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