IGE versus OGE
During a practical standards exam for a commercial license, after many questions which I had no clue what the answer was, the examiner asked what the difference was between IGE and OGE. Finally something I thought I could answer.
After a fine dissertation with references to Prouty, George Saunders, physics and the rest, he looked above his glasses and said "try again"
Hmm, I had no other ideas.
"look in the performance section for IGE"
Ah, there it was, on the bottom of the graph, in the performance seciton of a 500, IGE is defined as "Figure 5-4. Hover Ceiling VS Gross Weight, IGE, 3.5 foot skid clearance with a standard landing gear, Allison 250-C20B."
And as far as the FAA was concered "everything else is OGE"
So that may just be the "practical" answer for OGE versus IGE.