Vigilant Pilot:
The Minimum Eye Height over Threshold (MEHT), It is the lowest eye height over the runway threshold at which an on-slope
indication on the PAPIs will be seen for a given approach.
Reference Datum Height (RDH) is the height that the aircraft should pass over the threshold on a 3 degree approach (normally 50').
Professional Stude:
A Daysack is useable at night, however it should be referred to as a Knapsack at sunset until sunrise.
Co-capt: I could answer your first question but it would see the Geek-Police go into overdrive!!
I'll get my coat...