B737 - No Autoland with one power source

[172_driver]

The trickle down of information from senior colleagues and the internet has educated me that if you are down to one AC power source an dual channel approach will not be possible due to lack of redundancy, even if both FCCs are powered. This was quite well described in an earlier PPRuNe thread from 2002 I found with help of Google.

What I am struggling a bit with is to find a reference to this in the operation manuals. Nothing in the Limitations section of FCOM Vol. 1, no QRH checklists mentions it. The Systems Description of FCOM Vol. 2 is of limited help. The MEL/DDG which in this company is used to determine CAT 2/3 status, also inflight, has nothing.

It's remote, but still not that remote it couldn't happen, that you end up with one AC power source and destination weather is crap. I am tired after a long day at work, but am I so tired I've missed something essential here? I am basically wondering where this info can be found in Your operations manual? (if at all..)

[FlyingStone]

Check your AFM, should be in Section 3.1 (Normal Procedures). A bit tricky, yes.

It normally says something like this (same text for Cat II and IIIa approaches):

Two sources of electrical power. (The APU generator may be used as an independent power source.).

You are correct - no Cat II/IIIa operations on 737 with only one AC source.

[vapilot2004]

As FS mentions, this information is in your Normal Procedures - 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. For Autoland, we know this is due to redundancy requirements.

In your configuration, if you have only one AC power source, but have both FCC's powered, your electrical configuration has the Main AC BTB closed. In this config, there is no redundancy in cases of a further AC/bus failure.

Grab an engineer and ask to have a look at the AC power schematic for your aircraft. The operative phrase "dual redundancy" should become clear to you.

I agree, the books fail to provide a detailed, direct connection from Autoland requirements to ELEC reconfiguration scenarios, however an overall systems understanding will make that light appear above your head, 172.

[Wizofoz]

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.

My understanding is that it will.

[latetonite]

As you need two AP 's in CMD, getting info from two FCC's , and the BTB is open, how you want to do that with only one AC source?

[framer]

My guess is that it would do it fine, but obviously you would only do it if it was your last option.

latetonite As you need two AP 's in CMD, getting info from two FCC's , and the BTB is open, how you want to do that with only one AC source?

If you've lost one generator and the BTB has closed I can't see anything critical that would be left without power. What is it that you think will be left unpowered?

[latetonite]

Framer:

The BTB opens at GS interception. You have a different model 737?

[de facto]

172,if you are down to a single AC source,you need to proceed to a suitable alternate...crap weather may not be that suitable..,diverting to your alternate(cat1 or better) would be a better option than trying to land AUTO in crappy weather,(i understand crappy as below cat1?).
One source,i wouldnt even try AUTO land..

[latetonite]

Worst case scenario, fly the approach in VNAV. Just be prepared to manage the last 50' yourself. And do not forget to flare.

[de facto]

Yes please dont forget to flare
Thanks in advance.

[framer]

The BTB opens at GS interception. You have a different model 737?

I guess I do latetonite, my BTB's are open most of the time but in this scenario they will be closed. My Cross Tie Relay does tend to fly open on G/S capture though.
My thinking is that if you're operating on one IDG and your APU is u/s, both transfer busses will be powered, they in turn will be powering your DC busses through the rectifiers both before, and after glide slope capture. You will of course have lost a lot of redundancy and protection, but I still see no reason why it wouldn't work.
Don't interpret that as me thinking it's a good idea, if a divert was not an option I think I'd rather do it manually on a protected ILS. Good conversation though.....why do I get the feeling I'm about to learn something?

[latetonite]

Framer,

You confuse me now.
So tell me why, one one engine, and no APU AC available, I loose FD 1 or 2 upon GS capture.
Am willing to learn something new as well, you see.

[de facto]

Wheres DENTI when we need him?

[latetonite]

Must be in a meeting negotiating things.

[TypeIV]

Tips for your next sim session if you get some time to spare, keep both start levers on idle.

Single channel one Eng ils approach on one generator. As the FDs dissappear at 50ft monitor what happens without disengaging the autopilot. (hint: manual thrust)

Single Engine approach with both startlevers on idle. Engage the second AP as normally and observe. Same hint as with previous exercise.

Doing these will connect some of the dots. Even without a three axis autopilot the aircraft shows that the guys in Seattle did a very good job. None of these are legal to use in real life of course.

[framer]

The only thing I can think of is that when you have done it you have also had a TR failed at the same time, or you are in a classic. I think ( from memory) that F/D 1 is from DC bus one and F/D 2 is from DC bus two, both of these should still be powered through the two powered transfer busses and TR1 and TR 2 even though the busses become electrically isolated from each other when the relay opens at G/S capture.
Keep in mind that I may not be correct , this is just what I think and I have not specifically tried it in the sim. Denti I will no doubt show up soonish and confirm or deny, until then, enjoy your day or night, for me it is sleep time.

[latetonite]

Thanks Framer and TypeV

I can spare some sim time on this. And keep you posted.

[Centaurus]

You can do an autoland on one autopilot in the 737 Classic. Works beautifully. However it does not trim back at 400 feet but that makes no difference on the flare which is normal.

[flyingchanges]

-800 sim autolands just fine on 1 AP too.

[de facto]

However it does not trim back at 400 feet but that makes no difference on the flare which is normal.

Interesting,so how does the A/P actually flare? I admit I'm baffled on that one.