Loss of control

The pilot submitted a plan for a flight from Moorabbin to Corowa via Melbourne and Mangalore. When the aircraft was approaching Melbourne, air traffic control (ATC) issued a clearance to track direct to Corowa. About 15 minutes later the pilot reported that he entered intermittent instrument meteorological conditions (IMC). The aircraft was cruising at 7000 feet and no icing was present. Shortly afterwards, the aircraft entered continuous IMC and a short time after that ice started to form on the windscreen.

The pilot requested descent. Before ATC responded to his request, he adjusted the directional gyro (DG) to correspond with the magnetic compass. The autopilot had been engaged for some time before the DG adjustment was made. The pilot made the adjustment by pressing in on the heading bug knob while he pressed and turned the DG adjusting knob. This procedure was intended to prevent the aircraft from turning while the DG was adjusted.

The pilot said that immediately after the two knobs were released, the aircraft turned sharply to the right. He attempted to correct this manoeuvre, principally with rudder, but the aircraft persisted with the turn and began a rapid descent. From that point the pilot probably became disorientated. He made a mayday call and advised that the aircraft was inverted and that he had lost control. ATC assisted by providing advice on aircraft present position, lowest safe altitude, ground elevation and cloud base.

ATC then asked the pilot if the aircraft was autopilot equipped. This prompted the pilot to disengage the autopilot and shortly afterwards he regained control. Until that point, he had unwittingly been trying to override the autopilot. The aircraft was close to 4000 feet when control was regained. Recorded radar data and communications showed that the aircraft had executed a series of left turns and descended from 7000 feet to 4200 feet in about two minutes. Three and a half minutes after the mayday call the pilot advised that control had been regained. Altitude varied up and down during that time.

The pilot then elected to proceed to his destination, initially accepting ATC vectors via Mangalore. When the aircraft was later inspected by its home base maintenance organisation, no fault could be found with the autopilot. The aircraft had not been overstressed during the incident.

Factors

The following factors were relevant to the development of the incident:

• Control of the aircraft was lost in IMC for undetermined reasons.
• The autopilot was engaged when control was lost and the pilot neglected to disengage the autopilot, until prompted by ATC, while trying to regain control.

The pilot advised that, during the take-off roll, the aircraft's left main wheel entered a soft, muddy area and he lost directional control. The aircraft entered a ground loop during which, the right wing contacted the ground.

The pilot reported that the aircraft was hit by a gusting cross wind during the landing roll. The aircraft then ground looped turning through 180 degrees before coming to a halt, on the western edge of the runway.

Later examination of the aircraft by a local LAME revealed that there was damage to the left landing gear, the left wing tip and wing strut and the windscreen had popped out.

FACTUAL INFORMATION

The Boeing 737 was being radar vectored to intercept the final approach path for a landing on runway 16R from a right base leg, to follow a Boeing 747 which was already established on the instrument landing system (ILS) final approach. The B737 turned onto the ILS localiser track below the glideslope whilst descending to 2,500 ft some 10.5 NM from the runway threshold.  Shortly after the localiser track was intercepted, the aircraft experienced several abrupt changes in bank angle, both left and right, the most severe being a roll to the right through 51 degrees to a maximum right bank of 34.8 degrees.  A missed approach was carried out, followed by a normal approach and landing. A post-flight inspection found no defects which may have contributed to the occurrence. The aircraft was subsequently cleared to continue scheduled operations.

A review of recorded radar data and of information derived from the flight data recorders of the B737 and the preceding B747. It showed that the B737 was about 450 ft lower than the B747 had been at the same point in space, reaching that point some 127 seconds after the B747 had passed.  The longitudinal separation between the B737 and the B747 at that time was 5.5 NM.

Recorded wind data, as derived from the inertial reference system of the B737, indicated the wind direction varied between 165 and 185 degrees, at a speed of 8-14 kts.

ANALYSIS

The circumstances described in this occurrence are very similar to those of an earlier occurrence (9500460).  The following features were common to both:

Both lead aircraft were B747s which were established on the localiser as well as the glideslope. Both following aircraft were B737s which were given a radar vector to intercept the localiser, below the glideslope, at 2,500 ft. This resulted in both B737s passing the same point in space some two minutes later, but 500 ft lower than the preceding B747s.

Atmospheric conditions in the vicinity of the approach path at the time of both occurrences were conducive to the slow decay of wake vortices. As the localiser track is 155 degrees, there would have been little, or no lateral displacement of any wake vortices produced by the B747s.

Both following aircraft encountered uncommanded rolls consistent with encountering wake turbulence generated by the preceding B747.

United Kingdom Civil Aviation Authority wake turbulence studies (August 1994) have shown that B747 aircraft produce high rates of wake turbulence affecting following aircraft.

For sequencing purposes during VMC operations in the Sydney terminal area, most domestic aircraft arriving from the south are radar vectored to join a downwind leg when runway 16 is the duty runway. These aircraft are routinely cleared to descend to 2,500 ft whilst being radar vectored to intercept final approach about 6 NM from touchdown. International flights, however, must be established on final approach at least 10 NM from the threshold.  Many of these aircraft, such as the B747, are in the "heavy" category. This sequencing often results in the following domestic aircraft passing through the same lateral airspace as the preceding aircraft but some 500 ft lower.

The relative positions of respective aircraft, the provision of minimum wake turbulence radar separation, and meteorological conditions conducive to the formation and slow decay of wake vortices can make it possible for aircraft to experience wake turbulence encounters whilst such procedures are being implemented.

Consideration, therefore, of the vertical positioning of the following aircraft relative to the leader may provide the greatest potential for preventing accidents and incidents as a result of wake turbulence encounters.

SIGNIFICANT FACTORS

1. Atmospheric conditions were conducive to the slow decay of wingtip vortices generated by the preceding B747.
2. The B737 was sequenced by ATC to intercept the localiser for runway 16 approximately 500 ft below the preceding B747.

SAFETY ACTION

As a result of the investigation into this occurrence and a number of other occurrences, the Bureau of Air Safety Investigation issued interim IR 960101 recommendations to the Civil Aviation Safety Authority and Airservices Australia on 7 November 1996.

"1. The Bureau of Air Safety Investigation recommends that the Civil Aviation Safety Authority and Airservices Australia:

"(i) Evaluate the current wake turbulence separation standards. Consideration should be given to the evaluation of technology being developed to aid in the detection, tracking and forecasting of wake vortices as a further means of reducing the risk of wake turbulence encounters.

"(ii) Critically evaluate all current airport arrival and departure paths and procedures to identify and eliminate potential wake turbulence problems.

"2. The Bureau of Air Safety Investigation recommends that the Civil Aviation Safety Authority re-institute a wake turbulence education program. This education program should highlight areas of possible wake turbulence encounters and advise ways to minimise the effects of the encounters".

The helicopter was being landed at the completion of a training exercise. The weather was reported as being fine and clear, with little or no wind. During the landing, shortly after making a normal touchdown, the helicopter commenced to shake violently. The transition from normal operation was very rapid and did not allow the instructor time to take any remedial action before control of the helicopter was lost.

The most probable reason for the loss of the aircraft resulted from the rapid onset of ground resonance. This problem is associated with fully articulated rotor systems and is the result of geometric imbalance of the main rotor system. This imbalance of the rotor causes an oscillation which is transmitted throughout the entire helicopter, giving movement from side to side, as well as fore and aft. This action can become violent enough to cause the helicopter to roll over or incur major structural damage. Although not determined in this investigation, the onset of ground resonance can be aggravated by a number of factors, including incorrect landing gear strut inflation pressures, and incorrectly adjusted friction type blade dampers.

The pilot reported that he and his 13 year old son were going for a short flight from Maryborough airport. All necessary preflight checks were carried out and sufficient fuel was on board for the intended flight. After take-off from runway 17 at between 100 and 200 ft the engine stopped completely. Witnesses saw the aircraft enter a left turn at a low height. The aircraft then appeared to stall and hit the ground inverted. The landing gear was in the retracted position.

The pilot stated that the engine failed because the magneto switches were bumped to the off position. Both switches are mounted close together in the centre of the instrument panel with other switches below them. The pilot stated that he was told in hospital that his son was waving to a person on the ground and had bumped the magneto switches off.

The pilot reported that he was giving another pilot instruction for the purpose of endorsement on type. A few seconds into the landing roll the left wing lifted, and the aircraft veered sharply left towards the runway edge. Both the instructor and the student applied opposite rudder resulting in the aircraft turning quickly right. This was followed by a series of sharp turns left and right down the runway until the aircraft eventually entered a ground loop to the left before coming to a halt.

The pilot reported that during take-off with a full load, and 15 knots of crosswind, he lost directional control and ground-looped the aircraft about one third of the way along the strip. The right gear collapsed, and the right wing was damaged when it contacted the ground.

Boeing B737-476, VH-TJK, was being radar vectored onto the Sydney 16R localiser from the right, to follow a B747-438 already established on the localiser. The pilot in command was flying the aircraft with autopilot "A" engaged, heading mode selected, and VOR/localiser capture mode armed. The aircraft turned onto the localiser, descending through an altitude of 2,500 ft, some 2.3 dots below the glideslope, about 10 NM from the landing threshold. As the B737 began to intercept the localiser track, maintaining an airspeed of about 218 kts, the bank angle was progressively increased to 29.5 degrees before the aircraft abruptly rolled further to the right. This uncommanded event coincided with initiation of trailing edge flap and leading edge slat extension from the stowed position. The aircraft reached a maximum bank angle of 62.9 degrees before the roll was stopped, and the wings levelled by the handling pilot. A missed approach was carried out, after which the aircraft was landed normally without further incident. At the time of the occurrence the surface wind was 210 degrees at 10-15 kts, with scattered cloud at 1,800 ft and light rain in the area.

VH-TJK immediately underwent an extensive ground inspection program, followed by a flight test, in accordance with recommendations from the manufacturer. No defects were found which could have contributed to the occurrence.

An examination was carried out of recorded radar data, and of information derived from the flight data recorders of

VH-TJK and the preceding B747. It was found that the B747 had intercepted the 16R localiser at an altitude of 3,000 ft, approximately 11 NM from the landing threshold. The aircraft maintained 3,000 ft until intercepting the glideslope at about 9.5 NM and carried out what appeared to be a normal ILS approach to runway 16R. There was no reported turbulence.

VH-TJK intercepted the localiser at about 10 NM from the landing threshold, some 500 ft lower and about 115 seconds later than the B747 had been at the same lateral position. Recorded wind data from the inertial reference systems of both aircraft indicated the wind direction varied between 155 and 165 degrees, at a velocity of 25-35 kts. As the localiser track is 155 degrees, this meant there would have been little or no lateral displacement of any wake vortices produced by the B747.

Research has shown that for a typical jet transport aircraft, the wake descends behind the generating aircraft at approximately 300-500 ft/min for about 30 seconds. The descent rate decreases and eventually approaches zero at

500-900 ft below the flight path. The decay process of the wake is complex and is strongly influenced by atmospheric conditions (Boeing Airliner/Jan.-Mar. 1995). UK CAA wake turbulence studies have also shown that B747 and B757 aircraft produce higher Category A incident rates than other aircraft, where Category A incidents correspond to the development of uncommanded roll angles 30 degrees or greater to following affected aircraft. The same studies showed that aircraft with the highest incident rates of encountering wake turbulence behind B747 and B757 aircraft were BAC-111, B737 and DC9 types.

CAA wake turbulence radar separation standards are described in the Australian Manual of Air Traffic Services and are based on three categories determined from the maximum certified take-off mass of the aircraft. B747 aircraft are categorised as heavy, while B737 aircraft fall into the medium category. The standard, which is based on distance, shall be applied when an aircraft is crossing behind, or operating within 0.5 NM laterally of another aircraft's flight path at the same level or less than 1,000 ft below. In this case, a medium behind a heavy required a minimum separation of 5 NM. Examination of the recorded radar data showed that the separation between VH-TJK and VH-OJC was approximately 5 NM at the time of the occurrence.

The recorded trailing edge flap and leading edge slat data for VH-TJK indicated that the uncommanded roll occurred whilst the devices were extending from their fully stowed positions. It was not possible to determine from the recorded data whether this was responsible for the uncommanded roll or contributed to its severity. However, ground inspections and subsequent flight testing did not reveal any defect in the operation of those systems.

It is therefore considered that the onset of the uncommanded roll resulted from an encounter with wake turbulence generated by the preceding B747.

Significant Factors

1. Atmospheric conditions were conducive to the slow decay of wingtip vortices generated by the preceding B747.
2. VH-TJK intercepted the localiser some 500 ft below the altitude of the preceding B747.
3. The maximum longitudinal spacing between VH-TJK and the precedingB747 was 5 NM.

The student pilot was conducting a series of stop and go landings. On the fourth approach, Tower instructed the pilot to carry out a touch and go landing due to a following RPT aircraft. The aircraft bounced on landing and the pilot lost directional control. During the subsequent ground loop, the propeller and right-wing tip struck the ground.