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# B737 Descent Questions

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# B737 Descent Questions

18th May 2018, 18:38

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B737 Descent Questions

Hey. 2 quick questions regarding descent planning in the B737.

1) In the VNAV descent forecast page, it says: "Enter the average ISA deviation for descent in °C (+/–XX°C) or °F (+/–XX°F)". What is meant by that? How does one calculate the average ISA DEV for the entire descent? At my outfit, the normal is that we'd put the destination's ISA dev and some others opt for putting in the average of ISA DEV at cruise and ISA DEV at destination.

2) During descent, once vectored off our planned route, we normally do:

"If displaying the arrival procedure is not desired, perform a “DIRECT TO” or “INTERCEPT COURSE TO” the FAF, OM, or appropriate fix, to simplify the navigation display. This provides:
• a display of distance remaining to the FAF, OM, or appropriate fix"..
However, once we do this, the VNAV profile changes rapidly and might show that we're too high (normally). Why is that? Does the VNAV descent profile consider that we're flying directly to the FI, therefore we're high? Would appreciate if you could highlight the reference to the explanation please.

Many thanks.
18th May 2018, 19:38

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The answer to your question one: you calculate the average ISA deviation from TOD to landing, by averaging them. Isa at TOD +20, ISA deviation at destination +10, so +15 degrees. This will influence your TOD, your VNAV Path and fuel consumption. Or fuel remaining at destination if you will.

Question 2 I will not answer. The answer is simple as well, but you do not want my explanation, you want a reference. Google will be your best friend.
19th May 2018, 09:11

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Does the VNAV descent profile consider that we're flying directly to the FI, therefore we're high?

Indeed, because from your description you have told the FMC you are flying a shortened route. If you were on VNAVPTH at the time of the change then informing the FMC you are flying s shorter distance will cause VNAV to calculate you are high; but you may not be and so it needs good SA to manage your descent path and remain in idle and on a sensible path.
20th May 2018, 18:08

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So why is it then that when I am flying directly to the FI and then execute the intercept course to the FI the vertical profile varies greatly. Shouldn't it be the same then if that was the case? I think would would help me to understand this more clearly would be if I can know what is the "from" point.... the "to" point is obviously the FI but where is this profile starting its calculations from?
21st May 2018, 03:29

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Originally Posted by RAT 5
Does the VNAV descent profile consider that we're flying directly to the FI, therefore we're high?

Indeed, because from your description you have told the FMC you are flying a shortened route. If you were on VNAVPTH at the time of the change then informing the FMC you are flying s shorter distance will cause VNAV to calculate you are high; but you may not be and so it needs good SA to manage your descent path and remain in idle and on a sensible path.
Didn't last time this question came up (and the time before that) we decided that the VNAV PATH transposes you laterally to the final approach course but at your current altitude? (i.e. If you are 5 miles direct to the FAF and 4 miles to the final approach course, VNAV thinks you're 3 miles 3-4-5 triangle)
21st May 2018, 14:10

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Originally Posted by 100 above minimums
So why is it then that when I am flying directly to the FI and then execute the intercept course to the FI the vertical profile varies greatly. Shouldn't it be the same then if that was the case? I think would would help me to understand this more clearly would be if I can know what is the "from" point.... the "to" point is obviously the FI but where is this profile starting its calculations from?
CMIIW vnav calculates as if you were flying straight in approach...a simple trick is simply push lnav when you are on intercept heading vnav will recalculate your profile accurately.. sorry for my english its not my 1st language
ps: all based on my exp flying the 738/739
21st May 2018, 14:51

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Originally Posted by ImbracableCrunk
Didn't last time this question came up (and the time before that) we decided that the VNAV PATH transposes you laterally to the final approach course but at your current altitude? (i.e. If you are 5 miles direct to the FAF and 4 miles to the final approach course, VNAV thinks you're 3 miles 3-4-5 triangle)
Agree. By arming LNAV it goes back to proper calculations.
22nd May 2018, 12:13

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Originally Posted by sonicbum
Agree. By arming LNAV it goes back to proper calculations.
i did not know that, gonna try out next time I’m flying, however VNAV will also calculate your speed at the faf at Vref40+20. As we don’t normally do this that will also cause the indication to be high!!
22nd May 2018, 12:42

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Yea, do try it out and let me know. I'll also try it out on when I get the chance to. But either way, not sure that this is a viable solution; one wouldn't want to arm LNAV while they're only being vectored on a HDG SEL. Would cause a huge mess if during such a phase, with high workload, they inadvertently arm and forget to disarm LNAV.
But more importantly, is there no reference for such thing in the manuals?
22nd May 2018, 19:53

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If you are on base and are in HDG SEL, the VNAV PATH will show you as high, correcting as you approach the final. If you select LNAV at the same location, you will get an accurate indication on your path.

Alternately you can simply select DIR to the FAF or
Simply put the FAF/Crossing Alt in the DESC page to get a Vertical Bearing.
22nd May 2018, 22:54

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Not sure if its the right idea to press LNAV while on HDG Sel getting radar vectored. You would not want to get things messy in terminal area during high work load.

For VNAV calculations while not in LNAV is tricky. I have not seen any recommended procedure for this as well which is surprising esp from manufacturers because 737s just dont descend. So what are the hard and fast rules? Greatly appreciate if someone can list one. Mental calculation is there that multiplication of three but it can be sloppy and misguided because we are not getting the exact track miles all the time.
22nd May 2018, 22:57

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Originally Posted by ImbracableCrunk
If you are on base and are in HDG SEL, the VNAV PATH will show you as high, correcting as you approach the final. If you select LNAV at the same location, you will get an accurate indication on your path.

Alternately you can simply select DIR to the FAF or
Simply put the FAF/Crossing Alt in the DESC page to get a Vertical Bearing.
Hello,

Is it possible that you describe elaborately on the second paragraph. VNAV already calculating direct to FAF when selecting INTERCEPT TO FAF in FMC LEGS page from my understanding.
23rd May 2018, 08:01

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Originally Posted by sonicbum
Agree. By arming LNAV it goes back to proper calculations.
This statement is correct.
However you must be aware that arming LNAV in such a situation will result in intercepting the final approach course. But sometimes we are not cleared for that as ATC may need us to cross the final track and intercept from the other side for separation purposes.
23rd May 2018, 08:29

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Why not just select VS and use your brain and the DME to achieve a smooth intercept of glide slope at the same time as the localiser? Just extend the centreline from the glideslipe intercept so the go around will be available in LNAV.

Or is there a good reason to be head down messing with the FMC at 4000 ft / 15 miles from touch down that I've somehow
missed in the last thirty years?
23rd May 2018, 13:05

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Originally Posted by excrab
Why not just select VS and use your brain and the DME to achieve a smooth intercept of glide slope at the same time as the localiser? Just extend the centreline from the glideslipe intercept so the go around will be available in LNAV.

Or is there a good reason to be head down messing with the FMC at 4000 ft / 15 miles from touch down that I've somehow
missed in the last thirty years?
Knew it was a matter of time before I get a policing comment such as this. The problem here isn't that I am looking for an alternative on how to descend without using my FMC but merely trying to understand my aircraft's FMC system more profoundly. And I see nothing wrong with that. There is something I am curious to know, couldn't find a reference to, therefore I asked. The FMC is there to serve a reason and is used to help, and not replace, the pilot; and for it to do so, I need to understand it fully. Again, nothing wrong with that. If you have any input that would help me do so, I'd be eternally grateful.

And to alleviate any worry you might have, I do know how to use the V/S/LVL CHG and DME to adjust my profile during descent. But again, that was not what I was asking.
23rd May 2018, 21:22

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The 737 FMC user guide says over the adding ISA DEV: This has a negligible effect.
24th May 2018, 02:06

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I m sorry to disturb you; but you learn that during the TR ?
24th May 2018, 09:40

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100 above minimums

" The path is built from the lowest constraint upwards, assuming idle thrust"

That's a quote from one one of the Boeing 737 manuals...I'm not going to give you a reference, maybe whilst looking for it you might learn some other useful stuff, I'm assuming you work for an airline and have the manuals available to you.

Thinking of an example, if you have a long SID and fifty miles to the FAF which is probably the lowest altitude constraint, then you will be at an altitude which matches that distance for an idle path descent, if your programming on the descent forecast page is accurate. If you suddenly get a short cut and put DTO the FAF and now have thirty miles to go instead of fifty then it's going to tell you you're high, because you are.

It's all in the manuals, try reading and learning instead of asking here, or ask the next training captain/check airman you get to fly with, or email your training department. Unfortunately you can never be sure that pprune users are even real pilots nowadays, I could have just made all of that up.
24th May 2018, 09:45

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keep an eye on your track miles to go, even ask air traffic, X that by 3 for a rough height you should be, keep an eye on the range of the aircraft ahead, if on the same profile never let him get less than 500 below you, you'll be too close.
http://www.b737.org.uk/
24th May 2018, 09:54

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Originally Posted by 100 above minimums
1) In the VNAV descent forecast page, it says: "Enter the average ISA deviation for descent in °C (+/–XX°C) or °F (+/–XX°F)". What is meant by that? How does one calculate the average ISA DEV for the entire descent? At my outfit, the normal is that we'd put the destination's ISA dev and some others opt for putting in the average of ISA DEV at cruise and ISA DEV at destination.
The average ISA deviation has no effect on descent profile unless you operate in extreme latitudes.
Average ISA cannot be determined unless you can verify ISA at multiple altitudes during your descent.
If you have ISA deviation at say every 10,000' then you can average the TOD and intermediate ones and find their average.

As mentioned above and by some proviso posters, the average ISA in DESC page in FMC has little to no impact unless you operate in atmospheres of extremities.
Think of -10C and +15C average during descent, not at landing but during descent.
If you do not operate in these conditions, there is no need to use it.