A320/1 Green Dot QRH/PFD Difference

[Eurotraveller]

Hi,

In the cruise, I understand that comparison of the green dot speed indicated on the PFD with that given for the current aircraft weight in QRH 4.01 "Operating Speeds" can provide an indication on whether the aircraft is in fact heavier than you believe it to be.

My question is, how reliable is this indicator? And is it possible to equate the difference in knots to a specific weight? For example, if you work out that your green dot speed should be 235 knots according to the QRH, but it indicates 250 knots on the PFD, can you equate the 15 knot difference to a specific weight and say "we're x tonnes heavier than we think we are"...

Thanks in advance,
ET

[Bus Driver Man]

You can always look up the mass that the FAC's have calculated with aerodynamic data via the AIDS pages in the MCDU.

Look for "Alpha Label" and enter GWFK (kg) or GWFL (lbs) and compare it to GW (ECAM, FMGS or also with the GW Alpha Label)


However, I don't know how accurate this is. I've seen GWFK being 2 tons more than GW.

[Chris Scott]

Eurotraveller,

Going into AIDS, as Bus Driver Man suggests, can be a bit inconvenient. There was a thread about Green Dot in October, which may be of interest:

http://www.pprune.org/tech-log/466436-green-dot-speed-concept.html#post6757000

For the A320, Green Dot = ([GW in tonnes] X 2) + 85 + (1kt/1000ft in excess of FL200)

So a discrepancy of 15kt would equate to 7.5 tonnes. That would be excessive, and suggest a gross error in the loadsheet. I never saw more than 10kt (5 tonnes). There is sometimes a discrepancy between the two FACs, but I never saw more than about 3kt. This suggests that the FACs (and AoA probes) are fairly consistent.

[Eurotraveller]

Chris Scott and Bus Driver - thanks to both of you for your help.

ET