B737-300 QRH SG Failure. Do not enage the autopilot.

[Centaurus]

Page 10.2 of the B737-300 QRH is headed "SG Fail"

After taking the QRH actions there is a line that states "Do NOT engage either autopilot" The word NOT is in capitals indicating a serious matter.
No explanation is given and I am unable to find out why the autopilot(s) should not be engaged. Appreciate ideas.

Also in RVSM airspace you need an autopilot. Otherwise altitude penalty applies. If critical on fuel this could be embarrassing. If say the captains SG fails, you may be faced with the situation (however unlikely) that it may be essential to not operate the EFI transfer switch in order to theoretically have one autopilot engaged and still meet RVSM requirements. In other words accept that the captains EADI and EHSI stay blank. Of course that brings with it more problems re flight instruments for the captain - but worse still could be fuel consumption at the lower altitude if forced out of RVSM.

[Cough]

but worse still could be fuel consumption at the lower altitude if forced out of RVSM.

Even worse is the subsequent failure!

A flight restricted to FL250 due pack failure increased the flight plan fuel by 250kg from BCN to blighty...FL280 isn't going to be as much!

The reason for the no autopilot is that the independent sources of data cannot be monitored. i.e. Capt SG fail, if 'A' autopilot is engaged you do not know what modes it is operating in as you cannot see the FMA's. For 'B' autopilot, a slow roll of the IRS would mean the aircraft would slowly roll too, along with ALL EFIS displayed AH's (watch the standby!). So if you are flying, you will detect the failure mode earlier...

Lastly, when you get the failure, after the QRH is done you may want to 'check' the other SG by going EFIS XFR Switch back to Norm. On 95% of occasions things will be back to normal...Happy days!

[gonso]

Also in RVSM airspace you need an autopilot. Otherwise altitude penalty applies. If critical on fuel this could be embarrassing.

And what exactly is wrong with diverting instead of trying to press on to destination?

[c100driver]

In that previous thread Sink Rate said;

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.

I really hope this guy is out of aviation by now!