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C I V I L 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

"9-0 ! ./

Civil Aviation Department,Accident Investigation Division,New Rodney Block, 1st floorf99t Queensway,Hong Kong.

March, 1972

Your Excellencyf

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. ThomsonChief Inspector of Accidents

His Excellency the Governor,Government House,Hong Kong*

CIVIL AVIATION DEPARTMENT, HONG KONGACCIDENTS INVESTIGATION DIVISION

Aircraft;

Engine:

Registered Ownerand 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 Airportf Kowloon, Hong Kong,Latitude 22° 19' 28*3"N, longitude 114 llf 3

Date and Time: 24th Octoberf 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 747f

causing loss of control at a height from which recovery was impossible.

- 2 -

1. 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 •A1.

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*

- 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 1 - -

None 1

1»3« Damage to Aircraft

Destroyed*

1*4* Other Damage

None*

- 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 1961. The Certificate of

Airworthiness No. 93 was first issued by the Director of Civil

Aviation, Hong Kong on 24th April 1969. The Certificate was

valid until 23rd April 1972* 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*

- 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, 1971.

This check was signed by the supervising engineer of the Club

and was valid for 5O hours or 31 days whichever came first* It

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 aircraft1 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

The weather over Hong Kong was dry with 14 Kilometres

visibility* 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 lightf

although wind speeds ranging from 5 to 14 knots were recorded

between 0526 and 0531 hours* 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*

- 6 -

1*11* Flight Recorders

Not required, none fitted*

1.12. 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*

- 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 rearf 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*

1.13. 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*

- 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 turbulencef

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.

2.1.2. 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

- 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.

2.1.3* 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.

- 10 -

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 wouldf 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 7̂ 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 7̂ 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.

-li-

lt 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* The

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* 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* He exercised

his judgement in shortening the circuit to avoid delaying a

following commercial jet aircraft and aimed to touch down at

a point on the runway five hundred feet beyond the threshold*

• /He

- 12 -

He did not observe the landing of the preceding Boeing 7̂ 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 7̂ 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 7̂ 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*

- 13 -

(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* HallamInspector of Accidents

Accidents Investigation Division,Civil Aviation Department,Hong Kong*

March 1972

C f c N T R - E t l N E o f A P P R O A C H LIGHTS

KITSCALE : I.:

Acc I P E N T

C I R . C U V 1 F L O W N f e y VR. -HAE

A c M PATH op B O E I N G 74.7

[HKP] 629.13255 H7 B71

XD1763D37

XQ1763037

TOC

K&*̂ 1400223629.13255

Hong Kong. Civil Aviation Dept«Accident Investigation Division.Civil aircraft »**/»•!/***«+