We know that now. They didn't know that then. Remember, this was well less then 5 years after the de Haviland Comet's started falling out of the air because the

We know that now. They didn't know that then. Remember, this was well less then 5 years after the de Haviland Comet's started falling out of the air because they used square windows instead of ovals, and metal fatigue at the corners of the windows were resulting in massively more rapid wear cycles then projected.