Narrative:

I was the first officer; and PF for flight from ZZZ to ZZZ1 on nov/xa/08. Approximately 100 mi southeast of abc intersection we encountered the following messages on the EICAS: presn automatic fail. Immediately; we could feel the effects of cabin depressurization in our ears; and noticed that the cabin was climbing at an alarming rate (approximately 3000 FPM). The most immediate thought that came to both me and the captain was to descend. We queried ATC immediately and notified them that we needed a lower altitude immediately. Unfortunately; ATC didn't respond immediately; and the cabin continued to climb. We started to descend at a rapid rate (4000 FPM) as we obtained clearance from ATC to descend from FL350 to FL250. The approximately time we descended without clearance was no more than 5-10 seconds. During the descent I called for the presn automatic fail checklist; and my captain proceeded to run the checklist. The pressurization system was switched into manual mode and the cabin pressurization was stabilized. ATC notified us that the supervisor would like to speak to the captain after we arrived; and proceeded to give us a phone number. Upon talking to them once reaching ZZZ1; it was determined that we should have declared an emergency in order to justify our altitude deviation prior to receiving a clearance. We were told that the ATC was not going to file any paperwork; as there were no aircraft within our vicinity. The supervisor was very kind and understanding; and both the captain and I agreed that declaring an emergency would have been the best thing to do. Looking back is just too easy. It was determined that the automatic pressurization unit failed when we received the EICAS message: presn automatic fail; and we felt the rapid ascension of the cabin altitude in our ears. It was evident that we deviated altitude as soon as I initiated a descent. Once we received the EICAS message; we determined the best corrective action was a rapid descent. I initiated a rapid descent by selecting a 4000 FPM descent in the fgcu; the autoplt was engaged throughout the entire maneuver. During the descent I called for the checklist procedure for automatic pressurization failure; and the captain read and complied with the listed material. The failure of the automatic pressurization unit was out of our control. The altitude deviation was a result of our own corrective action without first consulting the appropriate checklist. We got a bit too excited and did not wait for the appropriate ATC clearance before vacating our assigned altitude. After reviewing the checklist; it is evident that a descent is only required for the second part of the checklist; not the first; which is what we did. The problem with this situation was that the captain and I panicked a bit at the rapid rate at which the cabin altitude was ascending. Rather than calling for the checklist immediately as I should have; we initiated a rapid descent. The captain and I were afraid that the rate was going to continue to increase; therefore leading to the rapid maneuver. I think that this particular event and checklist should be stressed during the training process in order to prevent what we did. Also; in complying with the checklist; the captain and I discovered that there is a problem in the EMB145; in that the 2 guarded switches (dump pushbutton; and manual pushbutton) on the automatic pressurization system are unidentifiable. It is required in the checklist for this event that the manual pushbutton be selected; but the captain and I could not ascertain which button was the dump button or the manual button. We did not want to push either without knowing which was which. It took a conversation with maintenance control to ascertain which button was which. When the button is pressed; the function of the button is then illuminated. This needs to be visual before the button is pushed! I strongly recommend that the checklist be edited to say which button is which.

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Original NASA ASRS Text

Title: AN EMB145'S AUTOMATIC PRESSURIZATION CONTROL UNIT FAILED AT CRUISE. AN IMMEDIATE EMERGENCY DESCENT WAS BEGUN BUT THE CREW DID NOT DECLARE AN EMERGENCY. THE DUMP AND MANUAL PUSHBUTTONS ARE NOT IDENTIFIED ON THE PRESSURIZATION CONTROL PANEL.

Narrative: I WAS THE FO; AND PF FOR FLT FROM ZZZ TO ZZZ1 ON NOV/XA/08. APPROX 100 MI SE OF ABC INTXN WE ENCOUNTERED THE FOLLOWING MESSAGES ON THE EICAS: PRESN AUTO FAIL. IMMEDIATELY; WE COULD FEEL THE EFFECTS OF CABIN DEPRESSURIZATION IN OUR EARS; AND NOTICED THAT THE CABIN WAS CLBING AT AN ALARMING RATE (APPROX 3000 FPM). THE MOST IMMEDIATE THOUGHT THAT CAME TO BOTH ME AND THE CAPT WAS TO DSND. WE QUERIED ATC IMMEDIATELY AND NOTIFIED THEM THAT WE NEEDED A LOWER ALT IMMEDIATELY. UNFORTUNATELY; ATC DIDN'T RESPOND IMMEDIATELY; AND THE CABIN CONTINUED TO CLB. WE STARTED TO DSND AT A RAPID RATE (4000 FPM) AS WE OBTAINED CLRNC FROM ATC TO DSND FROM FL350 TO FL250. THE APPROX TIME WE DSNDED WITHOUT CLRNC WAS NO MORE THAN 5-10 SECONDS. DURING THE DSCNT I CALLED FOR THE PRESN AUTO FAIL CHKLIST; AND MY CAPT PROCEEDED TO RUN THE CHKLIST. THE PRESSURIZATION SYS WAS SWITCHED INTO MANUAL MODE AND THE CABIN PRESSURIZATION WAS STABILIZED. ATC NOTIFIED US THAT THE SUPVR WOULD LIKE TO SPEAK TO THE CAPT AFTER WE ARRIVED; AND PROCEEDED TO GIVE US A PHONE NUMBER. UPON TALKING TO THEM ONCE REACHING ZZZ1; IT WAS DETERMINED THAT WE SHOULD HAVE DECLARED AN EMER IN ORDER TO JUSTIFY OUR ALTDEV PRIOR TO RECEIVING A CLRNC. WE WERE TOLD THAT THE ATC WAS NOT GOING TO FILE ANY PAPERWORK; AS THERE WERE NO ACFT WITHIN OUR VICINITY. THE SUPVR WAS VERY KIND AND UNDERSTANDING; AND BOTH THE CAPT AND I AGREED THAT DECLARING AN EMER WOULD HAVE BEEN THE BEST THING TO DO. LOOKING BACK IS JUST TOO EASY. IT WAS DETERMINED THAT THE AUTO PRESSURIZATION UNIT FAILED WHEN WE RECEIVED THE EICAS MESSAGE: PRESN AUTO FAIL; AND WE FELT THE RAPID ASCENSION OF THE CABIN ALT IN OUR EARS. IT WAS EVIDENT THAT WE DEVIATED ALT AS SOON AS I INITIATED A DSCNT. ONCE WE RECEIVED THE EICAS MESSAGE; WE DETERMINED THE BEST CORRECTIVE ACTION WAS A RAPID DSCNT. I INITIATED A RAPID DSCNT BY SELECTING A 4000 FPM DSCNT IN THE FGCU; THE AUTOPLT WAS ENGAGED THROUGHOUT THE ENTIRE MANEUVER. DURING THE DSCNT I CALLED FOR THE CHKLIST PROC FOR AUTO PRESSURIZATION FAILURE; AND THE CAPT READ AND COMPLIED WITH THE LISTED MATERIAL. THE FAILURE OF THE AUTO PRESSURIZATION UNIT WAS OUT OF OUR CTL. THE ALTDEV WAS A RESULT OF OUR OWN CORRECTIVE ACTION WITHOUT FIRST CONSULTING THE APPROPRIATE CHKLIST. WE GOT A BIT TOO EXCITED AND DID NOT WAIT FOR THE APPROPRIATE ATC CLRNC BEFORE VACATING OUR ASSIGNED ALT. AFTER REVIEWING THE CHKLIST; IT IS EVIDENT THAT A DSCNT IS ONLY REQUIRED FOR THE SECOND PART OF THE CHKLIST; NOT THE FIRST; WHICH IS WHAT WE DID. THE PROB WITH THIS SITUATION WAS THAT THE CAPT AND I PANICKED A BIT AT THE RAPID RATE AT WHICH THE CABIN ALT WAS ASCENDING. RATHER THAN CALLING FOR THE CHKLIST IMMEDIATELY AS I SHOULD HAVE; WE INITIATED A RAPID DSCNT. THE CAPT AND I WERE AFRAID THAT THE RATE WAS GOING TO CONTINUE TO INCREASE; THEREFORE LEADING TO THE RAPID MANEUVER. I THINK THAT THIS PARTICULAR EVENT AND CHKLIST SHOULD BE STRESSED DURING THE TRAINING PROCESS IN ORDER TO PREVENT WHAT WE DID. ALSO; IN COMPLYING WITH THE CHKLIST; THE CAPT AND I DISCOVERED THAT THERE IS A PROB IN THE EMB145; IN THAT THE 2 GUARDED SWITCHES (DUMP PUSHBUTTON; AND MANUAL PUSHBUTTON) ON THE AUTO PRESSURIZATION SYS ARE UNIDENTIFIABLE. IT IS REQUIRED IN THE CHKLIST FOR THIS EVENT THAT THE MANUAL PUSHBUTTON BE SELECTED; BUT THE CAPT AND I COULD NOT ASCERTAIN WHICH BUTTON WAS THE DUMP BUTTON OR THE MANUAL BUTTON. WE DID NOT WANT TO PUSH EITHER WITHOUT KNOWING WHICH WAS WHICH. IT TOOK A CONVERSATION WITH MAINT CTL TO ASCERTAIN WHICH BUTTON WAS WHICH. WHEN THE BUTTON IS PRESSED; THE FUNCTION OF THE BUTTON IS THEN ILLUMINATED. THIS NEEDS TO BE VISUAL BEFORE THE BUTTON IS PUSHED! I STRONGLY RECOMMEND THAT THE CHKLIST BE EDITED TO SAY WHICH BUTTON IS WHICH.

Data retrieved from NASA's ASRS site as of May 2009 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.