There were five flights stuck on the tarmac for three hours or more in May, the first month under a new rule banning lengthy tarmac delays, the government said Thursday.

It will be several weeks to a month before any fines may be levied against the airlines for violations, as the Department of Transportation investigates. The maximum fine is $27,500 per passenger for airlines that do not return their planes to the terminal when they are delayed on the tarmac for three hours or more. There are exceptions for safety and security reasons.

Tarmac delays have fallen significantly since the government announced the new rule. While there were five flights stuck for more than three hours in May, that compares with 35 three-hour delays in May 2009. Tarmac delays also dropped in April compared with a year earlier.

United Airlines operated four of the five flights that were stuck this May. One of those United flights stayed on the tarmac for almost five hours. All four of the United flights were bound for Denver on May 26 when severe thunderstorms and hail swept through Colorado. Denver International Airport had 30-to-60-minute delays on average that day and limited use of runways.

United spokeswoman Jean Medina said all four of the flights were diverted to Colorado Springs where weather caused additional delays. She added that all the customers were given the chance to get off the planes.

Thunderstorms are one of the main causes of flight delays because they are difficult for airlines and airport officials to predict.

The fifth was a Delta Air Lines flight from Atlanta to Dallas-Fort Worth on May 28. That flight sat for two hours in Atlanta when the tarmac was closed for lightning. Delta spokesman Anthony Black said air traffic control denied the pilot's request to turn back to the gate, and the plane sat on the tarmac for another hour before it ultimately took off.

Overall the on-time performance of U.S. carriers declined in May from the same month a year ago. Flights were on-time 79.9 percent of the time in May, down from 85.3 percent in April and 80.5 percent in May 2009.

US Airways was the most successful major airline in getting travelers to their destinations on time, 85.3 percent of the time. Hawaiian and Alaska Airlines had the highest on-time rates overall in May.

Comair, which operates as Delta Connection, had the worst ranking in May with 67.1 percent of its flights arriving on-time.

Airlines also canceled more flights in May compared with the month or year before. Carriers canceled 1.2 percent of their scheduled domestic flights during the month, compared with 0.9 percent in May of last year and 0.7 percent this April.

Customers also appeared to be less satisfied with their air travel experiences during May. DOT received 801 complaints from customers about airline service in May, up 22 percent from a year ago.