This is a question that I have wanted to ask for some time....

If (for example), I reduce speed from 200 knots to 190 in the PMDG 737-600, it will frequently drop to around 185 before the autothrottle properly "kicks in" to return it back to 190. Is this accurate? I would have expected the autothrottle to keep a better handle on the speed that has been set on the MCP. I do not remember noticing this in the P3D version to such an extent, so is it something that is a bit "loose" in the MSFS version?

I think i saw that sometimes too, but I can't say whether that's accurate or not..

cheers 😉

Yes i noticed this as well  , also on the iFly MAX .

i dont know if wind gust is playing a role in the sim

Michael Moe 

Edited August 23, 2025Aug 23 by Michael Moe

I fly with hardly any wind, so it is not that. I am hoping that one of our resident real world 737 (or other) pilots will respond.

56 minutes ago, Christopher Low said:

I fly with hardly any wind, so it is not that. I am hoping that one of our resident real world 737 (or other) pilots will respond.

Wind is half the fun no?  Assuming you mean you don't do live weather?

5 hours ago, Christopher Low said:

I fly with hardly any wind, so it is not that. I am hoping that one of our resident real world 737 (or other) pilots will respond.

Before @Stearmandriver shares his expertise about the 738, I remember—if my memory is correct—if speed drops below 5 KTS in VNAV, you should see the THRUST REQUIRED message, but the A/T will not add thrust until 15 KTS below the UP speed (target). During the approach, the deviation is +10 or -5 KTS from the target. Another issue relates to poor flight dynamics with PMDG products. Unfortunately, these problems were not present during their earlier development. They need to spend more time on testing and quality control. 

Edited August 24, 2025Aug 24 by LRBS

The autothrottle will not add thrust until 15 knots below the target speed??  That is way worse than in the simulator, so maybe I should keep quiet? 

Edited August 24, 2025Aug 24 by Christopher Low

No, that's not quite right.  That refers to Autothrottle "wake up"; when the Autothrottle mode is "arm".  This occurs, for instance, when you're in a VNAV idle path descent.  The throttles will retard to idle and then the A/T will revert to "arm"; this de-clutches the Autothrottle servo and allows the pilot to manage speed while the aircraft pitches to stay on VNAV path.  The autothrottles will eventually "wake up" and add thrust if you get 15kts slow (or sooner if you're going to violate a minimum safe speed).  If this happens, the pilot flying has failed at their primary responsibility of controlling aircraft flight path and energy state. 

But when the autothrottles are in any kind of "speed" mode, they modulate according to an algorithm that seeks to maintain speed precisely, but also to smooth out the thrust changes.  So in smooth conditions, you'll maintain speed within a couple knots.  If you get into rapid airspeed fluctuations due to turbulence or aggressive wave, you'll see the autothrottles tolerate larger speed excursions as they'll avoid making abrupt back and forth corrections which can be destsbilizing.  Autothrottle corrections will always get more aggressive if they sense the need to protect against violating an over speed or under speed limit though. 

Perhaps this is configurable, but on our fleet, you'll never see the "thrust required" scratchpad message unless the airplane thinks it needs your action to prevent violating a minimum safe speed.  You won't see it just because you're slower than a commanded speed.

 

2 hours ago, Stearmandriver said:

If you get into rapid airspeed fluctuations due to turbulence or aggressive wave, you'll see the autothrottles tolerate larger speed excursions as they'll avoid making abrupt back and forth corrections which can be destsbilizing.

It's all about passenger comfort! Also a reason why you would disconnect AT on final in certain wind conditions and add +15(?) to landing ref if windsheer reported? 

If we had force feedback chairs for the sim I'm sure we'd appreciate this behaviour more! 

Doesn't a similar behavior happen at cruise with some jets? Where cruise altitude is allowed to wander up and down a couple hundred feet to prevent the pitch control and then AT from constantly lurching up and down, accelerate and decelerate which would fill your sick bags up pretty quick! 

10 hours ago, sloppysmusic said:

Doesn't a similar behavior happen at cruise with some jets? Where cruise altitude is allowed to wander up and down a couple hundred feet to prevent the pitch control and then AT from constantly lurching up and down, accelerate and decelerate which would fill your sick bags up pretty quick! 

No, that's not true.

Yeah, you wouldn't see an autopilot allow altitude fluctuations just for comfort.  That would bust separation in RVSM airspace pretty quick, and would never be certified for approach operations.

15 hours ago, Stearmandriver said:

Perhaps this is configurable, but on our fleet, you'll never see the "thrust required" scratchpad message unless the airplane thinks it needs your action to prevent violating a minimum safe speed.  You won't see it just because you're slower than a commanded speed.

 

Thanks for correcting me. Memory seems to play games. 

Yeah, on our airplanes, not the 737, we have so many options that they include a note on the dispatch FPL and company notams to check differences before flight. They wouldn't standardize or just bring all the software versions to a certain level. Beancounters strike again. 

Absolutely.  Plus I know there's a lot of subtle differences like that from one Boeing model to the next and you operated several of them.  I know I'd never keep them all straight.  The only Boeing I know well is the 737 (unless you count the Stearman 😉.)

19 hours ago, Stearmandriver said:

No, that's not quite right.  That refers to Autothrottle "wake up"; when the Autothrottle mode is "arm".  This occurs, for instance, when you're in a VNAV idle path descent.  The throttles will retard to idle and then the A/T will revert to "arm"; this de-clutches the Autothrottle servo and allows the pilot to manage speed while the aircraft pitches to stay on VNAV path.  The autothrottles will eventually "wake up" and add thrust if you get 15kts slow (or sooner if you're going to violate a minimum safe speed).  If this happens, the pilot flying has failed at their primary responsibility of controlling aircraft flight path and energy state. 

But when the autothrottles are in any kind of "speed" mode, they modulate according to an algorithm that seeks to maintain speed precisely, but also to smooth out the thrust changes.  So in smooth conditions, you'll maintain speed within a couple knots.  If you get into rapid airspeed fluctuations due to turbulence or aggressive wave, you'll see the autothrottles tolerate larger speed excursions as they'll avoid making abrupt back and forth corrections which can be destsbilizing.  Autothrottle corrections will always get more aggressive if they sense the need to protect against violating an over speed or under speed limit though. 

Perhaps this is configurable, but on our fleet, you'll never see the "thrust required" scratchpad message unless the airplane thinks it needs your action to prevent violating a minimum safe speed.  You won't see it just because you're slower than a commanded speed.

 

Correct me if I’m wrong here but I doubt I am. I see it all the time in VNAV PATH with the A/T in ARM and when the speed drops 5 knots or more below target speed, FMAs change to FMC SPD. 
 

like I said correct me.

1 hour ago, ahsmatt7 said:

Correct me if I’m wrong here but I doubt I am. I see it all the time in VNAV PATH with the A/T in ARM and when the speed drops 5 knots or more below target speed, FMAs change to FMC SPD. 
 

like I said correct me.

This may be a company selectable option as well, but in our aircraft it's definitely -15kts before Autothrottle wake up.  This is graphically depicted on the Max with a magenta speed band; the Autothrottles expect you to manage your speed within this band and will only wake up if you hit the edges of it.  In this shot you can see that the commanded speed is 279, AT in arm, and the bottom of the magenta band is 264, 15kts slow.  The NGs work the same way, they just don't have the magenta band.

Edited August 25, 2025Aug 25 by Stearmandriver