PASSENGERS boarding a plane are familiar with the configuration - first class at the front, business class in the middle, and economy at the back. But this layout is relatively recent. Neither has it always been easy to watch films, or have seats like upmarket hotel beds. Fifteen years ago, first-class travellers could expect as many unscheduled surprises as those in economy endure today. There were no state-of-the art 747-400 planes then. In the mid-1970s, Cathay Pacific used second-hand Boeing 707s, and the first of the Tristars arrived in 1975, before 747s came on stream in 1979. It was not until 1980 that Cathay Pacific introduced its business class, one of the first airlines in Asia to do so, said Stewart John, Cathay's engineering director. He remembered the introduction of first class in 1977, with seats that had ''relatively generous leg room, reasonable recline and no leg support''. First-class seat pitch then was 101.5 centimetres to 106.5cm - a far cry from today's 152.5cm to 157.5cm. In-flight entertainment has also changed beyond recognition. Gone are the earphones that Mr John described as ''suck and blow''. Those uncomfortable air-tube devices that fitted inside your ear are museum pieces now. Passengers today have Dolby sound and eight channels to choose from, including the BBC World Service live - a first for Cathay. ''It's all our own design and the other airlines are still trying to work out how we've done it,'' Mr John said. In-flight movies had also changed a great deal in his 161/2 years with Cathay, Mr John said. In the early days, the unpredictable often happened: the disaster movie Poseidon Adventure was shown on a flight. In the movie, a tidal wave overturns the luxury liner, throwing partying passengers into chaos - just at that point, the power to the aircraft's cabin died, and the lights went out. ''In the old days, on DC10s, there would be one dirty, great reel of 16 mm film on a projector at the back of the cabin,'' Mr John said. ''If it broke, which frequently happened, there was pandemonium with film flailing wildly about the cabin.'' By 1979, things had improved: films were shown with an eight mm projector in the hat rack, then, later, video cassettes were introduced. Now, first-class passengers face a confusion of choice: each has five or six channels on videotapes, a personal television monitor in the seat arm, and moving maps fed from the plane's navigation computers. Business-class first hit the skies in 1980. It surpassed the standards of the old first class. It had, in addition, retractable leg rests and meal trays in the seat arms, which replaced the type that flapped down from the seat in front. Aircraft cabin temperature control has improved greatly in the past 15 years. Each aircraft now has several thermostats, which allow the crew to adjust the temperature in discrete zones of the cabin.