Delay spool up and A/T bug

[Cptburgos]

Hello development team

Before the release of the new update, i think is pertinent that you see this little problem.

Conditions:

Airport: SKBO (Bogotá Colombia)

Hour/Temp/QNH: 20:00Z/17/30.33

Bug: At the moment of start engine the time to spool up is to high, since the fuel lever goes to idle, i counted 15 second to begin spool up.

A/T bug:

I follow all the steps to TOGA as the manual said but A/T just advance throttles to 81% remain there. (i put full fwd my throttle hardwarejust after press TOGA).

Thanks for your time.

[Litjan]

I haven´t tested this at your airport and those settings, but what you describe could very well be realistic behaviour.

When starting the CFM56 engine you have "20 seconds" between putting the start lever to IDLE and seeing EGT rise (not even spoolup!). It could take a long time for the engine to spool up, the starter time limit is 2 minutes (from engagement to cutoff). So as long as you stay below that, you are fine.

Also make sure to turn off all air-conditioning before you start the engines, this is a common mistake sim-pilots make.

At high density altitude, jet engines accelerate VERY slow. This is the reason for the "flight idle" setting which is considerably higher than the 21% or so you get on the ground. It would simply take "forever" to accelerate the engines from 20% while at FL200...

When the jet passes 80 kts during the takeoff roll, the autothrottle servo motor gets "cut off" from it´s electrical supply (shown by THR HOLD at 84kts). Now it can´t drive the thrust levers anymore. If the correct N1 is not achieved by that time, the pilot has to set it manually. This is normal.

To avoid that, follow correct procedure:

Hold brakes

Advance thrust levers until both engines reach 40% N1

Click TOGA

(for IXEG: Advance both hardware thrust levers full forward)

Release brakes (note: If you are at a very high airport, you may consider holding your brakes until N1 passes 60% or so - this should ensure that calculated N1 will be set by autothrottle before the aircraft is passing 80 kts. You can even hold your brakes until N1 is achieved ("static takeoff") but it is not recommended as it places a lot of stress on landing gear and wheels and also increases the chance of foreign-object ingestion and adverse flow in the engine in high crosswind situations.

Cheers, Jan

 

 

Edited April 18, 2020 by Litjan

[Cptburgos]

1 hour ago, Litjan said:

I haven´t tested this at your airport and those settings, but what you describe could very well be realistic behaviour.

When starting the CFM56 engine you have "20 seconds" between putting the start lever to IDLE and seeing EGT rise (not even spoolup!). It could take a long time for the engine to spool up, the starter time limit is 2 minutes (from engagement to cutoff). So as long as you stay below that, you are fine.

Also make sure to turn off all air-conditioning before you start the engines, this is a common mistake sim-pilots make.

At high density altitude, jet engines accelerate VERY slow. This is the reason for the "flight idle" setting which is considerably higher than the 21% or so you get on the ground. It would simply take "forever" to accelerate the engines from 20% while at FL200...

When the jet passes 80 kts during the takeoff roll, the autothrottle servo motor gets "cut off" from it´s electrical supply (shown by THR HOLD at 84kts). Now it can´t drive the thrust levers anymore. If the correct N1 is not achieved by that time, the pilot has to set it manually. This is normal.

To avoid that, follow correct procedure:

Hold brakes

Advance thrust levers until both engines reach 40% N1

Click TOGA

(for IXEG: Advance both hardware thrust levers full forward)

Release brakes (note: If you are at a very high airport, you may consider holding your brakes until N1 passes 60% or so - this should ensure that calculated N1 will be set by autothrottle before the aircraft is passing 80 kts. You can even hold your brakes until N1 is achieved ("static takeoff") but it is not recommended as it places a lot of stress on landing gear and wheels and also increases the chance of foreign-object ingestion and adverse flow in the engine in high crosswind situations.

Cheers, Jan

 

 

Thanks commander, i'll try out that.