valves until self-sustaining speed is achieved. GREAT STUFF!

The GE-90 is an engine that I don't have any experience working on or around, but it's not incredibly different in design, simply bigger. The fan is VERY slow to accelerate due to its large diameter, although it is pretty light due to composite construction.
All in all, the GE-90 is a bit of an unusual engine... it's like a CF6, but with some interesting differences that make it unique.

RE: 777 Engine Startup - Why Such A Low Sound?

Posted: Fri Jan 22, 2010 8:10 pm
by edoca


Quoting Comorin (Reply 3):
is this the famous mooing sound?

"Moo" is the perfect transcription. A roaring, very low (frequency) but loud noise that gradually increases in pitch until the engines seem to be up and running, then it quickly disappears.

In any case, long live the GE90 and thanks for the insights!

RE: 777 Engine Startup - Why Such A Low Sound?

Posted: Sat Jan 23, 2010 4:01 pm
by jetlife2
Every engine design has its own characteristics. Each module (fan, turbine, etc) has its own acoustics and each is prevalent at different operating conditions, and each is perceived differently depending on where is the observer (i.e position outside: distance, angle, in front or behind the engine, etc; position inside: fwd, aft, L-R). It is a fascinating science all its own.

In the specific case of the GE90-115B, there is a distinct low frequency tone that the engine passes through briefly during the start sequence. This is the combustor. It is affectionately referred to as "growl".

Hope that helps.