At what altitude might a pilot be able to see at least the brightest stars during the day?
SR-71 pilots saw the stars, but of course there is no account of the exact height, they started to see them. They were flying around at 80000 feet at some point in their mission profile. Basically you have to get above most of the atmosphere. So you should start to see stars above 43000 feet. Of course there are some other options, you see them during an eclipse or above the Arctic circles in the right season. You can always see the stars, if you know where to look and if you have a telescope, but I presume you mean to see them with the naked eye?