Speed out of control.

[adribar 0]

Hi.I'm flying with PMDG B747-400. On FMC I set cruising speed .800 Mach. When this speed has been reached, it has continued to rise slowly until the current indicated by .924 Mach. Any changes to the settings on the FMC are irrelevant. The following message appears on the display: "> AUTOTHROT DISC" in yellow. Yesterday I made an intercontinental flight with the PMDG 777-300ER. The same problem was presented. The FMC did not control the speed and the aircraft went into permanent owerspeed. Deactivating the autothrottle and lowering the handcuffs, the aircraft began to dive. What is the cause? Thank you.

[skelsey 765]

10 minutes ago, adribar said:

What is the cause?

 

10 minutes ago, adribar said:

The following message appears on the display: "> AUTOTHROT DISC" in yellow

Have you tried (re)engaging the autothrottle at all?

[Captain Kevin 586]

Be advised that as per PMDG rules, users are required to sign their posts with their first and last name. In any event, in the case of the Boeing 747-400, your speed is not being maintained by the auto-throttle because the EICAS message you got is telling you that the auto-throttle has been disconnected. I'm not entirely clear what's going on in the case of the 777-300ER. When you say you disconnected the auto-throttle, did you simply flip the switch on the MCP? Because if it was showing the same message, the auto-throttle was never controlling the speed in the first place since it was disconnected. Need more information here.

[Fabo 213]

Do you, by any chance, have set up autothrottle override in your settings?

[Budbud 227]

Hi,

It's not clear to me at which stage the AUTOTHROT DISC EICAS caution message is displayed.

Is the message already displayed when you have the issue with the speed? If yes, the autothrottle is disconnected and therefore cannot control the thrust, thus the speed.

Reason for AUTOTHROT DISC is either the A/T arm switch is off or you have pushed either A/T disconnect switch on the thrust levers.

Another reason is an automatic disconnection due to a failure of both FMC or a fault in the active autothrottle mode.

If the issue with the speed occurs without the message displayed, then check the behaviour of the thrust levers. If they don't move backwards to command a thrust reduction, check:

_ the FMA to verify the autothrottle mode: in cruise it should read SPD. If for any reason it reads HLD (can figure out why it would happen though), then the autothrottle doesn't command the thrust.

_ whether your throttle quadrant doesn't override the autothrottle thrust command. You can display the position of your throttle quadrant command on the upper EICAS with an option in the PMDG setup>simulation to check if it matches the commanded thrust. And you can also check the override option in the same menu (in hold mode only, always, never). Try to set it to never to prevent any interference from your hardware from altering the thrust control.