In preferred practice, {TAIL ~ the pairs} of {HEAD ~ injectors} 524, 526 and 528, 530 are operated in such a manner that, one of the injectors of each pair is permitted to defrost while its mate is in active service 
In preferred practice , { TAIL ~ the pairs } of { HEAD ~ injectors } 524 , 526 and 528 , 530 are operated in such a manner that , one of the injectors of each pair is permitted to defrost while its mate is in active service
OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL OTH HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH