B737 Pressurisation-Setting FLT ALT lower than actual

[housecarl]

We got a crack in one of the windows outer pane on our B737 classic during climb. We stopped climb and performed QRH which says no further action, resume normal operation. Now to the point, captain then selects a lower FLT ALT on the pressurization panel. He says because of the window damage. I understand you want to reduce the stress on the window, but to select a lower FLT ALT in AUTO mode on the pressurization panel, will this really reduce the stress?

The cruise mode is activated when the airplane climbs to within 0.25 psi of the selected FLT ALT. During cruise, the controller maintains the cabin altitude slightly below the selected LAND ALT, if the differential pressure between the selected LAND ALT and FLT ALT is less than or equal to 7.8 psid above 28,000 feet or 7.45 psid below 28,000 feet. If the differential pressure between the selected LAND ALT and FLT ALT is greater than these values, the controller maintains a pressure differential of 7.8 psid above 28,000 feet and 7.45 psid below 28,000 feet

-B737 classic FCOM

[citizensun]

did you continue climbed to your flight plan level and set the FLT ALT to a lower altitude
or
you maintained the altitude you stopped climbing and reset the FLT ALT to a lower altitude?

[blackmail]

b737 presurization

hello housecarl,
i would suggest : 1) follow QRH & don't invent new or home made procedures. 2) maybe i'm missing something, but i would think the flight altitude on the pressurization panel should be HIGHER than your actual flt altitude, so the delta pressure should be LOWER than the nominal values & exert a lower strain on the window ? anyway, this was the case, as you stated that you correctly stopped the climb when the event occurred. i think your captain aggravated the situation by reducing the flt altitude on the pressurization panel BELOW the actual flt altitude
kind regards,
bm

[NSEU]

The cruise mode is activated when the airplane climbs to within 0.25 psi of the selected FLT ALT.

How sophisticated is the 737 pressurisation system? If the Captain has set a false cruise altitude, will the 737 even go into (pressurisation) cruise mode?
This statement from the manual seems to be saying that the aircraft is looking for actual pressure altitude being +/-0.25psi of target pressure altitude. If it never sees this, it may never go into cruise mode, but will still be in climb mode. This might be one of the reasons for not messing with the QRH.

I can perhaps understand what the Captain is thinking. If he sets the cruise altitude to below 28,000', then, if the pressurisation system transitions to cruise mode, the controller will pick the lower differential pressure of 7.45psid (which, on the surface, appears to be a good thing).

Is the aircraft sophisticated enough to recognise level-off as an interim cruise mode? (like later generation aircraft)?

Anyway, we probably need more details to make better sense of this.

e.g. initial set target altitude, level off altitude, new cruise setting, , etc.

Regards
NSEU

[despegue]

Ok, your Captain is wrong about several things here...

1) The outer pane is not a load-bearing pane (the INNER one is) and thus is not directly influenced by Delta P.

2) If you want to decrease Delta P. on the B737 Classic, go to SBY mode ( with the non-digital pressurisation controller) and change your cabin altitude to a higher one than the standard placarded value ( given for a Delta P of 7.4psi below 28000' or 7.8psi above 28.000'). Selecting a lower flight level than your actual cruising level in auto mode will cause greater delta P, going into the yellow band above 8.0 psi I think (but this I don't recall ever doing so I might be wrong here)

3) FOLLOW THE QRH!!

[EW73]

Having operated in STBY mode several times in various airplanes, who in there right mind would intentionally go there if they didn't really have to?

That is, the QRH didn't direct you to that mode!

crazy....

EW73

[Loose rivets]

With a more serious failure, we used to:

Turn off Windshield Heat to stop the hot glass spitting at us.

Shut the taps, breaks out and nose down.

Screw the cabin to beyond 9k or until the knob fell off.


One night, I complimented my oppo on the smooth pre-checklist actions. 'Third one I've had this month.' Says he. Flippin' £3,000 quid a shot in those days...the price of a house.

[RobinR200]

As i understand it. U planned say FL330. Crack at FL210. Level off at FL250. QRH procedure. U remain in FL250. Cpt resets FL ALT indicator. That´s very very reasonable. Cause due to ur level off the deltaP rose further (since the auto cruise mode was not yet reached). Adjusting the FL ALT assures the correct deltaP again.
However if u climbed further again and then resetted the FLT ALT to a lower level, that´s no good. See previous comments.

[blackmail]

b737 presurization

robin,
if you stop the climb of the aircraft, the cabin altitude stops climbing & the delta pressures stops as well.
bm

[RobinR200]

u r correct...
nevertheless, it was an ok action to avoid the unscheduled press light.

[blackmail]

b737 presurization

robin,
it was not. 1) it was not in the QRH. 2) a flight altitude equal to actual altitude would be ok to avoid the unscheduled pressurization caution light, when initiating a descent, but not lower than actual flt altitude.
bm

[RobinR200]

disagree.
it was an action besides the QRH...he was thinking ahead.
that is....if the FLT ALT setting was not below current ALT.

[BOAC]

We stopped climb and performed QRH which says ?no further action?,

- different QRH to mine!

I cannot fathom how we have got to 12 posts here when we do not know:

1) Which window
2) Which pressurisation Ctrl system
3) What actions WERE taken

Actions differ accordingly. Mind you, none say change the Flt Alt! QRH rules?

[blackmail]

b738 pressure

robin,
for the last time : flight altitude(not "land" altitude) set on the pressurzation panel, should never be lower than actual flt altitude. if it is, the delta pressure will eventually go up to 9.1psi, which is the safety setting of the two overpressure relief valves. the cracked window will appreciate.
bm.

[RobinR200]

i guess u cant understand my comments.
i allready wrote 2 times ...IF FLT ALT was not set below current.
Now its the third.
hope u get it.

[blackmail]

b738 pressure

robin,
it would help, if you try to write correct english, given or taken a few typo's,
bm,

[RobinR200]

funny, guess u have level 6

[NSEU]

Cause due to ur level off the deltaP rose further (since the auto cruise mode was not yet reached). Adjusting the FL ALT assures the correct deltaP again.

Why would levelling off increase the delta P?

Or did you mean that there was an increase in delta P due to the increase in altitude (between the time of the crack and the completion of level off)?

Is there are direct relationship between current altitude and pressurisation, or is there a sophisticated pressure schedule? i.e. a schedule based on the takeoff altitude and the previously set cruise altitude (and the controller follows this schedule irrespective of current altitude unless a Delta P is exceeded)?

Hopefully someone has a Component Maintenance Manual for the Controller. There seems to be a lack of data on this system (as well as a lack of data on the event).

Rgds
NSEU

[KristianNorway]

Hello housecarl

This was a lot of discussion when it seems people don't really read your posting. Don't get bothered, it's just part of the pprune package.

I'm not sure whether your captain is rationalizing or prioritizing correctly, but technically what he is doing is correct. Here is how I think:

If the differential pressure between LDG ALT and FLT ALT is more than 7.8 psid it will maintain this pressure on the inner window when flying above 28.000'. If you then select a FLT ALT below 28.000', it will regulate to 7.45 psid. In other words you get a lower differential pressure, and thus a lower total pressure on the inner window.

On the 738 we see this all the time during cruise climb. Up to and including 37.000' it regulates to 7.8 psid, and when setting FLT ALT to more than 37.000', say 38.000', you see the cabin descending to create the computed pressure of 8.35 psid. Looks somewhat weird the first times when you are expecting a climb.

The problem with this is that a lower differential pressure than the 733 was designed for at those altitudes gives you a higher cabin altitude. Maybe not significant, but still.
What I do get is that he wants to better his chances and reduce the load on the inner, load bearing window, but this is not supposed to be necessary, and it lowers the pressure in the cabin.

This is as you see a tradeoff. Whether one thing is better to do than the other I don't know, I haven't been in this trade for a very long time myself.

Personally, because of my lack of experience, I would not regulate the pressure because it is not mentioned in the QRH.

I hope that helped. Cheers

[housecarl]

Thanks Kristian,
My question was as the title says "setting FLT ALT lower than actual", the incident itself was just to give some info why I raised the question, and have no relevance. We followed the QRH accordingly.

Window crack or not, what happens if you select a FLT ALT lower than the actual FL in AUTO mode.
I agree that you should not try to invent the wheel again, but I now understand more what he was trying to do.

This was not the question but If you really need to reduce the differential pressure, our QRH says:

14 CAB ALT indicator . . . . . . . . . . . . . . . .9,000 feet
15 Pressurization mode selector . . . . . . . . . . . STBY
16 CABIN RATE selector. . . . . . . . . . . . . .As needed
17 Choose one:
Minimum safe altitude is below 14,000 feet:
>>Go to step 18
18 Descend to below 14,000 feet.
19 Maintain a cabin differential pressure of 2 psi or
less by limiting flight altitude to below 14,000 feet.

DO NOT USE FOR FLIGHT.

Thanks all for your inputs