B738 Descent Planning
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On this subject on the B737NG VNAV ....
I used to be a Captain with EZ on the -700, and rapidly developed the opinion that maybe Boeing had been a little lazy/cost saving with the -NG VNAV algorithms. Regardless of descent programming, the A/C would always go high on the energy profile .. more so than any other Boeings ... I put this down to possibly 2 things, and would be grateful for any thoughts from people with considerable experience on this aircraft ..
1. The software assumed instantaneous idle thrust at the T.O.D., and not the slow retardation of the thrust levers ...
2. They used the aerodynamic algorithms in the VNAV programming from the B737-300 Classic, which is a brick compared to the "glider" -NG. Thereby always ensuring that the new model struggles to loose energy in descent ..
Any informed comment would be appreciated ..
I.Duke
I used to be a Captain with EZ on the -700, and rapidly developed the opinion that maybe Boeing had been a little lazy/cost saving with the -NG VNAV algorithms. Regardless of descent programming, the A/C would always go high on the energy profile .. more so than any other Boeings ... I put this down to possibly 2 things, and would be grateful for any thoughts from people with considerable experience on this aircraft ..
1. The software assumed instantaneous idle thrust at the T.O.D., and not the slow retardation of the thrust levers ...
2. They used the aerodynamic algorithms in the VNAV programming from the B737-300 Classic, which is a brick compared to the "glider" -NG. Thereby always ensuring that the new model struggles to loose energy in descent ..
Any informed comment would be appreciated ..
I.Duke
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"Trust but verify" VNAV is only as good as the data it can work with. It does a decent job but you still have to monitor it.
High on profile with no speed constraints by ATC? Increase you airspeed is the most efficient method. Fly the airplane first, then update the VNAV descent page with the increased descent speed.
Landing gear is your friend if you have too much energy in close and need to reduce your energy state.
High on profile with no speed constraints by ATC? Increase you airspeed is the most efficient method. Fly the airplane first, then update the VNAV descent page with the increased descent speed.
Landing gear is your friend if you have too much energy in close and need to reduce your energy state.
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if you do just the distance to the airport from the progress page x 3 for verify if you are hight or low, can work? or being the rate of descent greater than the half gs you will be too low?
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I totally agree with Duke, I am willing to bet a monthly salary that even in no-wind conditions a vnav descent from FL390 down to minimums (setting flaps according FCTM while letting VNAV decelerate for the approach) will not function properly and will end in a high energy approach.... not entirely reliable 
What I do is that i usually enter a descent speed 10 knots lower than what I am actually planning to fly in order to compute a lower angle descent.
What I do is that i usually enter a descent speed 10 knots lower than what I am actually planning to fly in order to compute a lower angle descent.
TightYorksherMan
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Basically,
Give them FMC as much info as possible in regards to Descent Forecast page - remembering tailwinds are most important.
Put a fix around the airport of 10miles and 25miles (25 for MSA)
If your 210kts by 10miles your quids in and aim for this all the time.
When your in HDG SEL you can put the Centrefix 'CI' point on top or if your clever make yourself a little circuit with what you think you might get, and use the Vertical Profile indicator.
When you've got it figured, use the standby instruments to monitor how high on the glide you are - it's more accurate farther away.
Good luck, it takes time.
Remember, 210kts or Min clean (UP bug) by 10miles at the latest, and in the decent, tailwinds are your enemy.
Give them FMC as much info as possible in regards to Descent Forecast page - remembering tailwinds are most important.
Put a fix around the airport of 10miles and 25miles (25 for MSA)
If your 210kts by 10miles your quids in and aim for this all the time.
When your in HDG SEL you can put the Centrefix 'CI' point on top or if your clever make yourself a little circuit with what you think you might get, and use the Vertical Profile indicator.
When you've got it figured, use the standby instruments to monitor how high on the glide you are - it's more accurate farther away.
Good luck, it takes time.
Remember, 210kts or Min clean (UP bug) by 10miles at the latest, and in the decent, tailwinds are your enemy.
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Don't understand this
If you are expecting a 6 mile final, 4 miles out, then 6 miles to where you are downwind then you want to be around 5000ft at the end of the downwind.
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TAI input does change a lot. If one leaves it out when needed of course the result is a high energy approach. Put in all the info you can and VNAV aint all that bad. Since it is only average ISA deviation, QNH and expected TAI usage it is not all that much info. Descend winds are taken care off by the automatic route/descent wind download.
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I was using this workaround method when flying into airports that do not have stars or have stars but ATC radar vector instead, because of the high traffic volume!!, and the controllers vector you everywhere, I found this to be the best technique personally for a idle path 3 degree descent for some high traffic airports in SE Asia
To follow my logic here goes!!
Following the FMC programming from below, this technique is based on using the landing RWY threshold height from the legs page data to calculate a dynamic 3 degree descent profile Hope this helps and my explanation is understood! PS this technique takes a little practice but is great once the operator can master this technique (best practiced after line training!!) and will always give a 3 degree profile in relation to the rwy threshold
say we are at fl330 or whatever altitude (calculate the approx TOD mentally 3 * rule) set up the fmc des page for the anticipated arrival rwy, remain in vnav till the controller starts vectoring you for the approach or start the descent using the value of VB 3.1 approx on the FMC des page, when the controller issues a descent at pilot discretion to the intially ATC step down alt), then we are looking for a VB of approx 3.1 degrees on the DES page which equates to a approx 3:1 descent rate) us VS or FLCH to maintain this VB, by using the VS associated with this VB on the FMC des page. If you see that you are being vectored away from the airport for a while your VB will start to show a VB of say 3.0, 2.9, 2.8 etc.... to maintain the the VB of 3.1 reduce the VS initially to approx a value less than the current VS VB ie (say we have VB 2.8 reducing with a VS of 500FPM use say a value of 200FPM to maintain the VB 3.1 relation) use VS and or FLCH as your tools to refine the VS to equal the desired VB.
Similarly if you get vectored a short dog leg (short cut vector due traffic sequencing) on to final app course the VB trend will start to get high ie 3.1, 3.2, 3.3 etc... use FLCH and speed brake if needed as a way to get down back to the VB 3.1 profile. when you are starting to approach 2 thousand feet above the commencement of the approach, Make the VB 2.5 now to capture the g/s from 1 dot below (to prevent you from paralleling the ils G/S for capture) or use a VB of 2.0 = 2 dots below the ils g/s on the b737 classic and NG presentations, this will ensure a idle path des and that the throttles open up when you start adding more drag and the final flaps.
The key to remember is to try and maintain the VB3.1, so if you are slightly low ie (VB 2.7-2.9) use VS to fine tune, and say you are way high (VB 6.0 -8.0) use FLCH and speed brake. (practice will refine this technique)
The above technique really does help to always know the req ROD to achieve for any RWY and is particularly useful for a late runway change (ie parallel runway)
FMC input technique below
Example rwy: 27 threshold height from fmc 434’
To create a rwy vb using fmc u5.0 software b737 classic
1. Legs page
2. Enter rw27/-.1
3. Place above rw27 waypoint on legs page
4. Then from the entry above rw27 new waypoint (rw201)
5. Downselect this new waypoint (rw201) to the des page (lsk 3r)
6. Add 30’ to calculation ie… rw201/0464 (-.1 =1/10 nm in front of rwy threshold) des page lsk 3r or fix lsk 1l
7. Go back to legs page to erase the waypoint “(rw201)”
Example rwy: 27 threshold height from fmc 434’
To create a rwy vb using fmc u10.6 software (b737 ng)
1. Legs page
2. Enter rw27/0434
3. Downselect this new waypoint (rw27) to the des page
4. For the vb calc enter this format into the des page rw27/0434 (lsk 3r)
5. You do not need to worry about deleting the waypoint from the legs page like in the previous u5.0 example
Example rwy: 04 threshold height from fmc 56’ (made by the pilot from the nav supp) fmc d’base does not contain this rwy yet! Only the icao airport codes
To create a rwy vb using fmc u5.0/u10.6 software from a pilot made rwy input from nav/supp page
1. Legs page
2. Locate the rw04 waypoint
3. Downselect this new waypoint (rw04) to the fix page (lsk 1l) or des page (lsk 3r)
4. For the vb calc enter this format into the des page rw04/0056 (lsk 3r)
5. You do not need to worry about deleting the waypoint from the legs page like in the first u5.0 example, this also applies for u10.6 software
To follow my logic here goes!!
Following the FMC programming from below, this technique is based on using the landing RWY threshold height from the legs page data to calculate a dynamic 3 degree descent profile
Hope this helps and my explanation is understood! PS this technique takes a little practice but is great once the operator can master this technique (best practiced after line training!!) and will always give a 3 degree profile in relation to the rwy thresholdsay we are at fl330 or whatever altitude (calculate the approx TOD mentally 3 * rule) set up the fmc des page for the anticipated arrival rwy, remain in vnav till the controller starts vectoring you for the approach or start the descent using the value of VB 3.1 approx on the FMC des page, when the controller issues a descent at pilot discretion to the intially ATC step down alt), then we are looking for a VB of approx 3.1 degrees on the DES page which equates to a approx 3:1 descent rate) us VS or FLCH to maintain this VB, by using the VS associated with this VB on the FMC des page. If you see that you are being vectored away from the airport for a while your VB will start to show a VB of say 3.0, 2.9, 2.8 etc.... to maintain the the VB of 3.1 reduce the VS initially to approx a value less than the current VS VB ie (say we have VB 2.8 reducing with a VS of 500FPM use say a value of 200FPM to maintain the VB 3.1 relation) use VS and or FLCH as your tools to refine the VS to equal the desired VB.
Similarly if you get vectored a short dog leg (short cut vector due traffic sequencing) on to final app course the VB trend will start to get high ie 3.1, 3.2, 3.3 etc... use FLCH and speed brake if needed as a way to get down back to the VB 3.1 profile. when you are starting to approach 2 thousand feet above the commencement of the approach, Make the VB 2.5 now to capture the g/s from 1 dot below (to prevent you from paralleling the ils G/S for capture) or use a VB of 2.0 = 2 dots below the ils g/s on the b737 classic and NG presentations, this will ensure a idle path des and that the throttles open up when you start adding more drag and the final flaps.
The key to remember is to try and maintain the VB3.1, so if you are slightly low ie (VB 2.7-2.9) use VS to fine tune, and say you are way high (VB 6.0 -8.0) use FLCH and speed brake. (practice will refine this technique)
The above technique really does help to always know the req ROD to achieve for any RWY and is particularly useful for a late runway change (ie parallel runway)
FMC input technique below
Example rwy: 27 threshold height from fmc 434’
To create a rwy vb using fmc u5.0 software b737 classic
1. Legs page
2. Enter rw27/-.1
3. Place above rw27 waypoint on legs page
4. Then from the entry above rw27 new waypoint (rw201)
5. Downselect this new waypoint (rw201) to the des page (lsk 3r)
6. Add 30’ to calculation ie… rw201/0464 (-.1 =1/10 nm in front of rwy threshold) des page lsk 3r or fix lsk 1l
7. Go back to legs page to erase the waypoint “(rw201)”
Example rwy: 27 threshold height from fmc 434’
To create a rwy vb using fmc u10.6 software (b737 ng)
1. Legs page
2. Enter rw27/0434
3. Downselect this new waypoint (rw27) to the des page
4. For the vb calc enter this format into the des page rw27/0434 (lsk 3r)
5. You do not need to worry about deleting the waypoint from the legs page like in the previous u5.0 example
Example rwy: 04 threshold height from fmc 56’ (made by the pilot from the nav supp) fmc d’base does not contain this rwy yet! Only the icao airport codes
To create a rwy vb using fmc u5.0/u10.6 software from a pilot made rwy input from nav/supp page
1. Legs page
2. Locate the rw04 waypoint
3. Downselect this new waypoint (rw04) to the fix page (lsk 1l) or des page (lsk 3r)
4. For the vb calc enter this format into the des page rw04/0056 (lsk 3r)
5. You do not need to worry about deleting the waypoint from the legs page like in the first u5.0 example, this also applies for u10.6 software
Last edited by downwind; 18th Jul 2012 at 09:27.
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Quote:
Descend winds are taken care off by the automatic route/descent wind download.
Descend winds are taken care off by the automatic route/descent wind download.
That sounds nice. Where do I get one?
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KISS Quit complicating things with such things as the "Angle of the Dangle, Heat of the Meat and the Mass of the A$$".
3 for each 1,000 AGL, plus 15 miles. That's anticipated track miles. You have to realize that STARS are not always flown for their entire route thus reducing track miles, and the requirement for speed brakes.
Quit complicating things with such things as the "Angle of the Dangle, Heat of the Meat and the Mass of the A$$".3 for each 1,000 AGL, plus 15 miles. That's anticipated track miles. You have to realize that STARS are not always flown for their entire route thus reducing track miles, and the requirement for speed brakes.
