Green-Dot and GW
Quote from Feather Boa [Jan29/22:18, currently #11]:
The (almost!) standard practice in our operation is to add 8kts to the VLS calculated by the FMGC to ensure a 5 kt buffer over the VLS displayed on the PFD...
[Unquote]
Was a little concerned to read this, as an ex-A320/319 driver (6 years retired). If there is no significant discrepancy between the gross weights calculated by the FACs and the FMCs, this seems to be an unnecessary and potentially problematical practice.
Just in case the previous posts have not made it entirely clear, the quickest way I know of picking up any discrepancy is to compare the 2 FAC-displayed green-dot speeds (on the 2 PFDs. They should be roughly the same. Now, look at the FMC green-dot speed on the PERF page. If you are above FL200, add 1 knot per 1000ft to the FMC green-dot speed. [Or, do it below FL200, and as you slow down approaching the destination airfield. If you are at high IAS, of course, green-dot may be off the bottom of the PFD scale.]
If the difference between FMC green-dot is, for example, 10kts, there is a 5-tonne discrepancy between the FMC GW and the FAC GW on the A320/319. I guess it is the same on the A321, having the same wing when clean?
In my company, we used to ignore discrepancies of less than 5 tonnes, but I can’t comment on whether that would be appropriate on an A321, if the FMC GW was 5 tonnes too low. Perhaps not.
By the way, there’s the quick way of calculating green-dot speed on the A319/320, provided you know the true gross weight:
[GW (in tonnes) X 2] + 85; then add 1 knot for every 1000ft you are in excess of FL200.
Does that also work on the A321?
Apologies to those − including Feather Boa − for whom this may all be old-hat,
Chris