When to descent...

[Broomstick Flier]

Hi chaps,

I recently transitioned to the RHS of a CJ3, and the training capt. taught me the following technic:

While under vectors, but with a STAR loaded into the FMC and VNAV armed, and instructed to perform a descent, lets say from FL080 to 6000ft I should not descent immediately after receiving the ATC instruction, but put 6000ft on the altitude selector and wait for the VNAV path to be intercepted and let the aircraft descent by itself.

I think that somewhat odd, as I would descent immediately (is this not what ATC expect me to do?) and that was the way I made it on my previous type, where I flew single pilot. Is this acceptable, especially from an ATC point of view?

Thanks!

[STBYRUD]

If I remember correctly you should at least descend with 500fpm or so to comply with the ATC instruction (unless they say something like 'when ready descend to...' obviously)...

[BOAC]

You are correct, he/she is wrong. As STBY says, you are REQUIRED to commence descent on receipt of a descent instruction. You may do better on the ATC forum with this but hopefully a passing controller (if not asleep....:)) might pick up on your post.

[misd-agin]

Were you told - "descend and maintain...." or were you told "cross XYZ at 6,000'...." ?

In the first case you have to descend at least 1000' FPM. So his technique would be wrong. If the FMC TOD is a mile away delaying the start of descent is not a crime.

In the second case his technique is an efficient way to run the a/c(not for a 2000' descent but as a general technique). Nothing dumber than seeing guys descend well before TOD, especialy with a tailwind.

If the VNAV generated path is barely ahead delaying the start of descent for seconds, or starting a gradual descent until the VNAV descend profile is captured, are also acceptable techniques.

[BOAC]

In the second case his technique is an efficient way to run the a/c(not for a 2000' descent but as a general technique). Nothing dumber than seeing guys descend well before TOD, especially with a tailwind.

I assume you meant 'headwind' although of course TOD is TOD is TOD regardless of wind!

In ICAO land you ARE required to intitiate descent and not to 'wait' unless told 'when ready' as stby said regardless of any VNAV path (and 500 is enough, although ATC prefer 1000)

[STBYRUD]

Oh, and one more thing: Sounds like this is your first exposure to the black magic of VNAV - in the situation you described I would strongly advise you to descend to 6000 early enough to give you margin to the VNAV path in case ATC forgets to clear you for further descent ;) Otherwise you are stuck at 6000 while the VNAV path shoots down below you :D

[Broomstick Flier]

misd-agin, we normally do not get many "cross XYZ at 6,000'.." instructions around here where I fly, most of them are the "descent and mantain" type and I was always a bit unconfortable with this delay in descending, as the FMC TOD was normaly well beyond the 1Nm mark.

stbyrud, indeed that is the first time I use VNAV, but fortunately I still think the best VNAV is in my brain and I like to plan my descents ahead, but one has to pay attention on what the magic box is planning, better safe than sorry...

BOAC, "You are correct, he/she is wrong"
I thought so, now I have just have to think on a polite way to say that to the guy...:ugh:

Thanks for the inputs!

Cheers
BF

[AerocatS2A]

Where are you?

In Australia if you're told to "descend to 6000" then you do it straight away, if told "when ready descend to 6000" then you can do it when you get to your own top of descent point. Normally when under vectors you're in the terminal area and they want you to descend straight away. In the USA they use the phrase "pilot's discretion" to indicate you can descend when you want to.

If you're using VNAV and you're told to descend before the VNAV TOD then you should be able to use a VTO type function to link your present vertical position with the level you're descending to. This will start a descent straight away using VNAV but at a shallower angle so you still hit your next VNAV waypoint. If this doesn't give you at least 500 fpm (or whatever min rate of descent is required where you are) then you need to increase the descent rate and forget about VNAV for the time being until you can get back on profile.

Another option is to use the VNAV for guidance only, you may be able to get a VNAV display up on the PFD but use another descent mode to follow it, that way you can see where the VNAV thinks you should be but you can adjust the descent rate as you like to meet ATC requirements.

[bubbers44]

I have found some airports give you a star and then take you off for a direct clearance like Mexico City and Bogota. Local knowledge keeps you from having to do high dives sometimes. Then descending early is good, assuming the shortcut route in your planning. I usually get caught the first time but after that it it is my fault if the speed brakes are required.

[Broomstick Flier]

AerocatS2A,

I am currently flying in South America, under ICAO rules.

The VTO would be a direct to in vertical mode, right? If so I use this often, but not always, as I prefer to control the bird through the MCP rather than fiddling with the FMC below 10000ft, althought that is the job of the PNF anyways.

[Basil]

a polite way to say that to the guy..
I'm sure I'm teaching Grandmama to suck eggs, but if PIC is handling and you take the call perhaps "Is that descend now or at pilot's discretion?" would make the point to the boss.

[STBYRUD]

Even better is waiting for the day that ATC asks if you are descending or not when you clearly should be... ;)