I'm afraid we have an issue with the VNAV logic, and it needs lots of love.

In this picture, on the EWR-to-JFK flight, I have 5000 set for cruise. Note the T/C before the TEB wpt; I manually entered a restriction TEB 210/3000, and it returned an invalid entry. That entry should be accepted as typed and move the T/C after TEB. Not working well.

On a second try I changed cruise altitude to 9,000 and added a new WPT (TEB02) before TEB with a restriction TEB02 210/3000. Now it took the restriction, but see the legs page.

But now we have 4 problems related to the VNAV logic.

1. The UNABLE CRZ ALT should not be triggered. We have a T/C and a T/D.

2. At TEB, it shows correct acceleration and crossing at 218/4420 in climb to 9,000.

3. T/C symbol and computations are wrong; we cannot have the T/C there; it needs to be passed by TEB.

4. At wpt TEB01, we should be at 250/9000, NOT at the value we have in the legs page.

Thanks.

Do the predictions change to show correctly when airborne?
Have you have raised this with iFly support?

Good findings @LRBS ...

Now that my desktop has crashed for good and went for repair (( although I still think the culprit was Win11 latest update... because other desktops and laptops of users I know from work are suffering from similar symptoms... )) I can't even do what I would rush doing, just for the sake of having the satisfaction of uninstalling one or both of them 🤣after trying a similar scenario in the PMDG 737...

One of the most - well the most advanced ever flight simulator I was able to use so far is Aerowinx PSX. From time to time rw 744 pilots would comment on "deviations" from reality experienced under some rather weird scenarios... and Hardy would promptly cure them :-) with a lightspeed patch.

Edited August 1Aug 1 by jcomm

13 hours ago, cowpatz said:

Do the predictions change to show correctly when airborne?
Have you have raised this with iFly support?

Excellent question. And it is true: many times, when at the gate (not at the runway threshold), and if it is a tight crossing restriction, VNAV will compute from that position and might give you UNABLE NEXT ALT. When taxing/arriving at the threshold point and pushing TOGA, VNAV logic will sample that position and update accordingly. We are on the same page here.

Now let's pay close attention to the predictions on the legs page for TEB2; it is over 4,000 ft, and the restriction is only 3,000 ft, less than the prediction; there is no way to get the UNABLE CRZ ALT or UNABLE NEXT ALT.

And the nasty one is at TEB01; instead of being at CRZ ALT of 9,000, it shows 3,000 after crossing TEB, climbing through 4,420 ft, and then descending back to 3,000 ft.

That's why I'm saying the whole VNAV logic needs attention; something in the coding is off. 

Hhmm, Maybe this is why some SOP does not allow VNAV ARM before 800 feet?

Michael Moe

32 minutes ago, Michael Moe said:

Hhmm, Maybe this is why some SOP does not allow VNAV ARM before 800 feet?

Michael Moe

Some SOPs are quite different, but system-wise, after takeoff, if VNAV is armed (some do and others don't), VNAV automatically engages at 400 feet AGL. It has nothing to do with the logic in our case. 

33 minutes ago, LRBS said:

Some SOPs are quite different, but system-wise, after takeoff, if VNAV is armed (some do and others don't), VNAV automatically engages at 400 feet AGL. It has nothing to do with the logic in our case. 

Thanks , always a pleasure to understand some of the basics and its differenties :-)

Is there btw a timeline (or FMC version etc) for LNAV/VNAV ARM on ground ?

Michael Moe

Edited August 1Aug 1 by Michael Moe

1 hour ago, Michael Moe said:

Thanks , always a pleasure to understand some of the basics and its differenties :-)

Is there btw a timeline (or FMC version etc) for LNAV/VNAV ARM on ground ?

Michael Moe

You are welcome.

I don't know; it's very difficult to give a correct answer. What I noticed first is the airline's SOP, and next is the version. For example, on the 747, 757, 767, 777, and 787, we had LNAV/VNAV armed on the ground. Today, with so many 737 variants, the answer can be very different. Some don't; what is interesting is that new airlines, even if the aircraft is capable, don't arm it. Some on the 757 and 767 swear not to arm it.

I'm sorry, I don't have a good answer.