VNAV PTH Descent

[Guest]

Good Evening,

Having a play around today I noticed during descent VNAV PTH doesn't always command Autothrottle to RETARD then ARM and instead uses FMC SPD despite being on Path and Speed.

FMC SPD should usually annunciate as a thrust mode when the aircraft speed drops around 10kts or so to accelerate the aircraft to it's planned descent speed. The Autothrottle mode should then (see attached picture) return to RETARD then ARM when the aircraft speed is equal to the target speed.

I tried recycling SPD INTV and reengaging VNAV, changing the CI to fly a higher speed, but it always defaults to FMC SPD and not ARM, despite flying the Target Airspeed. Is this a setting I can change or is this how PMDG is programmed?

Secondly I notice if you're high on profile during descent and you engage VNAV the aircraft defaults to VNAV SPD, not VNAV PTH. (The aircraft as configured by my company will revert to VNAV PTH and do a bit of a dirty dive to recapture the profile whilst not caring about speed) Could any of the NG operators here confirm is this a configuration I'm unfamiliar with or is just the way the PMDG NGX behaves?

Either way it's workable! Would just like it to work to what I'm used to! 

Thanks in advance! 

 

 

[downscc 1,327]

I don't have the real world experience you are look for but FCOM 4.10.6 "autothrottle holds idle but can command FMC SPD mode if ground speed becomes too low to maintain FMC vertical path."  I've noticed when on VNAV PTH in the flight levels where we 280-310 KIAS it does just as you say and ends up in ARM.  You didn't show your engine instruments but I think you'll find the FF is a little higher than flight idle.  Regardless, I'm not aware of anything in the FCOM that says this is wrong.

[Captain_Al 66]

I am no expert in the 737 at all, more familiar with the 747, 757, 767, and 787, but the 747-8 does an off idle descent and does not show IDLE and HOLD, but stays in SPD for the A/T. This is different than the 747-400. Maybe the NG does the same thing, but just a guess since I am not rated on it like I am the others.

[Guest]

Forgot to mention, I tried playing around with the wind as well, I put a stonking tailwind to increase the ground speed and it still behaved the same way. I tired several different descent speeds too. 

Certainly in the real aircraft if you're on Path the system will revert to RETARD - ARM, unless you have a VERY strong headwind which will significantly lower your ground speed.

In this example the thrust was constantly changing to maintain FMC SPD as it should do in this mode but expecting it to revert to RETARD - ARM when back on path.

[Guest]

Just now, Captain_Al said:

I am no expert in the 737 at all, more familiar with the 747, 757, 767, and 787, but the 747-8 does an off idle descent and does not show IDLE and HOLD, but stays in SPD for the A/T. This is different than the 747-400. Maybe the NG does the same thing, but just a guess since I am not rated on it like I am the others.

Will it add thrust when you get below your target speed straight away? The NG will add thrust when speed drops around 10kts below to accelerate the aircraft to it's planned descent speed.

[Captain_Al 66]

No, the NG sounds more like the 400 then, the -8 always stays in SPD in an off idle descent, never goes to IDLE and HOLD, like most Boeing jets do. It will go from SPD | VNAV PTH at cruise to the same FMA's after crossing the T/D, so the NG is just like the 400, different FMA's but sounds operationally the same.

[JoeDiamond 67]

2 hours ago, flightdeck2sim said:

Secondly I notice if you're high on profile during descent and you engage VNAV the aircraft defaults to VNAV SPD, not VNAV PTH. (The aircraft as configured by my company will revert to VNAV PTH and do a bit of a dirty dive to recapture the profile whilst not caring about speed) Could any of the NG operators here confirm is this a configuration I'm unfamiliar with or is just the way the PMDG NGX behaves?

This is correct behavior.  If you are above the path VNAV will revert to VNAV SPD and the A/T will retard to idle then go to arm.  Once the aircraft is back on the path it will revert back to VNAV PATH.

[Guest]

28 minutes ago, JoeDiamond said:

This is correct behavior.  If you are above the path VNAV will revert to VNAV SPD and the A/T will retard to idle then go to arm.  Once the aircraft is back on the path it will revert back to VNAV PATH.

Interesting to see how it varies from operator to operator. When high on profile and you select VNAV our fleet goes to VNAV PTH, you get the message drag required and then a dirty dive for the profile which you have to keep your eye on it because it will happily put you towards VMO/MMO! Referring to that VNAV ALT post I made last month I guess we have some sort of old school logic in our FMC!

With regards to my screenshot would you expect it to revert to -RETARD - ARM?

I haven't flown for about two months so it's hard to recall! I'm doing about 80 hours a month in the sim. The only time I'm above 10,000ft is to practice Rapid Depressurization/Stalls and Mach Buffet! 

Cheers Joe!

[Green parks 24]

44 minutes ago, flightdeck2sim said:

Interesting to see how it varies from operator to operator. When high on profile and you select VNAV our fleet goes to VNAV PTH, you get the message drag required and then a dirty dive for the profile which you have to keep your eye on it because it will happily put you towards VMO/MMO! 

Cheers Joe!

From what I've read, here there and everywhere, that's a common trait of the NG's in VNAV descent. Pretty normal. 

[JoeDiamond 67]

2 hours ago, flightdeck2sim said:

With regards to my screenshot would you expect it to revert to -RETARD - ARM?

It depends on where you are on the VNAV path.  If it is on an idle descent segment then you would get RETARD, followed by ARM.  If it's on a geo path then you will get FMC SPD.  There isn't enough information in the screenshot to tell if there were any previous VNAV constraints that would have put it into geo path.

This is based on U10.8A and U12.0.

[downscc 1,327]

14 hours ago, JoeDiamond said:

If it is on an idle descent segment then you would get RETARD, followed by ARM.  If it's on a geo path then you will get FMC SPD. 

Exactly, I was looking for this in the FCOM but this is exactly what the developer told me a long time ago.  I think OP entered the geometric path when passing the 250 KIAS constraint.

[Guest]

On 3/17/2017 at 0:43 AM, JoeDiamond said:

It depends on where you are on the VNAV path.  If it is on an idle descent segment then you would get RETARD, followed by ARM.  If it's on a geo path then you will get FMC SPD.  There isn't enough information in the screenshot to tell if there were any previous VNAV constraints that would have put it into geo path.

This is based on U10.8A and U12.0.

 

10 hours ago, downscc said:

Exactly, I was looking for this in the FCOM but this is exactly what the developer told me a long time ago.  I think OP entered the geometric path when passing the 250 KIAS constraint.

I think I've found out what's going on, we believe (after discussing with colleagues with which I need to confirm with engineers) the VNAV geometric path descent option has been deselected as an option. it only become active (with our fleet) when the FMC transitions to on Approach logic. Up to this point idle descent segment is planned for the entire descent and will command a level segment after approaching an altitude restriction and stay in VNAV PTH until reaching the next TOD point. We have U10.7 and U12.0 on the newer aircraft and they both behave the same way.

I had a good play in the sim today and in VNAV PTH I could only get FMC SPD if low on profile and/or slow on speed regardless of any descent altitude restriction in the FMC. As soon as I was on the VNAV profile with VNAV PTH engaged I would get ARM/roll mode/VNAV PTH.

With regards to this screenshot which is a descent into Alicante (LEAL) I had the following descent restrictions. 

@VILNA (the active waypoint) FL70A @220kts

@D290T 5900A

@FI10 - 3300A

I've flown into this airport more times than I've had hot dinners and I've never seen VNAV behave the way it does like in PMDG. I'm 100% not saying it's wrong, I'm not used to the logic behaving like that and I've not seen it do that for the last 4000 hours! I'm not 100% sure myself and will confirm after talking to the engineers! This must be an option selected from the operator regarding geometric descent which has changed the VNAV PTH logic compared to the way you're describing to me!

This is the only reference I have from our FCOM explaining when VNAV PTH will change to VNAV SPD during the descent phase.

"when descending in VNAV PTH and the airplane speed is less than the descent target speed by a customised threshold amount loaded in the Loadable Defaults Database, the FMC will transition to FMC SPD as the thrust mode. The A/T mode will return to RETARD/ARM as soon as the airplane speed is equal to target speed and on Path.

I'm guessing because we don't have the option of a geometric path during descent I'm expecting it to behave this way every time and I was surprised it didn't return to RETARD/ARM as per the screenshot. 

I guess my operator didn't tick every box in the options list.....

Thanks for the info anyway, learn something new everyday! 

 

[jaguara5 1]

Look here

http://www.pprune.org/tech-log/382263-737ng-vnav-question-geometric-idle-descent.html

 

Gerasimos Stamatatos

 

 

[Guest]

Hi Gerasimos,

Thank you very much for sharing, although the post is 8 years old it points towards the fact you can change the GEO DES in the FMC to either FULL/APP pre-flight and I'll hopefully be able to confirm later! Looks like it's buried in the MAINT section of the FMC which an engineer told me once not to touch, pilots usually end up breaking things (Joe will vouch for me on that one )

Thanks Joe and Dan for letting me know about how the system works anyway and hopefully it's something they can make an option to change with PMDG in the future! 

[Fabo 213]

You can use forum search feature also to find posts from Rob or Ryan discussing the decision to incorporate Geometric mode.