THE ISRAELI defense minister - an aviator - is surely wise to warn of "a few clouds, a bit of sunshine, a bit of rain, a bump here and a bump there" on the way toward a Mideast peace. All of us flabbergasted spectators at the drama being acted out by Anwar Sadat and Menahem Begin had best keep that in mind. There will be turbulence along the way, and it will doubtless be reflected in dispatches from the peace front: You will hear, alternately, of "deadlocks" and "progress," "stalemate" and "breakthrough" as the necessary hard bargaining moves from principles to the practical bits and pieces of a comprehensive settlement.

But there is something else to keep in mind: As President Sadat and Prime Minister Begin prepare for their second summit meeting at Ismailia on Sunday, their course, or so it seems to us, is firmly, almost irreversibly fixed. It derives its thrust with overwhelming force, we think, from two extraordinary personalities, from the roles they have cast for themselves in concert, and from the relationship they have, accordingly, struck up. Mr. Sadat and Mr. Begin give every evidence of men gripped, even driven, by the idea of peace. Each conveys an unmistakable sense of the moment. Each has subordinated normal state calculations to a historical imperative. Each has made such an investment of self and policy as to become virtually the other's hostage. They have set out, we believe, on a road on which there can be no turning back.

It was always known what the elements of a settlement would have to be: an exchange of territory for real peace in a context including the Palestinians. Various combinations of violence, maneuver and persuasion were employed, in vain, to bring it about. It took an unforeseeable leap of courage and imagination for President Sadat to visit Jerusalem to transform the mind-set that had constricted Arab and Israeli policy for 30 years. Prime Minister Begin's response was no smaller thing. He set aside a lifetime's intellectual and spiritual baggage justifying retention of the West Bank and produced a position that is, we are convinced, within Arab bargaining reach. The key to it is the right of Jewish settlement, which allows symbolic satisfaction of Israel's biblical claims while opening the way to self-rule/self-determination for Palestinians, who would have a balancing right of settlement/repatriation in Israel itself.

At Ismailia, the two men should be able to move swiftly toward agreement in principle on an overall settlement. Negotiation of the Israeli-Egyptian part of that settlement can follow. With the territory-for-peace principle accepted in respect to the Sinai, it becomes possible for Israel to put it into effect in respect to the West Bank when a Palestinian negotiating entity emerges, and to work it out in respect to the Golan Heights whenever Damascus is ready to read the signs and relinquish its fantasies of a "greater Syria" including a Palestinian client state.

It is understandable for the Carter administration to speak guardedly about the prospects for a process that it can do a lot to influence - but also a lot less than it has been accustomed to doing. It is not easy for American policymakers to talk confidently about events that have very largely passed beyond their control, and we would like to think that this, rather than a fundamental misreading of the events, accounted in large part for the administration's early skepticism. In any case, it must be evident to Mr. Carter and his top aides by now that this is no hit-or-miss initiative, but a calculated and controlled effort by the two most important countries in the area. It could, perhaps, be stopped by the removal of one or the other principal figure - but, no, we doubt even that would do more than slow for a time the accelerating move toward settlement. The outlook is genuinely thrilling, and all the more so for taking place at the Christian season of hope. The peace for which the Middle East has waited and suffered is coming to be.