Boeing 737 descent without VNAV
 

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!

It sounds like you're doing just fine. What you need is practice, and no matter how long you've been doing it you'll never get it perfectly every time (because you're right about there being an element of magic).

In addition to the math(s), I like to keep a Vertical Bearing to the FAF in the Descent page. Some of my co-workers use the RWYXX/ on the Descent page instead of the FAF. Both will give you an idea of your energy state.

3 x altitude plus the middle digit of the airspeed works pretty well, e.g. 5000’ at 250 kts = around 20 nm. 6000’ at 290 kts = 27 nm. And so on. Obviously apply common sense for 300 kts or more…

Would agree with this. If you can’t achieve a CDA on the day for whatever the reason, it’s not the end of the world …. treat ATC track miles to touchdown (if given) as a general guide as opposed to the gospel truth! I think your maths is good enough…it’s pretty much what most people are doing.

Why do you use level change to slow down?

Hi guys,

thanks a lot for your replies.

As I can see all the techniques run about the same:
ALT x 3 = NM to run (+ some NM for deceleration)
NM x 3 = ALT to be (minus some FT for deceleration)

Seems that both are working, with the difference between the 2 calculation getting smaller and smaller as we go closer to the ground.

I personally prefer to judge the distance to go and to get an altitude to be, because in the other way around as I feel slow sometimes, and I prefer to say “I’m 1500 ft high” rather than “I need 5 NM more”.
That’s personal of course.

To answer why I use LVL CHG to slow down I meant to say: I am in LVL CHG at 250 kts and I bug to UP speed, in this way the plane will reduce V/S to gain the speed.

You'll find it very difficult to perform a good CDA doing that. It's a bit agricultural and priorities speed over descent rate. Not great for CDAs. Youre better of using VS and then you have direct control on your ROD and speed at the same time rather than a wishy washy variable rate because the AP is going for speed first.

on Airbus but the principle is similar. I don’t like much NM*3 at long distance because it’s a bit misleading as the rate is steeper than 3° with more than 250 knots. In this way you can arrive low. That’s why I prefer altitude*3 (or distance*3 + 10%).

Hi,
never heard about Distance x3 + 10%.
I will have a look on it, thanks!

But yeah you are right, at long distances the NM x3 can lead too low, but at low altitudes, especially during vectors, I find it the fastest way for me to understand my profile.

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.

I never did that but, if I did, I'd set vs -500 not zero in controlled airspace as levelling off would not be complying with your descent clearance.

A true CDA includes minimum thrust too remember, which is very much unachievable in a lot of places today. "Speed up" right after you've commenced your descent throws any proper CDA right out the window from the get go. Then "slow down", out comes the speed brake, and you wonder why we bother.

Anyway ..

Am I low? I go V/S.

Am I high? I go LVL CHG

Under vectors and have some capacity to doodle a realistic LNAV path, or is my PM willing to ... then great, VNAV it is (unless I'm expecting even shorter miles, I'd stay below the VNAV profile a bit)

Is it totally perfect? no. Though, I don't think I have ever levelled off unintentionally with the above approach. There is a book out there with all sorts of formulae that prove it can be an exact science.I personally half can't be assed and half don't want my brain zapped away doing all manner of formula whilst I'm flying and trying to free up capacity.

As long as I comply with the restrictions on the STAR, I am good. Also, almost always it was because ATC wanted the speed first. But yeah, could have worded that better.

Not quiet right. Below the transition altitude you are free to use whatever descent rate you like.

The 500'/m is only relevant above the TA or holding. CDAs are much easier if you're aware of that ;)

That's interesting. Is that just in the UK or in Europe too?

Never been yelled at for using V/S values less than 500 ft, especially when cleared for approach.

How many of us are guilty of flying a 300fpm "CDA" to avoid the ALT CAP and keep the fuel efficiency bean counters happy? The irony is that when you are already below the glide the most fuel efficient thing to do is fly level.

It's an ICAO thing.

Yep , it’s basically 3*Altitude but in this way you get altitude as outcome. So easier for you to compare how high you are (or low)