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Descent planning tips 737

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Descent planning tips 737

Old 9th Oct 2020, 09:53
  #1 (permalink)  
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Descent planning tips 737

Hi,

Iím 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.

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

Any tips much appreciated.

Thank you.
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Old 9th Oct 2020, 11:25
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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?
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Old 9th Oct 2020, 13:44
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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.
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Old 9th Oct 2020, 14:03
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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.





Last edited by Derfred; 9th Oct 2020 at 14:16.
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Old 9th Oct 2020, 16:31
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Join Date: Oct 2011
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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. ​​​thatís 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 👍
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Old 9th Oct 2020, 16:46
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Or just push vnav and cross your fingers :-p
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Old 9th Oct 2020, 17:52
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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?

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Old 9th Oct 2020, 18:41
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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 90įI 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 youíll 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 Iím 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 donít be afraid to have good margins (obviously within reason cheers
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Old 9th Oct 2020, 21:48
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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, 6į down is 3600ft/min (approximately 5% of error with this calculation)
At 180kt, 3į down is 900ft/min (real number 954fpm)

Then, the required downward angle is simply ΔFL/distance. Example : 6000ft, 20 miles : 60/20 = 3į. 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.
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Old 9th Oct 2020, 22:24
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descentmanagement.com - Home

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.
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Old 9th Oct 2020, 23:31
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Originally Posted by Paulm1949 View Post
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.
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Old 9th Oct 2020, 23:57
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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 Boeingís Iíve flown.

Oh, and never forget. Youíre still learning so never beat yourself up if it doesnít 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!
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Old 10th Oct 2020, 06:05
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Originally Posted by Paulm1949 View Post
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 ďwhere would you useĒ 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 youíve 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 youíve 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 itís slow to descend because all it cares about is the speed before giving your good descent you wish. ďIt doesnít go down and slow down at the same timeĒ. 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 itís the auto throttle holding the speed and not the AFDS, so you donít see the planes nose fluctuating as youíve told it to hold what ever descent rate youíve chosen. Your find the auto throttle canít 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 ďV/S to finesseĒ I.e. thereís 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 havenít waded through all the posts. Goodluck.
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Old 10th Oct 2020, 07:58
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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.
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Old 10th Oct 2020, 21:36
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Originally Posted by Derfred View Post
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 5000í, you really need to request extra track miles or just put the gear down early. Going F5 at 249kts wonít 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 wonít gain the experience, because youíll never know why it did or didnít work!

Set a couple of ďgatesĒ. Obviously 3000í 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 5000í. 20NM at 230kts and reducing. If you are doing 250kts at 5000í 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). Iím old. I learned to fly jets before NDís, so I used DMEís 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 1500í you had failed. That was the old days. We donít encourage that sort of dick-measuring these days. When NDís with range rings were invented I couldnít believe how easy it became, and Iíve 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 ďwell, my next gate is 5000í, 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, thatís 900í, so I should be 5900 feet. That might sound like a lot of mental maths, but youíll get good at it after practice. That is a bit conservative, but thatís 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 - itís 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 wonít dare ask for tips from the F/O!).

If you want to be a child of the magenta line, you can pre-plan a ďbaseĒ 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 ďdirect-toĒ base, and you will have an approximate VNAV profile. Donít engage VNAV. Use the ďabove/belowĒ 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 wonít achieve what you want. When using V/S, donít let the thrust come up unless you know you are low on profile. Donít become a VNAV sucker, it doesnít 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, donít let ATC screw you - if they cut you in and shorten your track miles unexpectedly, donít accept it unless you know based on your experience that itís achievable. Just. Say. No. Your safety is more important than their efficiency. If they didnít 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.
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Old 10th Oct 2020, 21:54
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You cannot learn descent planning from reading tips on the interweb. Don't worry, it will come with experience.
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Old 11th Oct 2020, 01:09
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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.
Thatís 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
ďPositioned on extended downwind with 22 nm to run , at 1500í AGL , 320 kts. Objective : stable in landing configuration at 1000í. Handing over......Ē
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.
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Old 11th Oct 2020, 06:07
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Originally Posted by IRRenewal View Post
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.
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Old 11th Oct 2020, 08:15
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My top tip: don't calculate the X3 table, just push the meters button and use that.
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Old 11th Oct 2020, 14:02
  #20 (permalink)  
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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
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