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