Most experts say airlines should board aircraft randomly if they seek maximum efficiency. But that's a problem since most carriers want to reward their best customers with early boarding. Under the circumstances, airlines do the best they can.

United Airlines has been around for nearly 100 years, and yet the airline, like most of its competitors, has not learned how to most efficiently board an airplane while letting its best customers have first crack at overhead bins.

Five years after United streamlined its boarding process, creating five groups, each with its own special lane - similar, some say, to a cattle chute - the airline is testing changes in Los Angeles, one of its seven North American hubs, an airline spokeswoman said.

United's test does away with chutes for most customers. United's premium customers still board first, in groups 1 and 2, but they're asked to stay seated until they're called.

Once they're on the plane, United is calling one coach group at a time, asking everyone else to wait. Coach customers will board through the same walkway as group 2.

The airline still will let some customers board early if they need extra time.

On paper, United's current boarding strategy seems passenger-friendly enough, but it has had an unintended consequence: Passengers often line up in their boarding chutes long before departure, overcrowding gate areas. In a town hall meeting last year, United President Scott Kirby told employees that when the airline asked customers why they lined up so early, many said they only joined the queue because they saw others in it.

During the current trial, there's "no need to line up before your group is called," United told passengers. "Simplified lanes with new signs will allow you to relax at the gate or within the airport longer."

A United spokeswoman said, "Our customers have told us they want a better experience when boarding and we are looking for ways to improve it for them."

CONSTANT REFINING

United is the latest airline to try to refine the boarding process. Last year, American began calling nine boarding groups, up from four, not including special calls for premium customers. Meanwhile, in October, JetBlue Airways introduced boarding groups, rather than calling customers by rows. Other airlines constantly tweak procedures.

For airlines, boarding is a tricky proposition. While they'd like to board planes faster, that's not the only priority. They must also reward their best customers with early boarding, and often that slows the process. High-value customers prefer the front of the plane, and since they board first, it can be tough for travelers in the back to reach their seats. Airlines also must let families board together, even though it may slow boarding.

Under the circumstances, airlines do the best they can. Some use an algorithm to randomly assign zones, while others, particularly outside the United States, allow all non-elite coach customers to board at the same time - essentially a free-for-all. United is known for its WilMA approach, boarding windows first, then middles, then aisles. But that doesn't account for all the premium customers and elite frequent flyers who board first.

When it introduced its chute-based system in 2013, United thought it had a winner. "We're going to be making several changes to address crowding and consistency and, frankly, to help you just understand where you should stand as you wait to board," a United executive posted on Flyertalk, a message board for frequent flyers.

But almost from the beginning, customers have complained the process treats them like farm animals. United has occasionally tested possible replacements, including a similar trial late last year in Cleveland. So far, though, nothing has emerged as a winner.