FACTUAL INFORMATION

A Boeing 737 aircraft, VH-CZM, inbound to Sydney from Brisbane was cleared to descend to FL190 and instructed to enter a holding pattern at YANGO. YANGO is a holding point to the northwest of Sydney.  A following Boeing 737 aircraft, VH-CZG, inbound to Sydney from Cairns was cleared to descend to FL200 by Brisbane air traffic control (ATC), with a requirement to reach FL200 by YANGO. The crew of VH-CZG then transferred from Brisbane to Sydney ATC and reported on descent to FL200. The Sydney controller instructed the crew of VH-CZG to enter the holding pattern at Yango and to maintain FL190. The crew readback the level and continued descent to FL190. Shortly after, the crew requested confirmation from Sydney control that the previous requirement to reach FL200 by YANGO was still necessary. The requirement was confirmed by the Sydney controller.

During a subsequent scan of his traffic, the controller observed that the radar display altitude label of VH-CZG was indicating FL190. The controller requested confirmation from the crew that the aircraft was maintaining FL200; the level indicated on the flight strip. The crew confirmed maintaining FL190 and stated they had been cleared to FL190 with a requirement to be at FL200 by Yango. The Sydney controller instructed the crew of VH-CZG to climb immediately to FL200 due to VH-CZM being in the holding pattern at FL190. The crew acknowledged and climbed the aircraft to FL200. Radar separation was maintained while vertical separation was less than the standard separation of 1,000 ft. There was no breakdown in separation.

ANALYSIS

The air traffic controller manning the Sydney radar position had recently been re-rated after an absence of approximately two years and was consolidating in the position at the time of the incident. Air traffic was light and he was endeavouring to ensure all aspects of the position were conducted in accordance with current procedures.

Despite the flight strips for both aircraft being annotated with different levels to provide vertical separation of 1,000 ft the controller instructed the crew of VH-CZG to maintain FL190. This was the level assigned to VH-CZM which was already established in the YANGO holding pattern. The controller may have confused the levels notated on the flight strips of VH-CZM and VH-CZG when he acknowledged the initial call by the crew of VH-CZG. This may have caused him to inadvertently transmit to the crew of VH-CZG to 'Maintain FL190' when he actually meant 'Maintain FL200' a level which would have ensured vertical separation between the two aircraft.

While the crew of VH-CZG had only been assigned descent to FL200, the instruction from the Sydney controller to "Maintain FL190" would have strongly suggested to them that the aircraft could continue descent to the lower level.

However, as the controller did not prefix the clearance for the lower level with "Descend to ..." the crew of VH-CZG should have queried the instruction. "Maintain (a level)" is only used to restrict a previously assigned climb/descent level or to confirm a requirement to maintain a current level. The term is not used to assign changes of level. A challenge of the instruction by the crew may have alerted the controller to his error.

By regularly scanning the radar display the controller was able to quickly identify and rectify the situation after observing the radar display altitude label of VH-CZG was the same as VH-CZM.

FINDINGS

1. The crew of the VH-CZM operated in accordance with their air traffic control instructions.
2. The controller inadvertently instructed the crew of VH-CZG to maintain FL190.
3. The crew of VH-CZG misinterpreted the instruction to maintain FL190 as a clearance for further descent.
4. The controller observed that the flight level on the radar display altitude label for VH-CZG was the same asVH-CZM and undertook corrective action.
5. Radar separation was maintained until the standard vertical separation of 1,000 ft was re-established.

The pilot and an approved testing officer were engaged in a commercial licence flight test. An aircraft landing area inspection of Wagin runway 06 and a touch-and-go landing were completed. This was followed by a period of instrument flying. The testing officer then gave the pilot a simulated engine failure, overhead Wagin airfield at 3500 ft. The pilot flew the aircraft to a short base leg for runway 34 at 1000 ft above ground level. Following the pilot's comment that the approach was high, the testing officer instructed him to do something about it. He lowered the nose and allowed the aircraft to accelerate whilst continuing the descent towards the strip. Speed over the runway threshold was faster than optimum and the aircraft landed, following a bounce, 600 m into the 1000 m strip.

As the pilot applied power to go around the testing officer observed powerlines across the departure end of the runway. He instructed the pilot to keep the aircraft on the ground, and they attempted to stop in the runway remaining. The runway surface included loose gravel and de-acceleration was poor. Main wheel skid marks were apparent for 300 m. The aircraft ran off the end of the runway and collided with a raised road surface and a water pipe. The left main and nose landing gear legs collapsed. The aircraft came to a complete stop 100 metres from the end of the runway.

The powerlines were not observed during the landing area inspection because it concentrated on runway 06 and not 34.

The testing officer later indicated that he had tried too hard to introduce some realism into the exercise. He had allowed a simulated situation to continue without ensuring that the aircraft could overshoot safely or stop within the runway confines, should it become necessary. The pilot had not taken independent action because he was under pressure to meet the test requirements and expected the more experienced testing officer to maintain a safety watch.

Factual Information

The aircraft had just completed a landing on runway 21 when the nose wheel collapsed. The engine cowl and propeller struck the ground, and the aircraft came to a stop.

The aircraft had a history of nose wheel shimmy on take-off and landing.

Analysis

The manufacturer's design specifies a Cessna 185 tail wheel as the nose wheel for this aircraft type. The nose wheel is fully castoring with no controls and is free to castor during ground operations. A fixed fairing over the nosewheel prevents any pre-flight inspection of the mounting.

This aircraft has experienced two previous nose wheel collapses. The aircraft owner is considering modifications to convert the aircraft to a tail wheel configuration.

The pilot attempted to land his lightly loaded aircraft uphill on his 425-metre private airstrip with a slight tail wind. Threshold airspeed was slightly fast. The aircraft touched down long. The pilot soon realised that he could not stop the aircraft before the end fence. When he decided to go-around and applied power, the aircraft was too slow to become airborne before the fence. He then closed the throttle, and the aircraft ran through the fence.

Numerous aircraft were inbound to Sydney, including a foreign Boeing 747 (B747) aircraft on a Rivet Three Standard Arrival Route (STAR) and a Fokker (FK50) aircraft on an Oakdale Two STAR. Both aircraft were at about the same distance from the airfield, with the FK50 sequenced to overfly to the east for a left circuit to runway 16L. The B747 was sequenced to remain to the west of the airfield for a right circuit to runway 16R. The FK50 was assigned 7,000 ft and the B747 assigned descent to 6,000 ft, with a requirement to reach that altitude by 14 DME. This was in order to achieve vertical separation before losing lateral separation between the two routes as the B747 was turned downwind, through the track of the FK50.

At 18 DME the B747 was observed on radar by the Approach South controller to be passing through 6,900 ft, and to make an authorised left turn. The aircraft was then expected to track 049 degrees until given radar vectors, but the B747 took up a track of 030 degrees, towards the track of the FK50. An instruction was given to the B747 to turn right immediately onto 090 degrees, and traffic conflict information was passed on the FK50. Because the B747 was initially slow to execute the turn a further instruction to turn immediately was passed. As the B747 turned away from the FK50, the lateral separation had reduced to 2 NM, with a vertical separation of 800 ft. The minimum required separation standard was 3 NM laterally, or 1,000 ft vertically.

The B747 crew were of the understanding that a clearance for a visual approach had been given and hence turned the aircraft onto 030 degrees to position the aircraft for a left circuit for runway 16R. The pilot in command of the B747 had the FK50 in sight and assessed that there was sufficient separation.

Helicopter VH-AUQ was substantially damaged in a ground resonance occurrence in November 1995. It was rebuilt and released, as serviceable, on 13 April 1996. The helicopter was immediately flown from Australia's east coast to Fitzroy Crossing, arriving there on 24 April 1996. During the flight to Fitzroy Crossing, which took 24 flying hours, the pilot/owner reported that, on one occasion, there had been some vibration in the rotor system. The vibration occurred after touchdown and immediately prior to shutdown at Mt Isa, it was minor and had only lasted for a short period.

The helicopter was fitted with a single set of controls. On the day of the accident another pilot attempted to fly the helicopter, whilst the pilot/owner sat in the passenger's seat. After start up, and prior to lift off, it began to vibrate. The helicopter was shut down and inspected. No reason for the vibration was found. After some discussion it was decided that the vibration had been caused by incorrect positioning of the cyclic control stick during start up. The pilot/owner re-phased the main rotor blades, i.e.. positioned the blades evenly around the rotor head, before attempting another start. The pilots then switched seats and strapped in. The pilot/owner started the engine and completed a power check. He reported that there was no vibration and everything appeared normal.

The pilot flew the helicopter to a 1 m hover and commenced a turn to the left. As the helicopter started to yaw a severe vibration started. The vibration was so bad the pilot was unable to maintain a reasonable degree of control and he reduced power and allowed the helicopter to touch down. The vibration continued, increasing in amplitude. This made it difficult for the pilot to reach the mixture control lever. However, he was finally able to shut the engine down. During the vibration after touchdown, the bubble fractured across the top and bottom frames and fell forward. The pilot/passenger's seat belt came undone, and she was ejected forward of the helicopter, falling onto a grass area. By the time all movement had stopped, in addition to the bubble damage, the tail boom had been cut off, most of the airframe structure had been fractured or severely distorted and the main and tail rotor blades destroyed. The pilot/owner was assisted from the wreckage by an observer.

The investigation found that lockwire and spacing washers were missing from the torque-nuts on all three main rotor blade dampers. Each of the torque-nuts was found in a different position on its shaft. None of the nuts were positioned near where they were expected to be on a newly overhauled blade damper. Evidence indicated that all three dampers were overhauled prior to being fitted during the rebuild. Two of the blade dampers were severely damaged and their torque setting could not be checked. The third damper was undamaged, and its torque was found to be 140-inch pounds counterclockwise and 135-inch pounds clockwise. The manufacturer's minimum torque requirement is 230-inch pounds, in both directions on installation and 200-inch pounds in service. The internal bearings and friction plates, in all three dampers, were in good condition. All three damper arm uni-ball bearings, which had been replaced during the overhaul, were found to have excessive play. Two of three bolts, which were used to secure linkages between the trailing edge of the main rotor blades and their respective dampers, were found to be badly worn, as were their corresponding lower bolt holes. Whilst the play in the uni-ball bearings was probably the result of wear during the accident-flight vibration, the extent and condition of the wear on the bolts indicates that it had been there for some time.

The lack of torque and the wear on the bolts and holes probably led to partial and uneven damping of lead and lag in the main rotor blades. This, in turn, led to the vibration which caused the accident.

The organisation which overhauled the blade dampers reported that it had been completed in accordance with the manufacturer's requirements. The dampers had been delivered to the rebuild-organisation with spacers and lockwire fitted and torqued to the correct setting. The rebuild-organisation reported that they had fitted the dampers, as delivered, and had not had to make any adjustments to them prior to releasing the helicopter to the owner. The pilot/owner reported that the lockwire had been removed during adjustment and ground testing, after the dampers had been fitted to the helicopter and before it was released to him. He had assisted whilst adjustments were made to the dampers by maintenance personnel. The spacers and lockwire were not refitted.

It could not be determined how worn bolts came to be fitted to the damper linkages. Neither the overhaul- or the rebuild-organisation believed it had fitted the bolts.

It is possible that the dampers were correctly torqued when the helicopter was released to the owner. Although no evidence of a similar occurrence is available, it is possible that the absence of lockwire allowed the torque-nuts to back off gradually, whilst the helicopter was being flown to Fitzroy Crossing. The reducing damper torque went unnoticed by the pilot/owner, probably because of his inexperience. The different positions of the torque nuts, at the time of the accident, indicates that, if the nuts had backed off, they had done so unevenly. The alternative explanation is that the blade damper torque-nuts were incorrectly set during adjustments made before the helicopter was released to the pilot/owner. However, had this been the case any significant variation in damper torque would have been evident as vibration in the rotor system, much earlier than reported. Therefore, the first alternative is considered the more probable explanation and the uneven and reducing damper torque probably reached a point, on the last two flights, where the vibration became excessive.

An inspection of the pilot/passenger's seat belt tongue and buckle indicated there was a significant amount of movement between the tongue and the buckle when the tongue had been pushed into the locked position. This made it possible, at some angles, for the tongue to be pulled out again, even though the belt appeared to be locked.

During a cattle mustering operation, the helicopter suffered a total power loss at about 80 ft AGL. The resultant heavy landing spread the skids, and the main rotor severed the tail boom. Both occupants were able to exit the aircraft safely.

Post flight examination found there was no fuel in the helicopter's tanks. The tanks were intact.

The pilot has not responded to a request for information.

On arrival overhead, the pilot observed sheep on the flight strip. He made two low passes in an attempt to clear the sheep. During the third pass the remaining sheep cleared the runway so the pilot decided to discontinue the pass and make a straight in approach for a landing.

The pilot did not complete his pre-landing checks, and the landing gear was not lowered. The landing gear warning horn did not sound during the approach, possibly because of a higher than normal power setting.

The aircraft was landed with the landing gear retracted.

The pilot reported that he was distracted by the need to chase the sheep of the strip, and he forgot to lower the gear.

The Mitsubishi MU-2 was to be flown on a cargo flight from Bankstown to Wagga Wagga. During his pre-flight inspection the pilot cleaned the windshields with warm water to defog them. The Swearingen SA226 was also to be flown on a cargo flight from Bankstown to Cootamundra. The pilot of that aircraft wiped the side windows with a chamois, as the front windshields were already clear.

The MU-2 taxied at 0630 for runway 29C, followed some three minutes later by the SA226. The MU-2 proceeded along taxiway X, then across the 29 engine runup area towards taxiway S1, which also required crossing taxiway K. The SA226 proceeded along taxiway K, towards taxiway S1. As he taxied through the runup area the MU-2 pilot was unaware of the presence of the SA226 on taxiway K. The SA226 pilot had seen the MU-2 to his left, taxiing through the runup area, and assumed that the pilot of that aircraft would give way to him. He reported that his attention was mainly directed ahead and to his right, as he monitored departing aircraft.

As the MU-2 was turning left out of the runup area to cross taxiway K, the pilot felt a lurch to the left as his aircraft was struck from the rear right. The wing of the SA226 had passed under the right-wing tip fuel tank and then into the propeller of the right engine, resulting in substantial damage to both aircraft. The pilot of the SA226 had been unaware of the close proximity of the MU-2 until the impact pulled the nose of his aircraft to the left, and he saw the other aircraft. Both pilots conducted emergency shut-downs and escaped from their aircraft without injury.

Pilots of adjacent aircraft witnessed the accident and reported that it was a clear morning with no obstructions to visibility. Immediately prior to impact, the witnesses observed the aircraft to be converging at an angle of approximately 50 degrees and thought it was going to be close. When the impact was seen as inevitable, they were unable to give a warning call as the ground radio frequency was congested.

The two aircraft were on crossing tracks. VH-NJG was cleared to descend initially to flight level (FL)180 to maintain separation with the VH-EKH at FL170. The controller noted successive radar returns indicating NJG was at FL177 and assumed the pilot had continued descent through the cleared level. He decided that the most appropriate course of action was for the aircraft to continue descent through the level of EKH while maintaining separation laterally.

He therefore cleared NJG to descend to FL115 and instructed the aircraft to alter heading 15 deg to the left. However, the groundspeed and descent rate of NJG did not proceed as anticipated and it became evident to the controller that vertical separation would not be established before lateral separation was lost. Accordingly, the controller instructed NJG to expedite descent through FL160 but did not pass traffic information to either aircraft when separation was lost. Radar separation reduced to three miles, instead of the required five miles, before vertical separation was established.

The investigation revealed that when the controller noted NJG's radar return indicating FL177, he assumed the pilot had continued descent through the cleared level. He then decided the conflict would be most quickly resolved by the aircraft continuing descent to below FL160, rather than reversing its flight profile to climb back to FL180. This was why he cleared the aircraft to continue descent to FL115. A replay of the radar tape indicated that the aircraft actually levelled out at FL180 but in doing so had gone down to FL177 in the level out manoeuvre (there were three returns indicating FL177 before FL180 was indicated). Hence the descent instruction resulted in the exact effect the controller was trying to avoid which was a delay due to reversal of flight profile while vertical separation did not exist.