Narrative:

Just prior to a descent clearance to FL290 I heard a click behind me followed by a caution message AC ess bus; followed by a number of status and caution messages that were associated with the loss of the AC ess bus. We evaluated the messages; and I as pilot not flying; referenced the pom (pilot's operating manual). We were directed to press the ac essential transfer switch on the overhead panel; which should have transferred the bus to bus 2. However when the switch was pressed; it did not light up and did not transfer the bus to ac bus 2. We referenced what items we had now lost due to the bus not being powered. I looked at circuit breaker's (circuit breaker) and there were no cbs popped. We analyzed the status messages and caution messages. We had a display cool message associated with some inoperative fans ... Therefore I referenced pom which directed us to turn the display fan to standby and select full cool in the flight deck with the temperature controls. We gave it a little time but the display cool caution message persisted ... Pom guidance told us we had about 30 minutes before the screens would shut down due to overheat. We showed ourselves about 17 minutes from landing; and had been working on the abnormal for about (estimated) 3-5 minutes. I decided that any undue delay could take us to the 30 minute limit ... And we needed to get on the ground as soon as possible so as not to allow a situation that would be much worse be created. We wanted to take advantage of all resources to assure the best possibility to a safe and uneventful landing with abnormal conditions. ATC cleared us direct to a VOR thence direct the airport which helped shorten the time as well.when we were 8 miles or so on final; the display cool message extinguished. This was my main concern as the remainder of the messages was not of a critical nature. I wanted to make sure that ATC was fully informed; so I let them know that the message that prompted my emergency declaration had extinguished; and that we did not anticipate any assistance necessary once on the ground. ATC thanked us for the information and cleared us for landing. The landing was uneventful; we stopped clear of the runway to reanalyze our messages and confirm that we were good to continue to the gate. We also contacted the flight attendants to let them know that there were no further problems; and that we would continue to the gate.we talked with the mechanics afterwards; and they could not figure out what happened; apparently they cycled the left generator and everything came back normal. Possibly this could be part of the procedure if the ac essential transfer switch doesn't work.

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Original NASA ASRS Text

Title: A CRJ-900 declared an emergency and diverted when they suffered the loss of some Essential AC power and equipment cooling fans.

Narrative: Just prior to a descent clearance to FL290 I heard a click behind me followed by a caution message AC ESS BUS; followed by a number of status and caution messages that were associated with the loss of the AC ESS BUS. We evaluated the messages; and I as pilot not flying; referenced the POM (Pilot's Operating Manual). We were directed to press the ac essential transfer switch on the overhead panel; which should have transferred the bus to bus 2. However when the switch was pressed; it did not light up and did not transfer the bus to ac bus 2. We referenced what items we had now lost due to the bus not being powered. I looked at Circuit Breaker's (CB) and there were no CBs popped. We analyzed the status messages and caution messages. We had a DISPLAY COOL message associated with some inoperative fans ... therefore I referenced POM which directed us to turn the display fan to standby and select full cool in the flight deck with the temperature controls. We gave it a little time but the display cool caution message persisted ... POM guidance told us we had about 30 minutes before the screens would shut down due to overheat. We showed ourselves about 17 minutes from landing; and had been working on the abnormal for about (estimated) 3-5 minutes. I decided that any undue delay could take us to the 30 minute limit ... and we needed to get on the ground as soon as possible so as not to allow a situation that would be much worse be created. We wanted to take advantage of all resources to assure the best possibility to a safe and uneventful landing with abnormal conditions. ATC cleared us direct to a VOR thence direct the airport which helped shorten the time as well.When we were 8 miles or so on final; the DISPLAY COOL message extinguished. This was my main concern as the remainder of the messages was not of a critical nature. I wanted to make sure that ATC was fully informed; so I let them know that the message that prompted my emergency declaration had extinguished; and that we did not anticipate any assistance necessary once on the ground. ATC thanked us for the information and cleared us for landing. The landing was uneventful; we stopped clear of the runway to reanalyze our messages and confirm that we were good to continue to the gate. We also contacted the Flight Attendants to let them know that there were no further problems; and that we would continue to the gate.We talked with the mechanics afterwards; and they could not figure out what happened; apparently they cycled the left generator and everything came back normal. Possibly this could be part of the procedure if the ac essential transfer switch doesn't work.

Data retrieved from NASA's ASRS site as of April 2012 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.