Every time there is an airplane hijacking, somebody wonders why the Federal Aviation Administration doesn't require that passenger planes be equipped with an odorless, colorless gas that can released into the cabin to make everyone unconscious, so rescuers can board without tragic results.

In fact, the FAA and other government agencies have examined and, so far, rejected that solution.

"We don't want something that solves one problem while setting up a different hazard," a knowledgeable FAA official said. He cited three major areas of concern:

*No gas that will keep people alive knocks them out quickly enough to assure that terrorists would not have time to react violently before they are overcome. "It doesn't take long to pull the pins on a few hand grenades," the official said.

*The dosage necessary to incapacitate an individual is an individual thing, the reason "we pay anesthesiologists nice salaries," the official said. A typical commercial flight carries people of every age, from infants to octogenarians, whose physical conditions range from frail to robust. In other words, fatalities might result from a dosage that would guarantee at least the temporary incapacitation of everyone.

*No 100 percent guarantee has been found against accidental release of this odorless, colorless knockout gas. For a plane on a routine flight with no hijackers, even the remote possibility of knocking out all passengers and crew members is daunting.

SHORTHANDED. . . The National Transportation Safety Board, which is having its busiest year in terms of aviation accident investigations, has been without a full complement of five members since April 1984 and it looks like it will continue that way, at least until next spring.

The White House nominated three members this year, but only one of them, John Lauber, was reported favorably by the Senate Commerce Committee. Last week, Lauber took the seat held by Adm. G.H. Patrick Bursley. Bursley had been a big hit with the press because of his careful but straightforward answers to the questions that followed the Delta Airlines crash in Dallas last August.

Lauber was a human-performance expert with NASA and is regarded as highly qualified.

The other two nominees were Kenneth I. Hill, who worked for former presidential adviser Michael Deaver at the White House, and Vernon L. Grose, a systems safety specialist who served as a safety board member in 1984 on a recess appointment but never was confirmed by the Senate. Grose's tenure is best described as fractious, and he did not help himself by making it known that he wanted to be called "Dr. Grose," although his doctorate is honorary, or by holding early-morning prayer meetings in his office.

The objections to Hill seem to have more to do with his lack of technical experience, at a time when aviation safety has returned as a big issue.

It is unclear whether the White House will resubmit either Hill's or Grose's names. Of the two, Hill is regarded as having a better chance, assuming his nomination is accompanied by that of a technically qualified individual.

Meanwhile, Board Chairman Jim Burnett and Vice Chairman Patricia Goldman are splitting duty every other week on the board's "go-team," which must always be on call for a major accident.

CONTROVERSY . . . The safety board's investigation of the Nov. 10 collision of two private planes over Teterboro, N.J., has reopened an old controversy: FAA reliance on pilots to "see and avoid" each other's aircraft in clear weather.

That works for small planes at remote airports, but Teterboro is a high-traffic facility with flights that mesh with commercial planes using New York's LaGuardia and Kennedy airports.