The intensity or {HEAD ~ brightness} of {TAIL ~ a visual aid} could vary when the aircraft is operating on the approach phase of flight 
of
The intensity or { HEAD ~ brightness } of { TAIL ~ a visual aid } could vary when the aircraft is operating on the approach phase of flight
OTH OTH OTH HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH