This article is from the Final-Fantasy and other Square Soft Games FAQ, by firstname.lastname@example.org (Nick Zitzmann) with numerous contributions by others.
After getting the Highwind and getting Cloud back in your party, go to
Midgar. There will be a man out front who tells you that he lost his
keys. Go back to Bone Village and talk with the excavators. Choose any
method that you want in order to dig for items.
See the wrecked F-14 plane? Near the bottom of the screen, there is a
piece of metal sticking out. Go to it and press Right, then Up, so that
Cloud is firmly wedged next to the plane, below the piece of metal.
Choose to dig here and you'll get the Sector 5 Key the next morning that
lets you return to Midgar City.