If you thought the cabin air was a bit stale on your last jet flight, it was.

If it was a coastal flight, you might have wondered, a little nervously, if the life rafts were missing. They were.

And if the descent toward the landing strip seemed a lot like a roller-coaster dive, that's because it was.

The nation's airlines are in the throes of a ferocious campaign to save ever-more-expensive jet fuel. One way to do that is to lighten the load, so everything that weighs anything has been studied with a cold eye. Every maneuver or flight plan has been reviewed.

On big jets, ventilation has been cut a third. This means that more than half the humidity you feel is actually sweat, breath and other body moisture from fellow passengers. If you have ever wondered why pilots keep the cockpit door closed during flight, this is one important reason: ventilator pumps provide pilots 10 to 20 times more fresh air than passengers, and none if it is mixed with sweat or smoke from the passengers.

The amount of food served has been cut down. The drinking fountains are not full anymore. Where there once were three seat-pocket magazines, there is now only one. Metal seats are being replaced by plastic. Carpeting may soon be thinner and floorboards are scheduled to be shaved.

Computers now calculate for pilots the most gas-saving fuel burn for each stage of maneuvers. One airline has even taken to pressing out the tiny nicks and dings in the skin of the aircraft, in an effort to reduce the friction of air passing over its surface.

The most recent rumor is that wine will come out of its heavy little bottles and be put into light little cans.

The fuel-saving solutions range from the humorous to the serious, but for airlines the cutbacks have brought spectacular results. They are using one-third less fuel to carry one passenger one mile than they did in 1973. Despite an increasing number of flights and passengers, the airlines are expected to use no more kerosene in the 1981 than they did eight years earlier.

In the next 10 to 15 years, the airlines expect to double these savings, according to one spokesman.

The two most controversial, as well as most profitable, changes are dispensing with life rafts and reducing cabin ventilation.

According to the Federal Aviation Administration, 15 airlines have obtained permission to remove life rafts from coastal flights -- planes that fly up to 162 miles from shore between cities. Life rafts weigh a thousand pounds, and removing them can save an airline $1.5 million a year or more in fuel costs.

"We think this is a pretty shortsighted thing to do," said Rick Clarke, health and safety officer of the Air Line Pilots Association. "The life rafts may be heavy, but there is a reason for them. We sympathize with the airlines' desires to save weight, but this doesn't look like the way to do it."

Airline and FAA spokesmen said modern jet aircraft can easily make it to shore on a single engine from as far as 200 miles out, but airlines and the FAA both admit this does not take into account a situation in which the plane crashes into the sea, regardless of the number of engines working.

"The presumption there is that we would be close to shore, and would have readily at hand rescue vehicles, speedily available," a spokesman for one airline said.

But several planes have gone down in the coastal waters, in recent years. In two cases -- one off Los Angeles and one near San Francisco -- life rafts were used to keep passengers afloat until rescuers reached them. In a third case, a National Airlines flight that had removed life rafts days earlier crashed into the sea near Pensacola, Fla. Three persons died.

"They had the good taste to crash right next to a tugboat and barge that were lost in the fog," Clarke said. "Otherwise I think there may have been a lot more casualties."

He said that in a mock crash in the Potomac River -- a test of how long people would have to be in the water awaiting rescue -- the pilots' association found that "there are no resources to get all the people out of the water sooner than a few hours."

Nonswimmers, and even good swimmers in cold water, might not survive the wait, he said.

ALPA has filed a petition with the FAA to change the rules and equipment used for water "landings," as airlines call them.

Fresh air in airplane cabins has been a subject of perennial complaint. Since the fuel crisis began, airlines have instructed pilots on the bigger jets -- such as the DC10, 747, and L1011 -- to shut down one of three ventilator packs. That could save each airline $2 million or so per year.

Airlines maintain they have few complaints about cabin air, and that so long as cabin air is relatively safe and passengers are apparently comfortable, there is no reason not to conserve on "excess air" pumped into the cabin.

Flight attendants' associations, however, have reported far more complaints from their members about irritation and sickness caused by cabin air, and have negotiated with airlines to assure that ventilation packs can all be turned on when flight attendants request it.

There are no specific federal regulations on how much fresh air passengers must be given. But there are specific FAA regulations for pilots, because two decades ago pilots who had experienced eye, nose and throat irritation demanded a minimum level of fresh air. The FAA says pilots must get 10 cubic feet per minute; airlines actually supply between 70 and 150 cubic feet per minute.

Although airlines and jet manufacturers differ on the amount of fresh air passengers get, the average is probably between 6 and 10 cubic feet per minute, or less than a tenth of what pilots get, and no more than the minimum required for pilots.One airline has said it is attempting to reduce fresh air per passenger to five cubic feet per minute.

At four cubic feet per minute, passengers and flight attendants could begin to experience the first symptoms of suffocation, according to Boeing and Lockheed spokesmen. At least four cases of apparent "oxygen deficiency" among flight attendants have been reported to an accidents' union and the FAA. The cause of the incidents is not clear.

"I find it just amazing," said an FAA worker, "that the way airlines cut down on weight is by taking off life rafts and cutting down fresh air. Think of the things they don't take off -- the 250-pound liquor carts, for example. You can serve drinks without the carts.

"And most of the partitions in the cabin, like between first class and coach -- those are purely decorative. They must weigh quite a lot, but they stay. And the thick, colored carpeting on the walls.

"Someone ought to take another look at the priorities operating here."