B737 Loss Of Both Generators

[LEM]

One thing puzzles me, and since that never happened to me, I can't figure it out exactly:
In the LOSS OF BOTH ENGINE DRIVEN GENERATORS non normal cklist, there is no reference to controlling the pressurization manually, but that's what we are left with if on battery only.
In the damn 30 minutes you have to save the day, it is unlikely you are gonna remember this problem without a call in the cklist.

How could they forget to mention that?
What will happen to your pressurization if you just leave it alone while rushing for the runway?

[Flying_Tuur]

I think it's up to where you happen to be when you loose you're GEN's because if I remember well, the outflow valves will stay in the position they are in, unless moved in Manual Mode. So if you forget about your pressurization, the worst thing that can happen is that either your negative pressure relief valve will open, or your overpressure valve will open.
Since you will probably descent to land, and the outflow valves will probably not be fully closed, I expect you to catch up with the cabin altitude rather than the other case. If needed, the cabin altitude warning horn will sound anyhow, and you can gain control of the cabin, so no harm done...

Please coorect me if I got it wrong.

FT

[HotDog]

You got it right. When the airplane reaches cabin altitude, differential pressure will be zero and the cabin will descend at the same rate as the airplane. Hang on to your eardrums.

[wellthis]

Only three switches in the pressurization panel are AC: engine bleeds, and isolation valve, and they will stay where they were, which is normally open in flight for bleed air. The pack valves are DC, so they'll continue to work. When all AC power is intrupted to the AUTO system, it automatically changes to STBY which will continue to pressurize the airplane.

In the meantime, according to the checklist attempts are made to restore AC power. If unable to engage ENG driven GENs, you might be able to start the APU, and then it's almost normal operation.

If not, you'll land on STBY system, and prompted by AUTO FAIL light you'll do that checklist, position the switch to STBY to get rid of the AUTO FAIL light, and continue with the checklist which will look after your pressurization situation. The AC/DC STBY busses continue to be powered by BAT bus and static inverter. Upon landing, as any emergency landing, the FO will do the recall items at 60 kts, one of which is to select STBY on Bat to allow for power transfer to STBY busses on the ground. Just remember that with no AC system, the AP will stay engaged, but it won't work. Hope this helps.

[LEM]

Ok, wellthis, you are talking about -200.

On classics, engine bleed valves are DC (DC Bus 1 and 2).
If it is true that DC dies, AC lies, now I have another doubt: will we still have presurization without APU?

It is not clear 100%, but I think STBY system won't work on battery only.
In the All Generators Inoperative section (6.20.16), the Air System equipment that will continue to work are:
. A/C pack valves
. altitude warning horn
. manual pressurization control
. Pack trip off lights
. Bleeds trip off lights

Also, from a Bus equipment list given by an engineer (not official) I see the STBY Pressurization control in under DC Bus 2 and Transfer Bus 2.

So probably what we are left with is just Manual DC control.

Btw, I would set STBY Power on BAT on final before landing, to avoid a sudden blackout at touchdown.

[wellthis]

Well, your transfer bus #2 is powered by GEN1, so it won't be working, but check and see if DC bus2 isn't powered by BAT bus? And as for bleed valves, regardless of how they're powered, they'll stay where they were meaning open, and the airplane continues to be pressurized. As long you don't change altitude you pressure difference stays the same. (although you might have to descend to get a better APU start).

[LEM]

wellthis, DC Bus 2 is powered by TR2 (with "help" from TR3 also), not from BAT Bus (originals & classics are the same).

Hope I'll manage to find out exactly how the whole story works next time in the box.

Thankyou all :D

LEM