Descent time/distance/speed rules
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Descent time/distance/speed rules
I know the classic rule for working out distance to descend is:
(TOC-Platform Alt) x 3 but what speed does this assume and is this speed an IAS?
Any other hard and fast rules where descents are involved?
Thanks
(TOC-Platform Alt) x 3 but what speed does this assume and is this speed an IAS?
Any other hard and fast rules where descents are involved?
Thanks
Join Date: Mar 2000
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(TOC-Platform Alt) x 3 but what speed does this assume and is this speed an IAS?
Speed in the L1011, M.83/340 knots, indicated.
Join Date: Jun 2005
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TOD rules of thumb - 747
Well, the 3X rule seems to work with a speed of 340 KIAS...
I am of very low IQ... and for decent, I look at the trend.
xxx
If getting too high, INCREASE KIAS to 360 if your plane permits (increase drag) with speed.
Getting too low, stretch it by REDUCING KIAS to 300 or even 280...
xxx
I fly 747 Classics -
With heavy freighters, I would plan longer distance from destination for TOD.
Passenger airplane - average weight, I would use the standard 3X rule...
With light airplane (empty cargo plane) 3X rule, NO EXTRAS... maybe less.
xxx
Do not forget to look at your winds aloft.
If landing "circle to land" - I would start descent 10-15 NM closer to destination.
If "straight-in to ILS" - I would start a bit earlier (10 NM) to be at 250/30 NM out, 10,000 ft MSL.
xxx
With a 747, it takes 10 NM to reduce 100 KTS (level flight) -
Example - reducing 320 to 250 takes 7 NM... 350 to 240 takes 11 NM...
Whatever, try the above and adapt to your aircraft type.
xxx
Happy contrails
I am of very low IQ... and for decent, I look at the trend.
xxx
If getting too high, INCREASE KIAS to 360 if your plane permits (increase drag) with speed.
Getting too low, stretch it by REDUCING KIAS to 300 or even 280...
xxx
I fly 747 Classics -
With heavy freighters, I would plan longer distance from destination for TOD.
Passenger airplane - average weight, I would use the standard 3X rule...
With light airplane (empty cargo plane) 3X rule, NO EXTRAS... maybe less.
xxx
Do not forget to look at your winds aloft.
If landing "circle to land" - I would start descent 10-15 NM closer to destination.
If "straight-in to ILS" - I would start a bit earlier (10 NM) to be at 250/30 NM out, 10,000 ft MSL.
xxx
With a 747, it takes 10 NM to reduce 100 KTS (level flight) -
Example - reducing 320 to 250 takes 7 NM... 350 to 240 takes 11 NM...
Whatever, try the above and adapt to your aircraft type.
xxx
Happy contrails
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Just to be different............ I'll throw these words of wisdom into the pot:
DESCENT PROFILE MANAGEMENT
If the F-PLN were to be followed from TOD to touchdown, the descent profile
would be managed by the FMGS. However, ATC requirements or weather
avoidance may take the aircraft off the ideal profile. Consequently, it is important
to be aware of the aircraft's position relative to the ideal descent profile andthe
time available for any corrections to take effect. It is relatively easy for the aircraft
to correct from being 3000 ft above profile at FL 350, whereas being 3000 ft
above profile at 10000 ft will require a prompt, decisive correction.
All descent management revolves around the relationship between altitude and
distance to go (DTG) to touchdown. Consequently the F-PLN page must be
realistic. Ensure that the TO waypoint is in front of the aircraft and that the F-PLN
is representative of the expected route.
Before being able to assess the aircraft's position relative to the ideal descent
profile, it is necessary to have a method of calculating the profile. The following
method, illustrated with examples, provides a simple set of rules to monitor and
manage the descent profile. It assumes that the MCDU is updated to reflect the
expected arrival track.
TOD Cross-check
Multiply the flight level (in thousands of feet) by 4 to calculate the required
distance to go (DTG) to touchdown.
· At FL350, the required DTG is approximately (35 x 4) = 140 nm.
There will be factors for weight and wind but if the FMGC computed descent
point is within ± 20 nm of this figure, then it can be considered acceptable as a
gross error check of the FMGC computation.
Descent Monitoring
From top of descent to 15000 ft, multiply the altitude (in thousands of feet) by 4
to calcluate the required DTG.
· At 20000 ft, the required DTG is (20 x 4) = 80 nm
Below 15000 ft multiply the altitude by 3 and add 1 nm/10 kt above 150 kt.
· At 10000 ft and 300 kt, the required DTG becomes (10 x 3) + 15 = 45 nm
· At 5000 ft and 250 kt, the required DTG becomes (5 x 3) + 10 = 25 nm
· At 3000 ft and 180 kt, flap 2, the required DTG becomes (3 x 3) + 3 = 12
nm.
At this stage, the aircraft will be approaching the glideslope and, hence, a normal
3° slope
A
DESCENT PROFILE MANAGEMENT
If the F-PLN were to be followed from TOD to touchdown, the descent profile
would be managed by the FMGS. However, ATC requirements or weather
avoidance may take the aircraft off the ideal profile. Consequently, it is important
to be aware of the aircraft's position relative to the ideal descent profile andthe
time available for any corrections to take effect. It is relatively easy for the aircraft
to correct from being 3000 ft above profile at FL 350, whereas being 3000 ft
above profile at 10000 ft will require a prompt, decisive correction.
All descent management revolves around the relationship between altitude and
distance to go (DTG) to touchdown. Consequently the F-PLN page must be
realistic. Ensure that the TO waypoint is in front of the aircraft and that the F-PLN
is representative of the expected route.
Before being able to assess the aircraft's position relative to the ideal descent
profile, it is necessary to have a method of calculating the profile. The following
method, illustrated with examples, provides a simple set of rules to monitor and
manage the descent profile. It assumes that the MCDU is updated to reflect the
expected arrival track.
TOD Cross-check
Multiply the flight level (in thousands of feet) by 4 to calculate the required
distance to go (DTG) to touchdown.
· At FL350, the required DTG is approximately (35 x 4) = 140 nm.
There will be factors for weight and wind but if the FMGC computed descent
point is within ± 20 nm of this figure, then it can be considered acceptable as a
gross error check of the FMGC computation.
Descent Monitoring
From top of descent to 15000 ft, multiply the altitude (in thousands of feet) by 4
to calcluate the required DTG.
· At 20000 ft, the required DTG is (20 x 4) = 80 nm
Below 15000 ft multiply the altitude by 3 and add 1 nm/10 kt above 150 kt.
· At 10000 ft and 300 kt, the required DTG becomes (10 x 3) + 15 = 45 nm
· At 5000 ft and 250 kt, the required DTG becomes (5 x 3) + 10 = 25 nm
· At 3000 ft and 180 kt, flap 2, the required DTG becomes (3 x 3) + 3 = 12
nm.
At this stage, the aircraft will be approaching the glideslope and, hence, a normal
3° slope
A
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Just to be different again, with the 145 you can even use '2 x thousands of feet' for the descent. I was taught initially '3x plus none' but if there is any headwind or we are very light then this is too far in my opinion as the aircraft slows down very quickly and thrust is needed. Folk seem to tend to set the FMS guidance up for a 4 degree descent. When held high by ATC I have managed a 5 degree descent though pitch angle is very low and can't be terribly comfy down the back. Coming into LHR we are often at FL100 with 50 or 60nm to go which always seems crazy but what can you do about it at LHR?
Ball park figures I use for a normal day (no re-entries)
FL370 - 90nm
FL180 - 50nm
FL100 - 30nm
The main gotcha here is ice below FL100 where speed is relatively low, but thrust will be high due auto ice detection on the 145.
K.Whyjelly - out of interest what type do your figures work on?
Ball park figures I use for a normal day (no re-entries)
FL370 - 90nm
FL180 - 50nm
FL100 - 30nm
The main gotcha here is ice below FL100 where speed is relatively low, but thrust will be high due auto ice detection on the 145.
K.Whyjelly - out of interest what type do your figures work on?
