747 V3 TOGA Press Delay

[Lukas_Mathijsen 41]

You can't hear the servo in the cockpit. What you hear as the throttles move is the whine of the CF-6s spooling up.

Please, don't tell me what I can and cannot hear.

What I hear is DEFINITELY a servo moving.

[kevinh 409]

Please, don't tell me what I can and cannot hear.

What I hear is DEFINITELY a servo moving.

With respect Lukas, you can't hear the autothrottle servo on the 747 flight deck. So whatever you are hearing isn't that.

[rondon9898 84]

You do run into problems when you rely on YT vids for main experience because although I've been on a jumbo flightdeck, I haven't been on one for takeoff so I really do have to rely on the top brass at PMDG - after making a post on here before about how PMDG record their sounds, the sound chappy (awfully sorry, can't remember his name - can someone remind me?) convinced me that they are absolutely as pristine and realistic as you can get.

 

That said, Kevin, on this vid you can see both the delay after TO/GA switch depression AND hear the thrust lever servos

 

So I can see where these questions are coming from.

[Tristan Marchent 3]

I just hope PMDG can implement this even though it's now in Beta. Just add's to the way the aircraft has been designed and one of those PMDG Realistic 'Quirks'

[Heffron 29]

I'm with Tristan on this one. Also does not seem to depend on engine type. It is apparent in videos of aircraft equipped with GEs RRs and PWs.

[rondon9898 84]

It was my understanding that there was a delay too, but I'm happy to bow to superior knowledge.

 

Any rw jumbo pilots on the forum?

[FlyChuck 3]

There is a delay and you can hear the servo (some planes are especially noisy and the servo might be more difficult to hear).

 

The internet is full of proof:

 

https://youtu.be/ITqebF881uk?t=2m

https://youtu.be/2hbQH9J7LWE?t=55s

https://youtu.be/huhtY2OryF0?t=55s

https://youtu.be/v8xFaJ_-Re4?t=55s

[kevinh 409]

There is a delay and you can hear the servo (some planes are especially noisy and the servo might be more difficult to hear).

 

The internet is full of proof:

 

https://youtu.be/ITqebF881uk?t=2m

https://youtu.be/2hbQH9J7LWE?t=55s

https://youtu.be/huhtY2OryF0?t=55s

https://youtu.be/v8xFaJ_-Re4?t=55s

I agree the servo delay is consistent, and I'm happy to admit I was wrong about that. I was relying on memory, and the delay is more obvious when you are looking for it. It seems to vary slightly from aircraft to aircraft and some videos I've seen show a delay before THR REF annunciation too, others don't.  However none of those videos you link to have any throttle servo sound. Maybe some people's ears are more sensitive, but I've never heard it, even in a full flight sim with the sound system turned off.

 

Manually moving the throttles creates noise, so that would be apparent too if the servo is driving them. But I've never heard any servo motor sound.

[skelsey 765]

kevinh, on 14 Dec 2016 - 02:41 AM, said:

However none of those videos you link to have any throttle servo sound. Maybe some people's ears are more sensitive, but I've never heard it, even in a full flight sim with the sound system turned off.

To be completely honest Kevin, I could hear the servo in the KLM 747F video (not watched the others) and although I've not been in a real 747 flight deck at the appropriate moment, the bunch of real 747 bits I have operated does definitely have the same distinctive whirr when the A/T moves (the whirr is there every time the throttles move, but it's only really noticeable with TOGA application as it's the only time the thrust levers have so far to move).

 

There's no sound when you manually move the thrust levers, however -- the clutches prevent the servo from being back-driven (which is what would make the noise).

 

Edit: just to add -- the above is not to say that all A/T servo installations are created equal and it may be that some turn out quieter than others!

[kevinh 409]

To be completely honest Kevin, I could hear the servo in the KLM 747F video (not watched the others) and although I've not been in a real 747 flight deck at the appropriate moment, the bunch of real 747 bits I have operated does definitely have the same distinctive whirr when the A/T moves (the whirr is there every time the throttles move, but it's only really noticeable with TOGA application as it's the only time the thrust levers have so far to move).

 

There's no sound when you manually move the thrust levers, however -- the clutches prevent the servo from being back-driven (which is what would make the noise).

 

Edit: just to add -- the above is not to say that all A/T servo installations are created equal and it may be that some turn out quieter than others!

I've never heard any such whirring noise, the servo motor is buried inside the pedestal. I've listened to that KLM video several times and I can't hear any electric motor noise coincident with the throttles moving. I hear the wipers, the click of the TOGA switch, the whine of the engines, the roar of the exhaust, but no whirring.

 

The manual throttle movement noise I referred to is the mechanical sound of parts sliding against each other. You certainly can hear that in the cockpit when the levers are moved rapidly.

 

I thought the clutches were friction units only and always engaged, but I may well be wrong. There would be no problem in backdriving the servo if it wasn't powered. In fact you can oppose the throttle lever motion against the servo motor drive. The slipping clutch allows this.

 

There is some mechanical hysteresis in the system and that might explain at least some of the time delay, as the motor would have to take up the backlash before the throttles would move.

[MauB 74]

I can clearly hear a whirr sound (synchronized with throttle movement) after TOGA application on the KLM video.

[tcas_climb 45]

I hear it in all the videos. I can even hear it in the PMDG 777. I dont know how they work but could the delay be caused by the servos coming under load moving the throttles? Maybe they are de-clutched from the thrust levers themselves and they "park" at the idle stop. Then when you press TO/GA they move silently from idle until they catch the thrust levers (which are set at ~70%) and then the load of moving the thrust levers causes the whirr sound. Just a thought.

[Heffron 29]

I hear it in all the videos. I can even hear it in the PMDG 777. I dont know how they work but could the delay be caused by the servos coming under load moving the throttles? Maybe they are de-clutched from the thrust levers themselves and they "park" at the idle stop. Then when you press TO/GA they move silently from idle until they catch the thrust levers (which are set at ~70%) and then the load of moving the thrust levers causes the whirr sound. Just a thought.

Sounds very plausible to me. Slight variation in delay could be explained by the one captain setting 75% N1 and the other 65% before pushing TOGA.

[jcomm 5,886]

Honestly, i'd be more interested in correct modeling of VNAV logic, and even simple / basic details like pitching moments due to thrust...

[Heffron 29]

Honestly, i'd be more interested in correct modeling of VNAV logic, and even simple / basic details like pitching moments due to thrust...

it's all about the little details!