UNFROZEN FRONTIER. Alaska's outback: a flightseeing sojourn to Arctic's summer climes

THE 49th state isn't a destination of whim. Alaska is the sort of place people think about for years before actually booking a flight to Anchorage or daring to test their cars' endurance on the beautiful stretches of the Alaska Highway.

Alaska isn't just off the beaten path; it's off the path, period. Eighty percent of the state is inaccessible by road, even with four-wheel drive.

But 20 percent of Alaska is more than 100 percent of other places. Even with a conventional sedan, you can drive (using Anchorage as a base) south on the Kenai Peninsula to Homer and Seward, east toward the Yukon, and north to Denali, Fairbanks, and beyond, even crossing the Arctic Circle.

That should be enough vastness and grandeur for a reasonable person - but being unreasonable, my wife, Kay, and I found ourselves looking at the tantalizing blank spaces on the map and wondering what it was like in there. To find out, we would have to travel the way Alaskans do: fly.

The time to start planning an Alaska fly-in trip is before you leave home, preferably several months in advance. The season for most recreational fly-in services in Alaska is short, and demand is high, so operators understandably demand firm reservations and deposits in advance. It is possible to book a flight into the bush on short notice, especially if you won't be requiring lodging or guide services once you reach your destination. But why chance disappointment when you're coming this far?

I was fortunate enough to have an advance guard already in Alaska to help with the planning: Kay, who had more of the summer at her disposal than I did, had driven to Anchorage and beyond months before I would be flying up to meet her. While I collected book knowledge in Massachusetts, she talked with tour operators and charter pilots on location. By August, we were all set to be flown to a 10-mile-long lake south of Denali that would be ours alone for three days.

We had discovered that our first step would be to decide what type of experience we wanted. At one end of the spectrum is the day excursion, generally a sightseeing trip along a set route at a fixed price. This is ideal if, for instance, you are interested in a close-up at Denali (Mt. McKinley) but wish to return to your lodgings at day's end. A typical excursion, offered by several charter services flying out of Talkeetna (115 miles north of Anchorage, accessible by highway or rail), is a 1-hour flight around the Denali area, including a landing on a glacier. Planes usually carry four to six passengers, with costs averaging $50 to $100 a person, depending on group size.

The more ambitious Alaska air trips involve drop-off and pickup, with a stay of several nights, or weeks, in the bush. A few decisions are needed: Do you want to camp on your own once you've been left at your destination, or do you want guide services, perhaps even a base camp with meals provided? Will you be picked up at the same place the pilot left you, or - assuming you have maps, adequate supplies, and know what you are doing - will you be instructing the pilot to meet you at a second prearranged spot?

Many of the larger charter services can accommodate either unguided campers carrying their own equipment, or book the wilderness traveler into a group trip for which equipment and full meals are provided. Several of the outfitters flying into the Brooks Range out of Bettles, a town just north of the Arctic Circle 180 miles northeast of Fairbanks, offer bunkhouse accommodations at base camp, tents while the group is traveling, and canoes, rafts, or kayaks for trips of five days to two weeks along the tributaries of the Koyukuk River. Some trips are even arranged as fly-in, raft-out excursions, ending in Bettles, at the confluence of the John and Wild Rivers with the Koyukuk. A typical trip includes flight out of Bettles, accommodations, food at base camp, and use of a raft for a five- or six-day float back to Bettles, for a cost of roughly $1,200 a person. You'll also have to fly from Fairbanks to Bettles and back, at $80 to $90 a person one way.

A few of the flight services also rent wilderness cabins, for those whose idea of a sojourn in the bush doesn't involve trekking from Point A to Point B but rather staying put and enjoying day hikes or canoeing. Even where accommodations as luxurious as cabins aren't available, bush pilots can often recommend lakeside campsites (float-plane accessibility is a must in these situations) and even rent folding canoes to use while camping.

To enjoy this kind of total seclusion, it isn't necessary to fly into the high Arctic. We opted for a 40-minute fly-in from Talkeetna to our remote lakeside tent site just south of Denali National Park, a trip that cost about $200, plus $40 for the four-day rental of a folding canoe. We enjoyed all the scenery and seclusion we could have wanted. On the return trip, the pilot flew low over the Kahiltna Glacier, and even doubled back at one point to give us a better look at a grizzly bear clambering along a mountainside.

A word about canoes: When you're flying to wilderness destinations in Alaska, folding canoes make the most sense. A rigid canoe will require either a plane large enough to fit it inside cargo space - say, a twin Beech chartered for up to $800 an hour, as opposed to $200 or less for a smaller Cessna 185 - or a pilot with a permit to carry an external load. Not all pilots have them.

Canoes are just one aspect of your trip to discuss with an air charter company before you make back-country plans. The experienced pilots and guides at licensed firms can tell you how much weight their craft can carry, what sort of clothing and equipment to bring, what the weather is likely to be, and whether the excursion you have in mind would be best undertaken alone or with a guided group. The representative of the firm should also be able to quote prices based on per-mile, per-hour, or flat rates, including or excluding guides and equipment. Whatever fly-in plans you make, you're in for a rare opportunity to see part of the other 80 percent of Alaska - the part that roadbound travelers can only imagine. Practical information

For a complete list of licensed air charter companies in Alaska, and other travel information, send for ``Alaska and Canada's Yukon 1987 Official Vacation Planner.'' Write the Alaska Division of Tourism, PO Box E-007, Juneau, AK 99811. The 100-page booklet is free.