View Full Version : 737 FMC CG

15th Dec 2001, 17:41
Here is a question for all those who flies
How many of you do put in the C of G into the FMC PERF INIT page(GW/CRZ CG) and why?
In my airline,it is left at defaulted 15%,
anyone with a different figure? :)

15th Dec 2001, 18:17
According AOM you should enter actual CRZ COG. We normally put in 18%(737-800), default is 5%. Higher values give you increased MAX CRZ ALT on the CRZ page. Very high entries (25% when actual is less) can give you an unrealistic MAX CRZ ALT.

[ 15 December 2001: Message edited by: Track ]

15th Dec 2001, 18:39
On the 76,im sure it cant be a huge difference, it also gives you a more accurate optimum altitude..

17th Dec 2001, 18:34
It really only makes sense to enter the c of g figure when both wings are full and the centre fuel tank is being used. This essentially gives a better fmc max cruise level and performance for the aircraft. When only the wing tanks are used then the default is close enough to the actual one to make little or no performance difference.

19th Dec 2001, 15:18
On the 300 if you don't enter the actual CG it's affect as well the 1,3 G margin. If you change the CG in cruise at high level you'll see the lower yellow bar on the speed tape moving up or down. For that reason it is essential to enter the actual CG on the FMC before departure to have correct information on your speed tape.

19th Dec 2001, 17:22
I've tried to play around this in my -400 today.CG valid entry range from 4% to 32%,
it affect the MAX ALT a little but I didn't
notice the change in buffet on the speed tape
(may be not heavy or not high enough).I agree
that it affects the G load margin which in turn affect the MAX ALT.
-400's weight & balance datum is at 17.5% MAC
(648.5") from "0" station,thus 18% entry sounds good.
In most of my sectors,CG moves ~2% from T/O
to Top of Climb(index moves ~3 units and burn
~1600kgs of fuel).I remember some time ago there was a posting saying further aft CG requires less elevator down forces thus less drag => more economy.Now the question is,how
significant is it?Can we deliberately load the A/C to way aft CG(within limit)to safe
some fuel?How much?Hope there are experts
could help.Thanks. :)

24th Dec 2001, 01:39
Before I started flying I worked as a flight operations officer for a european major. We were instructed to prepare the loading instructions in such a way that the CG was as close to 23% MAC as possible, as this was supposedly the most economic MAC value for short-haul on the 737(-300/-400). We were told that if 50% of the flights were within +/- 1% of 23 it would save the company 1 million dollars worth of fuela year. I don't know how this value of 23% was generated but there are certain considerations as to why you would'nt want to operate with a MAC very close to the aft limits:

- More aft MAC means more,or all luggage and cargo in the aft hold, this means longer turnaround times as only one loading-crew has to load all the stuff in one hold.

- Chances of the a/c being out of trim at the last moment when planned with a aft MAC. (This occasionally happened with the 23% planning aim).

- I'm not a performance expert but, aft CG means more pitch up attitude during cruise and more trim. I think there is a turning point somewhere at wich increased drag of pitch attitude is higher than the gain of reduced drag on the stabilizer.

24th Dec 2001, 11:05
I used to fly the A330 which has a auto function that it would tranfer fuel from the main wing tank to the stabilizer(trim) tank for further aft
CG then automatically pump back to main tanks just
before descent or certain level of fuel remaining.
It was meant for trimming for fuel economy.Airbus
drivers,anything to add?
By the way,Thanks,Track.