737NG EFIS Control Panel failure

[FlyingTinCans]

On the 737NG an EFIS control panel failure will give a 'DISPLAYS CONTROL PANEL' on the PFD.

However it also blanks the altimeter on the failed side and displays the ALT flag until you move the CONTROL PANEL select switch to the controller that hasn't failed.

My question is why does the Altimeter blank? It is the EFIS controller that has failed both DEU's are functioning normally and providing independent, correct data.
Why remove primary flight data for no reason except as a bizarre attention getting exercise (The Amber Caution on the PFD should be enough). Is there a black magic reason behind it or just another Boeing thing?

[Denti]

The EFIS panel controls which baro reference is used, with a failed EFIS controller the information which baro reference should be displayed is no longer available. Why the system doesn't revert to STD as the failed state is anybodies guess.

[RAT 5]

Is it the EFIS control panel that determines which side to take primary data from or the selection of the FD switch; the 1st one selected being MA? Usually, in pre-flight the PF selects their FD first to make their side the master. Would this not be the factor that determines which side all the parameters are taken from? Thus PF, with FD = MA, and with a failed EFIS panel will also lose more than PNF. However, why just the ALT? If the AFDS or FCC decides there is inaccurate data then why not remove all parameters that can feed into the AFDS system e.g. IAS, HDG, VSI?
It's a question not an answer.

[FlyingTinCans]

Didn't think of the Baro reference, thanks Denti!

I can see the logic in it being removed rather than switching to STD.

RAT 5, The FD controls are not part of the EFIS control panel so are unaffected by the fault. You are thinking of the MCP.

[737aviator]

The FD is affected by a Displays Control Panel fault. If the control panel fails on the side of the MA FD (i.e. the PF) then the selected roll and pitch modes will disengage and the flight directors disappear from both sides.

If the fault occurs on the PM side then the PF keeps his/her flight directors and roll/pitch modes (and of course altimeter), and the failure is much less obvious. :)

[FlyingTinCans]

Is that referenced in the Boeing books anywhere?

I thought what you describe happens with a 'DSPLY SOURCE' not a 'DISPLAYS CONTROL'

[737aviator]

I've seen it with my own eyes. Was surprised myself!

[nick14]

Display source is different and depends on aeroplane altitude.

We are training this failure at present and it is quite a simple failure to manage.