Descent planning tips 737

[Paulm1949]

Hi,

I’m struggling with descent planning in the 737NG. I know about 3x table and calculating yourself the rough track miles, using the green banana or asking atc for track miles.

It’s really for the last 7000ft whilst on headings for the ILS and trying for a CDFA. It’s difficult. Trying to make it a science rather than an art.

Any tips much appreciated.

Thank you.

[Uplinker]

On the 737-300/400 FMC, can't remember which page, there is a read-out of the average V/S required to get you from your present altitude down to the threshold.

Very useful for CDAs; I imagine it is still there on the later models?

[Banana Joe]

It's in the descent page. You can put any waypont with any altitude you want or just RWXX/ and it default to runway elevation +50 ft. It will tell you what RoD you need to make it at the selected waypoint at the desired altitide.

Or just put the FAF on top on the legs page and ask the PM to execute periodically as you fly away from the magenta line.

[Derfred]

You are on the right track.

3x tables at 250kts or below can be a little optimistic for the NG, because it is very slippery until Flaps 10 or Gear Down. Flaps 1 and 5 offer almost no extra drag, and the speedbrake is almost useless below 250kt. If you find yourself in trouble with too much altitude/energy below 5000’, you really need to request extra track miles or just put the gear down early. Going F5 at 249kts won’t help (and it is very bad practice). With experience will come finesse. But you need to be always thinking about your energy vs NM to run or your won’t gain the experience, because you’ll never know why it did or didn’t work!

Set a couple of “gates”. Obviously 3000’ at 10NM flaps 5 at 180-200 kts is a good one, but if you are moving from a tailwind to a headwind, even that might require early gear. Be aware of the wind. Flaps 25 can be your friend also, as it has a high placard speed compared to F30 and provides significant extra drag over F15.

The next gate would be 5000’. 20NM at 230kts and reducing. If you are doing 250kts at 5000’ then you probably want 22NM. I fly a lot in high temperature environments, these gates vary with conditions, so work out what works for your operation.

Judging track miles to run:

Where I fly, ATC advice of track miles to run as about as accurate as as a muzzle loader fired from the hip in a duel.

Calculate your own track miles to run. You have a beautiful map in front of you with range rings - use them, with the abovementioned gates, and your 3x tables (minus a bit). I’m old. I learned to fly jets before ND’s, so I used DME’s and VOR/ADF to estimate NM to run when being vectored for final. It was amazing how accurate one could be without a map. We used to challenge each other to get it right so accurately that if you had to touch the speedbrake or thrust levers before 1500’ you had failed. That was the old days. We don’t encourage that sort of dick-measuring these days. When ND’s with range rings were invented I couldn’t believe how easy it became, and I’ve probably lost the skills I used to have after 30 odd years of map displays.

Interpolate the gates. If you have 25NM to run, 250kts, what should your altitude be? I would be saying “well, my next gate is 5000’, 230kts at 20NM, so I need to lose 20kts, and descend, so lets knock off 2NM for the 20 kts of IAS, that leaves 3NM, that’s 900’, so I should be 5900 feet. That might sound like a lot of mental maths, but you’ll get good at it after practice. That is a bit conservative, but that’s not a bad idea on the NG, and depends a lot on weight, temperature (allow more NM for high temp), and wind changes.

Be a bit conservative when you are new on type. If your colleague next to you has more experience on type, ask them! (Regardless of which seat you are in - it’s quite common in my airline to have a new Capt with zero B737 hours who has transitioned from widebody F/O who might be sitting next to an F/O with 3000 hrs on type. But they are so keen to prove their new Capt status, they won’t dare ask for tips from the F/O!).

If you want to be a child of the magenta line, you can pre-plan a “base” waypoint in the FMC (if ATC generally vectors you on a similar path), and connect it to the IAP. When you get on a downwind vector, execute a “direct-to” base, and you will have an approximate VNAV profile. Don’t engage VNAV. Use the “above/below” profile as a guide, and use LVL CHG and V/S as necessary. Use V/S when you are trying to achieve speed gates, as LVL CHG won’t achieve what you want. When using V/S, don’t let the thrust come up unless you know you are low on profile. Don’t become a VNAV sucker, it doesn’t know about the wind changes and potential vectors. Use the Progress page for track miles to run, and go back to your gates and mental maths.

Finally, don’t let ATC screw you - if they cut you in and shorten your track miles unexpectedly, don’t accept it unless you know based on your experience that it’s achievable. Just. Say. No. Your safety is more important than their efficiency. If they didn’t give you any notice of reduced track miles, make it their problem not yours.

[Fly26]

Hey P1949

Just remember these simple gates and the rest will fall into place. Keep it simple. Happy flying, glad your flying in such difficult times. 😎

Draw the following rings from the Runway threshold in the fix page Eg RW25

/120nm
/25nm
/10nm

120nm ring- start descent (works generally for all alts above FL300. If your using the TOD VNAV green circle, see where the ring is in relation to it...gives you a great idea of LNAV track mikes in relation to straight in. Also good for en-route alternates if you have an emergency- you know when to descend just by looking at the ND)

25nm - 10000feet/ reduce to 220kts unless ATC controlled

10nm - 3000 feet 180kts flaps 5.

(if coming downwind abeam the runway aim for 6000ft /220kts abeam) still use the 25nm and 10nm gates.

Use the MCP modes V/S and LVL CHG​​​​ to achieve these gates. ​​​that’s it ....the rest will make sense based around this.

the nice part is if you have weather you see where it is in relation to your approach instantly because it paints over the distance rings and the 220kts allows you to take initial Flaps and gear with ease 👍

[V1777]

Or just push vnav and cross your fingers :-p

[Paulm1949]

Thank you for the replies very helpful.

Using v/s for the speed gates could someone please explain?

Where would you use V/S instead of Lvl chg? And vice versa? Someone mentioned the thrust levers moving whilst in v/s? Is this bad?

[easymxp]

Flying Airbus, but the concept is quite similar. I always try to keep it simple and use an easy reference to get distance. This could be the direct distance to the rwy thr (on Airbus you can put this on the Prog Page ie LIMC35R) if you are arriving straight. If coming from 90°I tend to use the ND and range rings to estimate the distance to go to the point where I expect to be cleared at a certain point (generally I take the IF or even the FAP(F) ). Or if I expect a sort of cut downwind I may use the ND or again the distance to the rwy adding the expect track miles for downwind, base.
With time you’ll be able to compute this distance quite quickly and then its only a matter of energy management using your 3x rule plus allowance to decelerate. This is normally the minimum distance you can expect to fly, if the airport is busy you can keep a bit high (I suggest initially being conservative) and in case recover using s/brakes.
generally I tend to use open descent (the equivalent of level change), when I see I’m drifting low I revert to V/S to get a lower ROD.
basically I use LVL chg, If I need to reduce my ROD I use V/S. I also use V/S when approaching platform altitude to achieve a smooth continuos descent. Just try but at the beginning be conservative. One good captain take your time, take your distance and don’t be afraid to have good margins (obviously within reason :) cheers

[KayPam]

Two computations that can be useful :
From your ground speed, derive your number of miles per minute : 120kt is 2, 180kt is 3, 240 is 4, and so on.
Then, note that the vertical speed that corresponds to 1 degree is your miles per minute x 100.
At 360kt, 6° down is 3600ft/min (approximately 5% of error with this calculation)
At 180kt, 3° down is 900ft/min (real number 954fpm)

Then, the required downward angle is simply ΔFL/distance. Example : 6000ft, 20 miles : 60/20 = 3°. There is the same error of about 5% with this calculation.

Even better, if you use the calculations the way I described them, the errors will cancel each other out !
If you descend from 6000ft at 900ft/min and 180kt, you will have flown exactly 20 miles.

Now two things : if you're at ease with remembering things like I wrote them, then fine you will just have a 5% error on the slope you believe to follow
You can also use the formulas the other way around, with a little 5% error which is negligible in real life.

[Banana Joe]

descentmanagement.com - Home (http://descentmanagement.com/)

A guide on steroids!

I keep it simple - (FLXXX/10)x3 to calculate my TOD. Apply the applicable corrections for wind and deceleration.

During the descent I simply multiply miles to go by 3 and subtract 2000 if speed is 300 KIAS or 1000 ft if flying at 250 KIAS. 50nm at 300 kts is 50x3=15000. Subtract 2000 and you get 13000 ft.
20 miles to go x3 is 6000 ft. At 250 kts subtract 1000 ft and you get 5000 ft. And this my gate: 20nm to go at 5000 ft at 250 knots, decelerating with a RoD of around 700-800 ft. I do use the V/B function in the Descent page and works a treat. You need to be more conservative if you have a tailwind.

If at 10nm you're at 3000 ft AAL at a speed of 200 knots or less with F1 you're good. You need F5 at GS interception.

[George Glass]

Thank you for the replies very helpful.

Using v/s for the speed gates could someone please explain?

Where would you use V/S instead of Lvl chg? And vice versa? Someone mentioned the thrust levers moving whilst in v/s? Is this bad?

The trap with Lvl Chg is that you cant slow down and go down at the same time.
Best L/D is near flap up manoeuvre speed so clean at 220 kts above profile is a bad place to be.
Use V/S to drive the aircraft down . Speed is your friend IF you are BELOW profile. As previously said , speed brake useless at low speed.
Everything Derfred said is accurate. I would add as well that being below profile and a little fast is a lot more comfortable than being slow above profile. Company speed gates and 250 kts below 10.000 make life difficult however. I know this will drive training departments crazy but in years gone by the preferred method was to get well below profile, keep the speed on as long as possible , and lose speed in level flight. Its not as dramatic as it sounds but will get you tea and bikkies if you try it now. Worth remembering though.

[4468]

Where would you use V/S instead of Lvl chg?
Just remember in Lvl Chg, your tail plane is only controlling the speed. Nothing else is being controlled, other than the thrust levers being closed. (in a descent!)

In v/s the autopilot is controlling your rate of descent, and a speed which may or may not require auto thrust to activate, and the thrust levers to move.

If you want to use v/s but want to be sure the thrust levers remain closed, then select your desired v/s, but wind the speed bug back to 20 or 30 kts below your indicated speed. (Or even back to your next target speed!) Excess speed is no longer controlled, because you are commanding a rate of descent! (This is very important!!!!) However you can be sure the thrust levers will remain closed. You can use speed brake if the speed starts to run away, but obviously that will have no effect on your rate of descent!

I found this particularly useful on Airbus aircraft with non moving TLs, but it works just as well in the many Boeing’s I’ve flown.

Oh, and never forget. You’re still learning so never beat yourself up if it doesn’t work out. That just makes it ten times worse! Just smile, and take it on the chin. Soon enough it will just become second nature and all these calculations and ring drawing will be superfluous!

[Fly26]

Thank you for the replies very helpful.

Using v/s for the speed gates could someone please explain?

Where would you use V/S instead of Lvl chg? And vice versa? Someone mentioned the thrust levers moving whilst in v/s? Is this bad?


Try not to think of it as “where would you use” each one, think about what each mode actually does and apply them to what you need.

so Level change coordinates pitch and thrust to make climbs and descents to what ever altitude you’ve selected at selected speed. In Descent it holds idle thrust and selected airspeed using the AFDS (auto flight director system)

Vertical speed will allow you to select a rate of descent and the auto throttle engages in speed mode to hold selected airspeed.

So if you press level change with the aircraft at 250kts, the thrust will close and you descend at that speed probably around 2500 feet per min. Great. But soon as you reduce to 220kts, the nose rises as the aircraft slows. But the system cares about is the speed you’ve commanded it to make, and your descent planning goes out the window. So you need to factor this in. Your notice when the speed is a couple of knots above your commanded speed, when you press level change it’s slow to descend because all it cares about is the speed before giving your good descent you wish. “It doesn’t go down and slow down at the same time”. If you match the speed, say 220kts to 223kts your see the plane will immediately descend.

Now vertical speed gives you what ever rate of descent you want, but now it’s the auto throttle holding the speed and not the AFDS, so you don’t see the planes nose fluctuating as you’ve told it to hold what ever descent rate you’ve chosen. Your find the auto throttle can’t hold the airspeed above 1800 feet per minute ROD roughly even with the thrust closed, laws of physics. The speed will begin to run away. 220...221...223..225 etc, any V/S ROD above 2500ft and the speed will run quite quickly. Not what you want. (When clean)

So going back to your descent planning, if you want to come down relatively quickly your looking to use Level change for larger altitude changes. Then once your down to where you need to be we can bring “V/S to finesse” I.e. there’s no need to keep plummeting below the profile, so v/s 750 nicely maintains your profile. Another time you might use V/S is if your very low on profile or extended miles downwind, V/S 500ft nicely brings you down. Another maybe ATC assigned rate of descent which is normally not more than 1500ft min.

So apply these modes to make your gates. It will come as your practice more. Apologies if this is repeated above I haven’t waded through all the posts. Goodluck.

[tom775257]

I would add the following minor points:
-If you are a bit high, the aircraft does descend very well Flap 5, 220kts and speedbrake (some might disagree on the technique).
- If you are in a non-sfp, flap 10 does offer a bit more drag if required... remember 210 kt limit.
-If you enter QNH and descent winds in descent forecast it will help with the aircraft's calculations being more accurate (definately enter correct QNH for a Vnav approach!!)
-On base leg for an ILS, turning the standby attitiude indicator onto apporach will give a much earlier glide path indication than the pfd. Can help with judgement wih regards to CDA whether high or low.

[4runner]

You are on the right track.

3x tables at 250kts or below can be a little optimistic for the NG, because it is very slippery until Flaps 10 or Gear Down. Flaps 1 and 5 offer almost no extra drag, and the speedbrake is almost useless below 250kt. If you find yourself in trouble with too much altitude/energy below 5000’, you really need to request extra track miles or just put the gear down early. Going F5 at 249kts won’t help (and it is very bad practice). With experience will come finesse. But you need to be always thinking about your energy vs NM to run or your won’t gain the experience, because you’ll never know why it did or didn’t work!

Set a couple of “gates”. Obviously 3000’ at 10NM flaps 5 at 180-200 kts is a good one, but if you are moving from a tailwind to a headwind, even that might require early gear. Be aware of the wind. Flaps 25 can be your friend also, as it has a high placard speed compared to F30 and provides significant extra drag over F15.

The next gate would be 5000’. 20NM at 230kts and reducing. If you are doing 250kts at 5000’ then you probably want 22NM. I fly a lot in high temperature environments, these gates vary with conditions, so work out what works for your operation.

Judging track miles to run:

Where I fly, ATC advice of track miles to run as about as accurate as as a muzzle loader fired from the hip in a duel.

Calculate your own track miles to run. You have a beautiful map in front of you with range rings - use them, with the abovementioned gates, and your 3x tables (minus a bit). I’m old. I learned to fly jets before ND’s, so I used DME’s and VOR/ADF to estimate NM to run when being vectored for final. It was amazing how accurate one could be without a map. We used to challenge each other to get it right so accurately that if you had to touch the speedbrake or thrust levers before 1500’ you had failed. That was the old days. We don’t encourage that sort of dick-measuring these days. When ND’s with range rings were invented I couldn’t believe how easy it became, and I’ve probably lost the skills I used to have after 30 odd years of map displays.

Interpolate the gates. If you have 25NM to run, 250kts, what should your altitude be? I would be saying “well, my next gate is 5000’, 230kts at 20NM, so I need to lose 20kts, and descend, so lets knock off 2NM for the 20 kts of IAS, that leaves 3NM, that’s 900’, so I should be 5900 feet. That might sound like a lot of mental maths, but you’ll get good at it after practice. That is a bit conservative, but that’s not a bad idea on the NG, and depends a lot on weight, temperature (allow more NM for high temp), and wind changes.

Be a bit conservative when you are new on type. If your colleague next to you has more experience on type, ask them! (Regardless of which seat you are in - it’s quite common in my airline to have a new Capt with zero B737 hours who has transitioned from widebody F/O who might be sitting next to an F/O with 3000 hrs on type. But they are so keen to prove their new Capt status, they won’t dare ask for tips from the F/O!).

If you want to be a child of the magenta line, you can pre-plan a “base” waypoint in the FMC (if ATC generally vectors you on a similar path), and connect it to the IAP. When you get on a downwind vector, execute a “direct-to” base, and you will have an approximate VNAV profile. Don’t engage VNAV. Use the “above/below” profile as a guide, and use LVL CHG and V/S as necessary. Use V/S when you are trying to achieve speed gates, as LVL CHG won’t achieve what you want. When using V/S, don’t let the thrust come up unless you know you are low on profile. Don’t become a VNAV sucker, it doesn’t know about the wind changes and potential vectors. Use the Progress page for track miles to run, and go back to your gates and mental maths.

Finally, don’t let ATC screw you - if they cut you in and shorten your track miles unexpectedly, don’t accept it unless you know based on your experience that it’s achievable. Just. Say. No. Your safety is more important than their efficiency. If they didn’t give you any notice of reduced track miles, make it their problem not yours.


i agree with a lot of your response. The exception is the flap 5. I found flaps 5 to be great for getting the 73 down. 230 and flaps 5 will let you get down at over 2000fpm.

[IRRenewal]

You cannot learn descent planning from reading tips on the interweb. Don't worry, it will come with experience.

[George Glass]

One of the things that has changed over the last thirty years or so is much greater regulatory control over speed on descent and approach.
That’s a good thing because rushed approaches and runway over-runs are obviously a major risk factor.
But, and its a big but, it means that many new pilots have no idea about what their aircraft can actually do.
A mandatory exercise in every endorsement program should be
“Positioned on extended downwind with 22 nm to run , at 1500’ AGL , 320 kts. Objective : stable in landing configuration at 1000’. Handing over......”
Repeat till proficient.
Not possible on the line nowadays sadly but it would give trainees confidence by showing what the aircraft can do.
Would mean at lot less anxiety on the line when things go off the rails.

[Papa_Golf]

You cannot learn descent planning from reading tips on the interweb. Don't worry, it will come with experience.

Thank you for sparing me the effort of writing down the very same concept.

[tttoon]

My top tip: don't calculate the X3 table, just push the meters button and use that.

[Paulm1949]

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

[Banana Joe]

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.

[misd-agin]

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.

[Paulm1949]

Thanks for all the tips. I’ve 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 ?

[ImbracableCrunk]

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.

[Paulm1949]

Thanks. But what V/B numbers seem to work best for a CDFA idle descent? There must be an art to this. That’s what I’m trying to figure out.

[FlyingStone]

There’s 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 “FAF/[platform altitude]” 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,that’s not too bad. If you are doing 320 kts, that’s 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.

[tttoon]

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 :-).

[Paulm1949]

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?