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Aviation History
1948
1948 - 0369.PDF
MARCH I8TH, 1948 FLIGHT 309 Standard Approach Procedure Design for Safety in Landing Technique at State-owned and Joint-user Airfields AS the volume of air traffic in the U.K. has increased, so it hasbecome necessary to introduce a measure of standardization^ in Instrument Approach and Holding Procedure. Representa tives of the technical branches of the Ministry of CivilAviation formed a committee to produce standards acceptable to operators, for application to airfields in the U.K. equipped withcurrent approach aids. Agreement has been reached and the recom- mended Standard Instrument Approach and Holding Procedures forS.B.A., I.L.S., B.A.B.S., Radio Range, Ground D/F and Radio Beacons will be published in Notices to Airmen. Application of theagreed clearance areas and surfaces to all State-controlled and certain joint-user airfields in the U.K. with approach aids will bethe immediate task of the committee, and procedure diagrams and PRnstructions will be subsequently produced. Whereas it was a simplematter for the committee to formulate procedures for each approach aid by consideration of the physical aspects such as angles, charac-• teristics of beams, positions of markers and localizers, in relation to the task for which the aid was designed whether for homing, 4N.M. have indicated that allowance to be adequate. The width of1,000ft on either side of the centre line of the beam was considered to be the maximum deviation from the centre line during theinitial stage of an overshoot and that was supported by the sug- gestion of such a width for I.L.S. in the relevant I.C.A.O. document.The overall length of the approach section was determined by an assessment of the distance covered by an aircraft flying the initialstage of the Standard Procedure at 150 knots true air speed .with a wind component oi 60 knots, making due allowance for time anddrift during the procedure turn. The length of Sector A was fixed by an assessment of the distance covered by an aircraft flying at150 knots with the same wind velocity, therefore a head wind when descending from 1,400ft to the standard Outer Marker height oi600ft at a rate of descent of 600ft per minute. The length of Sector B was determined by the inner boundary of Sector A and the positionof the Outer Marker. A different speed was used for calculating the minimum length of Sector C, which was determined by thedistanc<ScQ><«red by an aircraft flying at 90 knots into a 6e knot . ,N 4HM A plan view of the clearance areas around the Approach, Airfield and Overshoot Ssctions. descent through cloud or blind landing, it was illogical not torelate the necessary ground clearance in each stage of the procedure to conform to an acceptable safety standard. As recommended, the landing procedure for S.B.A. is to com-mence the procedure over the Main Beacon or localizer at a height of 1,500ft and fly out for one minute beyond the Outer Marker onthe Q.D.R. of the beam, descending at the same time to 1,400ft. A procedure turn is then made on to the Q.D.M. of the beamreducing height to reach the Outer Marker at 600ft and the Inner Marker at 100ft above the airfield level. In deciding the outer limitslikely to be encompassed by such a landing procedure and the minimum clearance desirable within those limits, the committeerecommended that the approach section of the S.B.A. (Front Beam) safe clearance path should be considered as symmetrical about thecentre line of the beam and to vary uniformly in width from 2,000ft at the Inner Marker to four nautical miles at a distance oisix nautical miles beyond the Outer Marker. The sides of the Section were considered toextend parallel from that point for a further two nautical miles. The overshoot sectionof the safe clearance path was also con- sidered to be symmetrical about the centreline of the beam and to vary in width from 2,000ft at the Main Beacon to four nauticalmiles at a distance of seven nautical miles nd it. The approach and overshootons were considered to be joined by the airfield sector which extends from the InnerMarker to the Main Beacon and which is also symmetrical about the centre line of-the beam and to have a uniform width ol 2,000ft ' - The whole of the safe clearance path isdivided into Sectors as illustrated in the diagram and the clearance surfaces .of eachsector are as follows: Sector A: A surface inclined upwards from 300ft at the common boundary of Sectors A and B to 600ft at the outer boundary of Sector A Sector B : A horizontal surface at 300ft above airfield level. Sector C: A. surface inclined upwards from airfield level at the common boundary of Sectors C and D to 300ft at the common boundary of Sectors B and C. Sector D • A horizontal surface at airfield level. Sector E: A horizontal surface at airfield level. Sector F: A surface inclined upwards at I in 50 from airfield level at the Main Beacon to the common boundary of Sectors %F and G.Sector G: A surface inclined upwards at 1 in 30 extending from the line of intersection of the 1 in 50 slope with the common boundary of Sectors F and G, to the outer boundary of Sector G. This is called the Permissible Terrain Profile. It was considered that four miles would allow for the maximumdeviation from the beam of an aircraft commencing the final approach after making the procedure turn, and the maximumwidth of the approach and overshoot sections was based on that assessment. G.C.A. aircraft monitored during S.I5SV. approaches head wind and descending from 600ft to theStandard Inner Marker height of tooit at .1 rate of descent of 600ft per minute. The lowerground speed was used to establish the sector length because the descent is started lromthe Outer Marker which is a fixed datum, and if it is necessary to raise height at theOuter Marker the length of Sector C will increase and the length ol Sector D willdecrease. Sector E extends irom the Inner Marker to the Main Beacon as it was con-sidered that provided overshoot action was taken not later than the Inner Markerposition there would be sufficient time for an aircraft to carry out the necessary vitalactions on the first stage of the overshoot. and to start to climb by the time the Main Beacon was reached.In the overshoot section the clearance path was determined by the distance which would be covered by an aircraft climbing at anangle of i in 30 in still air conditions through 1,400ft. Such an angle approximates to 300ft per minute rate of climb at 90 knots4,000ft at 120 knots or 5,000ft at 150 knots. The subdivision of the section was conditioned by the distance covered by an overshootingaircraft in attaining a height of 300ft above the 1 in 50 clearance from airfield level at the Main Beacon. Safety Height From the foregoing, it will be appreciated that the committeehas decided what is likely to be the flight path ol an aircraft flying the recommended S.B.A. Procedure, without adjustment, in worstconditions and has arranged fqr an adequate terrain clearance beneath it. There is, therefore, a^teJ«ft*ionship between the " Worst COMPLETION OF FINAI PROCEDURE TURN.' 0 SION THROUGHP,ERMISSJBL£iiERRAIN PROFILE AISE5BREAK -OFF HEIGHT TO ENSURE PRESCRIBED CLEARANCE • WORST FLIGHT PATH 5OO FT PERMISSIBLE i /Z9>. TERRAIN PROFILE AIRFIELD LEVEL v In this diagram the "Permissible Terrain Profile " cuts obstructions on the Approach and Overshoot Sections. In asuch cases, height at the Outer Marker would be raised to ensure 300ft clearance in Sector B. Relationship between the "Standard Flight Path" and the " Worst Flight Path " is apparent. Flight Path" and the recommended Standard Path. In genera,it follows that where obstructions penetrate the " Permissible Ter rain Profile," the " Worst Flight Path ".profile will need to be raised,at least in part, to ensure the necessary' clearance. This in turn will necessitate the raising of the Standard Flight Path by a com-mensurate amount. Where obstructions exist in Sectors A, B or C, it will sometimes be possible to raise the Outer Marker height andstill attain the standard height of 100ft at the Innner Marker provided that a maximum rate of descent is not exceeded. Obstruc-tions in the other Sectors which comprise the safe clearance path however, will affect the Overshoot phase of the Procedure and willraise the height at which overshoot action should be taken. The ultimate relationship between these profiles is that a heightwill be reached in an Instrument Landing Procedure below which a pilot should not descend unless able to do so visually, becauseby so doing he will be unable to maintain the specified 'clearance when attempting a landing. This height will be known as the" Break-off-Height " and will be specified for each Approach Aid at all airfields Procedures so produced will ensure a safe clearanceof terrain and obstructions.
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