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ConcordeAlpha
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PMDG 747-400 Autothrottle in VNAV

Tue Jan 23, 2018 3:15 am

Hi all,

I was just conducting a flight from RJCC to RJTT (not that it's really relevant) in P3D v4, and as I reached my top of descent, I was beginning to descend via VNAV. All of a sudden, my throttles changed to 50 (or so) percent N1. I checked to see what my flight-stick throttle was set to and it was exactly the same. To see if the autothrottle had somehow disconnected without me knowing (no alarms, nothing on the recall), I slightly adjusted my flight-stick throttle forward and discovered that I had full control of the throttles. I then disconnected the autothrottle entirely and then re-engaged it, and the 'problem' persisted (the problem being that I had full control of the throttles when in speed intervene, and the autothrottle was on). I don't know if being in speed intervene had anything to do with it, but when I switched to vertical speed (disconnecting vnav), the autothrottle kicked in and took back control of the throttles. I then re-engaged VNAV and it still wouldn't take control of the throttles. I have a feeling I have a keybind that is connected to the autothrottle that I don't know about, but this 'problem' has only been happening for the last few weeks. I'm flying from Sydney to Dubai in this aircraft tomorrow and I have been looking for a solution for a few days now. At first I thought it was just a simple mistake I had made and that it probably wouldn't happen the next flight considering i'm getting more used to the aircraft with each time I fly it, but I guess this isn't the case.

Thanks in advance.
 
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pilotkev1
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Re: PMDG 747-400 Autothrottle in VNAV

Tue Jan 23, 2018 4:03 am

If you look on the PFD it should show 'HOLD' on your autothrottle control mode. This mode tells the pilot to 'HOLD' whatever setting was last commanded by the Autothrottle (or whatever power setting adjust their descent).. On the PMDG 747 this effectively disengages authothrust, as of course your throttles aren't motorized. No worries though, because as you'll notice as you begin to level off at either a VNAV restriction or your MCP altitude (whichever's higher), or intercept a shallow VNAV path, the autothrust should revert to 'SPEED' mode.

Same lack of comprehension of the autothrottle control logic brought down OZ214 I'm afraid.

You'll also see 'HOLD' appear on your PFD as you accelerate past 80kts on the takeoff roll. The logic behind this is, you don't want your autothrottles to be overriding either a 'firewall' or idle thrust command in the event of a caution/warning item above/below V1 respectively.
- Kevin Boydston
 
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ConcordeAlpha
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Re: PMDG 747-400 Autothrottle in VNAV

Tue Jan 23, 2018 5:07 am

Alright, thanks for the reply, simple to understand now that I think about it.

The strange thing is, i've never seen this until recently, and i've been flying the aircraft for about six months now.
 
BoeingGuy
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Re: PMDG 747-400 Autothrottle in VNAV

Tue Jan 23, 2018 5:22 am

pilotkev1 wrote:
If you look on the PFD it should show 'HOLD' on your autothrottle control mode. This mode tells the pilot to 'HOLD' whatever setting was last commanded by the Autothrottle (or whatever power setting adjust their descent).. On the PMDG 747 this effectively disengages authothrust, as of course your throttles aren't motorized. No worries though, because as you'll notice as you begin to level off at either a VNAV restriction or your MCP altitude (whichever's higher), or intercept a shallow VNAV path, the autothrust should revert to 'SPEED' mode.

Same lack of comprehension of the autothrottle control logic brought down OZ214 I'm afraid.

You'll also see 'HOLD' appear on your PFD as you accelerate past 80kts on the takeoff roll. The logic behind this is, you don't want your autothrottles to be overriding either a 'firewall' or idle thrust command in the event of a caution/warning item above/below V1 respectively.


OZ selected an inappropriate Autoflight mode in the first place. They should never have been in HOLD. The accident was caused by their inattentiveness and lack of situational awareness.

Caution messages are inhibited at 80 knots and Warnings at V1, although your comment was correct in concept. You don’t reject above V1 unless the airplane in unsble to fly safely. There’s another reason why the Autothrottle goes into HOLD at 80 knots. So there no chance of any kind of system anomaly or failure moving the thrust levers at a critical time during takeoff.
 
Redbellyguppy
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Re: PMDG 747-400 Autothrottle in VNAV

Tue Jan 23, 2018 7:44 am

At top of descent, in vnav path, you will be in an idle descent so your autothrottles should go to idle...?
 
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ConcordeAlpha
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Re: PMDG 747-400 Autothrottle in VNAV

Tue Jan 23, 2018 10:37 am

Redbellyguppy wrote:
At top of descent, in vnav path, you will be in an idle descent so your autothrottles should go to idle...?


So why did it tell me to hold the throttle when it was at 50% N1?
 
Max Q
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Re: PMDG 747-400 Autothrottle in VNAV

Tue Jan 23, 2018 12:10 pm

Not sure about the 744 but on the 767 if you select ‘Descend now’ prior to a normal idle descent the autothrottles will reduce thrust to maintain a 1000fpm descent and then go into ‘hold’ where they are effectively declutched and can be freely adjusted.


The FMA is not telling you to ‘hold the throttles’ it’s telling you the throttles are in ‘hold’ or disengaged prior to the next mode
Change.


When the VNAV path is reached the autthrottles will go to idle and the aircraft will pitch down to follow the VNAV path
The best contribution to safety is a competent Pilot.
 
Max Q
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Re: PMDG 747-400 Autothrottle in VNAV

Tue Jan 23, 2018 12:15 pm

Max Q wrote:
Not sure about the 744 but on the 767 if you select ‘Descend now’ prior to a normal idle descent the autothrottles will reduce thrust to maintain a 1000fpm descent and then go into ‘hold’ where they are effectively declutched and can be freely adjusted.


The FMA is not telling you to ‘hold the throttles’ it’s telling you the throttles are in ‘hold’ or in standby prior to the next mode
Change.


When the VNAV path is reached the autthrottles will go to idle and the aircraft will pitch down to follow the VNAV path
The best contribution to safety is a competent Pilot.
 
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SaveFerris
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Re: PMDG 747-400 Autothrottle in VNAV

Tue Jan 23, 2018 2:41 pm

Max Q wrote:
Not sure about the 744 but on the 767 if you select ‘Descend now’ prior to a normal idle descent the autothrottles will reduce thrust to maintain a 1000fpm descent and then go into ‘hold’ where they are effectively declutched and can be freely adjusted.


The FMA is not telling you to ‘hold the throttles’ it’s telling you the throttles are in ‘hold’ or disengaged prior to the next mode
Change.


When the VNAV path is reached the autthrottles will go to idle and the aircraft will pitch down to follow the VNAV path


The -400 has effectively the same logic except that the target decent rate is 1250 FPM (splitting hairs, I know).

Back to the OP’s post, I can’t comment on the PMDG programming but in the real airplane you will see the autothrottle FMA change from SPD to IDLE then finally end up at HOLD when you’ve reached the top of descent. Once you see HOLD the autothrottles are able to be repositioned by the pilot without reverting back to their original position. The key to HOLD is that the autothrottles will remain in HOLD until the speed drops to 15 knots below commanded at which point they will reactivate and go back to SPD in an attempt to bring the airspeed back up to target. The entire time this is happening the VNAV FMA should be VNAV PTH. Also, a caveat with this logic is this is assuming you are remaining in VNAV the entire time and beginning your descent at top of descent.

Hope this helps.
 
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ConcordeAlpha
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Re: PMDG 747-400 Autothrottle in VNAV

Tue Jan 23, 2018 9:26 pm

Thanks for the help, and yes, I did start my descent at the top of descent and remained in VNAV the entire time.
 
BoeingGuy
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Re: PMDG 747-400 Autothrottle in VNAV

Tue Jan 23, 2018 9:29 pm

ConcordeAlpha wrote:
Redbellyguppy wrote:
At top of descent, in vnav path, you will be in an idle descent so your autothrottles should go to idle...?


So why did it tell me to hold the throttle when it was at 50% N1?


You're both right. When the airplane starts the decent in VNAV PTH, the thrust levers well come back to IDLE. If the airplane is slightly ahead of speed and or altitude, the autothrottle will continue to drive the thrust levers back against the aft stop. This will cause it to go into HOLD.

Most often you are doing a VNAV PTH descent with the Autothrottle in HOLD. Sometimes in IDLE if the conditions are just right.

The another poster's point, it's doesn't "declutch" when you go into HOLD. The Autothrottle removes power from the Autothrottle Servo Motors.
 
Fabo
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Re: PMDG 747-400 Autothrottle in VNAV

Sun Feb 04, 2018 12:39 am

As per the original issue... not sure why this isn't in Aviation Hobby...

all the guys gave you (naturally) the correct explanation as per what happens in the real aircraft.

The difference here is the PMDG simulation is very good, but the hardware is not the real hardware. The devs know this, and have implemented autothrottle override modes, of which I suspect you run afoul... having the A/T override set to "In Hold mode" as opposed to "always" and "never".
Ergo what happens: A/T goes into idle, as normal when beginning VNAV descend. Switches to HOLD, at which point the simulator logic decided to invoke the override, bumping the throttle up to half advanced.
There are two possible reasons why you haven't seen this before (and, possibly, since)
1. You have thus far kept your levers at idle. This is likely if you just used TO/GA mode to advance throttles on takeoff. This time, maybe you did a manual spoolup...
2. The internal logic only reacts when there is a change in hardware throttle position. So either you bumped into the levers, moving them ever so slightly, or just happened to park them somewhere there was a little noise, just enough to register for the simulation as ever so slight movement of the lever.
The light at the end of tunnel turn out to be a lighted sing saying NO EXIT
 
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AirKevin
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Re: PMDG 747-400 Autothrottle in VNAV

Sun Feb 04, 2018 10:11 pm

Best way to avoid this happening is pulling your hardware throttles back to idle once you start your descent.
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