Chicago - Delta, and United have already said goodbye to their last Boeing 747 jumbos. It seemed the end of an era for the Queen of Skies. But now something unexpected happens. Parked 747-400s are coming to the maintenance hangars to return to the skies.

The demand for retired superjumbos of Boeing is increasing because the global freight market is booming.

Freighter operators are buying Boeing 747-400s from the years 1993-2009 if there is still one available.

Demand for the discarded 747-400s has picked up,
says William Flynn, head of the world's largest operator of Boeing 747 freighters, Atlas Air Worldwide.

The wet-lease company is about to add six more 747-400 freighters to its fleet.

But there are only a limited number of aircraft,
says, Flynn. After all, airlines are very much considering whether to invest in a 403 million dollar 747-8 freighter.

The investment in used Boeing 747-400 appears profitable for many freighter operators. In 2017, the Chinese SF Airlines bought two newer 747-400 in an online auction via the Taobao Internet platform for $50 million each. Its low price compared to a brand new 747-8 makes the used superjumbos highly profitable even at high fuel prices.

Almost every airworthy 747-400 is being brought from parking places to the maintenance hangars for the full D-Check program which costs $3 million, experts say.

The conversion of a passenger version of the 747 to the freighter, costs around $30 million per aircraft.