A former Boeing 747-400 passenger jet converted into the world's largest firefighting airplane has spent much of wildfire season sitting at a Colorado airport.

Right now, the reasons seem largely bureaucratic as the Interagency Air Tanker Board spends up to three months evaluating the latest series of tests of the airplane's ability to drop fire retardant.

In a statement from the U.S. Forest Service, the agency says it granted a "six-month interim approval that expired June 15 to allow them to make modifications in their tanking system, which earlier tests indicated needed adjustment to ensure property delivery of fire retardant."

Those tests were carried out on June 19.

The agency says it scheduled to do another test in the fall of 2016 "to determine if the issues had been addressed."

But a Forest Service spokesperson says the company elected to take an overseas firefighting job in the meantime.

With fires around the West, the issue has attracted the attention of Congress.

U.S. Rep. Krysten Sinema, a Phoenix-area Democrat, who in a letter to the forest service says such interim approvals typically run 18 months, three times longer than what Global SuperTanker Services was granted.

The airplane has yet to be used by the Forest Service or any other federal or state wildland firefighting agency. But the airplane has been used to fight fires--overseas. One job was in Chile, the other in Israel.

What can it do?

It can drop nearly 20 thousand gallons of water, fire retardant or firefighting gel. It can also distribute two of the three at the same time.

While the company says it can fly to anywhere on the planet in less than 20 hours, its largest market is here at home in the U.S. Global SuperTanker Services eventually wants four to five planes, and believes there is a market for them.

The forest service says as it plans for the 2018 fire season it will solicit "call when needed" for very large air tankers, those capable of carrying 8,000 gallons or more.

Converted DC-10 aircraft that carry more than 11 thousand gallons have been common sights over fires, including those fought during the record Washington state fire seasons of 2014 and 2015.