Volume 16 No 1 Feb 1965.pdf - Lakes Gliding Club

LEE WAVE uGEN" FOR AEROPLANE PILOTS•• DETECTION A~DAVOIDANCE OF MOUN-TAIN WAVE SYSTEMS - SAFETYHEIGHTS OVER HIGH GROUND" is tbe titleaf a Civil Aviation Information Circularissued by the Ministry of Aviation andnumbered 92(1964. It introduces thesubject with an account of how a LockheedLightning pilot arrived in theBishop Wave region of California tofind that a dust storm prevented hislanding below: so he soared his 13,500lb. machine in the upcurrent of a wavefor over an hour until the landingground cleared. He estimated the upcurrentat 8,000 fL/min.. and the circularpoints out that, if he had not been soknowledgeable about waves, he couldhave got into an equally strong downcurrentwith disastr:>us results.The circular then gives general informationabout waves, remarking that theirpresence is revealed by the aircraft'saltimeter and vertical speed indicator,and points out that "a height coupledautopilot. when in use, will attempt tocounteract any height fluctuations inducedby wave motion, thus bringingabout marked variations in airspeed. Itis therefare advisable either to disengagethe height lock, or to revert to manualcontrol in severe conditions, because thespeed variations may otherwise be somarked as to bring the aircraft to astalled condition."Then comes a warning about rotorsand an account of how a Hoeing B-52re:cently lost most of its fin and rudderin One. Here we have the first mentionof "safety heights", for which, for theavoidance of rcters, the recommendedclearance margin above high ground is"at least half the height of the mountainsabove the surrounding terrain".This is immediately followed by a warningthat, to counteract the downcurrentsof waves, a much higher "safety heightmargin" might be needed.The next section. "Downdraughts".deals at once with this problem, sayingthat the: effects of vertical currents dependlargely on the aircraft's performanceand on the direction of its trackrelative to the alignment of the highground which initiates the waves. Whatshould the pilot do if caught in a con­~muous downcurrent., The answer givenIS:"Unfortunately. because of the manyvariables involved, no hard and fastrules can be laid down, but generally thebest course would be to turn upwind ordownwind and seek rising air in anotherpart of tbe wave system. If the aircraft'sterrain clearance is already marginal itwill be desirable to turn downwind, i.e.away from the initiating high ground."The reader is referred to an Appendixfor more detailed information.. Three more hazards of flying overhigh ground are then mentioned. Firstair forced up a slope will c.ool at th~adiabatic rate and eventually becomecolder than other air at the same heightso its freezing level may be lowerelSecond, winds over mountains may deviat~in direction and vary in speed. Third,alttmeters may not read correctly: however,the errors due to airflow effectsare small.ALL.OVER THE WORLD4,000COSIM VARIOMETERSARE NOW IN USENational &- International Records"Cook" Compasses with pivotedmounting"Cook" Electric Variometers"Irving" Venturies & Water Traps"Burton" Total Energy Units"Cosim" Best Speed ScaleHoldersloolle'. I,OfIICobb-Slater Inst. CO. Ltd."COSIM" WORKS, DARLEY DALE,MATLOCK, DERBYS.TeJephone O.tley 0.1. 213825