One possibility is in the small print of FCOM1, page SP4.1. If you change the target altitude after engageing Level Change, it will engage V/S mode:

 

Note: If a new MCP altitude is selected while in ALT ACQ, the AFDS engages in V/S and the existing vertical speed is maintained.

I'm fully aware of the effect that TAI will have on the descent, and the correct use of the Descent Forecast page. My query is with the fact that the aircraft's performance seems incorrect.

 

Screenshot...

 

Doesn't look like your at flight idle. Hit F1 and see if that changes idle and check throttle trim settings.

 

Are you starting from a saved flight?

Also, there's no need for A/I if you don't have any visible moisture.

Doesn't look like your at flight idle. Hit F1 and see if that changes idle and check throttle trim settings.

 

The throttles were definitely at idle and I did confirm this by pressing F1. Obviously the engines are spooled up to approach idle, not flight idle.

 

Also, there's no need for A/I if you don't have any visible moisture.

 

This was a test with 'clear skies' to prove that it's not weather related.

 

I'd be interested if somebody could post a screenshot of their -800 descending, in the same conditions (220KIAS, clean, LVL-CHG, passing 4000 with TAI on).

Hello, In my humble opinion your selected altitude and speed plus the descending altitude displayed of 4000ft are all wrong considering you are showing 44miles to go to the next waypoint in the ND. I feel you are too much ahead of where you are going. Richard Welsh.

I think that this picture is only for demonstration of problem and shouldn't be analyzed for pilot's errors. Right now I'm en route EPKT-LEIB (it's little to warm on Ibiza right now), but tomorrow morning (it's over 9pm here) I will fly that route back to EPKT and will check that for you, if no one else will till that time.

All of those things were already discussed in the past, sometimes it was related to the N1 issue, few of us experienced this issue with old ngx versions. I also told to the support that trying to forecast a full descent with TAI ON, the VNAV was unable (with clear sky) to mantain speeds without using spoilers. But they worked and hopefully fixed with sp1c, I tested it only once and it was better than the previous releases. I'll try that condition again in these next days and report if works.

The throttles were definitely at idle and I did confirm this by pressing F1. Obviously the engines are spooled up to approach idle, not flight idle.

 

36.9% N1 is too high for idle at that altitude. There could be several things that cause it, but that is the "what." I'd expect to see 36.9 in the mid 30's.

The throttles were definitely at idle and I did confirm this by pressing F1. Obviously the engines are spooled up to approach idle, not flight idle

 

The throttles should be flight idle, not approach idle in this setup. Approach idle only comes in to play after gear down, or I think flaps 15 (not sure on this one). Although, 36.9% N1 does sound about right to me for idle at 4,500' MSL and TAI on. I usually see around 36% N1 on my descent to the final cleared ILS altitude without TAI.

 

Eric Szczesniak

The throttles were definitely at idle and I did confirm this by pressing F1. Obviously the engines are spooled up to approach idle, not flight idle.

 

 

 

This was a test with 'clear skies' to prove that it's not weather related.

 

I'd be interested if somebody could post a screenshot of their -800 descending, in the same conditions (220KIAS, clean, LVL-CHG, passing 4000 with TAI on).

 

I just tried it - it descended at 500fpm throughout the entire descent from 6,000ft to 3,000ft.

 

The throttles should be flight idle, not approach idle in this setup. Approach idle only comes in to play after gear down, or I think flaps 15 (not sure on this one). Although, 36.9% N1 does sound about right to me for idle at 4,500' MSL and TAI on. I usually see around 36% N1 on my descent to the final cleared ILS altitude without TAI.

 

Eric Szczesniak

There are 3 idles, one for ground, 2 for air.

As the TAI on will give an higher idle, flight idle with TAI on will change in approach idle. In that condition extending gears or flap will not increase idle anymore.

Is it due to the fact that as you on LVL CHG the r.o.d is adjusted to match your selected speed?

Is it due to the fact that as you on LVL CHG the r.o.d is adjusted to match your selected speed?

 

Well yes, but LVL CHG will always give the maximum rate of descent that won't result in an increase of airspeed.

I'm fully aware of the effect that TAI will have on the descent, and the correct use of the Descent Forecast page.

This is the internet so the only thing we know about you is what you tell us. You have told us that you either dont read or dont respect the forum rules (all posts require your full name). You have told us that you are not familiar with, or do not respect, standard operating procedures (If you always fly with clear skies, then you never need AI). You have told us that you had read the small print (but only after it was pointed out to you).

 

So, lets assume you do know that LVL-CHNG works by using pitch to maintain IAS at target speed. So what will happen if your IAS is above target? As yours is according to your screen shot?