Narrative:

Began our initial descent into [destination]; having left FL350 for FL280. While level at FL280 and preparing for descent into known icing we selected eng and wing anti-ice on. We immediately received a level one alert; air sys 3 off. Center gave us a descent clearance. We declined the descent and asked for holding to work a problem. Completed with QRH and saw the air system display page had the 3 air manifold highlighted in red. As well as red manifold light on overhead air system panel. The QRH level 1 air sys 3 off consequences stated if the manf light remains illuminated but the air manf 3 fail alert is not displayed (and it wasn't) the air system is depressurized. Consequence; wing anti-ice not available.so we knew landing in [destination] not an option. So we satcom dispatch. As we discussed the situation with the dispatcher; she conference in maintenance control. He believed we had a manifold 3 failure. We didn't have the air manf 3 fail alert or the aural waning; but as a precaution reviewed QRH procedure. No action was necessary; for the bleed and isolation valves were closed and manifold pressure was at 5. Consequence; do not repressurize system; avoid known or forecast icing and land at nearest suitable airport. Additionally; ran abnormal non alert operation with less than three bleed sources.again as a precaution we [advised ATC] and stated we needed to divert to an airport without icing conditions. Dispatch really struggled to find suitable airport. First they said ZZZ; so we informed ATC and headed that way. Then they discovered icing pireps. Dispatch asked us to turn around and head south west while they search. We turned around and climbed to FL360. They couldn't make up their mind. Finally; ATC asked if they could help and we told them we were looking to land at airport without icing. They suggested ZZZ1 and we told dispatch and they agreed; so we diverted and landed at ZZZ1 without incident.

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Original NASA ASRS Text

Title: Air carrier Captain reported after selecting engine and wing anti-ice the crew immediately received an alert indicating a system fail.

Narrative: Began our initial descent into [destination]; having left FL350 for FL280. While level at FL280 and preparing for descent into known icing we selected Eng and Wing Anti-ice ON. We immediately received a level one alert; Air Sys 3 OFF. Center gave us a descent clearance. We declined the descent and asked for holding to work a problem. Completed with QRH and saw the Air System Display page had the 3 Air manifold highlighted in red. As well as red manifold light on overhead air system panel. The QRH level 1 Air Sys 3 OFF consequences stated if the MANF light remains illuminated but the AIR MANF 3 FAIL alert is not displayed (and it wasn't) the air system is depressurized. Consequence; Wing Anti-ice not available.So we knew landing in [destination] not an option. So we SATCOM Dispatch. As we discussed the situation with the dispatcher; she conference in Maintenance Control. He believed we had a manifold 3 failure. We didn't have the Air Manf 3 Fail Alert or the aural waning; but as a precaution reviewed QRH procedure. No action was necessary; for the bleed and isolation valves were closed and manifold pressure was at 5. Consequence; do not repressurize system; avoid known or forecast icing and land at nearest suitable airport. Additionally; ran abnormal non alert operation with less than three bleed sources.Again as a precaution we [advised ATC] and stated we needed to divert to an airport without icing conditions. Dispatch really struggled to find suitable airport. First they said ZZZ; so we informed ATC and headed that way. Then they discovered icing PIREPs. Dispatch asked us to turn around and head south west while they search. We turned around and climbed to FL360. They couldn't make up their mind. Finally; ATC asked if they could help and we told them we were looking to land at airport without icing. They suggested ZZZ1 and we told Dispatch and they agreed; so we diverted and landed at ZZZ1 without incident.

Data retrieved from NASA's ASRS site 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.