This is the standard method if you are converting an AP to work with a Wireless LAN Controller (WLC).
Even with the correct file, issues arise. Here are the top three problems and their solutions: Ap3g3-k9w8-tar.153-3.jpo.tar
I have disabled NTP and set the clock on the 5508 back to before the certificate expires. Cisco Community Understanding Access Point OS Images - Cisco Community This is the standard method if you are