I found this too to be ambiguous The Hedge. No where could I found the exact meaning of a "unexplained discrepancy". So where/when is this limit reached? In IFR terms the best I could find/reason is that it is somewhere around 60 - 75 feet. I self taught myself the IFR theory, and from the material i used (an outdated Bob Tait study book) and by searching the docs I couldn't find an answer, the reason I say 60-75ft is because of two things.
1. All practice questions under this margin where allowed, all those over where not.
2. It is the upper allowable instument tolerance that must be checked pre flight.
To actually *know* the correct margin/meaning would be great?
What would the powers at be say in a court of law I wonder?
"You where on G/S with an altimeter reading 100 ft below that indicated on the chart, doesn't this constitute a unexplained discrepancy Mr Pilot"