Descent Clearance

[Tambaran]

I fly for an Asian Airline & it is our policy to ask for descent when the FMC tells us to reset the Altitude window. There is a delay of at least 3 minutes before the aeroplane will commence its descent. Is there not a requirement to begin descent within 1 minute of being cleared ? :confused:

[willadvise]

Yes your are correct. You have one minute to commence a change of level once instructed to do so, unless the controller has prefaced it with "when ready". If you are worried that the controller has forgotten to give you descent I find "ABC123 top of descent 3mins" will prompt the controller to give you "when ready, descend FL..." as opposed too just "ABC123 request descent" which the controller will almost always respond with "descend Fl...".

[eyeinthesky]

From a UK ATC point of view, I would comment that while it may suit you to set the level and then wait for the VNAV to start the descent, it is one of the very annoying things when someone asks for descent and then sits there for another 15-20 miles. Thi is because by asking for descent you may well have caused the controller to make a plan based upon you descending when cleared, and you inconvenience him and other aircraft by not doing so as you may engineer a conflict which he had avoided by clearing you for descent.

In short, you are Pilots, not computer operators. If YOU ask for descent, then descend! If WE tell you to descend, whether prefixed by 'when ready' or not, then follow that instruction too, please.

[HEATHROW DIRECTOR]

Best guys I ever worked with in this respect were the USAF. You only had to clear them to a lower level and they instantly replied "Out of xxx (FL)".. and they meant it. Oh for a return to pilot-driven planes!

[NextLeftAndCallGround]

What puzzles me is why aeroplanes that will be driven by the computer rather than the pilot ever got certified with a system like this.
Didn't the people who programmed the computers (or specified what the computers should make ther aeroplanes do) think about making them fit in with the other parts of the aviation system?
I presume that no-one would design a computer that requires an aeroplane to sit on a runway for three minutes after being given a take-off clearance (would they?) - so why three minutes before the darned thing will descend???? :confused:

[terrain safe]

Nextlaeftandcallground you have never claered an AN124 for immediate take off have you!!!! ;)

[NextLeftAndCallGround]

You know, I nearly put something in the post about the AN124 - I should have known someone would pick me up on it!

But seriously, the 124 doesn't do it because it has a computer that makes it do it.

I've heard it said that the 124 will sit on the runway at take-off power (or close to) for three minutes before rolling because if the engines keep going that long, there's a fair chance they'll keep going for the rest of the flight. I gather the more correct reason is to ensure that engines are suitably lubricated etc and it takes that long at high power to do so.

It's obvious from your post that the way in which the 124 operates has stuck in your mind and so makes my point - no-one with any sense would design that into an aeroplane. Maybe there's a good reason for the 124 to operate in this way but why would a modern aeroplane need three minutes to set off downwards when a pilot can do it in seconds?

I guess with all the powerful computers available these days we'll just have to get used to this sort of progress.

[Stan Woolley]

Some background info might help.

Once the FMC (Flight Management Computer) has been programmed it will produce a top of descent point.Around fifteen miles before this point is reached the FMC will produce a reminder to 'Reset MCP Altitude'if this has not already been done.It is at this point that most people ask for descent in the Boeings I fly.The MCP altitude window will normally be at the aircrafts cruise level eg 330.Once a lower cleared level has been received from ATC this level is put in the MCP.

Once the TOD point has been reached the aircraft will commence its descent in VNAV as long as the overriding MCP(Mode control Panel)has been reset to the lower level.All this takes place pretty gently so as not to spill the drinks!

If we waited until we were closer to the TOD point before requesting descent you can guarantee that 'Murphy 123Alpha' will start blabbering half a second before you do,causing you to speed past said point with the Pilot Flying rolling his eyes.This is because the mode on the Autopilot will change to Alt Hold if the TOD point is passed with the current level still in the MCP Alt window. The aircraft will then not commence an automatic descent and a bit more fiddling with the knobs is required to get back to where we want to be on the descent profile.

There are lots of ways to make the aircraft descend but the preferred method is to use VNAV which is as described above.The FMC has a 'descend now' option which will commence descent when pressed, initially at 1000fpm until the computed path is intercepted and the throttles will then go to idle as normal.Britannia used this method as SOP at one time for an engine related consideration which I can't recall - maybe still do?

[Tambaran]

Thanks Folks, I have mentioned this point a few times to trainers but the attitude is that we have always done it this way & no one seems to mind (obviously they do). Nice to have some ammo for the next time I bring it up. Thanks again

[eyeinthesky]

Flanker: Thanks for your input. I do understand what you say, but I think it is important that you understand the relevance what you are doing has in the wider ATC perspective.

I would divide the three methods of achieving descent from an ATC position as follows:

1) Descend when ready: You programme the FMS and let it follow the profile it wishes. This is the system you are describing.

2) Descend now: We need you to start a descent perhaps before the ideal FMS profile to solve conflicts with other aircraft.

3) A/c requests descent: From an ATC point of view, if YOU ask for descent, I expect you to start descent IMMEDIATELY upon receiving the clearance. In most peoples' mind the act of requesting descent means you want down now, not in 15 miles' time. As I have said, by giving you clearance the controller might have formulated a plan to deconflict you with other traffic, and if you don't descend you muck up that plan.

Some airlines are starting to make things a bit clearer by saying 'Requesting descent in 15 miles' or some such. That at least gives us an idea of what you are planning to do.

If you want a short advice, I would say if, in UK airspace, your MCP panel prompts you to set a new cleared level and you request and receive descent clearance, then start down upon receipt, if necessary using the procedure already outlined above with reference to Britannia. Then at least we are all using the same plan.