Frontier Airlines is adding a second daily flight between Aspen and Denver on Oct. 4-6  days when runway extension work at the Aspen-Pitkin County Airport will prevent United Express from flying in and out.Frontier serves Aspen with one flight daily during the fall offseason, but with United unable to fly in and out for the three-day period in October, Frontier decided to boost its service, according to Bill Tomcich, president of reservations agency Stay Aspen Snowmass and the resort’s liaison to the airlines.

A one-way economy seat on the one flight Frontier would normally fly from Denver to Aspen on Oct. 4 was priced at $119.70; a one-way economy seat on the added Oct. 4 flight was listed for $172.70 on Wednesday.The shortened runway also affected United this week on Tuesday through Thursday and Frontier’s single flight in and out appears to be packed, said David Ulane, assistant aviation director, on Wednesday. Frontier did not add an extra daily flight this week, while it’s enjoying an Aspen monopoly for three days.

The runway extension is scheduled to finish in early November. The airport remains open during the construction work, but for the two three-day periods  this week and again in October the available runway is reduced to 6,000 feet. That’s too short for United’s jets.The extension of the runway — to 8,000 feet when it’s finished (it had been 7,000 feet long)  is expected to ease weight restrictions that force commercial airlines to leave seats empty in order to take off with sufficient fuel during warm weather. It will also provide opportunities to add flights to more distant locales, according to airport officials.

During the final 45 days of runway work, navigational equipment that has been taken off line will also affect United flights into Aspen whenever a visual approach to the airport isn’t possible.As of Thursday afternoon, visibility had not resulted in the diversion of any United flights but a tailwind, in combination with a runway that had been shortened to 6,500 feet, led to the diversion of two of the airline’s flights, Ulane said. The navigational aid was taken off line on the evening of Sept. 8.