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