B737NG Pressurization question

[sire]

Here the scenario:

Flight plan filed is at FL360 on a 5hr flight plan. There are two step climbs later to FL 380 and 400.

I always put the filed flight level (FL360) into the overhead cruise altitude selector. When we later climb to FL380 I put that in there.

I just flew with a guy that ask told me another guy told him (...and so forth) that if you right away put in your final cruise altitude (FL400 in this case) while still at the gate you will not get any pressure bumps when you step climb later at altitude.

Now, I never noticed any pressure bumps when doing it my way, but he said that his way the computer would know the final altitude and avoid the climb of the cabin when step climbing.

What do you think?

[reivilo]

I was teached and prefer to select the final cruise altitude straight away, so FL400 in your example. Some captains I fly with, prefer to select the initial cruise altitude. When I ask why, they say that it is to prevent an 'OFF SCHED DESCENT' annunciation when you don't make it to the final filed level.
However when you do it with reselecting the flight alt and you step up from at or below FL370, climbing to FL380 or higher, you will get an annoying pressure 'bump' because the pressurization controller will have to reschedule a new differential pressure (from 7.8psi to 8.35psi) which causes the cabin altitude to change in an uncomfortable way, because it usually forces a cabin descent to increase the cabin pressure and thereby increasing the differential pressure...
When you step up from, for example, an initial FL380 to a final FL400, it will not give the annoying pressure bump, because then the differential pressure will remain the same

I think the reason of avoiding the 'off sched descent' is a bit too cautious... Usually you will always climb to the final planned cruise altitude. However if you do not, which will be a deliberate decision, you have plenty of time to reset the flight altitude window. And even if you forget to do that, the annunciation coming on doesn't harm anything, just reselect a flight altitude just below your actual level and the amber light will go out and the controller will regulate a smooth descent to your planned landing elevation.

[PCars]

My manual says FLT ALT...... Set desired altitude.

In your example, I would set FL400 because I fully intend to be at that altitude at some time during the flight. The auto pressurization controller schedules the cabin pressure to climb as I climb to FL400 and will level the cabin pressure when I make an intermediate flight altitude level off. The cabin pressure will resume the climb on schedule as I proceed on up to FL400. Worst case...if I never get to FL400 due to bad rides and I forget to change the FLT ALT at top of descent, the aircraft will remind me with an OFF SCHEDULE DESCENT and Master Caution; quickly and easily solved by changing the FLT ALT to my present flight level. I tend to watch the step climbs very closely for fuel efficiency. Other pilots may not.


What happens when you change the FLT ALT during the step? The cabin pressure is 'rescheduled' and the controller makes a cabin pressure schedule 'correction', then resumes the climb versus just picking up where it left off. Worst case...what would happen if you climbed to FL400 and left the FLT ALT at FL360? The cabin pressure differential may be forced well into the yellow band and possibly all the way to positive pressure relief.

My manual, in my opinion, would seem to give you the option to do it either way though. So, it's all up to you given the above considerations.

Regards

[Checkboard]

BOEING COMMERCIAL AIRPLANE GROUP

FLIGHT OPERATIONS TECHNICAL BULLETIN

NUMBER: 737-09-3

DATE: July 1, 2009


... Investigation of pressurization incidents in the fleet has indicated that some operators are setting the FLT ALT before takeoff to an intermediate altitude, and then resetting the FLT ALT one or more times during climb until the final cruise altitude or flight level is reached. Although this method of operation will also provide a safe and comfortable cabin altitude for the duration of the flight, there are some considerations which may make this method of operation less desirable:

a) Multiple adjustments of the FLT ALT unnecessarily increase crew workload
during climb.

b) Intermediate FLT ALT settings are likely to result in higher cabin altitudes than if the final planned cruise flight altitude is selected. This is because the pressure schedule for lower altitudes uses lower differential pressure limits.

c) If the flight crew does not reset the FLT ALT when appropriate, flying above the selected FLT ALT can result in an overpressure situation and has resulted in activation of the pressure relief valves. Boeing has investigated several events where the airplane altitude exceeded the selected FLT ALT. When this happens, the system will overpressurize the cabin. When the differential pressure limit is reached, the system will first try to relieve pressure by opening the outflow valve. Under these conditions the pressurization system will allow the normal cabin rate limitations to be exceeded which will result in high cabin rates. Depending on the magnitude of the difference between the selected and actual FLT ALT, the pressure relief valves may be activated.

So, that's Boeing's position.

[reivilo]

Great, thanks!
What source did you get that from?

[Checkboard]

It says in the quote The source is Boeing. Flight Ops Tech Specs (this one is number 737-09-3) are sent to the flight departments of operators of Boeing aircraft.

[reivilo]

Thanks checkboard, you're right, it was a silly question..
I'm currently operating the 737 and was looking through my fcom bulletins, but couldn't find this particular one, so thats why I asked. Its strange that my flight ops department has not included this bulletin, I might send them an email what their view on the subject is.