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