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In This question there is discussion about how to determine when to start the descent from cruise. There are several factors at play and it is usually calculated for maximum fuel performance.

Would this calculation not be done ahead of time, especially for aircraft cruising in class A airspace (presumably by dispatch for part 121 ops)? Why is this not included in the filed flight plan? Wouldn't ATC want to know when to expect this, especially if you lose coms?

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TomMcW
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  • For most of my professional flying I don't file, but I also don't have a descent point. I usually have to ascend to arrive at pattern altitude. – J W Jan 15 '16 at 13:00

2 Answers2

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Because the Top of Descent (TOD) is calculated in flight based on current weather data, STAR/Transition assigned, directs received during the cruise portion of the flight, vectors received taking you off your planned route, etc.. The FMC does the calculation for you with higher precision in flight.

For regular ops, ATC can use the rule of thumb 10.000ft / 30 NM to calculate an estimated TOD. In most cases, they will not need to do that however, as they can give instructions to aircraft to plan a certain STAR entry gate at an assigned flightlevel/altitude.

R: DLH123, when ready descend FL240 to be level\to reach at ARPEG

During lost comms, you are expected to maintain your filed or last assigned flightlevel/altitude until you reach the clearance limit. From there you will begin your lost comms procedures which usually include holding at the Initial Approach Fix (IAF), descending at the IAF to the approach altitude, and commending the standard approach procedure for the active runway.

SentryRaven
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  • So you stay st cruise alt until the IAF then descend in a hold? – TomMcW Jan 14 '16 at 22:22
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    Essentially, yes. [Leaving a clearance limit: 
    Fix from which approach begins (IAF): commence descent or descent and approach as close as possible to the EFC time if one has been received, or if one has not been received, as close as possible to the ETA as calculated from the filed or amended (with ATC) estimated time in route (ETE)](http://www.cfinotebook.net/notebook/flight-hazards-and-safety/lost-communication)
    – SentryRaven Jan 14 '16 at 22:26
  • @TomMcW See also here: http://studentpilot.com/interact/forum/showthread.php?19259-Descending-and-Lost-Comm-Procedures – SentryRaven Jan 14 '16 at 22:28
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    There is exception, based on jurisdiction. In Germany, the route terminates at the STAR or transition entry fix. This is also your clearance limit, so you would begin the descend there to the published altitude in the STAR or transition charts. Then from there you would hit the IAF (or fly the transition) and descend with the segments. – SentryRaven Jan 14 '16 at 22:31
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    So, basically, there are just too many unknowns to calculate TOD ahead of time? – TomMcW Jan 14 '16 at 23:17
  • I think maintaining cruise till the IAF will be very rare except in busy airspace. I have never experienced it. Have you? The link you provide is exclusively for lost comms. – J W Jan 15 '16 at 12:31
  • @JonathanWalters Well usually you are cleared to be at the correct altitude at the IAF or you are vectored around anyway. But when there is no other instructions and you are in lost comms... you maintain altitude until the IAF. – SentryRaven Jan 15 '16 at 12:33
  • @SentryRaven Are you speaking from real world experience? – J W Jan 15 '16 at 12:36
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    @JonathanWalters Yes. What else do you want to do? Start your descent in lost comms with ATC having no clue what you are doing? That's why there is lost comms procedures :D – SentryRaven Jan 15 '16 at 12:37
  • I was under the impression that the STAR or transition was part of the flight plan. Is that assigned later by ATC? – TomMcW Jan 15 '16 at 18:26
  • This is only true for some countries, in Germany you are required to file SID and STAR, in other countries it's not required or not even asked for, because it's ATC assigned at startup. – SentryRaven Jan 15 '16 at 20:55
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You can calculate ToD beforehand or let the FMS do the work. In reality though, you generally don't get to wait until your optimal ToD to start descending, especially flying into busy airports. If the airport has arrival routes, you'll be flying one and there will be crossing restrictions that are either part of the arrival clearance or expected clearances from ATC.

When you are flying the route you'll set the crossing restriction altitudes in the FMS and the FMS will generate ToD points to make those restrictions. You can also use the 1000' / 3NM rule to calculate your own ToD for each crossing restriction.

Going into smaller fields I often set my own target altitude of 10,000' 30 NM from the airport and used that to generate a ToD for descending from cruise. If you are landing in the direction of your arrival into a small airport and ATC is generous with pilot discretion descents you can sometimes manage an idle descent from cruise all the way to the FAF, but not often.

Finally, while the ToD might not explicitly be on the flight plan, it is accounted for in the planning. The dispatcher has to account for expected fuel burn for the flight so we have the right amount of fuel, and the performance calculations they do takes the descent from cruise into the terminal environment into account.

casey
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