Aaaargh not the dreaded Gillham Code again!!!! The most overworked Airworthiness Directive in recorded history.
The incident quoted on the web-site above resulted in a veritable snowstorm of unnecessary paperwork with two each ADs from the FAA, UKCAA, Netherlands RLD and Australian CASA across my desk in the past couple of months. Not to mention baffled inquiries from a number of airline customers of ours.
Gillham Code inputs into transponders are used in aircraft that do not produce a "digital" output from the altitude encoding device. In aircraft that have ARINC 429 databus transmission between the Transponders and the Digital Air Data Computers (ie. the majority of the current airline fleets) Gillham Code isn't an issue. Doesn't stop the Airworthiness Authorities panicking and swamping everyone with BUMPH though! I suppose their motto is "If you don't know what they are talking about, issue an AD"
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Through difficulties to the cinema