I noticed last night that I was at about 35% N1 at idle in descent in VNAV, throttle mode "ARM". I could pull the throttle back manually and the throttle position indication would change, then rather quickly work back to 35%. Is this correct?Thanks!George

Yes, it's correct. There has been a lot of discussion on the forums about this. Many say the flight idle setting is too high, especially for landing. This setting tends to let the airplane float down the runway on autothrottle. That's why I usually disconnect the autothrottle around minimum altitude, give the throttles a small manual "goose", (which lets N1 settle about 21%) and let the airplane gently land on the runway. No floating or bouncing.

Strictly speaking, Boeing recommend that when the autopilot is off, the autothrust is also off. When landing by hand, no autoland, it should be off.

Strictly speaking, Boeing recommend that when the autopilot is off, the autothrust is also off. When landing by hand, no autoland, it should be off.

True. But there is no such recommendation if the autopilot is on and the autothrottle is off. I should have mentioned keeping the autopilot on for an autoland. Of course, if both are off, then 21% N1 is still possible.

But you said ...This setting tends to let the airplane float down the runway on autothrottle. You won't be floating down the runway with an autoland, autoland works fine with autothrust on, autothrust should be on for an autoland... so it's logical to assume you were referring to a manual landing, with autothrust still on. Which is not proper procedure.From what I've seen in this forum, since the NGX was released, many of those individuals that are complaining about floating on landing, have the autothrust still on while hand flying the landing.

Edited February 26, 201214 yr by martin-w

True. But there is no such recommendation if the autopilot is on and the autothrottle is off. I should have mentioned keeping the autopilot on for an autoland. Of course, if both are off, then 21% N1 is still possible.

Flight idle N1 has nothing to do with whether autothrottle is engaged or not. Idle N1 is controlled by FADEC and increases depending on approach configuration. Some users experience a problem with flight idle with a/t engaged, others don't.

Thats true Kevin. The flight fidle issue is something specific to some users. The issue with floating and landing with autothrust still engaged is because despite the thrust automatically reducing to idle just before landing, it's not usually soon enough to limit floating.

Edited February 26, 201214 yr by martin-w

Ok, so then lets limit the discussion to descent phase. What's the N1 that we should expect to see when descending at ARM A/T?Regards,G

If I keep the autothrottle engaged, even with the autopilot engaged for autoland, I get 35% N1, as per the OP. If I turn everything off, and bump the throttles manually, N1 reduces to 21-25%. I like this behavior, but do I have a problem? Thanks for your input, Kevin and Martin.

35% is normal, you don't have a problem. It's designed that way by wise Mr Boeing.The manuals contain that much info it's probably hard to track down, but it is in the manuals.To be honest, I can't remember why, but it may be to decrease spool up time in the event of a go around.But anyway, PMDG got it right.

Edited February 26, 201214 yr by martin-w

Roger, thanks!

Approach Idle is always higher than Flight Idle !!Ground Idle is normally 20-21 %Flight Idle should be 31-32 %. The NGX tends to be very accurate while in Level Change but a little too high (36-38%) while in VNAV.Approach Idle could be as high as 45 % in case of a GA.Fred.

Edited February 26, 201214 yr by RYR738

This is maybe the 4th topic with "autothrust" word.The 737 is not an airbus os such a similar plane, it has autothrottle, not autothrust.Apart of that, flight idle and approach idle are greater than ground idle, and is not related to the autothrottle.In the NGX, keeping pushed F1 key will force idle to ground idle (21%), but is something that on the real cannot be done.Pilots have no control on the idle, its logic is determined by air/ground and gear and flaps.Fred, on my system in VNAV the idle is like the LVL CHG one. I found something strange about the idle when starting the descent, but I'm doing more tests before posting it.The problem is that if I start to descent (my controller lever is on idle) the throttles retard, but the idle is still a bit high (speaking about fl300, maybe around 40%), then just touching the controller and putting it back, the idle will lower to, lets say, 38% N1.I'll also probably wait the SP1C to see if the problem will stay or will go away.If it is still present I'll rwecord it and send a ticket.I also found a difference in vertical speed between VNAV and LVL CHG where with VNAV I can reach an higher vertical speed with a costant speed istead of lvl chg and same ias.I'm doing few tests right now, I want to wait sp1c wich will correct (as I understood) few things in that way.

The 737 is not an airbus os such a similar plane, it has autothrottle, not autothrust.

That is true, and yet the 737 has no throttles, it has thrust levers. At least the Airbus term is consistent. :-)

Approach Idle is always higher than Flight Idle !!Ground Idle is normally 20-21 %Flight Idle should be 31-32 %. The NGX tends to be very accurate while in Level Change but a little too high (36-38%) while in VNAV.Approach Idle could be as high as 45 % in case of a GA.Fred.

I tend to use the term Flight Idle because that is what I got used to years ago. Old habits die hard. In older Boeing's there was no such thing as Approach Idle, and Flight Idle was the increased setting with flaps extended. Any other increase in idle N1 in flight was down to the effects of forward speed and altitude, and the minimum rpm allowed by the FCU was still ground idle. Things have moved on with EEC and FADEC controlled engines so multiple idle settings are now possible.As for a difference between Flight Idle N1 in VNAV and Level Change I can't say I've noticed it. From a design point of view there's no reason why PMDG's engine model behaviour would be any different depending on the AP mode in use. However there may be a problem with the way the A/T switches between RETARD and ARM which might leave the thrust levers set above the idle stop.

Edited February 27, 201214 yr by kevinh