Narrative:

About 25 minutes after departure; we received an APU fire master warning on the EICAS. The [APU] was off; but because of the type of annunciator; we followed the APU fire procedure in the QRH which had us discharge the APU fire bottle. The APU fire light remained illuminated after the bottle discharged. We declared an emergency with mexican airspace (don't remember which center) and asked for help finding a suitable airport. One that had a control tower; decent weather; fire equipment and a runway longer than 5;000 ft. The airport suggested to us was 28 NM to the north. He asked if we had it insight and told him no we were in IMC and way too high. We asked for vectors and he gave us a turn for our descent. Initially into the 20's then down to 11;000 ft. At that point he told us to contact tower. I queried him and asked if the tower had radar and the language barrier got in the way and we just switched to tower. He immediately said cleared to land. The only response from the tower even after I asked him if he had radar to get us on the approach was; cleared to land. Told the tower that was not possible since we needed vectors for an approach. As I was setting up approach; I mentioned to the pilot flying that the airport is a mountainous airport sitting at 7;000 feet. As I was dialing in the MDA the radar altimeter popped up and said we were only 1;650 AGL and still descending. There was no ground proximity warning but I firmly stated to the captain; climb the airplane; climb the plane now. I immediately switched back to center and told him what happened. Again the language barrier was a big issue. He didn't understand what I was telling him. We were both visibly shaken at this point. Once we got back to altitude; we determined; based on that low altitude event and the lack of assistance from ATC; the best course of action was to get out of mexican airspace as soon as possible. We were 39 minutes from our intended destination (and the closest us airport) and decided going there was the safest course of action. We also had a good idea that based on the multiple warnings of the APU fire and the length of time the light was on that there was most likely not a fire and it was a false indication. About 180 miles from destination; I contacted the tower on the secondary radio and told him of our situation. Advised him we were an emergency aircraft and we would be stopping on runway to allow the emergency personnel to visually check the APU area for evidence of a fire. After given the all clear we taxied to customs.

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Original NASA ASRS Text

Title: B737 flight crew at FL350 experiences an APU fire warning with the APU not operating. QRH procedures are complied with but the fire warning remains and the crew elects to divert to the nearest suitable airport. The airport is deemed to be unsuitable after descending to low altitude; due to lack of ATC services and mountainous terrain. The crew then elects to continue to their original destination after determining that the fire warning is likely false.

Narrative: About 25 minutes after departure; we received an APU FIRE master warning on the EICAS. The [APU] was off; but because of the type of annunciator; we followed the APU fire procedure in the QRH which had us discharge the APU fire bottle. The APU fire light remained illuminated after the bottle discharged. We declared an emergency with Mexican airspace (don't remember which Center) and asked for help finding a suitable airport. One that had a Control Tower; decent weather; fire equipment and a runway longer than 5;000 FT. The airport suggested to us was 28 NM to the north. He asked if we had it insight and told him no we were in IMC and way too high. We asked for vectors and he gave us a turn for our descent. Initially into the 20's then down to 11;000 FT. At that point he told us to contact Tower. I queried him and asked if the Tower had radar and the language barrier got in the way and we just switched to Tower. He immediately said cleared to land. The only response from the Tower even after I asked him if he had radar to get us on the approach was; cleared to land. Told the Tower that was not possible since we needed vectors for an approach. As I was setting up approach; I mentioned to the pilot flying that the airport is a mountainous airport sitting at 7;000 feet. As I was dialing in the MDA the radar altimeter popped up and said we were only 1;650 AGL and still descending. There was no ground proximity warning but I firmly stated to the Captain; climb the airplane; climb the plane now. I immediately switched back to center and told him what happened. Again the language barrier was a big issue. He didn't understand what I was telling him. We were both visibly shaken at this point. Once we got back to altitude; we determined; based on that low altitude event and the lack of assistance from ATC; the best course of action was to get out of Mexican airspace as soon as possible. We were 39 minutes from our intended destination (and the closest us airport) and decided going there was the safest course of action. We also had a good idea that based on the multiple warnings of the APU fire and the length of time the light was on that there was most likely not a fire and it was a false indication. About 180 miles from destination; I contacted the Tower on the secondary radio and told him of our situation. Advised him we were an emergency aircraft and we would be stopping on runway to allow the emergency personnel to visually check the APU area for evidence of a fire. After given the all clear we taxied to customs.

Data retrieved from NASA's ASRS site as of July 2013 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.