AIRPLANES deteriorate with age. Parts wear out and quit functioning. Metal gets tired and breaks under loads it once withstood easily. Screws work loose and fall out. That's why frequent inspections of airliners are essential and why the Federal Aviation Administration is right in gearing up a new inspection program for the nation's older airplanes.

The boom in air travel since the end of rigid regulation had brought into being many new, small airlines and has brought out of the hangars many older airplanes. These are the planes on which the FAA needs to focus, although the recent accidents with the DC9s and DC10s show that not all airplanes have been doing their aging on the ground.

This problem of geriatric airplanes is a relatively new one for the FAA and the major airlines. Up until the 1970s, new generations of airplanes kept going into production so frequently that the airlines were retiring planes before they got really old. But the cost of new planes and the relative stability of the passenger market in the 1970s drove down the birth rate of new designs. As a result, the average age of the air fleet gets older and the need for rigorous inspections becomes more urgent. The first Boeing 707s have now been flying for 20 years and the DC9s for almost 15.

Fortunately, airplanes are less like people than like Model-T Fords in their ability to withstand the debilitating effects of age if they are properly cared for. The DC3s, although no longer in use on scheduled airlines, have been flying for 40 years and are still regarded by some air buffs as the safest planes ever built.

The FAA has turned to the right source by seeking advice from the Air Force on how to spot older planes in need of repairs and what to do about them. The early B52s have been kept in the air well beyond their life expectancy, and the Air Force has built up a wealth of data on how to keep them flying safely. With that data, the FAA should be able to come up with a sound inspection and repair program that will minimize the effect of age on the safety of commercial airliners.