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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
Posts: 493
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: 2,202
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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