sennadog - as you refer to M23 and trees and PAPIs you are referring to approaches to 28L/R... I usually find myself coming in a bit high on this approach for this reason: the pass over the motorway tend to lift you up a bit (warm air off the tarmac, I guess) but the area ahead of the threshhold with the trees always seems to give you sink of even greater rate than the previous climb... therefore, I seem to get very close to the trees if I follow the PAPIs exactly. Not good
I'm not saying the PAPIs are wrong - I'm sure it's my flying! But i find my landings are better if I come in on two whites until the sink over the greenery beford the 28 threshold...
Hersh