Tough B738 question
 

Hello

Here is a tough B737-800 question that I don't get:

When one experiences a sudden N1 seizure one looses the minimum and maximum buffet onset tapes on the associated onside speedbar and is given a 'SPD LIM' indication beneath the speedbar. Then, if one sets the engine start switches to 'cont', one gains them back.
Why is this?

The EEC alternator shuts down if N2 drops below 15% and is coupled to the transfer bus when the switch is set to 'cont'. But then I can't see why that should be applicable. The buffet onset tapes collect information on weight from the FMC and speed and AoA from the ADIRU and computes it in the SMYD before sending the information to the DEU.
How can a U/S EEC affect this?

A definitive answer would be of great interest :)

If engine 1 fails and N1 drops below 5% while N2 remains above 50% for more than 5 secs, the speed trim logic considers this a loss of N1 signal with the engine still running, and the SPEED TRIM FAIL amber light illuminates. SPD LIM appears on the PFD and onside AFDS pitch modes are not available. This is why this occurs only with a seizure or severe damage and not when there´s a flame out.

Placing the Start switch to cont re power the EEC and restores valid ENG FAIL signal. The SPD LIM flag is removed, the speed tape yellow and red bands are recovered and the speed trim fail amber lt extinguishes.

Thanks. One of the best answers I've received on this question yet.

One follow-up:
Why does it give you a SPD LIM? Switching the engine start switches only affects the EEC. Do the signals go from the SMYD, via the EEC to the DEU? In that case I get that the signalling stops when the EEC isn't energized.

@Sudden winds: I recon you too have the B737 MRG? A highly recommended book for any 737 driver. Carry it in my flight bag all the time.

Hello lospilotos

I even have it myself. It's great. But in this specific case it still doesn't tell you WHY this happens. Look it up yourself.

That's why I'm hoping to read a post with a plausible answer. It bugs me when I'm told "there isn't one..".