B737 - No Autoland with one power source

[172_driver]

Check your AFM, should be in Section 3.1 (Normal Procedures).

FlyingStone, I found it in the AFM. Thank you!

Autoflight section. It is also mentioned in the Systems Description of your FCOM stating (paraphrased) that for dual channel A/P operation, 2 AC generators are required.

vapilot2004, Your manual must be better than mine. Nothing in either volume of FCOM, unless I have overlooked it somewhere. But I had a good look.

I agree the engineers are a wealth of information.

But I think the question might not be is a Cat111 operation legal with one AC source, but rather will the aircraft actually DO an Autoland if coupled to an ILS and the A/P not disconnected.

In the first post I referred to a thread from 2002, here is a copy paste of a seemingly credible(?) poster. The info is from the maintenance manual.



During a dual channel operation, each FCC must be on an isolated power source. FCC A gets dc power from DC bus 1 and FCC B gets dc power from DC bus 2. Normally, the two dc buses are connected with the bus tie relay. If either FCC is in the active G/S mode (ie when glides slope captures),it will cause the bus tie relay to open. This will separate the DC power sources to the two FCCs.

What happens when the Bus Tie Relay Opens and why does it open?

When EITHER the 'A' FCC or the 'B'FCC is in the G/S active mode it tells the IFSAU (just a control and comparison computer)
to isolate the DC Buses. The IFSAU needs confirmation from the Standby Power Control Unit that the power is normal by providing a ground to the IFSAU.This causes the G/S engaged position to be activated in the IFSAU and cause the ground to be removed from the bus tie relay.

If the Bus Tie Relay has a ground from the IFSAU then the relay closes and the DC Buses are connected together, when the ground is
removed the rely opens and the buses are isolated.

The Bus Power Control Unit which comprises of the TR's usually get their power from different 115v ac buses (AC Bus 1 & 2).If the TR's suddenly get their power from one source ie when a transfer occurs the Bus Transfer Control Unit knows where the source of power is from (either the generators or the APU generator). It therefore knows that even though both buses are powered it is from a single source. It therefore sends a signal to both FCC's that the DC Buses cannot be isolated. Both FCC's now removes the flight director as it does not have its required redundancy for the Dual Channel approach.

[Denti]

I have never tried an autoland with just one power source available in real life. However the fail operational autopilot is of course able to do just that if the second power source becomes unavailable below Alert Height (200ft AGL). There will be no warning, no problem and a normal autoland and automatic rollout will follow, at least according to the system description and simulator experience. However, by that point the back trim is already in and the second autopilot does not disconnect.

As for starting the approach we have a reference table in our QRH (CAT II/III Requirements), but it is a company list and expressively states that it is for reference only, we have to check the MEL in any case as authoritative document. For CAT II/III operation it says that 2 generators are required (two out of three installed). The same is true for autopilots, two are required.

Interestingly, the MEL only states in the classic section (-300/-400/-500) that both generator busses have to be powered during CAT II/III operation, but not in the NG (-700/-800) section. However it states under the NG section that only one engine driven generator is needed as long as the APU is running throughout the flight which pretty much leads to the same thing (except for ER operation).

I heard stories since my classic days that one autopilot is enough for an autoland, but never tried it as it was a pretty pointless exercise in the middle of europe with alternates available every 10 flight minutes anyway.

[Centaurus]

The flare is via RA reading.

[Jwscud]

Another little gotcha is with one RA inoperative, the autopilot will disconnect and the FDs will disappear 2s after GS capture.

If you've lost one during flight and weren't planning an autoland, might come as a bit of a surprise to Bloggs and Smith as the aircraft banks and pitches when capturing both LOC and GS at once then disconnects at 30° AoB. Ask me how I know...

[framer]

why do I get the feeling I'm about to learn something?

It therefore knows that even though both buses are powered it is from a single source. It therefore sends a signal to both FCC's that the DC Buses cannot be isolated. Both FCC's now removes the flight director as it does not have its required redundancy for the Dual Channel approach.

And there it is. I'l follow up on this with our base Engineer but it would explain everything quite nicely if correct, especially Latetonites disappearing flight directors.

[latetonite]

small correction Framer: only the failed side FD looses roll. ( goes CWSr).

[Mats Hannesson]

It therefore knows that even though both buses are powered it is from a single source. It therefore sends a signal to both FCC's that the DC Buses cannot be isolated. Both FCC's now removes the flight director as it does not have its required redundancy for the Dual Channel approach.

Hi guys! As I understood we wouldn't have FD guidance at all since G/S is in active mode regardless of 1СH or BOTH due to required redundancy? Thanks.