"We will start our descent in the next 5 minutes…"

stratobee

stratobee

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stratobee
Ever heard the captain say that when you fly commercial? I have, about a million times. And I always wonder - how does he know that? ATC might keep him high, or descend him early - there's no way of knowing.

How do they know? Just experience on that route?
 
stratobee said:
Ever heard the captain say that when you fly commercial? I have, about a million times. And I always wonder - how does he know that? ATC might keep him high, or descend him early - there's no way of knowing.

How do they know? Just experience on that route?
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FMS has a descent point planned in. ATC will usually give a "pilot discretion to FLXXX" allowing the crew to take advantage of the flight planning.

Some arrivals have crossing restrictions, so you usually know at what point ATC will start you down.

And on other occasions, ATC will request a descent ahead of time for traffic, weather, etc.
 
If I give a PA and say that, it's because we already have our discretionary clearance and know when we're planning on starting down. That said, we usually don't light up the seat belt sign and give the goodbye PA until well into the descent, turbulence permitting.
 
Sometimes experience but usually you calculate(or the FMS calculates) a decent point. Or like they said, you get pilots discretion, or "cross 60 miles south of XYZ at 20,000".
 
its not for you, its for the flight attendants so they know its time to start packing up. . .
 
How do they know that, it's pretty easy really.
Most arrivals have at least one or more crossing restrictions, it'll say something like cross xyz intersection at FL 250. If you're at 350 you know you have to lose 10,000' to make that restriction. A normal descent rate is 2000 fpm and you would need to start down roughly 30 miles before that intersection.
Obviously some centers start you down early and others may keep you up longer than you like, so there're some variables there, and weather and traffic plays a part too.

You can do the math in your head or newer avionics will do it for you in that either it will show you what the required descent rate needs to be in the FMS and/or you'll have something like a glideslope indicator on the side of the ADI to follow to make the crossing.
Experience will play a part too, if you fly a certain route many times, you'll have a pretty good idea when ATC is about to start you down.
 
Simple math version --> (Alt to lose x 2) + 10

In the post above (10K x 2)+10 = 30 miles.

If you've got a sporty tail wind, maybe you add an extra 10. Nice headwind, maybe you don't add 10 at all...

Generally if given a PD to the terminal area I'll use the total distance to the airfield...so from 35K you need to start your descent around 80 miles.
 
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Don't over think this.....

next time look out the window.....they're following contrails down. :D
 
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wlawr005 said:
Simple math version --> (Alt to lose x 2) + 10

In the post above (10K x 2)+10 = 30 miles.

If you've got a sporty tail wind, maybe you add an extra 10. Nice headwind, maybe you don't add 10 at all...

Generally if given a PD to the terminal area I'll use the total distance to the airfield...so from 35K you need to start your descent around 80 miles.
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I've always used the 3 to 1 to figure descents manually, such as if you are at FL300 and the field is at SL, start down at 90nm out, and adjust for headwind/tail wind situations. This will give a constant 2000 fpm descent.

FMS is a better tool for this and takes into consideration many factors such as crossing restrictions, wind, economy, speed reductions, etc.
 
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If you have been paying close attention, that announcement and the similar "we are starting our descent into..." come well after the airline has been descending from it en route altitude for quite a while. I started noticing that years (decades?) ago when I would listen to Channel 9 on United and have noticed it on every airline flight I've been on ever since.
 
midlifeflyer said:
If you have been paying close attention, that announcement and the similar "we are starting our descent into..." come well after the airline has been descending from it en route altitude for quite a while. I started noticing that years (decades?) ago when I would listen to Channel 9 on United and have noticed it on every airline flight I've been on ever since.
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Depends upon the airline. Our manual requires us to make the announcement before TOD.
 
In addition to winds, I'll also adjust the descent point based on which direction I'm approaching the field from. If I've got to drive all the way to the other side of the terminal area, I'll stay higher longer.

I'm always worried about gas, so I'd rather stay higher longer, descend later, and hopefully not touch the throttles until I'm leveling off to intercept GS or hit the initial. 99 times out of 100 I'm just gonna do what ATC says.

3 to 1 is another excellent method though.
 
midlifeflyer said:
I guess I haven't been on yours.
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As I said, depends upon the airline and SOP's and GOM. The majority of carriers have the procedure written out, even down to what to include in the announcements. The common procedure is to do this before TOD before the workload begins increasing.

Of course, traffic, weather, etc play into the equation as well. Sometimes it works out on cue, sometimes it doesn't.
 
RotorAndWing said:
Depends upon the airline. Our manual requires us to make the announcement before TOD.
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Our 737 fleet does too. However, there was no such guidance on the 777 fleet. Unless it was one of the many changes that I didn't notice as a result of the merger.:rolleyes:
 
RotorAndWing said:
Our signs (seat belt) don't go on until 10,000, unless there is turbulence. The cabin call at 10K is "Cabin Crew prepare for arrival".
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ya but....that was like 30 years ago when you were flying DC-3's or was that 9's?:yikes: :D
 
wlawr005 said:
In addition to winds, I'll also adjust the descent point based on which direction I'm approaching the field from. If I've got to drive all the way to the other side of the terminal area, I'll stay higher longer.

I'm always worried about gas, so I'd rather stay higher longer, descend later, and hopefully not touch the throttles until I'm leveling off to intercept GS or hit the initial. 99 times out of 100 I'm just gonna do what ATC says.

3 to 1 is another excellent method though.
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I can't out-plan a modern VNAV system. :)
 
Greg Bockelman said:
Or three to one. Three miles to lose 1000 feet.
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I use 3X the altitude (in 1000's) for the descent point and 5X the groundspeed for the descent rate. If I'm doing 500 kts across the ground and I need to lose 40,000 feet I'll start down 120 miles out at 2500 fpm.
 
or just start your descent .......30 minutes out.:rolleyes2:

the math is much easier in a time continuum.
 
(120 / 8.3) x 2500 gives you 36,144...3800 feet high at the airfield. I'd say that's pretty f*in close man, nice swag.
 
I'm not a commercial pilot, however If they were given or expecting an arrival procedure to follow, they would easily know. Take a peek at them in ForeFlight or search for them online.
 
JasonM said:
I'm not a commercial pilot, however If they were given or expecting an arrival procedure to follow, they would easily know. Take a peek at them in ForeFlight or search for them online.
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I've noticed Centers don't often go by the vertical planning on STARS
 
He knows. That's why He's the Captain!

Standard rate of descent is 500fpm (for me). ATC and I know that. So we start my descent so I will go down 500fpm and cross the fix at the right altitude.
 
Gucci Pilot said:
I've noticed Centers don't often go by the vertical planning on STARS
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I want to share a funny testy exchange a few months ago coming in on a STAR :lol:

I think it was an American airlines jet and the controller told him to cross a certain fix at a certain altitude and he said "that ain't gonna happen!" and then said he hadn't been told to expect that. The controller then told him the STAR says to EXPECT that altitude and that he should have been prepared for it. The pilot said "no it doesn't say to expect". The controller then said "well haven't you done this before? You always cross this intersection at 7000 or 9000 feet". The pilot says "nope first time here". Anyway the controller gave him a new request involving a vector and a new crossing restriction elsehwere. The American pilot continued to complain probably because he was now getting delayed with a vector and then self incriminated himself by saying something about his experience on a different arrival followed by the controller saying "I thought you said this was your first time here!"

:rofl:

I had a look at that STAR and it definitely said to expect that altitude.:dunno: I don't get why some people have to argue with the controllers.
 
Quick and dirty when riding as self-loading cargo - typically I note top of decent about 1/2 hour out from expected time at the gate. YMMV, but that seems about right. And, for those who do their calculations based on 500 fpm, that works for our spam cans, but I think the airliners are more in the range of 2000 fpm for their initial decent out of the high flight levels. I know that when I get a PD clearance I base my start of decent on 500 fpm, number of thousands of feet to lose, etc. Cruising at 120 kts. Not hard at all.
 
Back to the original question... We know when we NEED to descend, and so does ATC. It's really pretty easy to guess when we'll get our descent clearance since they don't want to have us stuck too high anymore than we want to be stuck too high.
 
This is what I use and it works to a T. ALT to lose times 3. Ground speed times 6. PD Clearance to cross xyz at FL200 and your at FL 300 doing 500kts ground speed. 10000x3 = 30 mi 500x6 = 3000 FPM.

Then re check with 5000 to go to adjust for your decreased or increased ground speed for a new FPM down
 
RotorAndWing said:
As I said, depends upon the airline and SOP's and GOM. The majority of carriers have the procedure written out, even down to what to include in the announcements. The common procedure is to do this before TOD before the workload begins increasing.

Of course, traffic, weather, etc play into the equation as well. Sometimes it works out on cue, sometimes it doesn't.
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They don't like you guys just making it up?

"We're going down! Yeeeeeee hawwwww!!!", might bother someone?!

LOL! ;) ;) ;)
 
Well if you fly a Super Cub its easy to figure if you go down 1000' a minute and go 60knots, that is one mile a minute. If you have 10,000' to lose its gonna take 10 minutes and 10 miles.

If you fly something else, then multiply your speed by how many 60's it is, and youre there if you go down 1000fpm. If not, multiply your descent rate by how many 1000's it is too. If you go 120 then it takes 5 minutes and 5 miles. And if you descend at 2000 then its 2.5minutes and 2.5 miles.

Its not easy to do in the head, but its doable. Probably need to make a chart for YOUR airplane that makes it easy.

ATC does this all the time for you.
 
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I think in minutes not miles. How many thousands of feet to lose? 9,000 ft? Ok then I'll wait until I'm 18 minutes out. Might add a few minutes if I expect to gain lots of speed in the descent or want to be down a bit early. Precise enough for anything I've flown.
 
jesse said:
I think in minutes not miles. How many thousands of feet to lose? 9,000 ft? Ok then I'll wait until I'm 18 minutes out. Might add a few minutes if I expect to gain lots of speed in the descent or want to be down a bit early. Precise enough for anything I've flown.
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:yes: That is exactly what I do.
 
My guess is that the OP has only dealt with "descend and maintain" rather than PD descents or crossing restrictions.

Once you're aware of those last 2 things, it's less of a mystery as to how you'd know when you're going to initiate your descent.
 
My pet peeve is when we're passing through 10,000ft, there's the "chime" from the front, then the FAs make a PA saying "Ladies and gentlemen, we've been cleared to land at Los...."

There's NO WAY we've been cleared to land from 10,000' and way out. Cleared for the approach, perhaps. Cleared to land, doubtful. It just grates me to hear "cleared to land" and we're nowhere near the airport, and many minutes away from actually landing.
 
Mike I said:
My pet peeve is when we're passing through 10,000ft, there's the "chime" from the front, then the FAs make a PA saying "Ladies and gentlemen, we've been cleared to land at Los...."

There's NO WAY we've been cleared to land from 10,000' and way out.
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Yep..
 
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