Industry giants United Airlines and Delta Air Lines, as well as discounter AirTran, have abandoned the traditional back-to-front boarding of their coach sections. America West has made the move, too.

The motive: Time is money for airlines trying to stick to a schedule and to keep their planes flying as many hours as possible. Traditional back-to-front boarding clogs the cabin by drawing too many people into a confined area at once, says Eitan Bachmat, a professor of Israel's Ben Gurion University of the Negev.

"If we were infinitely thin — that is, if people were made of cardboard — then back-to-front is wonderful," says Bachmat, who has, in studying boarding systems, delved into time-and-space issues like those involved in Einstein's theory of relativity.

The recent changes:

•Window, middle and aisle. United now groups passengers at the gate according to their seat's letter designation. The carrier boards window passengers first, followed by those in middle and aisle seats. Passengers traveling in pairs or groups are kept together. Its boarding is now four to five minutes faster, saving the company about $1 million a year, says Robin Urbanski, a spokeswoman.

In 2003, America West, now part of US Airways, started boarding window-seat passengers before those in middle and aisle seats.

The procedure saves about two minutes in a typical 20-minute boarding process, says America West spokesman Carlo Bertolini. The carrier merged with US Airways in September and is in the process of integrating operations under the US Airways brand. It will introduce the boarding procedure on all US Airways flights by early 2007, Bertolini says.

•Alternating zones. AirTran's new procedure, introduced last July, boards passengers in the back four rows first, followed by the front four rows. The process is repeated until the cabin is full.

Delta Air Lines uses a variation in its new system, introduced in early 2004. The zone that includes the back few rows boards first, then a middle section followed by a front section. It then goes back to a rear section.

Letting passengers board without assigned seats, like discounter Southwest, is very efficient, Bachmat says. In being spread out throughout the cabin, passengers can "sit independently of each other." Even airlines with assigned seating would benefit from random boarding, he says.

Frequent flier Bill Haughton, a sales executive in San Diego, says the new methods don't impress him. Reducing the amount of carry-on baggage is the best way to speed the process, Haughton says.