B737 Descent Questions

[100 above minimums]

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.

[latetonite]

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.

[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.

[100 above minimums]

Thank you for your inputs.
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?

[ImbracableCrunk]

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)

[Valdarez]

Thank you for your inputs.
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

[sonicbum]

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.

[flyburg]

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!!

[100 above minimums]

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?

[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.

[carnival30]

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.

[carnival30]

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.

[poldek77]

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.

[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?

[100 above minimums]

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.

[D-OCHO]

The 737 FMC user guide says over the adding ISA DEV: This has a negligible effect.

[Piloto Maluco]

I m sorry to disturb you; but you learn that during the TR ?

[excrab]

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.

[memories of px]

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/

[Skyjob]

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.

[100 above minimums]

Thank you for all your inputs, appreciate it.

@excrab

Thank you for trying but again, off topic. What you explained is quite basically what would happen if you get a shortcut on a STAR, something I'm familiar with already. Look, this is the scenario I am talking about for example: you're 10 miles abeam the runway on a downwind when you do a Direct Intercept course to the FI. In this case, you're on Heading Select and your track is not even intercepting the active flight plan and you're not flying to any waypoint. So, at this point, the descent profile is giving you indications of too high/too low based on how many track miles when you are not really flying on, to, intercepting the active flight plan? Hope this makes my question more clearer.

It is correct, I do work for an airline with manuals at my disposal, thankfully. And while I also have other people who I could ask, I do consider PPRUNE as a valuable resource since there are too many colleagues here with far more experience than I have and I see no shame in asking them. And my ego does allow me to learn from someone with less experience or even not a pilot AS LONG AS he/she is able to provide a reference for what they say, something repeatedly asked for.

[ImbracableCrunk]

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.

What's wrong with asking here? That's kinda the point of this thread. You don't like the thread, don't read it, crankypants.

[100 above minimums]

It is indeed very ironic for him to say that one should not ask for help on PPRuNe while he himself has a thread started looking for someone to lend him a hand. I'm sure someone ought to have taught him to "practice what you preach" in his 30 years of flying.

[sonicbum]

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.

I'm discussing the technical side of it, not techniques/SOPs.

[RVF750]

Generally, using VNAV all the way and keeping your head inside while being vectored is a depressing consequence of the modern way of teaching. You never see a chap or lass who has come up via turboprops, instructing or GA who do this. Because they can actually fly an aeroplane, not a computer game.

As stated above, the 737 FMC does it's calculations in reverse and once under vectors and human guidance from the ground, it isn't really that good at it. The further you deviate from the magenta line, the worse it gets, but constantly tapping away is just poor airmanship. Intuition and common sense should suffice, but it doesn't seem to anymore.Finally,V/S and LVL CHG are mutually exclusive. Yo can have one or the other.......

[RAT 5]

but constantly tapping away is just poor airmanship. Intuition and common sense should suffice, but it doesn't seem to anymore.

Indeed; and as most airfields have a DME on site one would hope a competent pilot wold be able to make a reasonable calculation about the glide path proximity when off the magenta line. But as you say "it doesn't seem to anymore." One reason could be that some operators insist/SOP on LNAV/VNAV update to the OM at all times when off the STAR. i.e. the pilot has been discouraged to use their brains and so like other piloting skills it has become decrepit. I saw it many times, when VNAV gave some nonsense about path error, that the F/O sat there wondering questioning how this could be so, but doing nothing. There were occasions where it said we were high by quite a bit and their hand was reaching for the speed brake, only for it to be refused. "Yes but, it says......." Have a look at the altimeter & DME and think again."...."Yes but, how can it say that????" ......"No idea, but it ain't correct is it." ....."Oh yeah."

[flyburg]

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?

very interesting, tried it out, indeed when programming a extended centerline vnav shows a picture as if you where on that centerline, often depicting you high as you are probably still on a base leg(this I knew). When arming lnav it gives you an accurate picture! Going back to hdg select it goes back to the original picture! I did not know this behavior! Couldn’t find anything in the FCOM referencing this (and interested if anybody does have a reference) but it makes sense!

mind you, when arming lnav make sure the wings are level, it then continues on the heading until intercept, if you arm Lnav when in a turn to a specific hdg it starts doing it’s own thing!

learn something interesting from this board all the time.

p.s. Brought up with small aircraft and turboprops and always check all available means to cross check alt v distance but interesting nonetheless!!

[RAT 5]

mind you, when arming lnav make sure the wings are level, it then continues on the heading until intercept, if you arm Lnav when in a turn to a specific hdg it starts doing it’s own thing!

It will go on to a 45 degree intercept. As you say, if on ATC HDG's wings level then LNAV to intercept the centreline will keep the ATC HDG.