737NG Control Surface Position Indications

[SimWes]

Hi,

This is in regards to the 737NGs that have the Control Surface Position Indicator option on their Display Units.

The following has been observed on our machine.

'Control deflection display on lower DU systems page showed a small rudder deflection when tiller was turned to full deflection'

This is what we found

When the Yaw Damper is engaged and the Tiller is turned you will get an indication of Rudder deflection as well as a Yaw Damper indication.

When Yaw Damper is switched of, no deflections are shown by either indications.

What I need to know if this is correct or what should really happen.

Any help would be most appreciated.

Thanks,

WEs

[prop jocket]

SimWes:

The following is based on a foggy and addled memory of the course I did about six years ago, so PLEASE check out for yourself if what I'm about to say is still correct. There was some debate being had at Boeing at the time regarding whether this is a desirable function, or whether it is totally unnecessary. I don't know what the outcome was, sorry.

The rudder is responding to the apparent out of balance condition that the aircraft is in when turning it on the ground. There is no angle of bank, so it interprets the tail being out of line as an uncommanded yaw or the onset of dutch roll, and puts a rudder input in to try to correct it. As the airspeed is very low, (if you're using the tiller it is well below the computational airspeed for the yaw damper system) the yaw damper is limited to a maximum deflection of 3 degrees if memory serves. This will happen irrespective of autopilot or IRU alignment status.

Of course if the yaw damper is switched off, it won't respond, hence why no movement is shown on the SPI under those conditions.

Hope this helps, and would you let me know if my memory is still good?

Rgds

P J

[SimWes]

P J,

Thanks for that.

Sounds like our system works as advertised. When I get the chance I'll have to check how much actual deflection we get.

Much appreciate your help in this matter.

Best regards,

WEs