Loss of control

The pilot stated that he was spreading "Urea" on a cotton crop. Prior to commencing a procedure turn the aircraft overflew a stand of trees. When the aircraft was about three quarters of the way through the turn, the pilot felt the aircraft begin to stall. He lowered the nose and increased power, but the aircraft descended into the trees. It continued to fall between the trees and slid for 70 metres before coming to rest. The pilot stated that during a previous landing he had experienced a willy-willy which had originated from the stand of trees. He was conscious of downdrafts near the trees. During previous turns he had flown well past the trees before commencing a turn. On this occasion he feels that the aircraft was affected by a downdraft.

The pilot was tasked to fly parachutists to altitude where they would exit for a display at the Melbourne Masters Games.

Prior to any jumps, the pilot had held a safety briefing for about 50 parachutists involved in the event. According to the Safety Support Officer of the Australian Parachuting Safety Council, the safety briefing stipulated that every jumper would climb out onto the helicopter's landing gear skids and then leave one at a time at five second intervals without pushing off.

On about the eighth jump run, four parachutists climbed out onto the skids of VH-JGE. This placed two parachutists on the left skid and two on the right. The pilot flew from the right front seat, which is normal practice in a Bell 206.

When the helicopter was at 3,500 ft and 50 kts over Olympic Park, the target area, one parachutist from the left rear position departed the skid, immediately followed by the second parachutist from the left side. With the two remaining parachutists still standing on the right skid, plus the pilot in the right front, the helicopter's lateral centre of gravity limits were exceeded. According to the pilot, he was unable to prevent the helicopter from rolling to the right. At some point during the roll, the two parachutists standing on the right skid also departed. In the opinion of the pilot, the right side parachutists pushed off rather than stepping off and that the push aggravated the rate of roll. The roll continued through 360 degrees. The helicopter recovered at 70 kts after a height loss of six or seven hundred feet. The pilot advised that torque and rotor RPM limits were not exceeded during the incident.

After the incident the pilot landed and inspected the aircraft. He discovered evidence of a slight mast bump. He then flew the helicopter to Essendon where further inspections were performed by engineers. The main rotor mast was removed and checked for ovality and runout. The manufacturer was consulted during the inspections. When no fault was found with the helicopter, it was returned to service.

Just prior to the incident there had been some reorganisation of jump loads due to a six place helicopter arriving. The parachutists involved in the incident had, in rapid succession, been assigned to a four place load, then a six place load and finally to a different four place load. The Safety Support Officer subsequently determined that the two parachutists on the left side thought that all four parachutists were going to exit simultaneously on an exit count of `ready set go' given by the parachutist on the front left. The parachutists on the right side had previously briefed with a different group which had planned to exit alternatively left and right at five second intervals; they believed this was the standard briefing and had not discussed it with the left side jumpers after being reassigned to the Bell 206.

Since the incident, the Australian Parachute Federation, via a News Sheet and News Letter, has reinforced the importance of the pre-jump safety briefings, and the exit practice. Also, parachutists have been reminded of the need for extra care at special events where normal exit procedures may be varied due to operational requirements.

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

1. The pre-jump safety briefing was probably inadequate.

The pilot was tasked with spreading urea on dry rice crops.

He took off to the south in calm conditions. At an altitude of about 30 ft in the initial climb, he commenced a left procedural turn while continuing to climb at 70 kts to approximately 100 to 150 ft. The pilot then noticed that the airspeed had decayed to about 60 kts so he lowered the nose. Airspeed continued to decay so he stopped the turn while heading northwest and lowered the nose to increase airspeed. The aircraft continued to sink and touched down in a ploughed paddock. The pilot maintained climb power during the ground roll, hoping that the aircraft would take-off again. After a ground roll of 300 to 400 metres through the ploughed paddock, the aircraft collided with a low earthen embankment and overturned.

No fault with the engine or airframe has been reported to have contributed to the accident. The pilot admitted that the inflight turns were somewhat tight. It is probable that the aircraft stalled causing it to mush to the ground. It is also probable that the drag effects of the ploughed soil prevented the aircraft from accelerating enough to become airborne.

Significant Factors

The following factors are considered relevant to the development of the accident:

1. The pilot probably allowed the airspeed to decay during a tight procedural turn.

2. The pilot did not attempt to stop the aircraft in the ploughed paddock.

3. The pilot did not select full power for the attempted take-off from the ploughed paddock.

The pilot reported that, prior to take-off, the windsock indicated nil wind. He advised that the take-off roll appeared normal and that the aircraft had became airborne at approximately 50 kts. After lifting off, at a height "well above" the windsock, the aircraft suddenly appeared to lose airspeed. The aircraft then descended to contact the ground nose first and bounced again before coming to rest on the western side of the runway. The total distance from commencement of the take-off roll to where the aircraft came to rest was approximately 900 metres.

Both the pilot and passenger were able to exit the aircraft safely.

Witnesses working near the runway advise the aircraft may have been effected by a gust of wind that passed through at the time.

The pilot reported that he had completed two touch-and-go landings on runway 22 left and had been cleared for a touch-and-go on runway 22 right which had a displaced threshold. The approach, which was being flown with 20 degrees flap and about 1500 rpm engine power, was shallower than the earlier approaches. The pilot said that, about two metres above the ground, as he was about to increase power slightly to ensure that the aircraft did not touch down before the displaced threshold, the nose dropped. The nosewheel struck the ground firmly and the aircraft nosed over. The pilot could not recall hearing the stall warning sound during the approach, although it had tested serviceable before the flight.

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.