Three key transponder codes are:7500 for hijack, 7600 for radio, 7700 for emergency.
The following mnemonic can be helpful in remembering the codes. All three codes end in zeros.
• 75 you need a 45, that is you need a gun as you are being hijacked.
• 76 you need a fix, that is your radio is broken.
• 77 you are going to heaven, hopefully not.
AIM 2018 4-1-20
AIM 2018 6-2-2