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160knots
28th Nov 2009, 10:37
Presently doing a conversion course on the B777. Why is there a need to change the CG from the default of 7.5% to 30%? What is the advantage?

Thanks 160K

MrBernoulli
28th Nov 2009, 11:06
Hmm, not something that we touch in the airline that I fly the 777 with. You need to find Old Smokey, somewhere here on the forums. He is a very experienced 777 skipper (with Cathay?) and has test and similar experience, as well as pretty close links with Boeing, I believe. If he can't answer your question, then you can pride yourself on having asked a real difficult one! :ok:

5LY
28th Nov 2009, 11:11
30% seems high. We take 10% off of the T/O c of g and replace the default 7.5 with that. This usually results in a value of about 20% give or take. (This on the 300ER). Never did it on the other variants that I flew.

What you're doing is telling the computer that your c of g is further aft than the default, therefore allowing a higher cruise level than would have been calculated with the default c of g value.

The default is very conservative. The buffet protection required for that value would limit your climb. This is just a way of communicating a more acurate cruise c of g to the FMC.

I suspect this is just ass covering by Boeing. or by their legal advisors. otherwise, why wouldn't the normal entering of the T/O c of g on the perf. page adjust the default automatically for you? With the default in there, the chance of an upset is pretty remote as you'll be cruising at a lower FL.

5LY
28th Nov 2009, 12:15
Further to my last: what I said above is my speculation and I'd be pleased to hear more on it from someone who knows the facts. Particularly, why doesn't our t/o c of g as entered adjust the cruise c of g accordingly.

Old Smokey????

oz in dxb
28th Nov 2009, 12:21
When the 300ER came out, it had some airframe vibration problems. The fix was to fly the aircraft at lower FL. So the CoG was moved forward to limit the maximum envelope. Initially it was 7.5% MAC, however it's been changed to 17.5% MAC.

Standard 200/200ER/300 is 30% MAC. The 200LR and 200F is 20% MAC.

rudderrudderrat
28th Nov 2009, 12:27
Hi, I'm not 777 qualified - but on 747 400 the Max FL was based on 1.3g buffet margin. Provided there was no turbulence forecast, we used some QRH tables to "fudge" the FMS %Mac to provide 1.2g buffet margin which then allowed us to climb higher earlier.

777newbie
28th Nov 2009, 12:41
We too use 30% on all our 777's (200,200ER,300,300ER). Only the 300ER is defaulted to 7.5 requiring a manual input of 30%.
My own observation: The 300ER flies low in comparison to the other models and also other aircraft. I am sure JT can explain.
Regards

160knots
28th Nov 2009, 15:12
Boeing obviously know that this is being applied or they are advising airlines to make the change. If so, why don't they come out with a technical explanation instead of us making assumptions.

777newbie
28th Nov 2009, 15:24
We were given one when we started ops on the 300ER. I am away from base so cannot access it. Will do so when I return but I am sure the legend JT will have it all sorted out before that.

Spooky 2
28th Nov 2009, 15:34
This value controls your max alt as well as your maneuver speed in realtion to buffett, but think you already know that so I'm a little uncertain about your question?

mach 84
28th Nov 2009, 16:07
a CG towards the aft of the limit is helping you to safe fuel, the stabilizer always is creating some nose down force with the lift it produces, if you think the stabilizer of a 777 has about the area of a 737 wing, so you could imagine what i am talking about, if you go for an aft center of gravity you might counter this force, a CG of 32 is not uncommon to me and on a long range flight (5000 miles or more) it could safe you a ton of fuel at least. some companies even implied an aft center of gravity policy!

5LY
28th Nov 2009, 19:04
Yes an aft c of g unloads the stab. We're adjusting default FMC values, not adjusting the actual c of g. The default assumes a forward c of g, therefore keeping you down lower.

160knots
15th Dec 2009, 07:32
I'm sure Old Smokey could enlighten us further but it seems he is away on a long smoko.