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