When a four-engine jetliner carrying 75 passengers crashes into the desert floor here soon, it will set off one of the most intensive federal investigations in aviation history.

The passengers will be human-like dummies, all strapped in their seats and wired to the teeth with electronic sensors.

They will be the victims of an $11.8 million government effort to learn whether people can be given a better chance to survive a crash than they have today.

If the experiment is a success, the Federal Aviation Administration will be under strong pressure to require jetliners to carry a chemical additive that will prevent or retard the kerosene jet-fuel fire that has proven deadly in otherwise survivable accidents.

ICI Americas Inc., the company that manufactures the additive, estimates that its use would increase the cost of jet fuel by as much as 6 percent and add $2 to $4 to the price of the average airline ticket.

Transportation Secretary Elizabeth Hanford Dole said recently that about 80 percent of the 914 fatalities in U.S. airline accidents since 1965 were caused by fire, smoke and toxic gases and the inability of passengers to evacuate aircraft in time to avoid the hazards.

Much testing remains before the additive could be required because changes would be necessary to ensure that the additive does not impair the operation of jet engines.

Technical problems with the additive have arisen during planning for this crash and have contributed to delays, according to John Reed, the FAA project manager. The original target date was in July.

The planned crash is an unusual joint effort of two sometimes competing agencies, the FAA and the National Aeronautics and Space Administration. Additional help comes from the Navy, Air Force and British and French governments. It has been four years in the planning.

If technical problems are solved in NASA's remote-control piloting system, which forced postponement last week, the final flight of a 24-year-old, four-engine Boeing 720 jetliner could take place within the next two weeks.

Airline crashes in recent years, though few, have happened largely in circumstances similar to what is planned here. The plane is supposed to fly into the ground at about 170 mph with its nose slightly elevated, its wings level and its landing gear retracted.

That approximates the attitude of jetliners when they approach a runway for landing and is commonly the attitude of airplanes that crash short of the runway.

It also approximates the speed and attitude of a jetliner in two other relatively common accident scenarios: a failed attempt to break off an approach for a landing and regain altitude or a failure to stop the plane on the runway after the pilot decides to abort a takeoff.

The speeds and impact forces in such accidents are frequently survivable. The problem for the passenger is getting out of the plane before it becomes a death chamber, usually because of a fire that starts with ignition of jet fuel.

In addition to the test of the fire-preventing additive, other experiments will provide information on the structural integrity of aircraft seats, floors and walls, including some new designs that have been installed side-by-side with equipment in use today.

If the primary experiment fails and a fire starts, new fire-retardant materials in the cabin will get a test. The management plan for the test states that: "The role of interior materials in post-crash, cabin-fire survivability is controversial because of the apparent overwhelming dangers from the fuel fire itself."

While that is true, there was no fuel involvement in the fire on an Air Canada flight that killed 23 people in June 1983. Another 23, including five crew members, escaped after the plane made an emergency landing in Cincinnati.

That accident prodded the FAA to take several regulatory steps it had long considered and delayed, including the requirement for fire-blocking materials in the seats and low-level exit lights to guide passengers to safety.

Those requirements will take effect over the next three years. Additionally, the FAA has promised Congress it will produce by the end of the year a regulation requiring smoke detectors in lavatories and galleys. The Air Canada fire started in a rear lavatory, although the exact source of ignition has never been determined.

When an airplane strikes the ground the way the FAA and NASA hope this one will, what usually happens is this:

Fuel tanks in the wings are punctured and jet fuel spews into the air in a fine mist that feeds a giant fireball ignited by any number of electrical or other sources.

The aircraft slams into poles, trees and landing-light standards, then grinds to a halt.

Seats come lose from their moorings, blocking aisles. Bulkheads and storage compartments shift and split, scattering carry-on luggage about the cabin. Those problems, plus confusion and possible shock, make it difficult for people to flee the plane in the few seconds remaining before fire, the greatest killer of all, strikes.

The fuel fireball sets ablaze the interior of the airplane, made of lightweight plastics and synthetic fibers. Synthetics in carry-on luggage and clothing fabrics add potential fuel.

Once ignited, those miracle materials turn deadly, giving off cyanide gas and carbon monoxide. As the smoke and gases rise to the top of the passenger cabin, it becomes increasingly difficult to breathe and see.

The smoke blots out the exit signs over the emergency escape hatches and the main doors. Heat builds inside the metal cylinder until the gases ignite in another fireball -- this one, called a "flashover," occuring inside the cabin. Whatever hopes of survival there were are now extinguished.

The chemical additive that is supposed to prevent this scenario has been given the trade name AVGARD. It is a high-molecular-weight polymer that is supposed to bind the fuel droplets to keep them from misting. That dramatically reduces the probability of fire, but also makes engine function impossible because jet fuel must be turned into a mist before ignition in the jet turbine.

Federal experts, with assistance from General Electric and Pratt and Whitney -- the two major U.S. jet-engine manufacturers -- have built a degrader that removes the polymer from the fuel just before it is injected into the engines for combustion.

The FAA-NASA test comes after years of prodding from Congress and aviation safety groups, which have been distressed by what they regard as a snail's pace in regulatory efforts to improve airline cabin safety.

The results of this crash will be studied by safety experts for months.

More than 350 electronic sensors and gauges are being placed throughout the airplane, on the dummies and in the flight controls. On board, high-speed cameras are to survive the crash and report what happened inside.

Reed said that when the airplane that his team has so carefully prepared crashes, "there will probably be a state of depression to overcome.