Airbus FMGS holding logic
 

Doees anyone on here understand the intricacies of how the A320 FMGS calculates holding patterns ?

My particular query relates to losing a specific amount of time during an enroute/approach hold. For example, an ATC instruction like "7 min delay required, adjust the holding pattern to depart at time 52".

Logic says I would allow for 2 x 2min turns and make each holding leg 1.5mins. Having sad that, every time I have tried it, it has never worked. Principally due to the timing of the turns not being predictable. I have seen every variation of turn from 1.5 to 3 mins, making it difficult to compute. (Its seems to depend on altitude - we usually hold somewhere near FL180-FL250).

FCOM is vague on a lot of this information.

Does anyone have any hints/tips/tricks on how to accurately manage a holding pattern to make sure it terminates as close as possible to a set time ?

Thanks !

It's not a perfect system... But this is what I use.

35000' = 3.5min per turn
30000' = 3 min
25000' = 2.5 min
20000' = 2 min

Then adjust the leg times

fmgs
 

once you remove the defaulted logic from the hold page, the FM will be erratic by sometimes over a minute..it just doesn't process quick enough.

I agree with Bula.

I have always used 5 minutes for one hold as a guide - though I cannot find it written anywhere. But at the FL's you mention it is possibly more....
So I would add half the extra time to the outbound time, SHIG, and shorten or extend by a tad if necessary.
Or, just SHIG without any changes, look at the predicted ETA at holding fix and extend using selected Track at the appropriate moment.

In the 777 a standard hold takes approx 6 mins to do. In the A330 due to the reduced AOB in turns it can take about 8 to 9 mins. (3 to 3.5 just for each 180 turn)
Although having said that the bus did the turns last week at 30 deg AOB!!?

The Bus seems to limit the AOB at high levels above FL200 whereas the 777 didn't.

( from memory :ok:)

...The 320 uses varying bank angle when turning inb or outb in a xwind (most
obvious in a strong one) to maintain lateral track in NAV. This has an impact
on the total time for one lap. In such circumstances one can fly raw data and
triple the drift for a 1 min hold etc, arriving over the hold fix on time in XW of
50kt or more clean, 40kt or more CONF1, at GD+5 and F+5 respectively.

I use the same scale as Bula.