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. :ok:

[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..:E,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:E
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?:E

[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.

[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.