737-8 SPD LIM flag
 

Hi All,

Can anyone shed any light on the causes of the SPD LIM flag on the PFD of the 73-8? I have looked in the manuals and find just a description of what it means. Not the actual cause of its illmination.

I know it seems to illuminate on the related side of a V1 cut in the sim. Someone suggested that it is linked to the Start Switch (Related Side) when it is put off after an engine failure??


Any ideas anyone?

L-P

You would get the speedtrim fail on the overhead panel and a speed limit flag on your PFD as you have probley turned the start switch off on the engine with that has failed (severe damage). By doing so you depower the failed engines EEC hence the speedtrim fail etc. No crew action required in flight (QRH) But watch out as if your using the failed side autopilot (CMD A or B) you will not be able to select a pitch mode unless you either swap autopilot or turn the start switch back to CONT and power up the failed side EEC. About all I know about it. Black magic...

Thanks a million. That is exactly what a TRE explained to me before now that I think of it. I couldn't remember what he had said.

Thanks very much. Good tech!!!

L-P

Hello LP

Good to see someone else asking the same questions that bother me :)
I presented this question myself some time ago. I asked here on tech log and on the engineers and technicians pages, and no one can give you a definitive answer.
Ok.. selecting start switch to CONT energizes the EEC, but what does that have to do with the SPD LIM? Speed limit bars are computed in the SMYD with info from the ADC and weight info from the FMC.
My best guess is that the EEC relays signals from many instruments, like the DEU, so that when it looses pwr the signals are cut.
In the same way.. why are the vertical mcp-modes u/s in this situation? Those go from the FCC via the FMC to give you flight director commands.

Please, if anyone gives you a verifiable and good answer please let me know. :ok:

If one of your DEUs doesnt get correct information from the ADMs/ADCs,you will get the DISPLAY SOURCE .(ENG master caution as EEC revert to SOFT mode).
To find out which side/which DEU is bad,the captain (DEU1) or FO(DEU2) will get the speed limit and HYDR low pressure(ANNUNCIATION only).

Hope that helps.