Hello,

 

A few days ago i took off from CYYZ runway 15L, with a full house to ZBAA. 

 

During the acceleration on the runway i noticed that the acceleration was not consistent. There was a headwind of 7-13 knots directly on the nose. But the acceleration was going up stopped and then continued this happened about 5 times on the takeoff run, this resulted in needing a way longer runway then was planned. 

 

I use REX for weather, but this never happened to me before.

 

I didn't use any odd numbers or so.

 

ZFW: 198.1

Fuel was something like 110.000

234 pax

19.020 KG of Cargo and baggage

Temp was +8 degrees celsius

 

and I derated the thrust to 93.0

 

Any thoughts on the cause???

Was the wind direction and speed consistent all the time?

 

Were the engines giving a stable constant take-off thrust throughout the take-off run?

 

Maybe you've set your throttle (hardware) to override the A/T. The A/T is trying to keep the power up but your hardware is pulling the throttles back.

 

I found the PMDG feature of showing the position of the hardware throttle as a blue line on the EICAS extremely helpful in matching both throttles, the real and the "virtual" one. I always try and keep them consistent during all phases of flight. It's extremely important to have both throttles matched above 80 knots on the take-off run, where the A/T reverts to "THR HLD" mode and the slightest change on the hardware position will affect the "virtual" throttles.

Was the wind direction and speed consistent all the time?

 

Were the engines giving a stable constant take-off thrust throughout the take-off run?

 

Maybe you've set your throttle (hardware) to override the A/T. The A/T is trying to keep the power up but your hardware is pulling the throttles back.

 

I found the PMDG feature of showing the position of the hardware throttle as a blue line on the EICAS extremely helpful in matching both throttles, the real and the "virtual" one. I always try and keep them consistent during all phases of flight. It's extremely important to have both throttles matched above 80 knots on the take-off run, where the A/T reverts to "THR HLD" mode and the slightest change on the hardware position will affect the "virtual" throttles.

 

Wind was consistent all the time, same goes for the speed.

 

Engines were stable yes.

 

I do not use the manual override for the A/T. Thus this cannot be the issue.

I find, irregardless if settings,on my Cirrus II,my throttles always override the aircraft throttles.

Hi,

 

 

 

 

Maybe you've set your throttle (hardware) to override the A/T. The A/T is trying to keep the power up but your hardware is pulling the throttles back.

 

Passed 80kt, the auto-throttle mode passes from THR REF to HOLD meaning that the A/T no longer controls the thrust but leaves it at t/o thrust if the pilot doesn't move the levers. From this moment in hold mode, if your throttles send a command, the engines will go to the new thrust commanded but the A/T won't try to set your engine back to t/o thrust.

So in any case you shouldn't have lot of different changes in your thrust except if your throttles keep moving.

From page 63 of Tutorial #1

 

"After you activate TO/GA, wait for THR REF to be annunciated and then push your physical joystick throttle fully forward, this prevents any mismatch between the throttle position and the A/T for the rest of the flight till descent"

 

Firewall the physical throttles you are using.

 

Michael Cubine

From page 63 of Tutorial #1

 

"After you activate TO/GA, wait for THR REF to be annunciated and then push your physical joystick throttle fully forward, this prevents any mismatch between the throttle position and the A/T for the rest of the flight till descent"

 

Firewall the physical throttles you are using.

 

Michael Cubine

 

I'm certainly not the one to contradict PMDG's tutorial, but won't firewalling the throttles bear the potential risk of the actual plane throttles firewalling as well once the A/T reverts to HOLD above 80 knots?

 

I'm just saying because one should keep his hand on the throttle till V1, and being humans and having a pulse and so on, you could unwillingly input an "extremely" slight change to the throttle, which will in turn override the plane throttles and cause an unwanted max-thrust.

 

Also, in the event that you had to reject the take-off, even though smashing the throttles closed takes less than half a second, during the millisecond in which the hardware is moving between max thrust and the position of the plane throttles, you'll actually be accelerating the engines. Of course this lasts for a few mili-seconds and the effect is negligible, but I consider it to be "conceptually" not right.

I'm certainly not the one to contradict PMDG's tutorial, but won't firewalling the throttles bear the potential risk of the actual plane throttles firewalling as well once the A/T reverts to HOLD above 80 knots?

It shouldn't do that as the EEC will limit the engine thrust to reference thrust. However it is possible to momentarily beat this limit and get an N1 or EGT exceedance. It happened to me a couple of times. The best thing to do is align the throttles with the rated N1. That way your throttle input matches what the A/T is commanding. To do this you need to enable the feature that lets you see what N1 your throttle lever will set. I have this option set to show up only when the throttle input is moving so it isn't visible most of the time.

The best thing to do is align the throttles with the rated N1. That way your throttle input matches what the A/T is commanding. To do this you need to enable the feature that lets you see what N1 your throttle lever will set. I have this option set to show up only when the throttle input is moving so it isn't visible most of the time.

 

Yeap, that's what I said before and what I do.

 

Excellent feature the blue reference lines. Now I miss them when flying the Level-D 767. What I do in that case is simply advancing my hardware throttles until they match the thrust reference, and THEN I engage TO/GA (or N1 on the 767). If I engaged it before I'd have no way of knowing where my hardware throttles are. This is an annoying limitation of flight simulation for me, but well, not too hard to work around it either