Frequent travelers at Atlanta's airport are familiar with the dread of seeing an escalator closed for repair, then heading for a slow, luggage-laden trudge up a set of stairs with dozens of other weary passengers.

They may be heartened to learn that the airport is in the midst of a $51.4 million upgrade of aging escalators, elevators and moving walkways.

Some of Hartsfield-Jackson International's escalators and elevators date to 1979, installed for the terminal's opening the next year. They have now "reached the end of their reliable service life," according to airport officials.

They said the rehab work is needed to avoid the "ever increasing potential" that breakdowns could inconvenience passengers and disrupt operations at the world's busiest airport.

As the oldest escalators and elevators turned 30 in 2009, a survey of the airport's equipment, including more than 100 escalators, 106 elevators and 28 moving walkways, found that repair downtime was rising.

The oldest escalators needed work or repairs three times more often than newer units.

Airport officials this month got Atlanta City Council approval for an agreement with the Atlanta Airlines Terminal Corp. worth up to $38 million to cover the third and final phase of upgrades. A total of 38 escalators, 45 elevators and 16 moving walkways will be upgraded, most of them from 1979. The project is being funded through airline-backed bonds.

The work is expected to take until August 2016 to complete. That's because it is done in phases to minimize the number of escalators and elevators out at any one time, according to Jim Drinkard, Hartsfield-Jackson's assistant general manager for planning and development. Taking equipment out of commission can cause inconvenience for travelers, and he said the airport is adding customer service representatives to answer questions and help with crowd control.