Does ATC know our TOD?

RyanB

RyanB

Super Administrator
Management Council Member
PoA Supporter
Joined
Jul 21, 2010
Messages
16,200
Location
Chattanooga, TN
Display Name
Display name:
Ryan
How does ATC determine when it’s time to give us our initial descent while IFR? I assume there must be some sort of algorithm that indicates when they need to start descending an aircraft in cruise, so they’ll get down in time to make their intending point of landing without any excessive descents or 360’s to lose altitude. How’s it work?
 
RyanB said:
How does ATC determine when it’s time to give us our initial descent while IFR? I assume there must be some sort of algorithm that indicates when they need to start descending an aircraft in cruise, so they’ll get down in time to make their intending point of landing without any excessive descents or 360’s to lose altitude. How’s it work?
Click to expand...
I have never seen or heard of a Controller 'crunching the numbers' for this. It's what Larry said, experience. There's a feel for it. Sometimes yeah, ya get hung high. That's sometimes a bad experience for the controller as well as you and other pilots who get jerked around while he's trying to dig himself outta the sheeter. Then he strives not to do that again.
 
Last edited:
They are very used to when to issue descent for airliners - performance is all roughly the same, and they learn about where to issue instructions.

For everyone else, my experience is you need to be proactive and ask for it. This seems especially true for aircraft that cruise in the low FLs, like pressurized piston aircraft.
 
They have some guidance for assigning an altitude for aircraft on unpublished routes conducting an IAP.

D5DC0992-4FE5-49C5-8832-628ED98F2996.jpeg

There’s also an enroute application for military jets.

A2FC31B3-9858-4B6B-A590-9542B7287BB8.jpeg

Mostly though, it’s just eye balling it.
 
Did atc respond here?
I always figured they have it displayed, because they seem to have it pretty closely pinned in my a/c. 1500fpm shows up on the VS required.... and not long after they are starting me down.
Except for the occasional time they forget me!
Or, if traffic below and they need to keep me up.

Only time I ever had VS-required read more than 10,000fpm was at the last fix, 5mi from home base, they had told everyone to clam up a ways back, because of an Airforce emergency.
Everyone in my area was just flying to the final fix at altitude and waiting it out in the hold.
 
RyanB said:
How does ATC determine when it’s time to give us our initial descent while IFR? I assume there must be some sort of algorithm that indicates when they need to start descending an aircraft in cruise, so they’ll get down in time to make their intending point of landing without any excessive descents or 360’s to lose altitude. How’s it work?
Click to expand...

Back when dinosaurs roamed the earth, the controllers used altitude plus 10k to estimate tod distance. I don’t know if that’s changed. Didn’t work very well for an e route descent in a B-52 for fuel conservation.
 
RyanB said:
How does ATC determine when it’s time to give us our initial descent while IFR? I assume there must be some sort of algorithm that indicates when they need to start descending an aircraft in cruise, so they’ll get down in time to make their intending point of landing without any excessive descents or 360’s to lose altitude. How’s it work?
Click to expand...

They don't. When I want to stay high, they always bring me down early.
 
Let'sgoflying! said:
Did atc respond here?
I always figured they have it displayed, because they seem to have it pretty closely pinned in my a/c. 1500fpm shows up on the VS required.... and not long after they are starting me down.
Except for the occasional time they forget me!
Or, if traffic below and they need to keep me up.

Only time I ever had VS-required read more than 10,000fpm was at the last fix, 5mi from home base, they had told everyone to clam up a ways back, because of an Airforce emergency.
Everyone in my area was just flying to the final fix at altitude and waiting it out in the hold.
Click to expand...
@Velocity and I are former Controllers. @PastZTL was a Center guy. Whats your two cents ZTL?
 
Last edited:
orca64 said:
Back when dinosaurs roamed the earth, the controllers used altitude plus 10k to estimate tod distance. I don’t know if that’s changed. Didn’t work very well for an e route descent in a B-52 for fuel conservation.
Click to expand...
That rule of thumb is still around. See post # 7 above. What's a normal Rate of Descent for a B-52?
 
RussR said:
For everyone else, my experience is you need to be proactive and ask for it.
Click to expand...

EdFred said:
They don't. When I want to stay high, they always bring me down early.
Click to expand...

I've found that when I don't ask, I end up getting slam dunked.

TOD for the Mooney (descends at about 180kts at 500fpm) is 6 times the altitude to be lost in thousands.

Example: Flying at 9000, pattern is at 2000, so I need to lose 7000. 7*6=42 miles out. I add ten to the calcualted as ATC doesn't always give you the descent immediately. So in this example I'd ask for descent at 52 miles out.
 
Bill said:
I've found that when I don't ask, I end up getting slam dunked.

TOD for the Mooney (descends at about 180kts at 500fpm) is 6 times the altitude to be lost in thousands.

Example: Flying at 9000, pattern is at 2000, so I need to lose 7000. 7*6=42 miles out. I add ten to the calcualted as ATC doesn't always give you the descent immediately. So in this example I'd ask for descent at 52 miles out.
Click to expand...

You're doing too much math! Do everything by time. Need to lose 7000 @ 500fpm? 14 minutes out start the descent. Then it doesn't matter if you're flying at 50kts or 250kts.
 
I was flying with a client in a Cessna 340 once, and we were up a FL210 enjoying a nice 100-kt tailwind. So, GS was over 300 knots. Which means we need to start on down really far out (relatively), and the controller wasn't expecting it. When we asked to start our descent, he asked "how low would you like to descend to?" Our answer was "well, all the way to the ground eventually!"
 
I got to thinking about the question in the thread title. Does ATC know our TOD. Very often they do. It's when you report leaving an altitude after getting a Pilots Discretion descent.
 
luvflyin said:
That rule of thumb is still around. See post # 7 above. What's a normal Rate of Descent for a B-52?
Click to expand...

Depends who’s chasing me, but max legal airspeed was 390 kias, as I recall.
Speed brakes help.
 
I typically end up having to ask, or send w gentle reminder "ready for descent"

PS - as noted above by Ed, some people way over math this. It's very simple and straightforward to just do it by time. If you fly a localizer approach that's a great way also to do your step-downs without doing the dive-and-drive thing
 
orca64 said:
Depends who’s chasing me, but max legal airspeed was 390 kias, as I recall.
Speed brakes help.
Click to expand...
But what does this actually mean as far as feet per minute. 390 knots could be straight down or level depending on plane/factors/etc

Related / if you are trying to get down in a short distance wouldn't you want to go slower? Sort of like a Vx inverse? If you're high on the approach the trick is to slow down first and then go down..
 
Tantalum said:
But what does this actually mean as far as feet per minute. 390 knots could be straight down or level depending on plane/factors/etc

Related / if you are trying to get down in a short distance wouldn't you want to go slower? Sort of like a Vx inverse? If you're high on the approach the trick is to slow down first and then go down..
Click to expand...

I’ve never flown an airplane that lets you go down and slow down. Some were worse than others. In some airplanes, if you were able to slow to 250 at altitude, pop the boards and point the nose down, you could get a very high initial rate of descent. For about 60 seconds. After that initial 6000++ fpm rate, most large jets would only give 3000-3500 fpm with spoilers above 10000 msl, maybe 2000-2500 below 10000. Anything higher than those rates, you’d hit redline or 250. Airspeed will always be your ultimate limitation in the descent, whether structural, procedural or ATC required. So plan ahead, know your winds and count on your fingers.

If I know my ground speed and my crossing restriction, it’s very simple math.

It helps if you’ve got a few extra fingers.
 
Last edited:
Tantalum said:
But what does this actually mean as far as feet per minute. 390 knots could be straight down or level depending on plane/factors/etc
Click to expand...
KIAS does mean anything. It is groundspeed that is important. Airspeed with vary considerably with altitude.

Most jets will plan a flight-idle descent. The Mach, then airspeed, of the descent will depend on the cost index. One flight might be M.80 transition to 300 KIAS while another might be M.76/280.

When I was flying the DC9, without VNAV, we'd plan our descent profile as 2.5:1, from cruise down 20,000' above the airport, then 3:1 below 20,000' above the airport, plus/minus 10% of the head/tailwind.
 
luvflyin said:
What about this FL+10 thing. Does that sound about right for the typical airliner?
Click to expand...

No. Way too close in. 300 fpm, or approximately 3 degrees at .8 Mach (no wind) is 2400 fpm. That’s comfortable, but any more and you’d need spoilers. So, if you’re at FL310 and you need to cross a fix 80 nm ahead at 10000 msl, you’ve got to lose 21000 feet in 10 minutes. 2100 fpm on the vvi. At 41 nm (FL +10 nm) you’d need to descend at twice that rate; nearly 4200 fpm. Unsustainable in most large airplanes.

This is for a manual descent counting on your fingers in vertical velocity mode. Not including slow down for ATC requirements and/or configuration. Profile mode or VNAV of Airbus or Boeing is a whole different animal, using aircraft and airline specific cost indices, with slow down distances for transition calculated by the magic box.
 
Last edited:
For me, as was mentioned earlier, it's experience (no calculations involved). Many of our traffic flows were repetitive, so it doesn't take very long to get the hang of it.

It helps to know whether an aircraft is a piston or not, as I would try to start them down a little earlier. It's amazing how many don't know the difference between a C206 and a C208. Also, from the enroute perspective, many of our descents are based on a Letter of Agreement, so a lot of crossing restrictions are given into an approach control, which gives the pilot a lot of flexibility in different descent profiles.

I would repeat what's been said before, don't wait on the controller, as you are approaching your TOD, ask for lower then. Nobody that I know of would be offended.

Then there was the DAL flight, in the middle of the DAL arrival push into ATL, only he was landing JFK. When he was given PD to FL240, he would (most of the time) say, "You know we're landing JFK, don't you? " Occasionally he'd just read back the clearance. You only tended to do that once.
 
orca64 said:
No. Way too close in. 300 fpm, or approximately 3 degrees at .8 Mach (no wind) is 2400 fpm. That’s comfortable, but any more and you’d need spoilers. So, if you’re at FL310 and you need to cross a fix 80 nm ahead at 10000 msl, you’ve got to lose 21000 feet in 10 minutes. 2100 fpm on the vvi. At 41 nm (FL +10 nm) you’d need to descend at twice that rate; nearly 4200 fpm. Unsustainable in most large airplanes.

This is for a manual descent counting on your fingers in vertical velocity mode. Not including slow down for ATC requirements and/or configuration. Profile mode or VNAV of Airbus or Boeing is a whole different animal, using aircraft and airline specific cost indices, with slow down distances for transition calculated by the magic box.
Click to expand...
That's what I figured. The FL+10 thing is in the part of 'the book' about penetrations to High Altitude approaches. Not so much any Turbo Jet, but the tactical ones.
 
Larry in TN said:
KIAS does mean anything. It is groundspeed that is important. Airspeed with vary considerably with altitude.

Most jets will plan a flight-idle descent. The Mach, then airspeed, of the descent will depend on the cost index. One flight might be M.80 transition to 300 KIAS while another might be M.76/280.

When I was flying the DC9, without VNAV, we'd plan our descent profile as 2.5:1, from cruise down 20,000' above the airport, then 3:1 below 20,000' above the airport, plus/minus 10% of the head/tailwind.
Click to expand...
thanks, I guess I was curious what kind of fpm you guys see

orca64 said:
After that initial 6000++ fpm rate, most large jets would only give 3000-3500 fpm with spoilers above 10000 msl, maybe 2000-2500 below 10000
Click to expand...
Cool, exactly what I was curious about.
 
orca64 said:
I’ve never flown an airplane that lets you go down and slow down. Some were worse than others. In some airplanes, if you were able to slow to 250 at altitude, pop the boards and point the nose down, you could get a very high initial rate of descent. For about 60 seconds. After that initial 6000++ fpm rate, most large jets would only give 3000-3500 fpm with spoilers above 10000 msl, maybe 2000-2500 below 10000. Anything higher than those rates, you’d hit redline or 250. Airspeed will always be your ultimate limitation in the descent, whether structural, procedural or ATC required. So plan ahead, know your winds and count on your fingers.

If I know my ground speed and my crossing restriction, it’s very simple math.

It helps if you’ve got a few extra fingers.
Click to expand...
Toes.
 
orca64 said:
I’ve never flown an airplane that lets you go down and slow down
Click to expand...
big jets vs small piston planes I guess. I'm aware of this being a Mooney thing and true with some slippery planes but at least in a big lopey Aztec that I'm flying now a few inches off the MP even in a descent can keep the airspeed stable or even come down a bit, assuming of course you're only at 500-700 fpm. More than that you'll need drag or make the engines unhappy

FWIW if the air is smooth I'll keep the power in on descent 20-23 inches (wherever it was in cruise MP) and let the airspeed come into the yellow. Won't do it near mountains, but if your getting stepped down along the coast where the air is usually smooth it's fun to make up some time in the descent
 
All this talk about boards and spoilers got me wondering. Any of you ever fly a DC-8? My I'll never do that again story as a Controller was the day I discovered, the hard way, that DC-8's don't have them and can't down fer sheet when you really need it.
 
luvflyin said:
All this talk about boards and spoilers got me wondering. Any of you ever fly a DC-8? My I'll never do that again story as a Controller was the day I discovered, the hard way, that DC-8's don't have them and can't down fer sheet when you really need it.
Click to expand...

Before the CFM-56 reengine, the DC8's would reverse the 2 inboard engines to slow down and descend simultaneously. A captain told me with the new engines, there was a lot more residual thrust at idle, so it made for a really bumpy ride. Since the passengers didn't like it, they wouldn't do it anymore.
 
PastZTL said:
Before the CFM-56 reengine, the DC8's would reverse the 2 inboard engines to slow down and descend simultaneously. A captain told me with the new engines, there was a lot more residual thrust at idle, so it made for a really bumpy ride. Since the passengers didn't like it, they wouldn't do it anymore.
Click to expand...
Yeah. Years later I was getting a BFR with a crusty old CFI who I found out used to fly them. He said they could use the reversers hauling boxes, but not passengers.
 
I flew to Korea in 1983 on a DC-8. We had a layover in Japan and the crew had to reverse 2&3 on descent into Yokota to get the airplane down.
 
You must log in or register to reply here.
Back
Top