Plagued by a wide range of mutual misunderstandings, the proposed sale of four U.S. airplanes to Saudi Arabia has escalated into a major test of Washington's reliability as an ally and a highly visible measure of American friendship for the Saudi royal family.

Judging from the remarks of Saudi leaders and diplomatic observers here, the storm blew up less because Saudi needs and U.S. requirements conflicted than because two different systems and cultures clashed head on. The issue of U.S. control over the aircraft, once it was clearly defined, became important to both sides even though neither really wanted it to.

If the crisis seemed to swell out of control, it was largely because the Saudi leadership generally does not understand the give and take of American democracy, and to a certain degree does not care to. Conversely, it was also because competing powers in Washington trod heavily for many months on the sensitivities of a Saudi ruling family rooted in royalty, Arabism, Islam, and, most of all, traditions sharply different from those of the U.S. Congress.

U.S. and European diplomats experienced in dealing with the Saudi royalty and its military say that from the Saudi perspective there really was no need to define the issue of U.S. controls. Without the public demands from Congress, they believe, the whole affair could have been finessed -- with Saudis happy owners of unrestricted AWACS and Americans confident that U.S. technicians and security experts would remain with the plane for the rest of the decade and beyond.

"It is unthinkable that there would not be Americans in the planes for at least 10 years," a European diplomat said. "You just should not have said it out loud. You could have had it for the asking, but quietly."

A knowledgeable U.S. source agreed, pointing out that the combination of delivery schedules -- 1984 at the earliest for the first plane -- and training requirements -- several years at least -- would have kept American personnel in the Advanced Warning and Control System craft through the 1980s in any case. By that time, he added, a new generation of radar likely would have made the AWACS too dated for either Israelis or Soviets to worry about.

Without the airing of demands for U.S. controls in Congress and the press, he said, American crewmen in AWACS could have played a behind-the-scenes but important role similar to that played by other U.S. military advisers, silent but present down to the squadron level in the Saudi Air Force and the battallion level in the Saudi Army.

Once the issue was joined, however, the mutual misunderstandings came into play. Diplomats here say, for example, that there is an attitude in the royal family that the White House is the U.S. government and that members of Congress consequently are of little importance.

"We will not negotiate with congressmen," a high Saudi official and member of the royal family said last weekend even as the Reagan administration weighed whether to send a congressional delegation here to discuss the AWACS sale.

Prince Sultan, the kingdom's defense and aviation minister and brother of King Khalid, told the Saudi newspaper Al Bilad yesterday that the AWACS dispute was not for him to talk about because "the matter is in the hands of the American administration."

At the same time, Sultan's son, Prince Bandar, left Washington, where he had been pushing for the sale on request from Secretary of State Alexander M. Haig Jr. Although his departure presumably was to make him available here for decisions on new proposals brought by U.S. Ambassador Richard Murphy, it nevertheless left the kingdom without any special envoy in Washington as Congress considered crucial decisions.

At the same time, Murphy took charge of an embassy whose staff of Arab experts was decimated by reassignments, focussing responsibility on him to explain the U.S. position to the Saudis.

Saudi underestimates of congressional power have been demonstrated before. The U.S. Embassy here had trouble last April arranging a royal audience for Sen. Howard H. Baker Jr. of Tennessee, the Senate majority leader who now is playing a key role in the AWACS controversy. According to sources here who dealt with him, Baker left the kingdom lukewarm despite a visit eventually arranged with Khalid.

In a nation where the rulers are still genuine absolute monarchs, commanding one of the world's greatest fortunes, a legislator from Tennessee has less status than elsewhere.

In addition, the Saudi diplomatic corps draws its personnel from outside the elite, Western envoys here explained. Consequently it has neither the influence at home nor the ability abroad to mount an effective and long-term lobby.

Profoundly Arab as well as monarchist, the Saudi leadership feels most comfortable in private contacts and decisions taken between governments far from public scrutiny. Ambassador Murphy's arrival here with decisive proposals for the AWACS sale, for example, has gone unreported in the controlled Saudi news media.

In what perhaps was the most vivid demonstration of the urge for privacy, the Saudi royal family reportedly turned to the Central Intelligence Agency rather than the U.S. Embassy last fall to communicate to then-president Jimmy Carter its desire to borrow U.S. AWACS planes to monitor the Iran-Iraq war.

Such discretion is the rule rather than the exception in the Arab world.

American attempts to proceed according to Washington's ways in dealing with Saudi Arabia frequently have grated the nerves of a people and leadership used to such reserve.

If Baker and his eight fellow-members of Congress had difficulty getting appointments, for example, it was in part because they followed by only 72 hours an orientation visit by Haig.

Haig himself had been preceded only weeks earlier by a delegation headed by Sen. John G. Tower (R-Tex.). Besides these officials, a long list of lesser visitors sought in the same period to meet with Saudi royalty on various "fact-finding" missions.

"We've had some real wing-wangs," a junior officer at the U.S. Embassy here said.

The frequent duplication prompted the English-language Arab News to ask in a recent editorial, "What do the Americans do with all these facts?"

Widely reported remarks by individual members of congress and lobbyists declaring the Saudi royalty too unstable to be trusted with AWACS planes also have irritated the ruling family profoundly, according to diplomats here.

A high Saudi official suggested in an interview what diplomats describe as a widely held belief in the royal famly: that such opposition to the AWACS sale is deliberately stirred up by supporters of Israel to drive a wedge between Saudi Arabia and the United States.

The Arab News, reflecting this attitude, editorialized: "These congressmen ...are voicing opposition to friendly relations with Saudi Arabia -- an act amounting to an aggression against U.S.-Saudi friendship. Perhaps they are serving their friend -- Israel."

In the eyes of many Saudi officials and businessmen, according to diplomats with wide contacts here, former ambassador Robert Neumann was fired for defending Saudi positions against what is also regarded as a strongly pro-Israeli tilt in the administration.

"What kind of effect do you think that has?" a European diplomat said.

Murphy, who replaced Neumann, has served in Saudi Arabia before and is highly regarded here. When he presented his credentials in Taif, the Saudi summer capital, early last month, the new envoy engaged Khalid in conversation in fluent Arabic, leading the monarch to send away his interpreter and keep Murphy in a private salon twice as long as the customary 15 minutes.

At the same time, Murphy's posting was precipitated by one painful issue and now doubts on a second. And the foreign service staff has been disrupted by key reassignments just as the AWACS issue headed for a crunch.

"Most of the embassy is worrying about getting shipments of home furnishings out of port," a staffer said.