They're making headway on the frenzied skies at big US airports

Seat upright and belt cinched against your stomach. You're waiting for takeoff and primed for the scream of jet turbines. But 20 or 30 minutes, even an hour later you're still - rather uncomfortably - waiting.

Traffic delays have become so bad at Chicago, New York, Denver, and Atlanta that last month the Federal Aviation Administration (FAA) told the airlines: Either produce a plan to cut down on delays or the FAA will impose one.

At press time, ''a tentative agreement'' had been reached by commercial carriers for Hartsfield Airport in Atlanta, La Guardia and Kennedy in New York, and Stapleton in Denver, according to Dan Henkin, spokesman for the airline industry's Air Transport Association. Agreement is close on Chicago's O'Hare, but progress on Newark, N.J., remains slow.

For more than a week, officials from some 50 airlines have been juggling schedules to reduce flights during rush hours: 7 to 10 a.m. and 4 to 7 p.m.

Competition for the business traveler's dollar and a surge in the number of new airlines since deregulation in 1978 have resulted in too many flights stuffed into these time slots.

To alleviate the problem, the airlines have agreed to spread out flights over a longer period. No flights are expected to be cut, except at Newark.

For customers, the schedule changes will produce fewer delays, a little less elbowroom on prime-time flights, and no increase in ticket prices, airline analysts say.

''You may also find that connecting times between flights are a bit longer, maybe an extra 10 or 15 minutes,'' says Mark Daugherty, an analyst at Dean Witter Reynolds, the big brokerage firm.

What effect will the changes have on the carriers?

''This is not going to make a huge difference to the airlines,'' says John Pincavage, airline analyst at Paine, Webber, Jackson & Curtis, another brokerage.

''The fact of the matter is, there are delays. The planes are all leaving the gate at 9 o'clock and then getting in line on the runway until they can leave. This just formalizes the practical reality. Instead of all going at 9 o'clock, some will leave at 9:05, some at 9:15, and so on. It'll just cut out the delays and make things smoother,'' he adds.

Robert Joedicke, an analyst at the Shearson Lehman/American Express brokerage , speculates: ''With fewer flights (in the peak hours) it could increase the load factor (the number of passengers) on those flights, which would improve cost efficiency.''

One airline in particular may get squeezed by the changes. People Express has been expanding rapidly into new cities. The scheduling agreements may put a crimp in expansion plans, not only out of Newark but into its planned new markets, such as Denver.

Throughout the talks, Newark has been a hub of contention. It is the center of People Express operations. Since the carrier launched its low-cost flights in 1981, traffic volume has ballooned.

With more than 300 daily takeoffs and landings at Newark, People provides about 50 percent of the airport's service. But competitors say People's growth has made it difficult to get flight clearance during peak hours.

Eastern Airlines, one competitor at Newark, is pushing for People to reduce flights. Other airlines flying out of La Guardia and Kennedy have also taken a hard line, saying traffic into Newark affects traffic at the other airports.

Could it be that People's competitors are ganging up on the young upstart?

''I don't think they're sticking it to them,'' harrumphs Mr. Joedicke at Shearson Lehman/American Express. ''They may be unhappy because People is expanding, but I also don't see why they should be pushed out to make room for the new kid on the block. . . . There's a limit on what the facility can handle.''

Finally, the airlines are saying that if they have to make flight changes, so must private aviation.

''As it stands now, a corporate jet with four or five executives can hold up a 747 with two or three hundred passengers,'' says Mark Daugherty at Dean Witter. Carriers would like the FAA to put peak-hour flight restrictions on general-aviation aircraft.

The business aviation community, a strong lobby in Congress, points out that it holds a small percentage of the time slots. Of the 68 arrivals and departures per hour allowed by the FAA at La Guardia, only six are for general-aviation aircraft.