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2 UNIVERSITY OF HONG KONG LIBRARY This book was a gift from Hong Kong Government Printer

3 CIVIL A V I A T I O N D E P A R T M E N T CIVIL AIRCRAFT ACCIDENT Report on the Accident to Chipmunk DHC-1 Series 22, VR-HGE at Hong Kong International Airport, Kowloon, Hong Kong on 24th October 1971 March f 1972

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5 Civil Aviation Department, Accident Investigation Division, New Rodney Block, 1st floor f 99t Queensway, Hong Kong. March, 1972 Your Excellency f I have the honour to submit the report by Mr* J.A* Hal lam, an Inspector of Accidents, on the circumstances of the accident to Chipmunk DHC-1 Series 22, VR-HGE which occurred at Hong Kong International Airport, Kowloon, Hong Kong, on 24th October, 1971* I have the honour to be Sir, Your obedient servant, T.R. Thomson Chief Inspector of Accidents His Excellency the Governor, Government House, Hong Kong*

6 CIVIL AVIATION DEPARTMENT, HONG KONG ACCIDENTS INVESTIGATION DIVISION Aircraft; Engine: Registered Owner and Operator: Crew: Chipmunk DHC-1 Series 22 VR-HGB* D*H* GIPSY Major 1O MK 2. The Hong Kong Flying Club* Instructors Mr. J*R* Allen - injured* Pupil : Captain P*A* Bush - uninjured* Place of Accident: Hong Kong International Airport f Kowloon, Hong Kong, Latitude 22 19' 28*3"N, longitude 114 ll f 3 Date and Time: 24th October f 1971$ at O531 G*M*T* (1331 hours, Hong Kong Standard Time) All times in this report are G«M«T* SUMMARY The aircraft joined the circuit at Hong Kong International Airport, downwind for runway 131 when a Boeing 747 was on short final approach and turned onto final approach when the Boeing 74? was clearing the runway at the end of the promontory* The Chipmunk's approach was normal until at a height of approximately 4O feet above the runway centreline at the threshold, the starboard wing dropped suddenly and the aircraft commenced a rapid turn to the right* The instructor took control and attempted to regain level flight by the use of rudder and aileron but the nose dropped and the aircraft's starboard wing struck the ground before control became effective* No evidence of p re-crash malfunction was found and the report concludes that the aircraft encountered a residual vortex formed at the port wing tip of the preceding Boeing 747 f causing loss of control at a height from which recovery was impossible.

7 INVESTIGATION!! History of the Flight Chipmunk DHC-1 Series 22, registration VR-HGE, took off from Hong Kong International Airport at 0503 hours on a training flight with an instructor and a pupil pilot* The detail was planned to include revision and general handling practice for the pupil who had previous experience of the aircraft type* On completion of the exercise the Chipmunk returned to the Airport and joined the downwind leg of a left hand circuit for runway 13» Aerodrome Control asked the Instructor if he was able to make a short approach to land behind a Boeing 747* then on short final 9 as further jet traffic was approaching from Green Island, approximately 4 minutes behind the Boeing 74? This was acknowledged and half flap selected for base leg, the instructor taking control for a period during the turn onto finals* Pull flap was selected to reduce height and when established on straight final approach, half flap was reselected and the pupil resumed control for landing* The circuit patterns flown by the two aircraft are shown at Appendix A 1. The Boeing 74? was observed turning off the runway at the penultimate stub taxiway as the Chipmunk completed its turn onto final approach and the Aerodrome Controller advised: "Golf Echo caution wake turbulence from 74?* You*re cleared to land behind, wind is 120 at 10 knots"* The Instructor acknowledged this call and final approach continued smoothly, the pupil's intended touch down point being approximately five hundred feet upwind of the runway threshold*

8 - 3 - When about 40 feet above the runway centreline at the threshold, the starboard wing dropped suddenly but smoothly, with no prior buffeting, the nose rose and the aircraft commenced a rapid turn to the right * The Instructor took over control and attempted to regain level flight by using both rudder and aileron but before any control reaction became evident the aircraft's nose dropped and the starboard wing hit the ground* The aircraft came to rest on the grass area to the side of the runway two hundred and seventy feet from the centreline, having turned through 180 from the line of approach. The cockpit canopy sprung slightly open but could not be opened further from the inside* Firemen of the Airport Fire Contingent were at the scene within thirty seconds of impact and forced the canopy open* Fuel and ignition systems were turned off by the occupants before they were assisted from the cockpit and taken to hospital* The Instructor in the rear seat suffered lacerations to the forehead and a fracture of the left ankle, as well as bruising, but the pupil in the front cockpit was uninjured except for bruising caused by shoulder harness restraint*! Injuries to Persons Injuries Crew Passengers Others Fatal Non-fatal None 1 1»3«Damage to Aircraft Destroyed* 1*4* Other Damage None*

9 - 4-1*5» Crew Information Mr. J.R. Allen, aged 45, learned to fly at the Plymouth Aero Club, United Kingdom in 1958* He joined the Hong Kong Flying Club in 1964 and the Royal Hong Kong Auxiliary Air Force in I960. In the first quarter of 1969 he completed an Assistant Flying Instructor course with the Hong Kong Flying Club* At the time of the accident Mr. Allen had accumulated over 1,400 hours experience, about 35O hours of which were on helicopters and about 10O hours on Chipmunk* With the exception of approximately 5 hours conversion for instructor rating the Chipmunk time was all flown in an instructional capacity. He holds a valid private pilot licence with an assistant instructor rating. Captain F.A. Bush, aged 28, learned to fly with the British Army Aviation Corps in 196?. He joined the Hong Kong Flying Club in 1971 to acquire current flying experience on aeroplanes for the issue of a private pilots licence. He has passed the medical and aviation law and procedures examinations required for the issue of a private pilots licence* At the time of the accident he had accumulated 1,037 hours experience of which 6O hours were on Chipmunk aircraft, the remainder on helicopters. 1.6* Aircraft Information The aircraft, a De Havilland Chipmunk T-22 Serial No* DHB/F/158, engine Gipsy Major 10 MK 2, Serial Number 11697$ was manufactured by the De Havilland Aircraft Company, Limited of the United Kingdom in February The Certificate of Airworthiness No. 93 was first issued by the Director of Civil Aviation, Hong Kong on 24th April valid until 23rd April 1972* The Certificate was Prior to its purchase by the Hong Kong Flying Club the aircraft was owned by the Singapore Flying Club. At the time of its purchase by the Hong Kong Flying Club it had completed 3733 hours 15 minutes flying* The aircraft was shipped in a dis-assembled condition to Hong Kong where it was re-assembled by the Flying Club and made ready for flight* The total airframe hours to the time of the accident were 4328 hours 30 minutes*

10 - 5 - The type of Maintenance Schedule had been changed within the last few months and the aircraft was being maintained to a General Purpose Category Schedule at the time of the accident. The last monthly Check "A" was carried out on 5th October, This check was signed by the supervising engineer of the Club and was valid for 5O hours or 31 days whichever came first* had completed 14 hours 30 minutes since this check* The last pre-flight check was carried out on the morning of the accident by Mr* J*R* Allen* The aircraft was last weighed on 15th April 1969 and its empty weight and centre of gravity position established* The centre of gravity position on the aircraft 1 s last flight was calculated as 2*13 inches forward of the datum and thus lay within the prescribed limits of 6*8 inches to.77 inches forward of datum* 1*7* Meteorological Information visibility* The weather over Hong Kong was dry with 14 Kilometres At the time of the accident) cloud cover at the Airport was 6/8ths Strato Cumulus at 2000 feet* Winds were variable between northeast and southeast and generally light f although wind speeds ranging from 5 to 14 knots were recorded between 0526 and 0531 hours* It Barometric pressure was 1O16 millibars and Royal Observatory records show that the air near the ground was unstable* 1*8. Aids to Navigation Not applicable* 1*9* Communications All VHP voice communication channels were operating normally and no reception difficulties were reported by aircraft or aerodrome control. 1*10* Aerodrome and Ground Facilities Not applicable*

11 - 6-1*11* Flight Recorders Not required, none fitted* Wreckage When the aircraft finally came to rest it was in a near horizontal position, although considerably damaged* The cockpit area of the fuselage was virtually intact as was the empennage including the vertical and horizontal stabilisers. The rear fuselage however had broken away to the left around the area of the front fuselage to rear fuselage attachment joint* The right side fuselage skin had sheared just forward of the fuselage front lap strap joint* The engine mount was broken due to the engine being forced to the left when it struck the ground* The propeller was bent rearwards but the extent of bending was consistent with low engine power at the time of impact* The throttle at the engine position indicated half open but this should not be taken as indicative of the throttle setting prior to impact* The left wing, including the aileron and landing flap, was virtually intact except for damage at the inboard end of the leading edge, although the wing structure showed signs of slight distortion* The outer portion of the right wing had become separated from the inner portion at a position in line with the inboard end of the aileron* The aileron itself had become detached when the bolts securing the centre hinge bracket to the wing had sheared during the impact* The inboard hinge had then pulled out from its ball race and the operating rod had fractured at the eye end at the aileron* The remaining inboard portion of the wing was twisted trailing edge up in relation to the fuselage* The front cockpit area was virtually undamaged but the instrument panel in the rear cockpit had sustained damage, possibly due to the impact of the pilot's head* The glass of the Air Speed Indicator had been shattered*

12 - 7 - Both safety harnesses were intact* However, the shoulder harness at the rear cockpit was attached to the dorsal rail assembly on the rear fuselage and when the rear fuselage broke away to the left this attachment was carried with it, providing less restraint on the left shoulder strap than the right* No fault was found in the flying controls except for damage consistent with impact* The flaps were in the UP position. When the right wing trailing edge folded upwards and to the rear f the bracket securing the flap cable pulley group at the right and under the rear pilot's seat collapsed, allowing the flaps return spring to pull the flaps to the UP position* It was concluded after a detailed examination of the wreckage that there was no evidence to indicate any pre-crash malfunctioning of any kind and it is considered that all damage sustained by the aircraft was caused by impact* Fire There was no fire* I«l4» Survival Aspects The accident was survivable but it is considered that very serious injuries to both occupants were avoided by the proper use and integrity of the full safety harness fitted. As it was, the slight lessening of restraint of the rearseat left shoulder harness, occasioned when the fuselage ruptured immediately behind the cockpit area, allowed the instructor's forehead to impact on the instrument panel*

13 - 8-2 * ANALYSIS AND CONCLUSIONS 2*1 Analysis 2. i.l«turbulence generated by large transport aircraft. It is known that the larger proportion of an aircraft*s wake or "slipstream" effect, is produced by vortex turbulence f generated at the wing tips of the aircraft as a side effect to the lift which the aircraft's wings are producing. These vortices are formed in flight by air in the region of high pressure beneath the wings, spilling around the wing tips into the region of low pressure which the aerofoil shape is producing above the wing surface. This motion, coupled with the forward movement of the aircraft, creates a vortex of air funnelling back from each wing tip. The vortices, which are in the nature of a pair of narrow horizontal whirlwinds, rotate towards the wing tip which produces them. As a result of their own motion, the vortices tend to settle below and behind the generating aircraft, but if the aircraft is close to the ground, as in an approach to land, the vortices will reach the ground and tend to fan out laterally. Since the energy of the vortex turbulence at its source is directly proportional to the wing span loading of the aircraft and inversely proportional to its speed, the most violent vortex turbulence will be generated by large, heavily laden, swept wing aircraft flying at low speed with all high lift devices extended, such as during an approach to land or immediately after take-off* When the aircraft's flaps and undercarriage are extended, however, it can be expected that the turbulence generated will cause a faster decay of the vortices which will, in any case, cease to be generated when the aircraft is on the ground and lift is no longer obtained from the wing Effect of wake turbulence on light aircraft operations. Light aircraft, with a short wingspan and consequent relatively small spacing between ailerons, are more susceptible to a loss of lateral control when encountering a wake vortex than a large aircraft, the ailerons of which may remain effectively free of the narrow vortex core. Air Traffic Control Officers /and

14 - 9 - and pilots in Hong Kong have been aware for some considerable time of the dangers of a vortex wake encounter. A two minute interval is required between the touchdown of a Boeing?4? and that of a succeeding aircraft and warnings of the possibility of wake turbulence are passed. Pilots of light aircraft normally will aim to land well beyond the point of touchdown of a preceding large aircraft, in order to remain above its flight path and thus minimise the risk of encountering wake turbulence * Observations. The pilot of Chipmunk VR-HGE concentrated on adjusting his position, altitude and speed in the circuit in order to fit his landing midway between the preceding Boeing 747 and the aircraft approaching approximately four minutes behind the 747 «Being unable to pinpoint the exact location on the runway where the 747 touched down, and conscious of the following aircraft and the need to clear the runway as expeditiously as possible, the pilot aimed to touchdown about 50O feet upwind of the threshold, believing this to be the best compromise. Mr. Allen states that the wind sock indicated a light wind of approximately 6 knots from left to rights an indication which he took to mean that by the time he arrived at the threshold, the wake turbulence of the 747 would have been dissipated off the runway to the right* On this assumption, he maintained his approach on the runway centreline. Both pilots emphasized the sudden dropping of the starboard wing when approaching the threshold. Mr. Allen, in the rear seat, also had a definite impression of experiencing a negative G sink. This could be attributed to his position relative to the centre of gravity datum when the aircraft pitched up. There is a tendency for the Chipmunk's nose to rise when the starboard wing drops due to the gyroscopic effect caused by rotation of parts of the engine and propeller. The direction of propeller rotation is anticlockwise when viewed from the cockpit, which is opposite to the direction of rotation of the aircraft rolling to the right.

15 It is considered unlikely that the upper part of vortices lying over the runway threshold would be higher than about 50 feet The height above ground when the sudden roll was experienced indicates that it would f in all probability, be the upper part of the vortex core which induced the roll. The roll to the right experienced in this case should, in theory, be caused by the vortex formed at the port wing tip of the Boeing 7^7* The chance of one wing only entering either side of the starboard vortex, with its anticlockwise rotation movement, is considered remote. Again, in theory, the starboard vortex should have cleared well to the right of the runway under the influence of the light cross wind combined with the normal lateral spread caused by its own motion. It is assumed, then, that the Boeing 747's port wing tip vortex was entered by the Chipmunk and a strong induced roll to the right resulted through the clockwise rotation of that vortex* However, the runway of Hong Kong being 20O feet in width and the wing span of the Boeing 7^7 being almost 196 feet, would suggest that the wing tip vortices were generated at the extreme edges of the runway and were subject to ground effect as soon as they formed in the threshold area. The lateral movement spread rate of the vortices when in ground effect in calm condition is estimated at 5 knots, so that a cross wind component of more than 5 knots from left to right would be necessary to move the port wing tip vortex onto the runway. In the event, the mean wind indicated between 0526 and 0531 was 120 at 9 knots, giving a mean cross wind component of approximately 2.4 knots. Thus, again in theory, the port wing tip vortex should have rolled to the left, away from the runway, a distance of nearly 400 feet, by the time that the Chipmunk approached the threshold approximately 90 seconds after generation.

16 -lilt has not been possible to establish the precise time interval between the?4? and the Chipmunk when overheading the runway threshold. It is known that the?4? was clear of the runway 20 seconds before the Chipmunk commenced its roll to the right. Observed times of Boeing 747 runway movements indicate that in similar wind conditions the time from overhead the threshold to clear of runway on the penultimate stub taxiway can vary between extremes of 55 seconds and 9O seconds, one operator showing a consistent 80 seconds* From these observations a mean time of 7O seconds has been calculated and when the known 20 seconds is added an approximation of 90 seconds is reached* It is concluded that it is probable that the Chipmunk's threshold time was at least 90 seconds behind that of the preceding Boeing?4?* The Captain of the Boeing 74? has stated that the nosewheel of his aircraft met the runway approximately?oo feet upwind of the threshold and very near the centreline* Vortex wake should thus have ceased to be generated at this point* headwind component of the indicated mean wind in this case should have tended to drift the terminal vortex wake downwind a distance of approximately one thousand three hundred feet, or six hundred feet downwind of the threshold, in ninety seconds* The anemometer from which the mean wind has been taken is sited five hundred feet west of the runway 13 centreline at the threshold position* The Although the official aerodrome wind is measured by the anemometer at the southeast of the promontory, the proximity to the accident site of the anemometer used is considered to give a more realistic indication of the wind effect in the threshold area, although quite large variations can be expected at surface level* 2*1*4* Summary of Analysis* The instructor is experienced in mixed traffic conditions and was well aware of the dangers of wake turbulence* his judgement in shortening the circuit to avoid delaying a following commercial jet aircraft and aimed to touch down at He exercised a point on the runway five hundred feet beyond the threshold* /He

17 He did not observe the landing of the preceding Boeing 7^7 and thus was aware that his chosen landing point may not have been in accordance with the recommendation that following aircraft should aim to touch down well beyond the actual touchdown point of the aircraft ahead* The required time interval of two minutes between the landing of a Boeing 7^7 and a following landing aircraft would probably not have been effected in this case, although it has not been possible to establish the precise timing* The instructor assumed from the Aerodrome Control report and visual wind indications that wake turbulence from the preceding Boeing 7^7 would have dispersed from the runway before the Chipmunk landed* From the known characteristics and theoretical movement of wing tip vortices, this effect should have been well clear of the centre line of the runway at the threshold at the time the Chipmunk suffered the strong induced roll that led to the accident* It is apparent that knowledge of the ground movement of wake turbulence is still incomplete and it cannot be stated that turbulence would not have been encountered had the instructor made a wider circuit and allowed a greater time interval between the landing aircraft* 2.2* Conclusions (a) Findings i) The aircraft had been properly maintained, and its documentation was in order* ii) The centre of gravity was within the prescribed limits* iii) The instructor was properly licensed* iv) There was no evidence of pre-crash failure or malfunction of the aircraft* v) While on final approach to landing and at a height of approximately 4O feet, the aircraft rolled rapidly to the right and struck the ground before control could be regained* vi) The occupants survived the accident without serious injury due to the use of full safety harness*

18 (b) Cause The aircraft encountered a residual vortex formed at the port wing tip of the preceding Boeing?47? causing loss of control at a height from which recovery was impossible* 3. RECOMMENDATIONS 3«1» In addition to the required minimum landing interval of two minutes, pilots of light aircraft following heavy, large aircraft types for landing should maintain an approach profile above that of the preceding aircraft and aim to touch down at least five hundred feet beyond the point on the runway that the nosewheel of the preceding aircraft is grounded. 3«2» The attention of operators of light aircraft should be drawn to the survival aspects contained in para* 1*14 of this report and to the advisability of fitting full safety harness for all occupants and ensuring that shoulder harness is used at all times during landing and take off* J*A* Hallam Inspector of Accidents Accidents Investigation Division, Civil Aviation Department, Hong Kong* March 1972

19 CfcNTR-E tlne of APPROACH LIGHTS KIT Acc I PENT CIR.CUV1 FLOWN fey VR.-HAE AcM PATH op BOEING 74.7 SCALE : I.:

20 [HKP] H7 B71 XD1763D37 XQ K&*^ Hong Kong. Civil Aviation Dept«Accident Investigation Division. Civil aircraft»**/»!/***«+ TOC

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