I just did a flight test of the NGX to compare mimumum N1 in LVL CHG and VNAV as well as look at approach idle logic in the NGX. Interestingly I found that at high altitude, regardless of whether LVL CHG or VNAV was in use the idle position set by the A/T was a little bit forward of the idle stop. Pressing F1 reduced the levers to idle but they immediately moved forward again. It's as if the A/T has a minimum thrust lever angle limit above the idle stop. At FL200 in a VNAV descent I tried again and this time the thrust levers stayed at the idle stop once pulled back there. The change in N1 was from 41.7% to 38.7%, i.e. 3% reduction. So somewhere above FL200 the limit on minimum thrust lever angle in A/T was removed, independent of whether the AP is in VNAV or LVL CHG.Idle N1 (recorded at 4000', 180 kts for consistency):Flight Idle 30% N1, Approach Idle 36% N1. The engines switch to Approach Idle if: [Gear down] or [Flap 15 or more] or [either Engine A/I ON]. I wasn't able to get Approach Idle to go to a higher setting, even on approach when the thrust mode automatically switches to G/A.Idle N1 increases with airspeed. Altitude had little effect on idle N1 up to at least FL200. At higher altitudes Idle N1 increases (though N2 does not).

I just did a flight test of the NGX to compare mimumum N1 in LVL CHG and VNAV as well as look at approach idle logic in the NGX. Interestingly I found that at high altitude, regardless of whether LVL CHG or VNAV was in use the idle position set by the A/T was a little bit forward of the idle stop. Pressing F1 reduced the levers to idle but they immediately moved forward again. It's as if the A/T has a minimum thrust lever angle limit above the idle stop. At FL200 in a VNAV descent I tried again and this time the thrust levers stayed at the idle stop once pulled back there. The change in N1 was from 41.7% to 38.7%, i.e. 3% reduction. So somewhere above FL200 the limit on minimum thrust lever angle in A/T was removed, independent of whether the AP is in VNAV or LVL CHG.Idle N1 (recorded at 4000', 180 kts for consistency):Flight Idle 30% N1, Approach Idle 36% N1. The engines switch to Approach Idle if: [Gear down] or [Flap 15 or more] or [either Engine A/I ON]. I wasn't able to get Approach Idle to go to a higher setting, even on approach when the thrust mode automatically switches to G/A.Idle N1 increases with airspeed. Altitude had little effect on idle N1 up to at least FL200. At higher altitudes Idle N1 increases (though N2 does not).

It is almost the same found, there is something in the "retard" at high altitude wich not moves the throttles to the full back position so the descent is not fully in idle, if you push momentarily F1 or move the controller, the idle will reset and will be good for the rest of descent. I also found that in LVL CHG this had no effect on my vertical speed, vertical speed that at high altitude, no winds, was still at or below 1500ft/min (-800) while on VNAV I can reach higher values with a costant speed.Waiting SP1c for more tests.

PS: As you see, N1 is not a costant while N2 has preset values, the EEC idle control is on N2, N1 is slave.

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

Boeing call it AUTOTHROTTLE, also if refers to thrust levers. Maybe it is the "old" name of it, and it remained as the working principle between them is completely different.The main difference between autothrottle and autothrust is the working logic. The autothrust logic is that the thrust levers will not move, the thrust will be automatically modified by the system, on the autothrottle equipped aircrafts the thrust is setted by phisically moving the levers with servo motors.

Boeing call it AUTOTHROTTLE, also if refers to thrust levers. Maybe it is the "old" name of it, and it remained as the working principle between them is completely different.The main difference between autothrottle and autothrust is the working logic. The autothrust logic is that the thrust levers will not move, the thrust will be automatically modified by the system, on the autothrottle equipped aircrafts the thrust is setted by phisically moving the levers with servo motors.

I don't think it's true to say that autothrust always means a system where the thrust levers don't move.

Edited February 27, 201214 yr by kevinh

It is almost the same found, there is something in the "retard" at high altitude wich not moves the throttles to the full back position so the descent is not fully in idle, if you push momentarily F1 or move the controller, the idle will reset and will be good for the rest of descent. I also found that in LVL CHG this had no effect on my vertical speed, vertical speed that at high altitude, no winds, was still at or below 1500ft/min (-800) while on VNAV I can reach higher values with a costant speed.Waiting SP1c for more tests.

Andrea, note that at high altitude the levers did not remain at the idle stop but return to the above idle position. Only at mid to low altitude can you get the thrust levers fully back and have them stay there. However as you say the effect on vertical speed is minimal so the effect of the levers not coming fully back may be insignificant.

PS: As you see, N1 is not a costant while N2 has preset values, the EEC idle control is on N2, N1 is slave.

Idle N1 changes a lot as you say, but Idle N2 also changes though much less so, it isn't constant.

I don't think it's true to say that autothrust always means a system where the thrust levers don't move.

I don't know planes using "autothrust" that have a computer moved power levers. Maybe that there are few, but if they works like the 737, they have an autothrottle. Only the name changes.On the airbus the throttles are set by pilots giving to the autothrust the relative mode/limits, the engine response to the angle given by the levers will probably differs from a direct input.

Andrea, note that at high altitude the levers did not remain at the idle stop but return to the above idle position. Only at mid to low altitude can you get the thrust levers fully back and have them stay there. However as you say the effect on vertical speed is minimal so the effect of the levers not coming fully back may be insignificant.

I played with it today, and apparently, for the first time I paid attention to it, when descending, gradually the throttles tries to fix the problem, so if you push after long time from the top of descent, you will see that the change in N1 will be smaller and smaller.Need more tests about it.Hope also that pmdg finds a way to retard the levers to the fully idle position so that they remains in position if not moved.I'm also not sure about the moment when RETARD becomes ARM, I paid attention to it only one time, but I'm pretty sure the levers where still moving to idle (not fully idle) when the AT mode changed. It could be the cause if the ARM stopped the levers before reaching idle.More tests are needed. :)

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.

I have this exact same issue, and in a long continous descent with the right wind you can find that few extra percent means the speed of the aircraft starts to increase, resulting in drag being needed, if you don't remember to readjust the throttles. What I have noticed however that it occurs more than once. I.e if at the TOD point the throttles reduce to idle, and then you manually reset the hardware throttles you see the drop by the 3% or so, however as n1 changes with airspeed and altitude etc, again later on you will see the need to reset the the throttles as a new lower n1 is possible that the system hasn't dropped to.While we are on the subject of thrust setting, applying manual throttle to 40% to stabilise the engines prior to hitting toga, is it me or does the time it takes to reach 40% seem too long? It seems to trickle from around 35% to the last bit.