A logo is part of the personality of an airline. We strongly associate it with the brand obviously, but it can also trigger an emotional connection, evoke a feeling of nostalgia and also often tells a story—including hinting at economical difficulties (SwissAir's disappearance in 2002) or seemingly surfacing corporate turbulences (how many times did Delta change its logo?!).
Just The Flight has done a nice job with this infographic displaying the evolution of a selection of airlines' logos.
KLM and Lufthansa have had theirs for more than 50 years now (!), surviving and resisting fads and looking oh so current—I personally find the German one an example of excellence in design. It's pure, reliable and optimistic, discreetly evoking trust and safety, assessing a strong sense of responsibility from the carrier.
And here's for Alex, the Virgin Atlantic logo evolution: