The ALT-55B contains a circuit designated as AID (Aircraft Installation Delay). This circuit compensates for differences in cable lengths and the distance from the bottom of the aircraft to the ground when on the ground. This is selected by program strapping during the installation.
ALT-55B's with dash number -011 have Service Bulletin 6 installed. This modification accomplishes the following:
In the event of an AID failure, the indicated altitude will increase significantly, and this increase will be undetected in flight. After incorporation of this modification, an ADI strap failure will result in a decrease of indicated altitude, thus ensuring safe terrain separation. Incorporation of this Service Bulletin is for safety reasons and is optional.
Service Bulletin 5 is the same modification but does not require changing the dash number from -001 to -011.