3 degree descent planning, Time vs. Distance
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Altitude to descend x 3 gives distance to descend on a -3 deg path
Ground speed x 5 gives ROD necessary to stay on the -3 deg path
Say you need to descend 30,000ft and your GS is 400kts. So it's 30 x 3 = 90 miles needed
400 x 5 = -2000fpm ROD required to descend within the 90 miles.
Of course the altitude will need to have the necessary QNH adjustments to it for the calculated distance to be perfect.
I'm not so clever so complicated formula don't work, keeping it simple has so far!
Ground speed x 5 gives ROD necessary to stay on the -3 deg path
Say you need to descend 30,000ft and your GS is 400kts. So it's 30 x 3 = 90 miles needed
400 x 5 = -2000fpm ROD required to descend within the 90 miles.
Of course the altitude will need to have the necessary QNH adjustments to it for the calculated distance to be perfect.
I'm not so clever so complicated formula don't work, keeping it simple has so far!
Only half a speed-brake
For small Airbus (when the VNAV calc works as designed):
- Honeywell: VNAV + 700 ft
- Thales: VNAV + 1700 ft
assuming IDLE factor 0.
- Honeywell: VNAV + 700 ft
- Thales: VNAV + 1700 ft
assuming IDLE factor 0.
Altitude to descend x 3 gives distance to descend on a -3 deg path
Ground speed x 5 gives ROD necessary to stay on the -3 deg path
Say you need to descend 30,000ft and your GS is 400kts. So it's 30 x 3 = 90 miles needed
400 x 5 = -2000fpm ROD required to descend within the 90 miles.
Of course the altitude will need to have the necessary QNH adjustments to it for the calculated distance to be perfect.
I'm not so clever so complicated formula don't work, keeping it simple has so far!
Ground speed x 5 gives ROD necessary to stay on the -3 deg path
Say you need to descend 30,000ft and your GS is 400kts. So it's 30 x 3 = 90 miles needed
400 x 5 = -2000fpm ROD required to descend within the 90 miles.
Of course the altitude will need to have the necessary QNH adjustments to it for the calculated distance to be perfect.
I'm not so clever so complicated formula don't work, keeping it simple has so far!
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Altitude to descend x 3 gives distance to descend on a -3 deg path
Ground speed x 5 gives ROD necessary to stay on the -3 deg path
Say you need to descend 30,000ft and your GS is 400kts. So it's 30 x 3 = 90 miles needed
400 x 5 = -2000fpm ROD required to descend within the 90 miles.
Of course the altitude will need to have the necessary QNH adjustments to it for the calculated distance to be perfect.
I'm not so clever so complicated formula don't work, keeping it simple has so far!
Ground speed x 5 gives ROD necessary to stay on the -3 deg path
Say you need to descend 30,000ft and your GS is 400kts. So it's 30 x 3 = 90 miles needed
400 x 5 = -2000fpm ROD required to descend within the 90 miles.
Of course the altitude will need to have the necessary QNH adjustments to it for the calculated distance to be perfect.
I'm not so clever so complicated formula don't work, keeping it simple has so far!
Sounds complicated enough to me when I’m heading down with much else to do. 3x FL plus 20 and keep reviewing. Plus 10 when back to 250Kts. Worked well for me for the last 25+ years
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Agree, on 32x family I go for 3xFL normally plus 10 and 5 when at 250. But if very light I can accept less margins...
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How do you calculate the ROD as a matter of interest?
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Oldie but a goodie Airbus ALAR
Section 4 is a must read.
Section 4 is a must read.
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Altitude to descend x 3 gives distance to descend on a -3 deg path
Ground speed x 5 gives ROD necessary to stay on the -3 deg path
Say you need to descend 30,000ft and your GS is 400kts. So it's 30 x 3 = 90 miles needed
400 x 5 = -2000fpm ROD required to descend within the 90 miles.
Of course the altitude will need to have the necessary QNH adjustments to it for the calculated distance to be perfect.
I'm not so clever so complicated formula don't work, keeping it simple has so far!
Ground speed x 5 gives ROD necessary to stay on the -3 deg path
Say you need to descend 30,000ft and your GS is 400kts. So it's 30 x 3 = 90 miles needed
400 x 5 = -2000fpm ROD required to descend within the 90 miles.
Of course the altitude will need to have the necessary QNH adjustments to it for the calculated distance to be perfect.
I'm not so clever so complicated formula don't work, keeping it simple has so far!
That's assuming your GS is the same? When does that happen in the descent? What if you have a 120 kt TW at FL410, it's 80 kts at FL250, 40 kts at FL180, and a headwind at FL150?
The box tries to figure that out but it only has the wind 'cuts' in the box. If mother nature is different, which it almost always is to some degree, the computation is off, so guys blame the box. But they'd never be able to compute an idle descent as well as the VNAV can.
In low traffic areas it wasn't uncommon to go to idle at TOD (typically FL410) and be at idle thrust until 1,000'. It used to be 700' until stabilized rules changed. Now it's about 2,000' due to configuration SOP's.
Minor deviations from the path could be corrected by slight increase in descent speed for going high on the VTI (VNAV path) or a slight speed decrease (better glide profile) if you got slightly low on the VTI (VNAV path).
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I don’t usually need to, idle thrust and x3 +- is a good enough guesstimate. The just keep updating as you go down. If low add thrust, high and getting higher, increase speed or speed brake. KISS
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And Felix, what mode do you descend in then? I thought this was a discussion about VS mode and fast rules for knowing what VS to set to keep a 3deg profile, is that Vnav mode your referring to with a magenta 3 deg vertical guidance indicator on the PFD?
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The objective for descent planning isn't to achieve a 3 degree descent path. It's to achieve the most efficient descent for fuel and time adjusted for altitude and speed constraints. In the MD80 you had to figure the stuff out. Modern jets, with modern FMC's, are more capable than the overwhelming majority of pilots. I've yet to see anyone grab a calculator and start comparing the descent winds to do the math the FMC does without having to ask it.
The 'rules of thumb' are for the old school flying or non FMC aircraft. They're a good back up, just like the 'trust but verify' aspect of FMC use, but the primary method to achieve efficiency is via the modern tools provided. Updated winds are a key component of the FMC's computing ability.
The 'rules of thumb' are for the old school flying or non FMC aircraft. They're a good back up, just like the 'trust but verify' aspect of FMC use, but the primary method to achieve efficiency is via the modern tools provided. Updated winds are a key component of the FMC's computing ability.
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Early MD-80's had a descent planning tool. I forget what it was called. Perf??? It was a box on the center console. Most guys didn't want to go to the effort of getting the better descent planning available from it. Why? Because they had to manually enter the descent winds at various altitudes. With FMC's it's a couple of button selections and the FMC uploads the information automatically.
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Adjustments during the descent as necessary, multiplying present gs x 5 always gives ~3deg descent profile, it’s never failed me so far. It was actually taught to me by a former MD80 pilot who swore by it during his career.
And Felix, what mode do you descend in then? I thought this was a discussion about VS mode and fast rules for knowing what VS to set to keep a 3deg profile, is that Vnav mode your referring to with a magenta 3 deg vertical guidance indicator on the PFD?
This may sound old school but we still have to think outside the FMC.
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Height to lose x 3 + 10 works fine to determine the TOD. Add to it an allowance for tailwind.
During descent I use DTG x 3 and subtract 2000' if at 300 kts and 1000' if at 250 knots. Works well.
As I said earlier, on the 737 the FMC determines the TOD by multiplying the height to lose till 250 knots by 2,5 and then 3.0. Sometimes that can be too tight, and despite having inserted the corrected descent winds it may still need some speedbrake.
During descent I use DTG x 3 and subtract 2000' if at 300 kts and 1000' if at 250 knots. Works well.
As I said earlier, on the 737 the FMC determines the TOD by multiplying the height to lose till 250 knots by 2,5 and then 3.0. Sometimes that can be too tight, and despite having inserted the corrected descent winds it may still need some speedbrake.
