737 Symbol Generator

[b737bbj]

With reference to the 737 Classic Non-normal checklist 'SG FAIL', does anyone know why the Autopilot is not allowed to be re-engaged with the loss of a SG?

Thanks.

[comxtek]

I'm sure this doesn't help you, but for now all I can say is GOOD QUESTION!!!

[eagerbeaver]

i trained on the 300 series, its been a while but i seem to remember something about a comparator between the 2 sg's and fcc's, but u can definitely engage one autopilot, just not dual channel approaches.

if i am wrong shoot me down, i will be researching deaper though, watch this space

[comxtek]

I think I may have figured it out. If AP "A" is in CMD, then the both Capts and F/O's flight director commands are coming from FCC-A (master).
Under normal conditions, the Left Symbol Gen is displaying info from the master FCC-A (Both FCCs send the same output data to each SG) and the Rt Symbol gen is also receiving data from FCC-A.
That way in a sense, you have 2 sources of the same info to compare.
Well let's say you lose the No.1 Symbol Generator and switch to BOTH ON 2, now you have info from FCC-A displayed on both sides.
Not as much redundancy.

That's all I can come up with.
But there is no question that the A/P can be engaged with a faield onside SG.

Any more....

[Sink Rate]

You can engage either autopilot BUT the (our) QRH says you are not to as there is no comparater.

If the SG fails on the side the the AP is engaged then it will drop out. But after selecting the EFU both on 1/2 switch then it can be re-engaged.

Having had it happen twice now I'm sure it is far safer to re-engage the a/p and monitor it very closely rather than hand flying the remainder of teh flight in possibly demanding WX.

From experience I can say though that often by selecting the EFI switch, giving it a minute and then switching it back to the normal position the SG normally springs back to life never to give a problem again!

SR

[pancho]

Look guys the QRH is quite specific, it says in this event DO NOT ENGAGE EITHER AUTOPILOT. The reason is you no longer have a comparator crosscheck. YES you can engage an autopilot but dont, even if you decide to monitor it closely. This is one of the famous Boeing "we dont provide checklists for multiple failures" but infact that's what they are saying in this checklist, if you select an autopilot and something else comes unscrewed, you dont have any warning except from the SBY instruments which can easily go unnoticed now that you have removed the THIRD (and defining) instrument(s).

[b737bbj]

Ladies/Gentlemen,

Thank you for your replies and thoughts. Some good info.

Regards.

[Sink Rate]

If you are hand flying you still don't have a comparator to cross check.

The two EFIS screens are giving the same info, whether it be correct or otherwise, as one is just "slaved" to the other.

I stand by my thoughts that it is best to engage an A/P thus giving you more capacity to cross check against the standby instruments.

I don't see how hand flying helps the situation.