The one classic airline slogan that could still work would be "Fly the Friendly Skies." It is a slogan so rich in history that it amazes me that United has not revived it. By the mid 1990s when they replaced it with the truely elegant and equally profound "Rising", it was showing its age, but now, it could easily be revived and would fit well within United's new brand positioning. That said, "It's Time To Fly" really is a great slogan as well. An ideal strategy would be for United to rotate Fly The Friendly Skies in as its slogan in a few years tine, when "It's Time To Fly" will no longer be relevant.
It's worth noting that slogans don't have as long a shelf life as other aspects of the brand. Only a few slogans have been able to last for more than a decade. The causes for this problem are threefold: as a company's business evolves, it can be increasingly difficult for a multi-word, verbal identifier to remain relevant, as time passes consumers will get bored with the slogan (unlike the logo or name, which only get stronger as the years pass), and many slogans are created by advertising agencies, which embrace a short term, campaign-oriented approach to marketing.
One exciting development that will hopefully establish itself in the airline industry is what I call an attached positioning statement. Examples of attached positioning statements include BP
's "Beyond Petroleum", and in the airline industry, Frontier's "A Whole Different Animal." As positioning statements as well as slogans, these go in a bit deeper than the traditional superficial slogan and reflect a managerial and organizational philosophy at the firm. Time will tell how long these last in use, but one thing is for certain: for the moment, these positioners are becoming an increasingly integral part of the brand identity. I think we will see more of them in the airline industry, replacing such irrelevant, meaningless slogans such as "Good Goes Around" and "We know why you fly."