737 Auto Throttle Speeds - Key Information

[fordgt40]

I have used a new thread for this as the information is so important to throttle builders, I did not want it lost amid old threads

I have on occasion corresponded with David C Allen who has a marvellous site www.737flightsim.com and he has previously been a great source of information, particularly on "control loading". I strongly suggest you visit his site for the wealth of info available.

He emailed me today with reference data for 737 throttle speeds that is comprehensive and surprising. I quote


In the real aircraft the autothrottle move at different speed's based on the MCP mode as an example:

In takeoff or go-around, the A/T moves the T/Ls at a rate of 13.5 deg/sec to a predicted position. For final adjustments, the A/T moves the T/Ls at a maximum rate of 5 deg/sec.

Using 13.5 degrees per second it would take 4.4 (60 degrees/13.5 degrees per second = 4.4 seconds) seconds to move from full thrust to idle. This time is based on using 737's 60 degrees of thrust lever travel. This is as FAST as it gets, every other mode is SLOWER.

In modes other than TO/GA, the A/T computer limits thrust lever rates to 3 deg/sec.

That's 20 seconds to move from full thrust to idle ! ! ! (60 / 3 = 20)

There are two A/T retard modes, descent RETARD and flare RETARD. They annunciate as RETARD on the FMA.

For descent retard from cruise flight, at FMC top of descent (TOD) the A/T computer commands the T/Ls to reduce engine thrust to idle.
In descent RETARD, the thrust levers move to the aft stops at 1 deg/sec.

That's a MAXIMUM of 60 Seconds to move from full thrust to idle ! ! !

Flare RETARD is used during landing flare. For flare retard, the A/T computer uses these inputs:
Speed mode selection
Flap position
Radio altitude
A/P flare.

The A/T goes into the flare retard mode during landing if on G/S and in MCP SPD mode. The A/T commands the T/Ls to move to the aft stops at 3 deg/sec. After landing, the T/Ls move to the idle stop at 8 deg/sec. During the flare retard mode, the T/Ls get to idle within 6 seconds

You see there is no RAPID movement of the thrust levers when the ATs are controlling the power.

I would like to see someone write a autothrottle program that matches the real world operation.


I hope you find this interesting and many thanks to David C Allen

David

[archen]

Fantastic information David! Thanks for sharing!

I'll save this topic as reference when I'm going for the TQ conversion project.

Anders


Sent from my iPhone using Tapatalk

[FredK]

David -

That is very interesting information!

If you are using an advanced avionics program (PM, SimA, ProSim etc.) it should be doable I think to program all the different possibilities.  Not easy though!  However with a target sweep speed of 20 seconds (or even higher) that makes it a lot easier to smooth out hesitations and synchronization in motion as has been debated in other threads here. You will need motors that operate in the range of 5 rpm or less to achieve that.  Such is available.

Fred K

[fordgt40]

Fred

I see no reason why we cannot programme this in sioc. We will need to pick up the two "retard" modes from the SA offset list, however, they do not appear to be there at present. I will email Mark and see if he can help us on this.

David

[jackpilot]

Very good thread David
Please expand.

The other Dave (Allen) is an incredible source of info and a quiet and efficient builder. (hello David!)

Jack

[Flying_Fox]

Thank you very much David for sharing this! I might as well implement this in my program.

Nick

[Sudden81]

Couldn't  more than agree! Very very good info. But do you know any thing abut acceleration speed or ramp up speed, ramp down speed.

[Flying_Fox]

My throttle right now runs full lever sweep in 8 sec. I run the motors at 50% speed. (well, I run right engine lever at 55% speed since there is more friction in it.  )

[fordgt40]

Fred

I see no reason why we cannot programme this in sioc. We will need to pick up the two "retard" modes from the SA offset list, however, they do not appear to be there at present. I will email Mark and see if he can help us on this.

David

Fred

I have had a response from Mark (Sim-Avionics), he will look into adding a "flare" entry into the THRUST_REF_MODE offset
Thanks Mark, good support as ever

David