FMC B737
 

which C.G should be put in performance page as cruse C.G. is not given on load sheet.

Manual L/S and a drop line will get you a crz CG.
or
Interpolate TO and LND CG.

Try various CG's and you will see only a minor difference to your ETA and FF.

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I would be more concerned about the adequacy of loading and stabiliser setting before worrying about what I should put in the FMC. Yes, it is correct that different CG inputs will yield only a minor change in your ETA, but you may get a nasty surprise at rotation ( either a tail strike or unable to get airborne at correct point/time while precious paved surface under you zips by).

The take-off MAC (from the loadsheet) minus four percent.I lerrn eet from ae boook!

As far as I'm aware your consideration when changing the cruise C of G is that you will change maneuver margins, Opt and Max alts. This is graphicly displayed on a speed tape or MASI. Our FMC software on the -800 is U10.3, the default cruise C of G is 5%. I maybe totally wrong (Pls correct me) but believe/heard, that this was to be left alone. A possibility I guess as the CAA maneuver margins are fixed at 1.3G or if you prefer 39 degrees of bank.

Best Rgds

[This message has been edited by Slick (edited 12 June 2001).]

najib.

Surely it is better to 'look through' the FD at rotation. I don't want to use it in anger until I'm happy it telling me sensible things - like don't stike the tail on take off!

We don't have an SOP as such to cover this, but my normal drill is to input loadsheet %MAC in raw form, and then once level in the cruise at econ speed, look down at the trim indicator and input the figure from there. It seems to work, though it's only of consequence at high levels when considering a further climb or pondering buffet margins in choppy skies etc.

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'Brighten my Northern Sky' Nick Drake R.I.P.

I never changed these default values. But if I would do it, than acc. to our hand made loadsheet.
For best loading I can recommend to you for a -400 (167 Pax version) as much as possible in Hold 3+4 and for a -800 (184 pax version) a standard load of 1/4 fwd and the rest in the rear hold.

At our airline we use MAC at take-off from the loadsheet less 4 because 4% is the most that the MAC would move in flight due to fuel burn.

A higher MAC will allow a higher cruise level & greater speed range.

Operate with Update 5, then you don't have to worry about it at all! :)

Sorry, Checkboard, I'm missing your point. Please expand....

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'Brighten my Northern Sky' Nick Drake R.I.P.

The Boeing FMC programming at the Update 5 level doesn't have an input for %CG location.

I was having a private whinge at operating old technology aircraft and avionics (and flying for a company that can't see the point buying an upgrade). Bit like using Windows 3.1 when everyone else is using Windows 2000.

FAA operators are not allowed to change the Crz CofG leaving the default value for flight. Pretty conservative and on occasion causes unnecessary concern over margins. CAA operators can use actual CofG's and therefore you should either work out what your CoG is or use takeoff CofG -5% which as mentioned earlier is the very most the CG will change in flight under the worst circumstances. As the CTR tank empties the CG moves AFT then FWD again as the Main tanks empty.
The picture on your Speed tape is more correct with the actual CG entered and thats what I care about when it comes to the crunch, or flying the A/C to the envelope limit should it ever be required.

Thanks, Checkboard.

I am pretty certain, though, that we have some U5.0 aircraft, and have never noticed an inability to input the %MAC. I'll check next time.

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'Brighten my Northern Sky' Nick Drake R.I.P.