Here is the situation: At FL380 cruise "Auto Fail" warning light at pressurization panel light up. ALTN pressurization system didnt work neither. Pressure was controllable via manual positioning of valve. Cabin altitude, diff pressure and cabin rate were OK, warning horn remained silent. Cruise altitude and airport altitude was entered correctly.

 

Tried to switch back to auto system several times without success, and at about FL120 auto systems started to work like nothing happened.

 

No failures registered by Aircraft CDU menu.

 

My question is why system failed at first place, how there was no failure indication in aircraft menu, and how system start to work again?

It has happened to me before when I forgot to set the cruising altitude in the pressurisation panel on the overhead. What I did was manually set the valve to regulate cabin altitude (using the placard to work out what the cabin altitude should be) - once I got the cabin altitude to what it should've been, both AUTO and ALTN pressurisations started functioning.

Assuming that the system was working (confirmed by no fmc messages) the only thing that causes the AUTO controller (and alternate wich is a second automatic feature) to fail is a cabin decompression. Now, in this case if the horn was still off we have a problem. Close the isolation valve and check that cabin is pressurizing, if cabin goes to a safe altitude the AUTO function will be restored.

tnx for answers,

Cabin was pressurized. Maybe problem is in the pressure difference? Can to much pressure at high altitude turn off auto system?

The controller will fail if it is not able to mantain the cabin differential (8.75PSID), this means that it must be in AUTO in that moment.

The controller will also fail if the cabin rate is above 2000Ft/min, this is probably your problem.

Why this happens?

Probably you set a low altitude on the control panel, this setting let the pressurization to remain almost stable in the cabin, then, when you were climbing, the maximum differential pressure was reached, the controller started to open the outflow valve to try to mantain the differential. In that moment the cabin was climbing with the aircraft vertical speed, or near it. If the vertical speed was over 2000ft/min, panel fails, if it cannot reach aircraft vertical speed, it will fail as it is unable to mantain diff. press.

It will also fail if cabin altitude reaches 15800fts.