This is probably me not doing something quite right, but it has happened three times now, so I thought that I would mention it. Basically, I am on final using an RNAV approach, and VNAV PTH suddenly switches to VNAV ALT, and there is a surge in engine power. Now, at this point my approach speed is perfectly normal (and the barber poles are miles away on the FMA). It is not something that I have experienced in the past, but three times in recent weeks is making me wonder if I am missing something. It has nothing to do with the MCP altitude setting, as I have that set to 1000 feet (I am not interested in go arounds, particularly when I have no AI planes actually flying in the simulator), and I generally disconnect the autothrottle and autopilot at around 1500 feet for a manual landing.

So, am I doing something wrong?

When the power increases does the plane keep descending or level off? 

What is the FMS planned vnav speed for current descent? (PROG PAGE). 

When winds and conditions are not as expected the plane may have descended too quickly so it certainly may increase thrust and slow descent or even level off while it is recovering forecast speed. 

PMDG may have changed the logic recently. Have you checked with them. 

So many variables at play here you might need to make a video of it happening. 

Btw even real pilots are flumoxed by 737 vnav behaviour sometimes! 

Here's a example. 

https://www.pprune.org/tech-log/530411-737ng-vnav-path-question-new-s-n.html

 

3 hours ago, Christopher Low said:

but it has happened three times now

same approach ...... 

Three different approaches.

Hi mate,

real world 737ng pilot here, on a rnav approach going into vnav alt isnt what you want, the aircraft always has to stay on path otherwise your workload increases 10 fold. It,s all got to do with your MCP and forcing the A/C to level off when you've got further decent to go.

Now what i think your doing is setting 1000ft and leaving it when crossing the FAF hence it'll go vnav alt right away.. just before the FAF you'll set your MCP altitude rounded up to  minimums e.g. set 600 ft on the MCP. if your minimums is 580 or so or 400 ft if your minimums are 390ft. That should in theory keep the A/C descending in VNAV path, then where you're around 300ft below MAA set your missed approach altitude in your mcp and your A/C will continue descending in vnav path

Edited April 27, 20251 yr by Bilal2104

OK, so as an example the approach that I conducted this morning was the RNP 08 approach at ESSA Stockholm Arlanda. Based on the Navigraph charts, I assume that the FAF for this approach is SA747, which is at a mandatory altitude of 2500 feet @ 7.4 nautical miles from the runway threshold? I had already set the MCP altitude to 1000 feet by this point, but why would it switch to VNAV ALT when I was nowhere near that? Why would the aircraft not keep descending in VNAV PTH mode until it reaches that 1000 feet?

Edited April 27, 20251 yr by Christopher Low

i've just pulled my plates up.

You are correct with the FAF distance and platform altitude.

I presume the a/c has all been setup correctly in the fmc?. also set 600ft 2nm before the faf and see what it does and report back? does it stay in path or revert to Alt.

Chris, I just checked on my end with the PMDG738 that approach. I don't experience your issue, IRL, while on vectors to intercept, and established, we set the MCP to minimums (other operators) set the rwy elevation (in this case to 108, round up to 200), and VNAV does its magic. In this way, you would be on VANV PTH. Usually, we set the missed approach alt,  300 FT below the crossing alt at the FAF to prevent VNAV ALT capture. This case is different. You took that arbitrary 1,000 FT, which basically restricts the airplane. Now, this particular approach is different because of the initial go-around altitude, which is lower than the FAF crossing altitude, and if you don't pay attention to how you set it, the airplane might level off at 1,500 in VNAV ALT. Anyway, try this: after you are established, set the MCP to the minimums. Passed FAX, while descending on VNAV PTH, 300 ft below 1,500 ft (1,200 ft), set your MCP to 1,500 ft, which is your initial missed approach altitude. That will fix your problem. Perhaps their ATC minimum safe vectoring altitude is 1,500, which we don't know, but in real life, without ATC, we would climb to 2,500 FT, which is also above the MSA. Some airport procedures are quite interesting. 

 

I will try again, but I am still not sure that I understand why it would be any different. If I set my MCP altitude to 1500 feet when I am at 1200 feet, that would be after the aircraft had switched to VNAV ALT mode in my previous flight. Are you saying that setting the MCP altitude to minimums (rather than 1000 feet) after getting established on the approach path will make all the difference??

Edited April 27, 20251 yr by Christopher Low

36 minutes ago, Christopher Low said:

I will try again, but I am still not sure that I understand why it would be any different. If I set my MCP altitude to 1500 feet when I am at 1200 feet, that would be after the aircraft had switched to VNAV ALT mode in my previous flight. Are you saying that setting the MCP altitude to minimums (rather than 1000 feet) after getting established on the approach path will make all the difference??

Yes, I don't have the 736 to verify it, but I will reach out to someone who does. 

44 minutes ago, LRBS said:

other operators) set the rwy elevation (in this case to 108, round up to 200)

Just to note, if you're setting field elevation in the MCP, that's the one time you round down instead of up.  So 100ft in this case.

As far as the OP issue, easiest thing to do to determine if this is MCP altitude related is just to set the MCP altitude to zero before your top of descent and see what happens.  If it still reverts to VNAV ALT at any point, you've found a bug.

31 minutes ago, Stearmandriver said:

Just to note, if you're setting field elevation in the MCP, that's the one time you round down instead of up.

Not to argue, our SOP, we will round up to the next value of the RWY elevation, not the field elevation. Different airlines and training departments. Some don't do it this way, and others prefer to reinvent the wheel. It's interesting to see how airlines employ different Standard Operating Procedures (SOPs). 

Had the FSX version of the PMDG 737 so not certain about the 2020 variety but I seem to remember reading somewhere that RNAV isn't modeled in the aircraft?

Well, somewhat annoyingly my second test flight with the same parameters as last time (MCP altitude reset to 1000 feet before T/D) resulted in a flawless RNP 08 approach! I guess that something must have happened on the first flight that was different to this one, but I have no idea what that could have been.

I will just have to wait to see if it happens again on future flights, and try to isolate the reason why. Thanks for all of the suggestions, guys 👍

Edited April 27, 20251 yr by Christopher Low

1 hour ago, speedyTC said:

Had the FSX version of the PMDG 737 so not certain about the 2020 variety but I seem to remember reading somewhere that RNAV isn't modeled in the aircraft?

Oh certainly LNAV / VNAV / ability to conduct RNAV procedures is modeled and has been since FSX.  You're probably thinking of specific RF leg ability, which is only a recent addition (doesn't work great, but at least it's there now.)

1 hour ago, LRBS said:

Not to argue, our SOP, we will round up to the next value of the RWY elevation, not the field elevation. Different airlines and training departments. Some don't do it this way, and others prefer to reinvent the wheel. It's interesting to see how airlines employ different Standard Operating Procedures (SOPs). 

Yeah, interesting!  Our rationale is to maintain stable flight guidance all the way to the runway, so we take the TDZE and round down.  I suspect the HGS may influence our decision in this; normally, it doesn't matter what the flight guidance does 100ft above the runway as a crew is visual by then, so I could see sticking with the standard of rounding up as we usually do with MCP altitudes.  But for a crew flying the HGS, it can be distracting to see the guidance cue all of a sudden jump up off the runway when in close, if it commands a level off.  I've seen crews reflexively start to follow it, especially captains that are new to a HUD.  We hammer them so hard to remain in the flight guidance and especially to follow the flare cue during an AIII that the suddenly rising guidance cue can cause a momentary reflexive back pressure.  Given the short and often contaminated runways we're doing these approaches to, any minor destabilization in close isn't great. 

Or it was someone's arbitrary decision 30 years ago.  That's... Equally possible.  😁