looks to me like the difference is in the reduction method. QNH uses
ISA temp and pressure gradient, while QFF uses actual station temperature?
Or in other words, the QFF value tells you what the pressure at sea level would be (useful for plotting isobar charts), and the QNH value tells you what to dial on the altimeter so that it reads airfield elevation at the threshold when you land (useful for, obviously, landing a plane)
Well, given that QNH only gives you the airfield's true altitude AMSL in ISA conditions (temp.), it must use an ISA reduction to MSL.
Given that if you set zero(0) on the altimeter on the airfield you are reading absolute pressure at that elevation (hence QFE), this must use local station temperature.
So, given QFF is a reduction of QFE to MSL assuming isotheric conditions, I'm guessing that this is using station temperature as well (whereas QNH assumes ISA temp).
So, the difference appears to be the assumed temperature, with QFF using actual temperature, and QNH using ISA.
Hmmmm.... guess thinking about it more cleared it up. Thanks.
Given this, it would seem if you knew the QFF below, you could set your altimeter to it, and get true altitude (meaning adjusted for temp. vs. indicated not adjusted for temp) if the amtompheric temperature gradients were only guarenteed to be standard. =)
Interesting.