Appearances may mislead. But in the plush, wood-paneled serenity of his office, Alexander Haig has the look and manner of, say, an airplane pilot who has bounced around in rough air, climbed through the worst of it, and leveled off at a reasonably comfortable cruising altitude.

That isn't to say there won't be more air pockets and crosswinds of the sort that have made his first eight months as secretary of state an exceptionally bumpy passage: the confirmation hearing confrontations over his harsh Cold War rhetoric; the flustered TV appearance when the president was shot; the "crisis management" flap with Vice President George Bush; the sounds of scuffling with the White House; the open conflicts with the secretary of defense.

On camera and in public, Haig still occasionally comes across as needlessly contentious, overly combative--and not always entirely intelligible.

But in private, in the course of an on-the- record, hour-and-a-quarter interview, you encounter, well, another Al Haig: composed, assured, reflective, taking the long view. Has he changed any of his views? "Not especially"; he's been in the business for 20 years. Have the Soviets changed theirs in the face of the new Haig- Reagan hard line? "It's far too early to say."

But he is reinforced by what he perceives, and thinks the Soviets also perceive, to be "an unprecedented consensus in America to rebuild America's military strength." What about the interim, while rebuilding proceeds? Is the United States at a dangerous disadvantage?

"I'm not one that believes it is either right objectively or sound policy to inflate our concerns about our own weaknesses," he replies. The "window of vulnerability" is a matter of "trends," not "contemporary reality," Haig insists. "In a contemporary sense, the United States is very, very strong and very, very capable, especially in the strategic area. Our systems are both more sophisticated and reliable and more technologically sound."

Does the second wave of budget-cutting, now reaching into the Pentagon, trouble him? He is, first of all, philosophical: "The viability of our economy is a fundamental aspect of foreign policy. When we are engaged in belt-tightening, our foreign policy and defense structures have to carry their share as well."

But he has also compared notes with Defense Secretary Caspar Weinberger: "We both remain optimistic that the president is going to hold to commitments which have now become an essential part of our hopes for success in foreign policy."

Moreover, by forcing the pace of the buildup, we give the Soviets "something to gain by reciprocal restraint." And that in turn increases the prospect for successful arms control negotiations and defense economies later on. The surest route to defense spending cuts, for the long haul, in other words, is to spend a lot more now.

Somewhat the same principle applies, says Haig, to a narrower arms-control issue: Theater Nuclear Forces in Europe. American deployment of these tactical nuclear weapons is tied by allied agreement to parallel efforts to negotiate with the Soviets removal of such weapons on both sides of the line.

Haig concedes there was significant European heel-dragging on deployment even before President Reagan's decision to start producing neutron bombs (enhanced radiation warheads) for stockpiling in this country--but for potential use in Europe. "I don't want to be naive and suggest this (the neutron bomb decision) hasn't complicated the thing," he says. "But I think we'll manage it."

Haig argued against that decision and was overruled. But his quarrel was "only on timing." He claims a "substantial" role as NATO commander in Carter administration decisions to move forward with development of the neutron bomb.

Leaving aside the overarching Soviet threat, how would he rate the areas of his greatest concern? Paradoxically, for a proponent of American activism in the world, his lists begin with two centers of turbulence the United States, he freely admits, can do the least about: Poland and Iran. The latter looks "more like chaos every day but it has its own historical imperative--there's little we can do."

Poland ranks first because of its "strategic importance and because it is a historically unprecedented evolution." But apart from economic help, "I don't think that's an issue we have the luxury of manipulating."

Not so, with his third-ranked concern, Central America. To mention El Salvador is to evoke flashes of early Haig: a new "White Paper" is in the works to prove, once and for all, "the activity of externally directed Marxist-Leninist insurgency." The commitment to counter Fidel Castro and the Communists is absolute.