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)

I was taught this when I flew the L1011, it worked on the B757 and now the B737. Caution: this is technique only.

Take your altitude loss required, ex. 30,000 - 10,000 = 20,000.
Divide by 1000, ex. 20,000/1,000 = 20.
Multiply by 3 = 60. Now 60 miles is the distance needed to descend 20,000'. In calm winds. Start your descent at least 60 nm prior to your crossing restriction.

Now, I'm sure you're asking, "but what speed?" It worked for me for Mach .84 to Mach .82 to Mach .78 with a transitions from 340, 320, 300 kts. because...

... you should plan to descend at a rate 5 times your ground speed. In calm winds a B737 will by going around 450-500 kts. 5 x 450 = 2250 fpm. This is roughly what a B737 will descend at.

So, it takes 4-5 nm to decelerate from 300 to 250 kts without using speed brakes. It takes about 4nm to decelerate from 250 to 210kts Try to plan NOT using speed brakes.

So (again) add this 5 nm to the 60 planned above, and you should theoretically be at your altitude 5 nm prior to the requirement. If you have to slow, you can slow down after you level off without speed brakes. With the desired altitude set in your MCP, use your altitude trend reference symbol, i.e. the "green banana," on our ND to monitor your progress.

Now, if you're trying to slow down while descending, know that the A/P is not very aggressive. Whether you're in LVL CHG or SPD INV in VNAV, the A/P will aim to descend at 500 fpm, which will allow it to descend and decelerate, theoretically.

Here's another gem. If you're in VNAV or with GS CAP on a 3º glide path/slope, you can maintain 250 kts until 12 nm out, calm winds considered, then you can throw your gear out, slow down and configure on speed to be stable by 1000' agl. The 1st time you try this, use the speed brake with the gear to slow and configure. This should give you confidence.

I would love to be able to fly like this, however 99.9% of captains would scream at me and any who allowed me to would end up in an office meeting explaining why they let me. Absolutely against all company SOP
(speedbrake with the gear is expressly forbidden too, an OFDM trigger).

Company SOP, what company has these kinds of SOPs? There is a standard practice, that does not mean it is an SOP?

Boeing does not talk about SOPs, it explains in the FCTM and uses words like "normal" and "not recommended". When it comes to landing gear it uses words as "passenger comfort" and "gear door life". It explains how to, or to avoid, use of flaps, gear and speedbrake as a combo. On the other side, the problem with these "demo's" is that people tend to use it as a standard because "they can". This is a training issue: you can show/demo an ability, that doesn't make it a standard practice. But against SOPs?

Simple case of RTFM.

There is NO need for anyone to use the speedbrake with the landing gear down in normal operations.

Alone, the landing gear will slow the aircraft down or help increase the descent rate with the drag if you keep the speed up.

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We didn't have FMS in my day.
As a rule of thumb, from light, jets throught to heavies, we used this. 3 x the altitude. 3 degree decent angle. So, if you were at 30,000ft x 3 = 90 miles plus 10, for TOD. If you were going down at less than less than 3 degs, and unable too reduce power (pressurization), increase speed, more drag.
Higher the A/C weight, the more distance required, for a given speed. (Same reason a glider carries water ballast). Keep a check, if you are on profile, during descent.
We didn't use calculators, as they weren't around then either. K.I.S.
As for the use of spoilers in the air. Dont use them, in normal procedures. Observe an a/c landing. As soon as spoilers are deployed, watch the horizontal stabliser. The movement, caused by disturbed airflow from deployed spoilers, is disturbing to watch. Imagine the movement at twice the A/S. With prolonged use in the air
If you bend a piece of metal often and long enough, it will fail.

I don't know what airline you work for mate, but we have an FCTM and an FCOM, and mentioned is the prohibited use of speed brake with the gear down.

RTFM, indeed.

The real question is why anyone would want to fly like this?

Some people enjoy it

5000ft 250kts at 20nm
3000ft 210kts at 10nm with Flaps 1
GS intercept 180kts with Flaps 5

Aim to meet these "gates" for stabilised approach.

What 737 type (I'm not familiar with the MAX)?

For all I know, you can do an emergency descent with the landing gear down... you're not allowed to use speedbrakes? Don't read that anywhere.

Are you suggesting repeated use of spoilers in flight could lead to some sort of structural failure?

https://en.m.wikipedia.org/wiki/1977...eing_707_crash

There was a.crew change at Nairobi. The incoming Captain, mentioned to the out going crew, they couldn't quite get the nose up trim on landing. They said they would take it, see what happens.
If there are any doubts, periodically, on the ground, it is worth winding the trim full forward, and do an external check of the stabliser position against the fuselage markings for assimectrics. Do the same, full nose up.

Some pilots used outboard spoilers, only in the air. I wasn't that keen, Putting the gear down was very effective.

Ultimately of course it will, sooner rather than later.

As will selecting gear and flap repeatedly at the limitng speeds. Higher stresses, higher vibrations, higher air loads, will all lead to premature metal fatigue.

Go and fly something aerobatic with one seat.

If ABNORMAL or EMERGENCY situation dictates to use landing gear during emergency descend- it’s CPT responsibility to use all available sources and actions to meet emergency (FCTM - non normal OPS, QRH- GEN/ NNC usage).

But in normal and expected conditions many SOPs restrict using some techniques (for example ALTN MCP technique, flaps policy, speed brakes policy etc.).

One of my operators doesn’t allow to use SPD BRKs with Flaps more than 15 (FDR trigger).

What about descend rules? To my mind it’s PF responsible (of course ur colleagues should understand your strategy).

There are many techniques, for example:
1.PROG distance*3-1000 ft
2.(PROG distance*3)- 10 nm
3.Cross check at typical ALT vs total distance remaining:
-10.000 FT= 37 nm (FPPM value for 0.78/280/250 descend with allowance for deceleration to F15 configuration at distance approximately 7 nm from touchdown)
-5.000 FT= 22 nm (same as above)
4. Set WPT/ALT (on DES page) probable shortcut waypoint (IF/FAF) or just RWY/elevation, observe VS required, and set VS= VS required+300…500 fpm)
5. Just restrict IAF,IF, FAF to “AT” and set at IF/FAF Vapp instead of 180B, 160B (for example) - ALWAYS COMPLY WITH STAR/APP/ATC RESTRICTIONS AND DO NOT MODIFY LEGS WITHOUT ATC PERMISSION. Set on DESC FORECAST page tailwind at base and final legs (it can help you by extending DECEL segment).

In all cases comply with your SOP, ATC,AIRSPACE and make sure your colleagues understand your intentions.

That’s fascinating. Never heard of that SOP before.

What do you do when you have the gear down coming into land and ATC say "reduce *now* to minimum approach speed, slower traffic ahead”? Or it looks like you need to lose a bit of energy to be stable at 1000R?

I used to fly the 737 and there was nothing then in the manuals about gear + speedbrake. On the 777 the speedbrakes are another primary flying control and are used on almost every approach; you can land with them fully deployed if you really want to.

It is a rapid descent explained in the maneuvers section of the FCTM. There is no "request" of Boeing to go "beyond the scope of the NNC" for an emergency descent. You would fail your recurrent check if you don't know how to apply it correctly.

Speedbrakes beyond Flaps 15 has nothing to do with landing gear limitations. There is a limitation you cannot go beyond flight detent in flight, there is a limitation you cannot use speedbrakes below 1000ft _RA_.

Big takeaway that might one day save your life: FDM TRIGGERS ARE NOT LIMITATIONS. Otherwise you can't even go-around. Triggering an FDM might one day avoid you get into deep ####. Triggering limitations means you are already are in deep #### as Boeing does not guarantee anything.