737 VNAV inaccuracy

[Pilot Pete]

Ok, when you set up the fmc for a VNAV descent, with reasonable descent fcst winds and LNAV descent restrictions, why is it that very often after a 'direct to' VNAV gets into a pickle? It often says you are 2000ft or so high or low, when using simple arithmetic you know you are on pofile?

I don't mean when you have put some very restrictive hard altitude that has been lost with the direct to, or unrealistic predicted winds, just when an update to the LNAV track causes VNAV to lose the plot. Any comments?

PP

[CaptainSandL]

Which version of FMC software are you using?

[Seat1APlease]

A "direct to" will alter the distance to fly, so the height profile will inevitably show higher than it did before as you have less distance to get rid of the height.

[BOAC]

PP - the FMC is really an 'advisory' system and works out its descent in its own way, with the LNAV constraints you mention. It is often not close to my mentally derived predicted descent path, but in deference I have always assumed it knows more than me - whether I let it carry on....................? It has often led me into uneconomic descents and sometimes (always on the VOR R35 PRN) abandoned me and left me to sort out its mistake.

In answer to to your 'implied' question, it is your choice whether you react to it - or even use it.:) It is not worth trying to outthink it. Best to 'monitor' it and be 'aware' of its advisories.

[Pilot Pete]

Guys thanks for the answers so far, maybe some misunderstand. I am completely aware of changes to the LNAV track adjusting the distance to go and hence altering my relative position with regards to the VNAV Path. My question is more about why such a change often screws up VNAV. For instance, an example would be with 60nm to go to touchdown, passing 18,000ft at about 270kts, with little wind component, I would say that that is 'in the ball park', but I have had VNAV tell me I am 2500ft off profile (high or low!).

It seems that the re-calculation once you have executed a 'direct-to' is what triggers this mis-calculation. I am NOT talking about the inevitable high or low on profile because you have lost or gained track mileage. I mean when the track mileage, speed, wind component, deceleration and everything else fit the passing Flight Level/ Altitude. FMC is U10.6.

in deference I have always assumed it knows more than me Not something I am prone to doing as I think, an FMC doesn't.;) sometimes ....... abandoned me and left me to sort out its mistake. These are the times I am asking about. I understand if it just can't do it due to a suddenly increased tailwind etc, but I mean when it really gets its knickers in a twist simply due to a forced re-calculation as the result of an LNAV update.

PP

[BOAC]

Not something I am prone to doing as I think, an FMC doesn't. - which is why I added "- whether I let it carry on....................".

As I said in my last para, I suggest you stop trying to 'understand it', use it as a useful information source/take note of what is tells you/and make up your own mind.:ok:

[Pilot Pete]

Thanks BOAC.

Anyone willing to help me understand it, as that was the thrust of my question? It would be nice to know what happens that causes it to malfunction in this way. I ask because I am shortly to become a trainer and understanding these things is important as I can't really just say to a new lad 'don't worry about understanding it'. I would like to be able to explain what is happening and why he should not put his blind faith in the VNAV profile (which many inexperienced pilots do on the line).

PP

[Jonty]

Not sure if yours is the same as ours but the key is the recalculation. The vnav path is like the Alt cap scenario. The computer comes up with a vnav decent profile at top of decent using the info available to it (current wind, its version of the standard atmosphere ect..) this is why you sometimes see it shifting as you approach TD, its recalculating the descent profile. Once your on the vnav decent it becomes fixed, hence the speed increasing or decreasing as it tries to keep its notional glide path. When you now put a direct to into the box it recalculates the vnav profile , again using its version of the standard atmosphere,current wind and the decent restrictions you put in the box. And so it can now say you are 2000ft high or low. Now weather that agrees with your mental model would be a topic of some discussion! ;)


Good luck with the course!

[flash2002]

10.6 is not good.

VNAV used to calculate idle descents but with 10.6 it only does this until the first altitude constraint. After the first altitude constraint it will calculate a descent with a constant angle (a non idle descent) instead of programming a level segment and then descending in idle.

If you fly level this will look to you as being high on the profile, while you know you are not.

You can work around it by programming a new cruise level in the crz page, dont forget to change the cruise speed and descent speed again, the fms will then recalculate a new idle TOD. And you are no longer to "high" on the profile.

Or just ignore it.

Tell your students to turn the meters indication (altitude) on. This will divide the altitude approx by 3. (only on the NG)
Since the height/3 + 10 is the distance in miles you will need for descent.
They can instantly compare the height to the trackmiles. (prog page)

[BOAC]

PP - congratulations and good luck. I really do think that the "be wary of it" line is the best you could offer. The only other 'simple' explanation I can offer is that there may have been some 'above/belows' in the original which then become irrelevant? Otherwise it is just unreliable black magic. :) If you get your trainees to keep their own mental plot on the profile you will have been very successful!

Edited to add: In my experience, NG and Classic, it doesn't even take a 'Direct to' to make it screw up the descent:). Even with correctly programmed winds etc it never quite takes the prize. In any case, let's face it, how often is your descent 'as planned' anyway? 3x +/- wins all.

If you want a laugh, ask your 'training' trainers to explain it.......................

[relax.jet]

U10.6 even U10.7 now is doing the same. If you have already passed first altitude restriction, it’s changing to “point to point” descent or constant angle descent if you wish.
Then, if you select “direct to” you will not get idle indication on your VNAV, I guess it will stay in some angle (not idle anymore).
I don’t think Boeing wanted it to perform like this. It looks to me like a bug in the software.

Also interesting. Check your speeds over some points with altitude restrictions and sometimes it looks like the “energy compensation” doesn’t work at all (too low speeds).
:confused:

[Pilot Pete]

Thanks relax jet. I was aware of the constant angle that you describe, but this is not the situation I refer to. I mean in an idle descent, just like in LVL CHG, when the 'computation' goes haywire and pushes out guff as a profile, which you clearly know is incorrect.

PP

[Rainboe]

This is a feature I have observed on most descents. You instinctively know when the descent information supplied is way off beam. Suddenly it just seems to go haywire and you get data that must be anything up to 7000' out. It is mystifying. Previously I flew the 747-400 with the same flight system. The descent data was consistently accurate and reliable, with hardly any incidences of faulty descent data. The 737NG descent information is largely a joke. So inaccurate at times that you can never depend on it at all.

[Pilot Pete]

Exactly Rainbow, that is the point I was enquiring about. Like you, I have flown other Boeings (75/76) which seemed nowhere near as prone to error (although they did get it wrong sometimes too). Just wondered if anyone knew why this happened so frequently on the 737. Obviously software related, but knowing what exactly it is would be useful when trying to explain to new guys the fallibility of the system, which many instinctively follow if they have little other experience.

PP

[relax.jet]

I wouldn’t say the VNAV is completely unreliable.
I can say just how I use it now. It is a bit unpractical when you don’t have enough time, and then forget it, but it work
Approaching any altitude restricted point, when I am sure we will make it, I just switch to LVL CHG or VS and I simply clear the first restriction, than you again have idle descent to next point.
To PP described problem. Maybe I didn’t notice it, because in “direct to” navigation, using the procedure described above, I didn’t have such a strange indication, like completely wrong VNAV.

Happy landings.

[stator vane]

our nanny state airline FORBIDS selecting meters!!!!
too much information will lead to flight crew flying into mountains and landing in the water!!!
oh my god, we're going to die----
but they do tell you to put a green circle at 10 miles out---so you won't forget the flaps!!!! and i think a green circle at 30 miles to know when you should be 250/10000 ft.
it drives me crazy----

VNAV is an advisor--i never use it directly except during climbs.

but nanny airline is starting a new VNAV descent and approach procedure that will especially apply to non precision approaches.

boeing help us---