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Descent planning tips 737

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Descent planning tips 737

Old 11th Oct 2020, 14:12
  #21 (permalink)  
Join Date: Jan 2018
Location: Amantido
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It depends on your company's SOP. You can keep that ECON speed but you have to descent below the VNAV calculated path to allow for deceleration back to ECON speed.
If you are allowed to change the descent speed do so, you might not need to dive at all.

Every situation is different.
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Old 12th Oct 2020, 03:09
  #22 (permalink)  
Join Date: Jan 2006
Location: US
Posts: 2,207
Originally Posted by Paulm1949 View Post
A econ descent is 245kts. So if I speed up to regain my path do I need to update the fms page with the new speed? Or can I regain the path and revert to back to 245kts
1. As previous posters said - good luck learning this on the internet. You need to do it, make changes/corrections and observe the results, and try to understand what worked, didn't work, or could have been done better. That's what beers afterwards are for. At times you'll learn more discussing it after the fact.

But if you think you'll be able to read it and understand it the answer to your question is - it depends. Why aren't you on the path? If the deviation is from winds returning to 245 kts might create the same problem you just fixed.

Here are some high energy 'gates'. At these energy points you should be able to stay at idle power until below 3000', and possibly 2000'. So if you're higher, or faster, than the following gates you have excess energy and need to figure out what drag (speedbrakes, flaps, gear) you need, how much, and when, to get back to a normal energy state - 10,000' / 40 nm / 250 kts. 5,000' / 20 nm / 250 kts.
If you're on a CDA and you're below those gates you'll need power to avoid leveling off later. On a CDA, with a 1000' to level off and no further descent clearance, I slow my sink to 200 FPM. At 500' to the unwanted level off I slow my descent to 100 FPM. That basically extends the last 1000' of descent to 20 track miles if necessary. How? Either V/S, which is 'safest' or by adjusting power if I'm hand flying or using FLCH for the descent mode.

Speedbrakes work at low speed. Just not as effective as at high speed. Clean each additional 10 kts of airspeed takes about 1 nm to slow.

Watch the guys you fly with. Find out who does a good job. Ask them what they're looking at or adjusting to. Ask them for advice when it's your leg. No one knew the answers on day 1. You'll still see guys screwing it up, at least compared to the good guys, after they've been doing it for years.

Think about that 5000' / 20 nm / 250 kt 'gate'. That's 1000' below the 3:1 rule. Because you're at 250 kts. Clean you're typically descending about 1300 FPM at low altitude. What's that if you're 1000' higher? .8 of a minute? Times your GS and it's 3+ nm. Each mile = approx 10 KIAS so 6000' and 220 kts will be close to the same energy. So you can be higher but you need to be slower to have the same energy. Or you can be 1000' lower and approx. 280 kts and have the same energy. It's not perfect because L/D is close to the UP bug (210 KIAS +/-) so slowing down is a better glider.

So 5000/20/250 is 1000' lower than the 3:1 rule. Apply that to the CDA. Do the mental math on your rate of descent, altitude to lose to level off restriction/clearance, speed (nautical miles per minute), and work to stay approx. 1000' below the 3:1 rule if you're at 250 kts. If you are slower you can be closer to the 3:1 rule. Do it enough and you'll get pretty good at it. Don't put the effort in, don't ask the experienced guys to help you, and it will be a long learning process. How often should you be updating your energy state vs the desired state? How about every mile to the runway? So 3-4x per minute. Or you getting closer to the 3:1 normal altitude, maintaining a steady altitude below that decent path, or getting lower and lower vs the desired/planned altitude. If you're high consider more flaps (not recommended IMO for small deviations), use partial speedbrakes for small deviations/corrections, and if you're low add a little bit of power (FLCH or manual thrust flying) or reduce your V/S slightly. Between slight V/S changes, using a couple of inches (2.5x to get centimeters) of speedbrake travel, and you can fly the entire CDA with minimal pitch, power, and speedbrake changes. The nervous passengers will appreciate it and over time you'll get a reputation of doing your job well. Good luck.
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Old 10th Nov 2020, 14:05
  #23 (permalink)  
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Join Date: Jun 2018
Location: Uk
Posts: 33
Thanks for all the tips. Iíve been using them to hone my descent skills. Any suggestions and tips for using the V/B on the descent page? Looks like a great tool for a CDFA. Say if I wanted a complete idle descent CDFA how could I incorporate the V/B ?

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Old 10th Nov 2020, 23:49
  #24 (permalink)  
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I grab the FAF/Crossing Alt and monitor it. If I can get to the FAF at the correct altitude, on speed, I can get to the runway.
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Old 12th Nov 2020, 12:27
  #25 (permalink)  
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Join Date: Jun 2018
Location: Uk
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Thanks. But what V/B numbers seem to work best for a CDFA idle descent? There must be an art to this. Thatís what Iím trying to figure out.
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Old 12th Nov 2020, 14:15
  #26 (permalink)  
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Thereís no fixed number when using V/B, as your FPA in idle descent can vary greatly - descending with 320kts into a 100kt headwind will produce a much steeper descent path (higher FPA) than descending with 250 kts and a 100 kts tailwind (lower FPA).

I normally put ďFAF/[platform altitude]Ē on the descend page and then monitor it throughout the descent. The V/B value simply tells you the required FPA to waypoint/altitude point in space from your present position.

For example, if your FPA is 3.0 and V/B is showing 3.0, that means that you will reach the FAF on your path - but at present speed. If your speed is 220 kts,thatís not too bad. If you are doing 320 kts, thatís less good.

So in practice, compare your FPA to V/B and adjust for difference between your actual speed and desired speed at the FAF. All this is achieved with practice, practice, more practice - and even then, speedbrakes, V/S, or simply asking ATC for more track miles are all tools you can utilise.

To be honest, VNAV on the 737 is excellent when it comes to descent management, and provided you have inserted accurate data, keep your lateral path up to date with what you will actually fly in the end and you have a good understanding of how FMC constructs the descent path from the runway (or E/D point) upwards - it will generally do a very decent job on its own.
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Old 12th Nov 2020, 14:26
  #27 (permalink)  
Join Date: Jan 2008
Location: Belgium
Age: 36
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Unfortunately, it depends. If you look in the FCTM under descent you can see the descent rates for various speeds, which gives an approximate descent angle. In general, the airplane will do close enough to a 3 degree glide with constant speed at up speed, 1 speed and 5 speed (slightly steeper with more flaps or higher speed). So a 3.3 degree V/B might work with 50NM to go and 290kts, but wont work with even a slight tailwind close in. I usually fly with the FPA on, so I can monitor actual descent angle and compare that to what I need (calculated with the times 3 table and/or V/B). Then manage the drag with speed if able further out, speedbrakes if needed and config close in. And if you completely cock it up the aircraft can do a double glide in landing config, so level off, get everything hanging out and see if you need to buy the other pilot a beer :-).
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Old 12th Nov 2020, 15:35
  #28 (permalink)  
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Join Date: Jun 2018
Location: Uk
Posts: 33
Thank you for your posts. I understand the E/D are constructed from runway and then back along the route. Any tips here for an accurate vnav profile?
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