HiI have just done two flight in the 737-800NGX Winglets aircraft and on both at one point or another in climb i got the cabin altitude warning horn. I checked failiures since i just wanted a routine flight and there was none, however it the horn was sounding. I stoppped the first flight and later did another that went the same way. Am I doing something wrong in the procedures/checklists? CRZ alt and DES alt was set correctly in the pressurization panel so what else should I be checking? Thanks

You didn't set the bleeds and/or packs correctly after your engine start. Work through that section of the checklist again on your next flight. Have fun!

Ok thanks, now I know where to look.

Did you set your cruise altitude on the overhead? By default its at 10,000 and if your cruise level is going to be 35,000 feet for example you need to set it to that. Also set the altitude of the airport your landing at.

Did you set your cruise altitude on the overhead? By default its at 10,000 and if your cruise level is going to be 35,000 feet for example you need to set it to that. Also set the altitude of the airport your landing at.

I don't think those will cause the horn to go off.

Did you set your cruise altitude on the overhead? By default its at 10,000 and if your cruise level is going to be 35,000 feet for example you need to set it to that. Also set the altitude of the airport your landing at.

Nothing to do with the ALT horn going off Paul.The horn will sound if the Cabin Altitude reaches 10,000.That would rarely happen in the RW.Fred.

They were set to 38000ft cruise alt and 150ft landing alt on both occasions. I think I just forgot to turn the L and R recirc fans back on after engine start. From what i can remember L and R packs were set to auto, ISO valve auto, bleed 1 & 2 on and APU bleed off and no illuminations out of the ordinary.

Leaving recirculation fans OFF would not prevent pressurisation. All they do is allow the packs to run at a lower airflow rate to improve fuel economy. They don't add air to the cabin.Kevin Hall

Also make sure ALTN is set to AUTO not MAN.

Nothing to do with the ALT horn going off Paul.

I see he was talking about something different now. Thought it was the other Horn he was talking about. The one that goes off when you forget to turn your packs back on or forget to set the cruise altitude. Since I am deaf I never hear any of these anyway lol.However I have experienced the following. I would set the cruise altitude to say 39,000 and gradually step climb up to that altitude. However sometimes I would forget and stay at 37,000. So when I reached the TD and started descending I got a yellow caution light and and unscheduled descent. Would this cause the the horn he is talking about to go off?

Edited January 8, 201214 yr by UAL115

The horn only goes off if the cabin alt goes too high. It has nothing to do with the cruise alt setting or the land alt setting. The auto/manual mode could cause this. Most simmers aren't messing with that switch, I would guess.

When in AUTO MODE, or ALTERNATE (ALTN is a second automatic function, same as the AUTO mode) there is no way to reach the cabin altitude warning pressure. At least in normal conditions.The controllers are programmed and designed to never exceed maximum differential pressure between cabin and external air, this thing takes its part in the pressurization profile that controllers make when they know the desired crz altitude and landing altitude.Wrong data are only uncomfortable for passengers and crew.Different is for the manual mode where the pilot manually moves the valve.In this case there is only the overpressure protection wich will open the safety valves when maximum differential pressure is reached.There is no protection in manual if the airplane is de-pressurizing (except for the warning)

There would be an audible alarm, though not cabin alt horn, but master caution alarm, if UNSCHED DES is present.