The stunning decision by Secretary of Defense Caspar Weinberger to put the MX missile in a futuristic 1990 airplane, scrapping Jimmy Carter's landbased system, will solve President Reagan's political problems with western senators but raises disturbing questions about Pentagon decision-making.

Although the report to Weinberger by his own committee of experts, headed by Dr. Charles Townes, was deeply divided, it gave Weinberger's airborne choice short shrift. The principal reason: high cost. Weinberger privately claims that the long-endurance aircraft -- capable of flying two days without refueling and a week with it -- will actually save money in the end.

Motivated partly by political considerations -- admittedly very tough ones -- Weinberger and deputy secretary Frank Carlucci have hinged the most important strategic decision of the Reagan administration on fear of an environmentalist outcry and the personal feelings of senators. Nevada's Paul Laxalt, President Reagan's closest friend in the Senate, and Utah's Jake Garn have led the campaign against Jimmy Carter's land-based system, planned for Nevada and Utah.

"Cap is mainly interested in the politics of the MX basing problem," one insider told us. "That and the budget." When Weinberger, Carlucci and Gen. David Jones, chairman of the Joint Chiefs of Staff, briefed Reagan in the White House last month on the MX and other new strategic weapons, Jones was not asked a single question. Consequently, he said nothing.

Similarly, in one significant MX session with outside experts, Weinberger and Carlucci had no Air Force officers present. Undersecretary of Defense Fred Ikle, considered by many to have the most experienced strategic brain in the Pentagon, was also absent.

Critics both inside and out of the Pentagon admire Weinberger's own intellectual capacity and his dedication to long hours of hard work, but they insist his lack of experience in the strategic arts could not be overcome in a brief six months as defense chief. Weinberger came to the Pentagon leaning strongly to a sumbarine-based system to make the MX invulnerable to a surprise Soviet nuclear attack against U.S. missiles. When that proved undoable, he began moving toward an airborne system.

There are also dangerous political problems with NATO in any American decision to safeguard the MX in airplanes (to be fired by remote control after being dropped into the air). The United States has been pressuring its European allies hard to move ahead rapidly with a new force of land-based nuclear missiles targeted on the Soviet Union. An airborne option in the United States is certain to intensify the public clamor against a new generation of land-based missiles for NATO. That could tighten the political squeeze on West German Chancellor Helmut Schmidt and other European leaders.

The decision to base the MX in the air with a plane not yet built or fully designed shocked the Pentagon brass. Word leaked soon after a top Air Force officer was told on July 14 that Weinberger had "made the decision." Weinberger wanted the political spadework started for a selling campaign on Capitol Hill, both for the 1990 aircraft and for a stopgap to protect the MX before the new plane is built.

The secretary's stopgap appears to be the C5A giant cargo plane, a conventional aircraft from an assembly line that stopped running in 1973. Lockheed, the manufacturer, has already been asked to submit detailed information on when the 100 new C5As could be built and how much they would cost.

But the 100 C5As that Weinberger is planning have all the vulnerability problems of the aging B52 bomber, and they could not be built until the mid-1980s. "To fill the gap before 1990," one Pentagon analyst told us, "the C5A is less than a finger in the dike."

The Townes committee, which is still conferring with Weinberger, has similar anxieties about a stopgap solution that puts the MX in conventional cargo planes. Pressures continue on Weinberger to adopt a sharply cutback mobil land-based system for 100 or fewer MX missiles (compared with Carter's planned 200). Even though the secretary leans strongly toward the cargo plane as a stopgap, insiders believe he may finally approve a small, land-based system, perhaps to be scrapped in 1990 when the new long-duration planes are ready.

That would leave Weinberger's basic decision intact, if he can sell it to the president and Congress and if its political implications for NATO's European members prove manageable. But the deeper questions continue to be asked: will it work and is it truly the best strategic answer to America's growing vulnerability to surprise Soviet attack? Reagan promised no less.