The Transportation Department fired back at two airline consultants who say the rule that puts a three-hour limit on tarmac delays is harming travelers.

DOT said the study released "offers a misleading and premature assessment of the impact of the new passenger protections." The consultants, Darryl Jenkins and Joshua Marks, say the rule is hurting travelers because it leads the airlines to cancel more flights in an effort to avoid fines. The DOT can fine airlines up to $27,500 per passenger for holding them more than three hours.

The study looks at data for May, the first month the rule was in effect. The DOT said the study is "far too narrow to yield defensible conclusions about future airline trends." But the consultants say the impact to passengers is clear: more cancellations, more grief and possibly a trickle-down financial toll from tarmac violations.

The government thinks airlines can avoid tarmac delays by scheduling better. But the consultants argue that a bulk of long tarmac delays are unavoidable.

Thunderstorms are one of the main causes of flight delays.

There were five flights stuck on the tarmac for more than three hours in May - all five were weather-related.

Tarmac delays have fallen since the new rule. The five flights stuck for more than three hours in May compares with 35 three-hour delays in May 2009. Tarmac delays also dropped in April compared with a year earlier.

Cancellations went up, and the consultants argue the ripple effect of cancellations are more harmful to consumers than extended delays. Airlines canceled about one-third more of their domestic flights in May compared with May 2009. Cancellations nearly doubled from April.

The consultants say that while 110,000 passengers a year will be spared long tarmac delays, at least 200,000 passengers will be on a flight canceled to comply with tarmac delay regulations.

They predict that in the long run the rule will hurt even more passengers. For every tarmac delay that is prevented by the rule, they forecast another four flights will be canceled.

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