Today, the trend is to increase {HEAD ~ the temperature} of operation of {TAIL ~ gas turbine engines} 
Today , the trend is to increase { HEAD ~ the temperature } of operation of { TAIL ~ gas turbine engines }
OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL