21.   Investigators were uncertain if the crash of a fourth passenger jet was part of the attack.

22.   It is an upgraded version of the Ground Proximity Warning Systems that are standard among commercial passenger jets.

23.   Last year, Boeing announced that it would enter the market to supply live Internet connections on passenger jets.

24.   New passenger jets are so automated that two pilots do the work that used to be performed by three, yet they have lighter workloads.

25.   Nor did either have the automatic collision-avoidance equipment installed on passenger jets, Black said.

26.   Officials from various wings of the military gave conflicting explanations for why the fighters got so close to the passenger jet.

27.   On the tarmac at the spacious new Havana airport, completed three months ago, I count commercial passenger jets from eight identifiable countries.

28.   One passenger jet on hold, three flight plans, three distinct and separate destinations.

29.   Outside, passenger jets dip slowly from the cloudless sky down to the newly expanded Palm Springs International Airport.

30.   One national aviation official, who asked not to be identified, said Wednesday that comparing military and passenger jets would be ludicrous.