Six Frontier Airlines aircraft that were damaged by hail at Denver International Airport are back in service.

The Denver Post reports the airplanes re-entered service Thursday after a thorough examination. DIA reported ¾-inch hail Wednesday, but no other airlines reported damage. One reason could be that Frontier’s planes have composite materials in their horizontal and vertical stabilizers — the fin and small wings found on the planes’ tail.

Because damage to composites is often not visible, crews use ultrasonic technology to test the condition of the internal structure, which takes longer than checking an all-aluminum aircraft.

Carbon composite material is lighter and more durable than aluminum and is gradually being integrated into aircraft manufacturing.

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Information from: The Denver Post, http://www.denverpost.com

Photo Credit: In this Feb. 22, 2010 file photo, Frontier Airlines jetliners sit stacked up at gates along the A concourse at Denver International Airport. David Zalubowski / Associated Press