Bart Jansen

USA TODAY

Two big airlines are bunching flights at their hubs in an attempt to tighten connections and raise revenue. For passengers, the move could shorten layovers and -- in some cases -- help airlines increase the number of connecting options between cities.

But a new industry report warns that this effort by American and United to "bank" their flights could lead to delays and cancellations.

The concern focuses on how the airlines are clustering their flights – known in the industry as "banking" -- rather than spreading them throughout the day as they had following the terrorist attacks in 2001, according to masFlight, a data company that analyzes airline and airport operations.

While the changes haven't led to delays yet, masFlight warned of the potential for flight disruptions for American at Miami and Dallas-Fort Worth airports, and for United at Washington Dulles and Houston Bush airports.

"Past flight banking has caused a significant increase in flight delays," said Tulinda Larsen, masFlight's president. If there is a delay or cancellation with relatively full planes, travelers could find it harder to book a new flight, she warned.

"These inconveniences can deter people from flying and could cost the U.S. economy billions of dollars," Larsen said.

Banking offers significant advantages. Travelers get shorter connections between flights. American expects to boost revenue $200 million by offering more feasible connections and getting people where they're going faster, although the airline has to manage busier staffing through the peaks and valleys of arriving and departing flights.

United Airlines found the scheduling changes advantageous for customers, according to spokeswoman Jennifer Dohm.

"Banking can help reliability, as we have fewer total departures, and a peaked schedule has valleys to allow our operations to recover quicker in the event of a disruption, so delays don't cascade throughout the day," Dohm said.

"While peaking can sometimes create longer taxi lines, we've built that into the flight schedules," she added. "However, we are not currently seeing any materially increased taxi times or enroute hold minutes due to rebanking."

Airports strive to coordinate staffing for busy times with services such as security and customs checks.

"Washington Dulles International Airport has flexible facilities to support and accommodate constantly changing airline schedules," said spokesman Chris Paolino.

In busy airports, tight connections could force travelers to scramble between distant gates. Crowded taxiways and gate areas have slowed the turn-around times at some hubs to get planes back in the air.

The time it took for American to resupply a plane increased 5 minutes at Miami and 8 minutes at Dallas-Fort Worth, according to a masFlight comparison of four similar weeks in 2014 and 2015. United's turns grew 10 minutes at O'Hare and 4 minutes at Houston, according to the study.

Regional jets that funnel travelers to the major airlines fared even worse, with turns taking 8 minutes longer at Miami, 16 minutes longer at DFW and 13 minutes at Chicago O'Hare, according to masFlight.

One reason delays of at least 15 minutes aren't materializing yet in Transportation Department statistics is because airlines pad their schedules with longer flight times, according to masFlight.

American's on-time rates in April were 78.2% at DFW, 83.1% at Miami and 81.7% at O'Hare, according to the department. For comparison, American's overall on-time rate that month was 80.2%, which improved on the 76% during the year leading up to that month.

United's on-time rates in April were 85.3% at Washington Dulles, 75.1% at Houston Bush and 80.7% at O'Hare, according to the department. For comparison, United's overall on-time rate that month was 79.4%, which improved on the 76.3% for the year leading up to that month.

Henry Harteveldt, a travel-industry analyst with Atmosphere Research Group, said the new schedules allow for more flights, which mean more seats and more connections to serve travelers during the busy summer months. But he suggested neither American nor United wants to undermine the momentum they've gained in improving their businesses and balance sheets.

"No question, though, the new hub structures increase American's and United's operational pressures," Harteveldt said. "American and United both know if they get their new hub schedules wrong, and delays and losses mount, they've essentially shot themselves in the foot."