US Descent Instructions

[Rotten Apple #1]

Chatting with a guy/gal the other day about receiving the typical nearing YYZ instruction from Cleveland ARTCC to "Descend and maintain 290". We discussed whether pressing ALT INTV to start a 1000 fpm descent was sufficient. I thought not, and endeavoured to find the relevant documentation. So here it is...(from the US AIM):

d. When ATC has not used the term "AT PILOT'S DISCRETION" nor imposed any climb or descent restrictions, pilots should initiate climb or descent promptly on acknowledgement of the clearance. Descend or climb at an optimum rate consistent with the operating characteristics of the aircraft to 1,000 feet above or below the assigned altitude, and then attempt to descend or climb at a rate of between 500 and 1,500 fpm until the assigned altitude is reached. If at anytime the pilot is unable to climb or descend at a rate of at least 500 feet a minute, advise ATC. If it is necessary to level off at an intermediate altitude during climb or descent, advise ATC, except when leveling off at 10,000 feet MSL on descent, or 2,500 feet above airport elevation (prior to entering a Class C or Class D surface area), when required for speed reduction.

The first emboldened (?) words refer to "optimum rate". One POV has that meaning "ALT INTV" on the NG, as that's the button on the MCP. I think it probably means more like LVL CHNG, so let's press on to the second bold section: then within 1000 feet descend/climb at between 500-1500 fpm (superceded by RVSM requirements obviously). By extension, they must want more than 500-1500 fpm in the first part of the climb/descent.

My guess is at least 1500 fpm wins in the case of the NG, and probably hitting LVL CHNG keeps 'em happier on approach into MCO, LAS etc...

Anyone else bored?

[J31]

I agree a 1000 fpm usually does not satisfy the controller. In the US or where ever there is a lot of traffic I have found that ATC is expecting at least 1500 fpm when they clear a flight lower. Usually you need to get into an idle decent to make some of the forthcoming crossing restrictions.

[grammar boy]

I've done that descent a bajillion times in ALT INTV (1000fpm) and can count on two fingers the amount of times we've been asked to pick up the rate of descent, usually with traffic as the reason. I'd say that if you are descending early for ATC requests (i.e. prior to T/D) then 1000fpm is "an optimum rate consistent with the operating characteristics of the aircraft"

If they need you down quicker, they will ask, methinks. I think they are more worried about our 260ish kt transition speeds than our 1000fpm early descents to be quite honest with you. My SOP in US airspace is to do at least 280kts going downhill, unless I know for sure someone is behind me, then I plug in 300 if it's reported smooth to 10K. Every time I've been PNF into a US airport where the PF hasn't adjusted the descent speed, we've either been asked to speed up (in an angry tone btw) or given a clearing vector to allow faster traffic to pass (this happened into MCO).

Unoptimized decent speed be damned, I say. The speed the FMC spits out is purely a function of cost index, and nothing else. Plug 22 into a -600's FMC with 50 people in it and a tailwind and an -800 with 166 into a headwind and you will get 263kts both times. It makes no sense.

I especially enjoyed our cost index of 18 today, which produced a blazing 259kt transition speed. Good thing it was only going into YWG...

[Rotten Apple #1]

Ahhh,to be an F/O again heh...this guy wants that,and that guy wants this. It must suck.

Grammar Boy, I'm with you on the descent speed, though. Given the descent instruction, for me it's LVL CHNG and transition at no less than 280 KIAS. And no guilt about excess emissions: at the end of the day, reduced vectoring off course might be the great equalizer in the fuel economy department.

I wonder if the Boeing/FMS sub-sub-subcontractor in charge of vertical profiles had too many guys book off sick late in the project and just went with a WAG as far as descent speeds...in my glider days we added the HW component to "still air best L/D speed", in order to achieve better penetration...

[Pygmie]

If they need you down quicker, they will ask, methinks. I think they are more worried about our 260ish kt transition speeds than our 1000fpm early descents to be quite honest with you.

Yes, we are MUCH more worried about the speed than the descent rate. To be able to work something like an RJ in behind a 737 in descent, we need on the order of 20-25 miles in trail. 100kt overtakes are quite common between a descending RJ and 737. Needless to say we almost always have to put speeds in place to solve the problem, as usually it's a hell of a lot harder to get in-trail traffic ahead in the descent.

[grammar boy]

Yes, we are MUCH more worried about the speed than the descent rate. To be able to work something like an RJ in behind a 737 in descent, we need on the order of 20-25 miles in trail. 100kt overtakes are quite common between a descending RJ and 737. Needless to say we almost always have to put speeds in place to solve the problem, as usually it's a hell of a lot harder to get in-trail traffic ahead in the descent.

Tell your supervisor, and tell them to forward this concern to WJ flt ops. Or, when you issue the initial decent clearance, add a "transition at XXX kts or better". This gives us time to type it in, the FMS to calculate a new descent profile, we stay at CRZ alt for a few miles longer (unless you say "start down now") you are happy, we are happy, we save gas and time and it's all good.

Technically we are not to alter the FMC calculated (aka "It's a Friday and this FMC program is like, so overdue dude, so let's just make something up and hit the bar!" as Jonny D hypothesizes) descent speed unless ATC requires it.

Oh, you've got me started now.

[Rotten Apple #1]

Good idea GB.

Pygmie, I take it that you're a YYZ low level guy? Good to have the dialogue. I've some friends in QM, and QX centers, and the exchanges are always informative. Keep the info coming when necessary.

[Pygmie]

Yep GB, that's exactly what we do. If I have any jets coming in behind a WJ 737, the first words out of my mouth will be "maintain xxx, 300 knots or greater in transition."

Would be ideal to issue transition speeds before you're started down, but trying to coordinate things like that with the high is hard to do when it gets busy, meaning you'll probably be descending before you're talking to me.

Out of curiosity, how much of a difference does it make getting the speeds assigned before you start down vs. in the descent? I've never really though to ask, as we didn't think the transition speeds affected anything until you actually hit the transition, but if it makes a big difference (i.e. if it means you'll hold it up and stay at cruise speed a little longer) it might be worthwhile to look at getting the high to start assigning some speeds.

[invertedattitude]

Considering I've seen you WJA boys doing 6000 down and 4000 up in high level airspace, id be curious about 1000fpm after a descent clearence, but really it's up to you as long as its a constant rate, if the controller wants you down faster he/she will tell you.

On that same note, if you ask ATC for lower, and he gives it to you, don't stay at your flight level for 20 miles then descend, if ATC says "Descend FLXXX" regardless weather you requested it or not, descend immediately at your normal rate. Unless of course PD is issued.

[Four1oh]

Out of curiosity, how much of a difference does it make getting the speeds assigned before you start down vs. in the descent? ... (i.e. if it means you'll hold it up and stay at cruise speed a little longer)

That's exactly right. I'd rather know before I started my descent, but having said that if it's to speed us up in transition it's far less of a big deal than slowing us down once we've started the profile. The other problem I've run into is one controller telling us to plan a high speed descent, then switch freqs halfway down to get told to sloooooow down. Not fun.

[grammar boy]

Out of curiosity, how much of a difference does it make getting the speeds assigned before you start down vs. in the descent? ... (i.e. if it means you'll hold it up and stay at cruise speed a little longer)

That's exactly right. I'd rather know before I started my descent, but having said that if it's to speed us up in transition it's far less of a big deal than slowing us down once we've started the profile. The other problem I've run into is one controller telling us to plan a high speed descent, then switch freqs halfway down to get told to sloooooow down. Not fun.

What he said

Speeding up after TOD usually means adding some thrust for a little bit, slowing the descent rate to 1000fpm to catch the new path (this is the VNAV default) and then back to idle and away we go, yeah it burns gas, but at least you are getting something for it (more speed).

Slowing down once you've started down means pulling out the boards and shaking the airplane for the majority of the descent depending on the severity of the speed reduction. It's annoying to the guests, to the crew, and the accountants because now you are pissing away energy (i.e. gas) that you could have saved by starting down a little earlier had you known that you would be required to descend slow.

Good thread! This is what boards like this are for.

[invertedattitude]

Interesting info to have for sure from the other point of view.

However:

You have to consider each controller has different needs.

High level Airspace in ATC terms anyway is FL290 And above for the most part in Canada, so you're going to have a different controller above and below this flight level.

Take for example arriving into YHZ around 1PM in the afternoon, put the jetstream in the right place and that one WJA airplane has to cross 20 airplanes going in the opposite direction at various different flight levels above FL290, below 290, there are little to no airplanes on that profile (but there are many on different types of flight routes).

So we may not be able to safely work you down because of various pieces of traffic, and we'll tell you it will be a late descent since in order to safely get you down around all the mass of traffic arrivals you are swimming upstream against. Once you start going down, and you are handed off to low level (FL280 and below) they likely have a totally different picture, all arrivals departures, low level enroute airplanes to work you around, and of course it depends on what runway is active etc.

My point is each controller has their own airspace and each airspace has its own sets of challenges and scenarios to deal with.

There's no malice in place or disagreement in how to handle your flights it's just simply because High and Low are two different worlds.

ATC will never hold you up if they don't absolutely have to, ATC's primary job is of course safety, but they're in place to provide you a service as well, however they will never sacrifice safety for any level of service

[po'dcjer]

except going into Montreal, and your right beside AC or ACJ (or slightly ahead, or if your in a 737 and your following a dash a few miles ahead, then your sure to put into a hold or vectored almost to Ottawa....

[Duster]

Nooo.. that never happens does it???

[LVLChange]

Interesting topic... I had done some investigating myself.

To keep it simple. Boeing is designed in the US.

ALT INTERVENTION, gives us 1000'/min descent rate.
ALT INTERVENTION, is an option for the A/C, and is a 20000$ option, if it wasn't to standard/regulations where its set at 1000'/min it would otherwise be 1500'/min.

From my ATC buddies, in Canada... they want to see at least 500' per min.. aparently their radar does not indicate a desent under 300' per min. If they want more of a rate they will ask, they want to see you descending if you request it. Same for the US. Its all about other A/C, they are doing there job.

[Rotten Apple #1]

LVLChange...points noted. Thanks for the legwork...

Workin' on some US ATC connections...

JD

[LVLChange]

LVLChange...points noted. Thanks for the legwork...

Workin' on some US ATC connections...

JD

Let me know what you find!