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Chart for calculating cruise CG in flight

Started by Will, Mon, 27 Dec 2021 04:20

Will

Hello all,

The FMC allows the crew to update the center of gravity during cruise. You'll find this option on the PERF INIT page, line 4R (in both the legacy and NG FMCs). The default cruise CG value is 20%. Updating the cruise CG will affect the maximum altitude and the computed maneuver margin to aerodynamic buffet.

The actual variance is low... for example, it will change the maximum altitude only by something like 500' at the most. Because the variance is so low, flight crews will often ignore the option to update the cruise CG, and will just leave the default 20% value in place for the entire flight.

However, in case you would like to fly with the most accurate FMC data possible, here is a way for you to adjust the FMC cruise CG for fuel burn-off during your trip. Below is a chart that can speak for itself, however, you can read on for an explanation. And then at the bottom of this post is a link to download a PDF with the chart, some simple instructions, and an example.



For this to be accurate, you'll need to know your CG with all your payload but without any fuel. For our purposes, this is called your "MACZFW center of gravity," where "MAC" means the CG is expressed, as always, as a percentage of mean aerodynamic chord, and "ZFW" is a reminder that this particular CG was calculated for a fully loaded aircraft without any fuel.

In real life operations, your MACZFW center of gravity would be given to you on your loadsheet.

In PSX, you can get it very simply even without a loadsheet. While you are doing your preflight activities, go to Instructor > Situation > Aerodynamics and slide your fuel to zero. The CG you see is now your MACZFW center of gravity. (At this point you can adjust the CG if you like.) Make a note of your MACZFW, then upload fuel in your usual way and complete your preflight activities. You'll want the box "CG varies with fuel distribution" to be checked, otherwise there's no point to any of this.

Any time you like, you can now update your CRZ CG on PERF INIT based on MACZFW, plus the correction you derive from the above chart. The red lines on the chart are an example: assume the MACZFW is 18%, and assume you have 300,000 lbs of fuel on board. Go into the chart at 300,000 lbs, move to the right until you intersect the heavy black line, and then drop down to the X axis to read your correction value. In our example, for 300,000 lbs you get a correction of 1.6%. Add that to 18% MACZFW, and you'll see that the CRZ CG with 300,000 lbs of fuel on board is 19.6%. Set this value in PERF INIT on line 4R.

Some caveats:

First, you could always just enter the CG that you see on the Instructor Page directly into the FMC. Using this chart is a way for you to bypass the Instructor Page once airborne, and to do something similar to what a real flight crew might do (they don't have Instructor Pages in the real aircraft, obviously).

Second, the chart shows you PSX data, not Boeing data. So while this chart may look different from charts that Boeing distributes, it has the advantage that I made it myself and thus it can be shared with the PSX community (i.e. there is no Boeing copyright). And since it's PSX data, your calculations will be spot on--the chart just shows you what happens to the CG within PSX as the fuel load changes.

Third, the chart is specific to the passenger variant with horizontal stabilizer tanks installed, and assumes normal fuel balance and distribution. If any of those assumptions are not valid in your scenario, then the chart won't give you accurate data. Also, your MACZFW will change any time you manipulate the CG slider... so be careful not to move it once you have recorded your MACZFW. (Changing ZFW in PSX doesn't affect the CG; the loaders are quite skilled!)

Lastly, the chart should be accurate within the PSX world, but if for some reason it doesn't match Boeing's data (which I don't have access to) or otherwise isn't accurate, try not to panic, because like I said above, the operational implications are so small that many real-world flight crews don't even bother updating the CRZ CG in the first place.

With that said, here is the same chart in a PDF version, with instructions and the example:

https://www.dropbox.com/s/ruzdb7r0kxwzokp/CRZ%20CG%20Chart%20PDF.pdf?dl=0

I hope someone finds this useful!
Will /Chicago /USA