B737NG pressurization
 

If you dispatch a B737NG with the, say, right pack inop (max FL250). Then during cruise the left engine fails. QRH says to turn Left Pack OFF. Question: In this case the isolation valve should be open, due to dispatch with right pack OFF. In flight, will/can the Bleed Air from the no. 2 engine be used by the Left Pack to maintain pressurization, which in turn means you should NOT turn the left pack OFF? Should you manually move the isolation valve switch to OPEN, or just leave it in AUTO?

Can the APU maintain pressurization if you descent to FL170 and turn the left pack ON, and in this case, in what position should the isolation valve switch be in?

Can't get my head around this. All good inputs are appreciated.

:confused:

b737ng pressurisation
 

hello gotabefun,

in order to deconfuse, let's get back to the kiss principle: keep it simple, st*pid.
first of all, boeing qrh does not consider multiple failures at the same time, which doesn't mean they can't occur of course.
in your example the right pack = u/s. with the right pack switch off, the isolation valve, in the auto position, will be open.
the isolation switch logic( in auto), looks only to the 4 corner SWITCH position, eg.pack valves & bleed valves.if any corner switch is selected off, the isolation valve, in auto, will be open. ok, so here you fly with 2 bleed sources for one pack, which is acceptable. (1 bleed source for 2 packs, in flight, would not).

now, having a bad day, comes the left engine failure. you follow the qrh faithfully, which amongst other things, asks you to switch the pack on the affected(LH) side off.
if you do that, pressurisation is lost.

but here is a typical situation, where common sense & system knowledge come into play. the qrh gives you a hint in the brackets on the next sentence.

QRH:

PACK SWITCH (affected side)..................OFF
[causes operating pack to regulate to high flow in flight with flaps up]

in your example the "operating" pack is the right pack. but is not available.
so in this case, it is best to leave the LH pack switch on, fed by engine n°2 through the isolation valve in( auto )open.
if you are unshure of the isolation valve logic in auto position, you can always ask yourself:" do i want the isolation valve to be open or not?" & switch it open or close manually.

as for the apu, you will use it to substite the generator n°1.

apu bleed & electrical limitations:

from sealevel to 10000feet: apu can supply bleedair & electrics.
from 10000ft to 17000ft: apu can supply bleedair OR electrics.
above 17000ft to 41000ft(structural ceiling): apu can only supply electrics.

hope this brings some light in the darkness & yes, please land at the first suitable airfield.

Even on ECAM equiped planes you may encounter situations that will adress the judgment of the crew in order to solve a multiple failure.
I can give you one other ex for the NG:assume a bleed trip off,which will call for pack trip off on affected side.Then follow this with another failure on the other side,which will affect the other pack,for ex pack trip off,or smoke,or..etc.
You'll have to think out of the box.

b737ng pressurisation
 

hello alexban,

in your scenario, you really have a bad day.
but,firstly, the b737ng has no ecam( electronic crew alerting messages).
and this is some kind of a problem for companies using the new boeing standard operating procedures,(sops), because these new procedures are now in line with other more advanced boeing models like the b777, which have the full ecam capability. by this i mean, the checklists are reduced to an absolute minimum(only safety critical items remain) & are common to more than one type of aircraft, which in itself is ok. but for the b777 these checklists have the back up of a full ecam system, whereas the b737ng has no such back up.

now, back to your original scenario. it's more or less the same situation as proposed by gotabefun: left bleed trip off with no reset possible, the checklist ask you to switch left pack off in order to have the right pack go into high flow, in flight, with the flaps up. if now that right pack trips off, also with no reset, then there is again no need to have the left pack off. so here once more some quick thinking & system knowledge are required : engine n°2 feed the left pack through the isolation valve.
for the smoke scenario, always first go in full protection mode(mask/goggles or full face mask) on oxygen & establish communication. then determine smoke origin(airco or electrical) & follow qrh accordingly, eventually followed by a smoke evacuation procedure & get the bird on the ground asap.
hope this explanation helps.

Blackmail ,the new procedures on the 737 does not affect the QRH,and in no way there is a reduction of the items required to complete a malfunction using the qrh.
Those procedures changed mostly the areas of responsability for both pilots,during different phases of flight.The items that existed before,are still there,but in some cases must be done by the other pilot than before.
The checklists does not require the backup of the ecam,actually,for the qrh,the ecam replaces the standard qrh book.
The QRH is very good on the 737,but ,like the ecam ,does not cover all the possible situations and multiple faults.Mostly on multiple failures,you have to know the systems,and use good judgment.The same is valid for the ecam,proved more than once on the bus ,for ex.
Why do you think there is no ecam on the 737? I think for commonality between dif series,as we know that the 737 dates long before the ecam..till 2005 the 737 did 76 milion flights compared for ex,with the 6 milion flights done by the A320 family (newer plane,state of the art ecam ). Considering there are still -200 flying,you have the reason for the ecam not being there. I wonder if the NG is ecam capable,if desired by any customer?

qrh
 

hello alexban,

you are absolutely right, the new boeing sops only concern the normal procedures & not the nonnormals.

i don't think an ecam is an option on the ng.
maybe an ecam warning, clearly visible in bright red on a screen, would have saved the day, e.g. in the helios accident, something like:
"oxy mask.............................done/100%"
"communication......................establish(boom/mask:mask)"
"cabin alt above 10.000ft........check pressurisation controller AUTO", instead of that confusing cabin altitude/config. warning horn.
but that's history now & that system is not avail anyway.