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| When exactly does ATC call for descent? http://www.ocs-support.co.uk/forums/viewtopic.php?t=2960 |
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| Author: | martinlest [ Wed Aug 08, 2018 3:38 pm ] |
| Post subject: | When exactly does ATC call for descent? |
| What is it exactly in the PF3 programming that triggers ATC to ask for the user a/c to start its descent? I thought it was always at the waypoint prior to the T/D, which is why I always try to find a waypoint close before and close after the TOD in my flight plan, but I am often asked to descend earlier. The problem is, PF3's T/D and the aircraft's T/D do not always coincide; they can be many miles different. If I am still well short of the T/D and PF3 asks me to descend (and I do so), this can rather mess up a managed descent (where the a/c will respect all the waypoint speed constraints), as you are well below the T/D (i.e. cruise) altitude by the time you get there. I seem to have the choice of either not descending when ATC tells me (unthinkable!!) and waiting until the a/c FMC shows me to be at the T/D before doing so, or just letting the a/c manage things when it finds itself a lot lower than expected when you start the managed descent (really talking about Airbuses here of course). Neither very satisfactory. Ideally, you'd need to find a way of getting the PF3 T/D (where ATC gives descent instructions) as close as possible to the aircraft's calculated T/D, but since the a/c's calculated T/D isn't known until you are in the sim setting up the FMC, that's not really feasible I suppose. What's the best way to set this up? Anything I can do in the waypoint altitude adjustment page? Thanks. |
| Author: | Dan77 [ Wed Aug 08, 2018 4:27 pm ] |
| Post subject: | Re: When exactly does ATC call for descent? |
| If your initial descent instruction is to descend at pilot's discretion, you do not need to start down immediately. You can wait until you reach your FMC's TOD or you get a further descent instruction that is not at your discretion. However, if you reach your FMC's TOD and you have not yet been cleared to start down, you can't just leave cruise without clearance. In the real world ATC may need to delay your descent due to traffic below you or other reasons and they aren't concerned with your FMC. One thing you can do is load your flight in your FMC and then adjust the waypoint altitudes on the PF3 waypoint page so there is fairly good agreement. |
| Author: | Dave March [ Wed Aug 08, 2018 4:30 pm ] |
| Post subject: | Re: When exactly does ATC call for descent? |
| As you reach waypoint 'A' you will be stepped down to the altitude set at waypoint 'B'. PF3 will also start your descent when you reach the TOD. As for your FMC, PF3 doesn't know anything about it and neither does ATC in the real world so it's doubtful the two would ever be in sync. Personally I wouldn't switch my VNAV on till ATC start me on my descent and would adjust it accordingly. |
| Author: | martinlest [ Wed Aug 08, 2018 6:04 pm ] |
| Post subject: | Re: When exactly does ATC call for descent? |
Thanks for the replies! Yes, that's an idea... When does PF3/ATC instruct to descend 'at pilot's discretion' and when is it just an instruction that is not 'up for debate'? In practice, having set the ALT Nag (in the ini file) at 600" (probably too long in retrospect), I usually have plenty of time to acknowledge and then (if not a discretionary direction) to descend at minimum rate (50'/min or whatever) until I reach the T/D; that way I am not too far below the expected altitude when I do get there. At the T/D I can switch to managed descent and the AP adjusts things accordingly. (If I try the same before T/D, I get selected mode descent, which kind of messes up the programmed descent profile, as I think I said). Martin |
| Author: | Dave March [ Wed Aug 08, 2018 6:38 pm ] |
| Post subject: | Re: When exactly does ATC call for descent? |
If your flight profile calls for a descent but you haven't yet reached your (PF3) TOD we will usually give a 'at pilot's discretion' descent call. |
| Author: | RALF9636 [ Wed Aug 08, 2018 8:39 pm ] |
| Post subject: | Re: When exactly does ATC call for descent? |
| You will hardly ever get the PF3 TOD and your FMC TOD in sync. Just as in real life. But you can still more or less follow your FMC in most cases: If PF3 tells you to descent early you can start to descend with a very low descent rate until you catch up with your planned descent path. If I'm not mistaken PF3 will not nag you if you descend at 200 FPM. If PF3 does not descend you in time you can request a lower altitude shortly after your calculated TOD, which will be granted in most cases. As a general rule I always set the altitudes within the PF3 flightplanning page rather lower than higher because I can always choose a low descent rate if I'm told to descent early but if I am told to descend too late I might run out of distance. |
| Author: | Ashun [ Thu Aug 09, 2018 12:30 pm ] |
| Post subject: | Re: When exactly does ATC call for descent? |
| If ATC ask for descent at pilot discretion before FMC TOD, there is no need to descend. When aircraft reaches FMC TOD point and ATC hasnt cleared for descent, in real life many times pilots contact ATC suggesting that they are ready for descent. It is up to ATC to allow them to descend or ask them to hold on to cruise altitude for late descent clearance. Is it possible to add a call from pilot to ATC either "requesting descent or ready for descent" ( instead of request lower altitude) Then ATC can ask them to descend to specific altitude or deny (this feature is already in place) Best wishes Ash |
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