The so-called very light jets are so much slower than larger planes that controllers say they must isolate them whenever air traffic is heavy, USA Today reported Wednesday.

"They don't mix very well" with larger jets in the air, says David Cook, who heads the controller's union at Jacksonville Center in Florida.

The three-passenger Eclipse 500 jets being flown in the Southeast by start-up DayJet have not caused any major disruptions to date. Federal records show that no serious accidents have occurred.

However, Cook and controller Steven Wallace of Miami say if hundreds of the small jets were in the air, it would be difficult to accommodate them without major delays.

Pilots are also concerned about the slower moving aircraft.

Flying behind an Eclipse 500 in a large jet would be "a bit like following a farm vehicle," says Capt. Rory Kay, safety chairman for the Air Line Pilots Association.