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# TOD calculation query

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# TOD calculation query

4th Aug 2019, 10:38

Join Date: Jan 2018
Location: Amantido
Posts: 837
TOD calculation query

I've been reading the FMC User Guide recently and I was wondering if any of you is multiplying the altitude to lose down to 10.000 ft by 2,5 rather than 3. This should take into account a steeper ROD at high altitude due to less dense air.
For example, from FL300 the FMC calculated TOD would be around 87 miles.
At a typical 280 kt descent (still wind) 30.000-10.000=20.000 ft so 20x2,5=50 nm. Then add 30nm for the last 10.000 ft and it would be 80. Plus a few miles to account for deceleration.

The classic rule of multiplying by 3 would give us at least 10 miles extra in this case. And to that, we still add miles for a deceleration segment.

Geeky stuff, I know, but I have got plenty of free time at the moment
4th Aug 2019, 11:44

Join Date: Jun 2019
Location: VA
Posts: 52
The "3 to 1" rule evolved because it was simple "pilot math" and produced reasonably close results as long as appropriate corrections were applied for winds. Adding another digit is fine if you can do the math and if it produces a more accurate result for your aircraft. In the example above, a still air descent of 30,000 total feet would take about 90 miles vs the FMC calculation of 87 miles. That's pretty close all things considered and the math is easy (keep in mind that you could be busy with other tasks so easy is good). The important point for any of these rules of thumb is to constantly update it during the decent to see how it is working out, so in reality you are doing this calculation multiple times as you adjust the descent along the way.

Now, while I admire your attempt at precision, I would simply point out that the penalty for getting down early is burning a little extra fuel while the penalty for missing a crossing altitude could be an ATC violation. Unless you are really short on fuel, I would suggest that erring to the conservative side might be more in your interests.
4th Aug 2019, 12:30

Join Date: Jan 2018
Location: Amantido
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I agree with you Tomaski. The few times I've had to calculate my own top of descent because I was aware of a huge ATC shortcut coming I multiplied alt to lose by 3 plus 10. Conservative and easy to use.
'll try to "experiment" this when inbound an airport with no altitude restrictions. Captain permitting of course.
4th Aug 2019, 13:32

Join Date: Dec 2001
Location: FL, USA
Posts: 2,351
Do you have a “metric” ( meters) setting knob?
Feet : Meters = 3:1 so it will do it for you.

in this case 130 NM

4th Aug 2019, 13:39

Join Date: Jan 2018
Location: Amantido
Posts: 837
I am currently only flying the Classic, and only some of our aircraft are equipped with an additional altimeter in metres. I will look at it if I remember.
4th Aug 2019, 16:50

Join Date: Dec 2012
Location: France
Posts: 96
Now, while I admire your attempt at precision, I would simply point out that the penalty for getting down early is burning a little extra fuel while the penalty for missing a crossing altitude could be an ATC violation. Unless you are really short on fuel, I would suggest that erring to the conservative side might be more in your interests.
I agree with that. I that case the best option is proper FMS preparation and monitor the descent with VNAV (or equivalent mode for other aircraft type).
The green banana is also a very useful tool, but I fly the NG so I don't know if the classic is equipt with this feature.

4th Aug 2019, 17:09

Join Date: Jan 2018
Location: Amantido
Posts: 837
Luckily yes, it is equipped. It also had V/B and FPA indication in the descent page which sometimes can be helpful.
4th Aug 2019, 18:01

Join Date: Mar 2006
Location: USA
Posts: 1,349
I've seen 2 methods to this.

1. Altitude to lose in thousands x 3 = TOD

Need to lose 10,000ft, so 10x3 =30nm

2. Altitude to lose in FL ÷ 3 = TOD

Need to lose 10,000ft, so 100 ÷ 3 =33nm

The first method obviously produces a steeper descent.
4th Aug 2019, 18:15

Join Date: Jan 2018
Location: Amantido
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I use the first method to calculate my TOD plus 1nm for each 10 knots to decelerate to 210 kts. During descent, I follow up the progress with distance to go times 3 and subtract 1000 ft if at 250 kts and 2000 ft if at 300 kts i.e at 60 miles to go at 300 kts my optimum altitude would be (60x3)-2000=16.000 ft.

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Last edited by Banana Joe; 4th Aug 2019 at 20:56.
4th Aug 2019, 19:53

Only half a speed-brake

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(60x3) - 2000 = 16.000 ft?
4th Aug 2019, 20:56

Join Date: Jan 2018
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Typo! Corrected!
5th Aug 2019, 09:22

Only half a speed-brake

Join Date: Apr 2003
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Age: 41
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There is a point. If we are capable of making silly mistakes here, sitting at home and typing, is very easy to imagine what sort of error can be achieved when at work.

For any method to work dependably then, it better be as simple as can be. Added precision comes at a cost in this respect. What you have is reasonable.
5th Aug 2019, 10:05
Moderator

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There is a point.

Which is why practice makes perfect (or thereabouts).

Back whenever, we routinely hand flew, raw data, the entire sector for fun and practice. All sorts of interesting games arose when the traffic and weather workload was reasonably appropriate. For instance, on the Electra, for shorter sectors, we might climb to the maximum height deemed appropriate to nose over and commence the descent without any cruise - close the throttles and, while observing all SOP spin up rules and the like, not touch the throttles again or significantly vary from standard descent speed profile until the appropriate point on final or in the circuit. Severe penalties for miscalculations were settled in the bar on the next crew overnight (or overday for these freighter operations, more generally).

Similarly, on the jets, using the throttles or boards was a matter for some frowning so far as descent planning and monitoring competence was concerned.

End result, of course, was that we all had near no problems with any descent variations which ATC or weather might throw at us in anger.

Do you know what your aircraft can do when ATC holds you dreadfully high for whatever reason .. and then asks for a near-impossible descent crossing requirement ? The 733, for example, goes down real fine, dirty, at one nautical per thousand .. Do you know what maximum height you can overfly onto downwind for a simple circuit and landing while tracking a normal line over the ground ? Again, the 733 does this real fine over the top at 10-11,000 feet - often a useful technique coming into Cairns from the west with a strong westerly-caused circuit turbulence. Do you know what height you can enter downwind and do a normal-ish sort of circuit. On the Electra, coming northbound into Sydney in the wee hours, with a displaced threshold for noise abatement FL300 abeam the upwind threshold worked pretty fine. And so on ...

And this was, of course, sans FMC, GPS and other JB kits. Sure we had to work hard at getting proficient and maintaining proficiency ... but knowledge, work and practice are the main weapons in the armoury of flying skills ... all within the limitations of then-SOP restrictions.
5th Aug 2019, 12:19

Join Date: Jan 2018
Location: Amantido
Posts: 837
Originally Posted by john_tullamarine
There is a point.

Do you know what your aircraft can do when ATC holds you dreadfully high for whatever reason .. and then asks for a near-impossible descent crossing requirement ? The 733, for example, goes down real fine, dirty, at one nautical per thousand .. Do you know what maximum height you can overfly onto downwind for a simple circuit and landing while tracking a normal line over the ground ? Again, the 733 does this real fine over the top at 10-11,000 feet - often a useful technique coming into Cairns from the west with a strong westerly-caused circuit turbulence. Do you know what height you can enter downwind and do a normal-ish sort of circuit. On the Electra, coming northbound into Sydney in the wee hours, with a displaced threshold for noise abatement FL300 abeam the upwind threshold worked pretty fine. And so on ...

And this was, of course, sans FMC, GPS and other JB kits. Sure we had to work hard at getting proficient and maintaining proficiency ... but knowledge, work and practice are the main weapons in the armoury of flying skills ... all within the limitations of then-SOP restrictions.
I would love to have this kind of inputs. I've been on the line only for less than a year now and ATC kept me very high only and then vectored me for approach only once. Abeam the airport slowing down to 210 kt at FL90, I extended flaps 5 and deployed speedbrakes and all worked out very well for a stable final to RW 02 in BCN at 170 kts.

Of course, I currently don't fly pax so I am not sure if that would've been uncomfortable for them and CC's.
6th Aug 2019, 07:08

Join Date: Apr 2004
Location: Planet Earth
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Nothing came down like a 727

Our rule was ‘if you can see it, you can land on it’
6th Aug 2019, 13:04

Join Date: Jul 2006
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If I’m on radar vector I use distance times 3 or altitude times 3 + 1nm per 10kt to slow down. Works like a charm on A320 family.
I know it should be altitude divided by 3 but it’s close enough.

Otherwise the managed descent on Airbus works very well.

Last edited by pineteam; 6th Aug 2019 at 14:14.
6th Aug 2019, 14:49

Join Date: Jun 2001
Location: EUSSR
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Banana Joe,

What airplane are we talking about?

The 787 FCTM mentions a factor of 3.5.
6th Aug 2019, 20:23

Join Date: Jan 2018
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Originally Posted by wondering
Banana Joe,

What airplane are we talking about?

The 787 FCTM mentions a factor of 3.5.
737 - currently I mainly fly the Classic, but I might get on the NG in a few months.
14th Aug 2019, 17:58

Join Date: Sep 2016
Location: USA
Posts: 430
Originally Posted by pineteam
If I’m on radar vector I use distance times 3 or altitude times 3 + 1nm per 10kt to slow down. Works like a charm on A320 family.
I know it should be altitude divided by 3 but it’s close enough.

Otherwise the managed descent on Airbus works very well.
It's not really that it "should be" one or the other. As Check Airman showed above, it's just a different approximation, with the "multiply by 3" one yielding a slightly steeper descent. Put in degrees, "multiply by 3" gives 3.18 degrees, while "divide by 3" gives 2.86. Also I'd consider the factor that it's much easier (at least for me) to multiply by 3 in my head on the fly.

Last edited by Vessbot; 14th Aug 2019 at 20:25.
14th Aug 2019, 21:29

Join Date: Dec 2013
Location: Milan
Posts: 35
I tend to agree with FlightDetent, and normally try to keep it very simple. I have flown with many captains, very good in this, and they made their mental math (A320) in order to get a very precise profile.
I see I can lose a bit of SA if I make it too complicated. So I simply make 3xheight, then depending on the speed I decide if I can accept the distance I get or If I should make more margins. The only time I have a fixed distance in my mind is when ATC keeps me high and stuck at an altitude different than the platform and close to the threshold. In that case when 3xht plus 3 matches my distance I start configuring.