When Defense Secretary Harold Brown decided he wanted to take his family on a year-end vacation to Puerto Rico, he didn't have to scour the advertisements to find out which airline offered the best deal. He merely told an aide to call his favorite airline, which is not Eastern, United or TWA, but SAM.

If you are a cabinet official, or a member of Congress, or even a general or an admiral, SAM - the Special Airlift Mission of the 89th Military Airlift Command - has the friendiest skies of all.

All SAM pasengers fly first class and, in most cases, for free.

Call a couple days in advance and the Air Force will dispatch a couple of stewards from Andrews Air Force Base to the local Giant supermarket for that special cut of beef, and, of course, your favorite liquor will be served. And there's no two-drink limit.

If you are in a hurry SAM will crank up one of its "alerts," which guarantees that you'll be airborne within 60 minutes of the time the call is received at Andrews.

"We can have ' wheel up' quicker than they can get here from Foggy Bottom," boasted one SAM pilot. In the first three months of this year, SAM responded to nine "alerts."

Although the airline's "DVs," or "distinguished visitors," fly for free, the entire operation costs taxpayers $8 million a year. Operating costs for a Lockheed jet star, such as the one the Browns used in December, run $891 an hour.

The round trip between Andrews and Roosevelt Roads Air Force Base, Puerto Rico, takes about seven hours, making the cost to the government $6,230. Brown reimbursed the Air Force $285 for transportation of his daughter, Ellen, but the other trip costs were written off to official business.

The reimbursement generally is figured at the cost of commercial airfare, plus $1, a Brown spokesman said. (Round trip coach fare via Eastern Airlines between Washington and San Juan is $275.)

The cabinet official, his wife and his bodyguard, Joe Zaice, traveled at government expense, "as is customary," the aide said.

Brown "spent a day" with local and regional military commanders at the Caribbean air base, the aide said, and then "he took a weeks vacation," before returning to Andrews on Jan. 8.

Defense Department regulations state that SAM planes "will not be used except when travel is in the national interest and commercial transportation is not available or capable of meeting the movement requirement." But, as one Defense official said privately: "It means whenever and wherever these guys want to go, they go."

SAM's most famous passenger is the president, and its most famous plane is called Air Force One. Over the years, various planes have been designated "Air Force One," which in Air Force jargon means any plane on which the president is a passenger. The present SAM fleet includes five Boeing 707's including one that normally serves as the president's plane and another as a backup. Several of the remaining 707s previously carried the designation Air Force One.

Just like a commercial airline, SAM has a motto: "Safety, service and reliability," and it has statistics to back up that claim.In 30 years of operation, SAM has notched 700,000 hours of accident-free flying, and it's on-time record is higher than 99 percent. (Of 479 arrivals and departures from Jan. 1 through March 31, only one was late).

SAM offers services not available on commercial flights, including copying machines, typewriters, telephones, tables for working or dinning, and couches that fold into beds. And it does it with fewer employees per plane than TWA.

The 89th Military Airlift Group, as SAM is formally known, has 1,568 members, with a ratio of 98 "employees" for each of its 16 jets, compared to a 163 ratio at TWA, which has 22 planes and 36,249 employees.

SAM's apparent efficiency is partly because military workers aren't unionized, and therefore can be called to work longer hours than their civilian counterparts, SAM officers say.

Because SAM planes often make quick out-and-back trips, a typical flight may carry a bigger crew than a commercial flight would. A hopscotching tour of Western Europe by a dozen congressmen earlier this year took a crew of 22, including seven airmen who did nothing but guard the plane round-the-clock when it was on the ground.

Traveling members of Congress have put SAM in the spotlight over the years. Former House Administration Committee Chairman Wayne Hays (D-Ohio) made 21 trips in nine years to London alone, and Sen. William L. Scott (R-Va.) has visited 36 countries nearly always on SAM.But cabinet officials are also enthusiastic customers.

Civilian officials of the Defense Department made 28 flights totaling 215 hours in a six-month period from Oct. 1 for which travel manifests were examined. In addition to his vacation to Puerto Rico, Secretary Brown and his wife twice went to Brussels and once to Naples and Rome.

Among the Carter cabinet, Treasury Secretary W. Michael Blumenthal was SAM's most frequent customer during that half-year, making eight trips of 65 airborne hours. Blumenthal's most ambitious journey was as the leader of a 36-member delegation, including several reporters, who went to Saudi Arabia and returned via London.

One of Blumenthal's predecessors, William Simon, took 58 trips in 10 months in 1975, causing Sen. William Proxmire (D-Wis.) to issue one of his Golden Fleece awards to Simon and SAM.

In the six months most recently examined, Secretary of State Cyrus Vance or his top deputies took nine trips, covering 43 hours; Energy Secretary James R. Schlesinger flew six times, for 41 hours; Commerce Secretary Juanita M. Kreps led a delegation of 25 to Warsaw; Attorney General Griffin B. Bell went to New Orleans; and Agriculture Secretary Bob S. Bergland went to Fargo, N.D., Amarillo, Tex., and Blytheville, Ark.

The Defense Department also was charged for 86 flights, totaling 936 hours, with about half of the trips, and two-thirds of the hours, involving members of Congress.

The same regulations govern congressional trips that apply to cabinet officials, but senators and representatives often put in writing that "comparable commercial flights" are not available, even when their destination is Kennedy Airport in New York.

Sen. Barry M. Goldwater (R-Ariz.) is one of SAM's loyal customers. He often manages to get invited by the Air Force to speak to the troops. He visited Offutt AFB, Neb. last Oct. 18, and went to his home state on Dec. 7 for a "Wing briefing" on the F-15 jet fighter at Luke AFB, Airz. On March 10, Goldwater traveled to Mac Dill AFB, Fla., and two days later he went to March AFB, Calif.

Goldwater often is accompanied by Maj. Earl Eisenhower, a nephew of the late president, who is a staff member of the Senate Intelligence Committee, and by Eisenhower's wife, Judy, who is on Goldwater's staff.

In the same six months period examined by a Post reporter, Goldwater along with many other senators, flew to Panama to inspect the canal, and to attend the funerals of former Vice President Hubert H. Humphrey and former Arizona Gov. Wesley Bolin, all on SAM slights.

When it comes to arranging official trips to their home state, members of the Mississippi delegation were among the most proficient. In the six months studied, Sen. John C. Stennis and Reps. Jamie L. Whitten, G. V. (Sonny) Montgomery and Trent Lott all took SAM trips home to Keesler, AFB, Miss. In contrast, Mississippi Sen. James O. Eastland has the reputation for being able to find a commercial flight, at his own expense, for nearly every trip he has made as a senator.

SAM carries an average of 8,848 passengers a month of whom 1,717 are classified as "distinguished visitors." Many of the others are secretaries, staff assitants, or military escorts, who provide everything from medical care to such menial chores as bagging totting, gift wrapping and tour guiding.

Until reforms followed recent congressional scandals, military escort officers took along so-called "black bag" funds that financed a myriad of purchases by VIPs, including everything from greens fees on golf courses to fancy meals and liquor. If those funds grew short, the congressional delegations could always call upon the local American embassy. There funds owed by the country as part of its debt from foreign aid were converted from dollars to the local currency on the theory that the foreign funds would never be repaid to the U.S. anyway.

Now the State Department advances $75 a day in local currency to visiting dignataries, and is reimbursed from a fund in the Treasury Department, which in turn bills the appropriate congressional committee.

"Codel" (congressional delegation) trips continue to be popular among elected officials during their many "district work periods," as vacations such as the current Memorial Day recess are called.

This week, a large congressional delegation is in Brussels, then Madrid, as delegates to a meeting known to some stay-at-home colleagues as "the annual ISS conference." ISS stands for "International Shopping Spree," the nonflyers said.