Sign in to follow this  
Tristan Marchent

747 V3 TOGA Press Delay

Recommended Posts

Help AVSIM continue to serve you!
Please donate today!

Is this 'delay' only found on GE engines? I have a vague idea of what you're talking about and I believe it's ony encountered on 747s with GEs, but I don't have a source to support that.

Share this post


Link to post
Share on other sites

Not something I've heard of. My closest experience to the real thing is in 747-400 full flight simulators (GE and RR power) but I don't recall any simulated delay there, either on takeoff or go-around.
 
Why should there be a delay?
 
Edit:
 
I found this in the FCOM I have
 
"The TO/GA switches are inhibited two seconds after radio altitude decreases
through five feet on landing. TO/GA is enabled again three seconds after radio
altitude increases through five feet for a rejected landing or touch and go.
"

That's the nearest thing to a delay I could find in relation to TO/GA switches

Share this post


Link to post
Share on other sites

I know the 747-400 has a delay in setting T/O power when the TOGA button is pressed

 

You know this from actually flying the plane, I assume...?

Share this post


Link to post
Share on other sites

You know this from actually flying the plane, I assume...?

I don't know this from flying the plane... but from watching an in cockpit video on youtube, it definitely shows this. But please correct me if I'm wrong?

 

https://youtu.be/5wjoF8-Oaew (Presses TOGA at 9:06, and there is a slight delay..)

https://youtu.be/2hbQH9J7LWE (Again here at 00:59 seconds, another delay)

 

Are you seeing the same as me? Might be a specific engine...

Share this post


Link to post
Share on other sites

ΤΟGA doesnt have any delay. Thats actually its job to set power asap especially on go around manoeuvre or in case of windshear  


I see what you are talking about the servos on the auto throttle. Thats different than the actual toga mode which is instantly visible on the FMA as THR REF or THR (in case of go around) mode. On both cases the engines are already spooled  up before the toga application that has to do with the servos on the autothrottle. Remember that this is irrelevant because the thrust on the 744 is fly by wire and fully controlled by the eec 

Share this post


Link to post
Share on other sites

I seem to recall one second from pressing the switches to THR REF and another second from then to the thrust levers actually moving. At least on the ground setting takeoff thrust, no idea about the go around.

 

(RR engines).

 

Why? I can give you no rational idea, and not sure it's in the books anywhere either. But I am 99.9% confident that is what the real aeroplane does.

Share this post


Link to post
Share on other sites

On both cases the engines are already spooled  up before the toga application that has to do with the servos on the autothrottle. Remember that this is irrelevant because the thrust on the 744 is fly by wire and fully controlled by the eec 

The Autothrottle servos still are in the loop, EEC or no EEC. Anyway a delay in activating the servo isn't exactly desirable (you certainly wouldn't want that on a go around).

 

In the first video (KLM, GE engines) THR REF is annunciated as soon as the captain operates the TOGA switch. If there is a delay in the aircraft I've never noticed it in the sim (which use aircraft parts for the servos and clutches). However he could be holding the throttles momentarily against the servo,

 

Why hold the levers back? There is only one servo for all four thrust levers, each lever has a slipping clutch. If one of the clutches fails you could get asymmetric takeoff power so it might be wise to hold the levers until all four start to move, Just a theory, mind you.

Share this post


Link to post
Share on other sites

Hmm interesting.

 

In the first video listed, if you listen carefully you can hear the movement of the throttle coincides with the sound of the servo's.

Share this post


Link to post
Share on other sites

Hmm interesting.

 

In the first video listed, if you listen carefully you can hear the movement of the throttle coincides with the sound of the servo's.

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.

Share this post


Link to post
Share on other sites

There's a definite delay between "THR REF" annunciation and the throttles moving forward in both videos.

Share this post


Link to post
Share on other sites

On takeoff, up until the point the TOGA buttons are pressed the servos have been disconnected whereas on go around the response of the throttles may be quicker due to servo already being connected. Just a theory...

Share this post


Link to post
Share on other sites

I'm sceptical that this is related to a specific engine. Qantas' 744s are equipped with RR's on the OJ* series frames, and KLM appears to use the CF6 fleetwide. 

Share this post


Link to post
Share on other sites

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.

  • Upvote 1

Share this post


Link to post
Share on other sites

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.

Share this post


Link to post
Share on other sites

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.

Share this post


Link to post
Share on other sites

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'

Share this post


Link to post
Share on other sites

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.

Share this post


Link to post
Share on other sites

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?

Share this post


Link to post
Share on other sites

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.

Share this post


Link to post
Share on other sites

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!

  • Upvote 1

Share this post


Link to post
Share on other sites

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.

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
Sign in to follow this