A Skywest Canadair CRJ-200 on behalf of United, registration N431SW performing flight UA-5071 from Denver,CO to Prescott,AZ (USA), was enroute at FL320 about 20nm north of Flagstaff,AZ (USA) and 61nm northeast of Prescott when Albuquerque Center called the aircraft on guard frequency to change to their frequency. After a second call on guard frequency the crew reported on the center frequency and was immediately cleared to descend to 10,000 feet. A short time later the crew declared Mayday after having donned their oxygen masks and reported they were now executing an emergency descent to below 10,000 feet, the controller advised they were still in an area of MSA of 11,000 feet and provided a vector to take the aircraft quicker to an area where he could approve a descent to 10,000 feet. After levelling off at 10,000 feet the crew advised no further assistance was needed and requested a RNAV approach to Prescott's runway 21L. When ATC queried whether they had got their cabin pressure back the crew, already without oxygen masks again, replied they did not have the pressure back but were now low enough. The aircraft continued for the approach and a safe landing in Prescott.
The aircraft returned to service about 16 hours after landing.