Originally Posted by
Maverick SKC
Hello everyone,
I’m a first officer on B737-800 and MAX and even tough I’m not struggling anymore with descents, there is still something that I would like to ask to you guys with more experience on type.
I try to use VNAV as much as possible but however sometimes it’s not working smooth or sometimes might not be accurate (for example during radar vectors). Thus, I am trying to have a mental calculation model other than VNAV.
The rule of thumb for level deceleration is 1NM for every 10 kts above UP speed, and even the FCTM only mentions level flight deceleration.
However in many companies included mine, we try to achieve continuous descent, so those rules can not fully apply.
I’m trying then to find some Rule of Thumbs that work for descent deceleration from 250 kts to UP speed.
Do you have any?
I am generally doing (when not in VNAV) ALT = NM x 3 and then I remove 1000 ft out of it to allow for deceleration (if at 250 kts) meaning 200 kts every 10 kts above UP speed (~200 kts).
Starting to slow down around 20 NM (5000 ft AAL), earlier if heavy weight or tailwind.
The slow down I do it in LVL CHG.
This way works quite good, sometimes ends up high or low on profile but the corrections are manageable.
I would like anyway to receive a more precise Rule of Thumb, with a brief explanation, because I feel like what I am doing has no logic behind and just works out of magic.
Or if someone can explain why that works would be also appreciated.
Thanks in advance for who wants to share his experience with me, it’s always good to gain some knowledge from you guys!
Just started on the 737, but was in the left seat of the 320 for the last decade. From what I have seen I feel it is similar enough to say this: Yes, 1nm gives 10kts in level deceleration. Know what the aircraft will do idle descend at different weight/speed, like 300 = 2000 fpm, 250 = 1500 fpm, 210 = 1000 fpm (not saying these are the correct numbers, I am new to the 737).
But what I (almost) never did was using LVL CHG (open descend) to change speed. If I get/want a speed change, I set the new speed in the window, press V/S, set 0 fpm (press to level off), and 10 kts before reaching the speed, press LVL CHG (pull for open descent). Much more predictable deceleration, so much more predictable path.