An external examination by the operator's engineers found damage to the left engine turbine blades and shrapnel damage to the exhaust nozzle. The operator removed the engine and sent it to an approved workshop for examination and repair. The Australian Transport Safety Bureau (ATSB) did not attend the examination, but requested that all damaged components be forwarded to the ATSB for detailed examination.

After the engine was disassembled, the ATSB was advised that the engine failure was precipitated by a failure of the turbine first stage disc rotating air seal. The rotating air seal's outer rim was missing and the downstream turbine components received damage consistent with fragments of the rotating air seal passing though the turbine. The failed rotating air seal, the first, second and third stage turbine wheels and nozzle guide vane assemblies were forwarded to the ATSB for further examination.

Engine and component history

The Allied Signal TPE331-12UHR-701G turboprop engine, serial number P-70210, had accumulated 9,139.8 hours and 15,585 cycles since new and 3,066.9 hours since overhaul. In May 1997, it had Allied Signal service bulletin TPE331-72-2002 incorporated, which detailed replacement of the inner baffle with a new inner baffle part number 3108039-2. Service bulletin TPE331-72-2030, that detailed replacement of the compressor interstage seal assembly support, was incorporated in July 1999, at 6,073 hours, during engine overhaul, after overhaul, the engine was installed into DMI where it remained until the failure.

The rotating air seal, part number 3103839-3, serial number 2-23315-945, appeared to have been installed in the engine since new as its time and cycles since new were identical to those applicable to the engine. During the engine overhaul in 1999, it was inspected in accordance with the requirements current at that time and found serviceable.

The ATSB was advised that cracking of the rotating air seals had occurred in the past, but that it was rare to see a cracked rotating air seal on engines that have the requirements of the engine manufacturer service bulletins TPE331-72-2002 and TPE331-72-2030 incorporated.

Rotating air seal examination

The examination of the rotating air seal and other components from the failed engine is detailed in the ATSB's technical analysis report number 40/01. The examination revealed that the entire outer rim of the rotating air seal had separated from the flanged section. About seventy percent of the rim circumference was recovered and most material from the outer ten millimetres of the plate flange was lost.

One location, where the loss of material was substantially greater, exhibited a short length of fracture showing evidence of fatigue crack propagation. Heat tinting over the area of fatigue indicted that it was present prior to the event failure. The seal had no evidence of material or manufacturing anomalies.

Examination of the turbine components

The turbine disks and nozzle guide vane assemblies showed evidence of random impact damage to the blade leading edges. The damage was consistent with the separated pieces of the failed rotating air seal passing through the turbine.

A copy of the ATSB's technical analysis report, number 40/01, is available on the ATSB web site at or from the ATSB on request.

Engine manufacturer's action

The engine manufacturer reported a number of documented in-flight shutdowns due to separation of the rotating air seal plate rim. Their investigation into the events concluded that cracking in the rim area was due to elevated rim operating temperatures, primarily due to hot gas leakage from deteriorated first stage stator assembly hardware. To alleviate the problem, the manufacturer introduced service bulletins TPE331-72-2002 and TPE331-72-2030 and revised the engine maintenance manual to improve inspection of the relevant components at hot section inspection with the intention of preventing hardware prone to gas path leakage from returning to service.

Shortly after take-off, and in accordance with standard procedure, the flight crew selected engine bleed air as the source of air for the aircraft's two air conditioning packs. Subsequently, the cabin staff reported a strong smell of fumes in the cabin. As there was also a smell of fumes entering the flight deck, the flight crew donned their oxygen masks in accordance with the non-normal procedure for suspected cabin air contamination.

The flight crew then proceeded to identify the source of the fumes using a contamination source location schedule. That procedure involved selecting different combinations of engine air and air conditioning packs. During normal operation, bleed air from engines one and two was fed to pack one, which in turn supplied conditioned air to the flight deck and cabin. Bleed air from engines three and four was fed to pack two, which normally only supplied air to the cabin. Additionally, bleed air from the Auxiliary Power Unit (APU) was used by either pack during the take-off and landing phases or when air conditioning was required during ground operations. It was determined that with pack two selected off, the fumes dissipated. The flight was continued with only pack one supplying conditioned air to the cabin and flight deck. The two cabin staff and several passengers were affected by the fumes with symptoms of sore eyes, sore throat and headache.

On arrival of the aircraft in Brisbane, a Licensed Aircraft Maintenance Engineer (LAME) addressed the reported defect in accordance with the Civil Aviation Safety Authority airworthiness directive AD/BAe146/086 and the British Aerospace Systems Information Service Bulletin (ISB) 21-150. The ISB required certain actions to be performed whenever a cabin air quality problem was identified which was suspected of being associated with oil contamination of the air supply from the air conditioning packs. The engineer's inspection of the air conditioning system, engines and APU revealed no signs of oil contamination or oil leaks. The defect was therefore cleared and the aircraft resumed service.

The following day, the same aircraft but with a different crew was involved in a similar occurrence. A strong smell of fumes was noticed, mainly in the cabin, during the cruise. The flight crew donned oxygen masks as a precautionary measure. Symptoms of sore eyes, sore throat and headache were reported by both the crew and passengers. The pilot in command, who had been affected by fumes on previous occasions, received medical attention after landing.

Engineering inspection revealed oil contamination in the number 3 engine bleed band. The defect was deferred in accordance with the aircraft's approved Minimum Equipment List that allowed the aircraft to be flown with only the number one air conditioning pack in use. No further fumes were evident during following flights. Subsequent engineering investigation of the number 3 engine revealed that the oil leak was a result of a worn number one bearing seal. The number 3 engine was replaced, and the defect was cleared.

Evidence from previous incidents of air system contamination on this type of aircraft had indicated that the fumes were associated with engine or APU oil contamination of the air conditioning system. As a result, operators have incorporated various modifications to the cabin air system, APU, and engines. They have also introduced improved maintenance practices to further address the issue. However, that action has not completely solved the problem. The air supplied to the air conditioning packs was protected from contamination by oil seals in the engines and APU. A technical defect arising in one of the seals can result in oil entering the cabin air conditioning system, with the first signal of the defect being an awareness of fumes by the members of the crew. The difficulty of identifying the origin of the contamination is exacerbated by the often-intermittent nature of the fumes events.

A Boeing 737-377 (B737) was inbound to Canberra for a landing on runway 35. The crew had established the aircraft on the extended runway centreline from 10 NM (18.5 km). At approximately 4 NM the aerodrome controller (ADC) instructed the crew that they were clear to land. As the aircraft was about 1.85 km from the runway threshold, the ADC saw a truck enter taxiway Delta. Taxiway Delta was approximately 1427 m from the threshold of runway 35. The ADC considered that the truck was unlikely to stop and instructed the crew to go around. The crew acknowledged the instruction and conducted a missed approach, a left circuit and then landed.

The driver of the truck had entered the runway strip without a clearance and had not complied with procedures. Later analysis of recorded radar data showed that the B737 had descended to 310 ft above the aerodrome elevation during the approach and go around.

Air traffic control

The air traffic control tower operating positions faced south-south-west, overlooking the Royal Australian Air Force (RAAF) Fairbairn aircraft apron, the intersection of the runways and the runway 35 threshold. The tower had three operating positions: ADC, tower coordinator (COORD) and surface movement controller (SMC). The ADC operating position was situated on the north-western end of the console with the COORD to the left and then the SMC position on the south-eastern end of the console. All positions were staffed at the time of the occurrence. The COORD and SMC saw the truck approaching and enter the apron from the south before turning left onto taxiway Delta. The ADC was conducting the final scan of runway 35 prior to the B737 landing when the SMC advised that the truck was going to enter the runway. Simultaneously, as the COORD suggested to the ADC to instruct the crew to go around, the ADC issued the instruction to the B737 crew.

The SMC saw the truck stop on taxiway Delta, after crossing the holding point and entering the runway strip, and then reverse as the B737 went around. The SMC notified an airport safety officer who then drove to Delta and spoke to the truck driver.

Runway 35/17

Runway 35/17 was bounded by the runway strip that was an area around the runway, up to 75 m either side of the runway centre line and included the runway clearways that extend 60 m from the thresholds. The purpose of the runway strip was to reduce the risk of damage to aircraft inadvertently running off the runway and to protect them when flying over it during take-off, landing or missed approaches. Gable markers indicated the limit of a runway strip. On taxiways the limit of a runway strip was marked as a holding point. A clearance from air traffic control was required before personnel, vehicles or machinery could enter a runway strip. Personnel, vehicles or machinery were normally excluded from a runway strip when a runway was in use or a pilot had a clearance that enabled an aircraft to use the runway.

Runway 35/17 was 45 m wide and the position the truck reached before stopping could not be accurately established. Reports from the driver and witnesses indicated that the truck reached a position somewhere between 35 m to 65 m from the runway centreline.

Canberra airport works

Canberra airport was halfway through a period of major works to construct:

- 7.5 m shoulders on runway 35/17;
- a turning node on the northern end of runway 35/17;
- drains along runway 35/17; and
- widened turn fillets at runway/taxiway intersections for runway 35/17.

The works also included the replacement of runway lights.

The airport operator had awarded the contract to a company that had recently completed similar major works at another capital city airport. That company was the prime contractor (contractor) and the task was scheduled for completion by 31 August 2001. Planning was fast tracked to meet the schedule and both the airport operator and the contractor believed it was achievable. The truck driver's company was a sub-contractor to the contractor.

Management of the works site was a joint responsibility between the airport operator and contractor. The contractor was responsible for managing the works area while the airport operator was responsible for managing the interaction of works and airport activities.

A method of work plan (MOWP) was prepared by the airport operator and was provided to the contractor to assist in the development of their work method statement and project management plan. The airport operator reviewed the project management plan. The MOWP detailed how areas would be delineated to ensure airport users and contractor activities did not conflict. The airport operator also arranged for aeronautical information circular (AIC) H7/01 to be issued on 14 June 2001. The AIC detailed the scope of the work during the four work stages, operational restrictions required during each stage and advised that the dates and times of activation of each stage would be advised by a notice to airmen (Notam). The AIC included draft Notams for each stage.

The contractor was working on stages 1 and 2. The MOWP stated that taxiway Delta would be unavailable during stage 2 between 2200 Eastern Standard Time and 0600 (overnight hours). The MOWP did not have any restrictions on the use of taxiway Delta during the period from 0600 to 2200 (daylight and early evening hours). Seven Notams, that described the work stages and the operational restrictions, were current at the time of the occurrence. Those Notams were in accordance with the MOWP except there was no Notam regarding taxiway Delta overnight.

The airport operator prepared and conducted a site induction brief for the contractor and associated sub-contractor staff prior to commencing the work. That briefing included aspects of site safety including movement of persons and vehicles, "no-go" areas, contractor vehicle routes and that contractor staff were to comply with any directions from an airport safety officer. Following the initial induction, the contractor assumed responsibility for ensuring that all persons employed on the works underwent a one hour induction briefing and that copies of the induction brief were made available to staff. The contractor was also responsible for maintaining a record of the staff who had been briefed. Those records indicated that the truck driver had received an induction brief on 27 June 2001. The truck driver later reported that he was aware of the need for vigilance when working on the airport and had often consulted the maps provided that detailed approved routes for works vehicles and staff.

Effect of the works on runway 35/17

A displaced threshold for runway 17 had been imposed to enable work on the northern end of runway 35/17. Taxiway Alpha, parallel to the runway and taxiway Foxtrot that linked Alpha and the runway were both closed to aircraft operations for this stage. The runway 17 displaced threshold was located south of taxiway Foxtrot.

The contractor had implemented a new route for works vehicles for the current stage of works. The route had been used for 10 days by trucks to cart soil from the runway 17 undershoot (at the northern end of the runway 35/17) to an area outside the airport at the south-western end (adjacent to the southern end of the runway 35/17). That route was marked with stanchions and used taxiways Foxtrot and Alpha, across the intersection of Alpha/Delta and the RAAF apron, along a gravel road parallel to runway 30, around the eastern end of runway 30 and then along the perimeter fence to the dump area. The route was sign posted with a speed restriction of 40 kph with a reduction to 25 kph in the area near the runway 30 threshold.

The segment of the route across the intersection of taxiway Delta and the RAAF apron was not marked. The contractor considered that the gap in markers at Delta was sufficiently small for staff to appreciate route continuation, and that the intersection was referred to in the induction brief and was marked on the maps. Taxiway Delta remained open to enable aircraft to use the RAAF apron. There was no restriction on vehicles crossing taxiway Delta but generally drivers of vehicles stopped or slowed when approaching that taxiway to check for aircraft before continuing.

Truck driver

The truck driver had been on the site for five weeks and generally worked a 12-hour day. Prior to the week of the occurrence he had had four days leave as a result of rain that prevented work being carried out. He had little previous experience of operating on airports. He had worked from 0700 to 1900 Eastern Standard Time on Monday, 0630 to 1730 on Tuesday and had started at 0600 on the day of the occurrence. The truck driver was supervising two work teams. One team was at the northern end of runway 35/17, while the other team was working outside the eastern runway strip near the intersection of runways 35/17 and 30/12. He started the team at taxiway Alpha near the northern end of runway 35 and then commenced a task himself at about 1700 in the northern area. Shortly after, he received a request to provide equipment to the team near the runway intersection. He drove to the north-western boundary of the airport to get the equipment and then travelled via the northern and eastern route to the team's location. He delivered the equipment and was returning to the northern area when he drove onto taxiway Delta and entered the runway strip.

The truck driver reported that he had a lot on his mind at the time. His last break had finished at 1330. The truck driver knew that he had insufficient time in which to complete the job he had started and that he had to supervise the clean up by his teams before finishing work that afternoon. He had been warned to watch for aircraft and consequently, whenever he was crossing a taxiway, would check for aircraft. As he turned onto taxiway Delta he was watching the approaching B737 through the passenger's window of the truck but thought he was turning onto the northern works area via taxiway Foxtrot. He became aware that there were no markers in the area and that he was on the wrong taxiway. He stopped the truck and reversed as quickly as possible off taxiway Delta back onto taxiway Alpha. The truck driver reported that he had used the designated route about 25 times previously on the day of the occurrence.

A Lockheed C-130J Hercules (Hercules) had conducted a practice Instrument Landing System (ILS) approach to runway 35 at Canberra followed by a touch and go landing and departure to the northwest. A Boeing 737-376 (B737) was simultaneously radar vectored for an instrument approach to runway 17. Both aircraft intended to use the same runway surface but in opposite directions. The Canberra Approach East (CBE) controller became concerned that the separation standard of either 3 NM laterally or 1,000 ft vertically would not be maintained between the aircraft and issued turn instructions to both crews. While complying with the instructions the two aircraft passed with approximately 2 NM and 100 ft separation. There was an infringement of separation standards.

The Hercules crew had requested an ILS approach to runway 35 at Canberra for instrument flight rules (IFR) flying training purposes. An ILS is a precision instrument approach that provides centreline and glideslope guidance to the pilot, aligned to the landing runway and is primarily used during periods of low cloud and/or poor visibility. Runway 35 was the only runway serviced by an ILS approach at Canberra. The En Route Supplement Australia (ERSA) required crews planning instrument training at Canberra to contact Canberra air traffic control to book an approach time slot for air traffic control traffic management purposes. The service provider reported that the Hercules was running late and had missed its booked slot.

The CBE controller planned to sequence the Hercules between the second of two aircraft taxiing for a departure from runway 17 and the B737, which was the first of a number of arrivals for that runway. The CBE controller was aware that the Hercules' practice ILS approach might be delayed by up to 45 minutes if he was unable to sequence that aircraft ahead of the B737. The CBE controller stated that he felt some pressure to provide a good service to the Hercules crew.

On first contact with the CBE controller the B737 crew was told to expect a Very High Frequency Omni-Directional Radio Range/Distance Measuring Equipment (VOR/DME) approach to runway 17. At various times thereafter, the crew was assigned radar vectors and a speed restriction of 270 knots indicated airspeed to position the B737 behind the Hercules and provide sufficient spacing to maintain the required separation standard. The CBE controller had asked the crew of the Hercules to maintain their best speed once established on the final approach track. That was a request only and the crew of the Hercules was under no obligation to comply. From that point, the Hercules was capable of maintaining an indicated airspeed between 10 to 20 kts faster than that which it would routinely maintain. The crew of the Hercules could not recall whether they had complied with the request.

The CBE controller was concerned about the application of separation between the B737 and the Hercules after the Hercules had completed its touch and go landing. The Letter of Agreement between the Canberra Tower and Canberra Approach Control stated that the approach controller was responsible for separation assurance during reciprocal runway operations unless it was assigned to the ADC "by mutual agreement". The CBE controller advised that his contingency plan was to instruct the B737 crew to terminate the approach if it became apparent that separation between the two aircraft may be infringed.

The CBE controller reported that he had based the traffic management plan on his expectation that the Hercules would be operated at a higher speed along the final approach path. The CBE controller also reported that the Hercules had commenced its turn later than he had expected following the touch and go landing. He could not see the Hercules on radar following the touch and go landing and was unsure of the position of that aircraft in relation to the B737. The CBE controller asked the ADC to confirm when the Hercules had commenced the turn and was visually clear of the inbound path of the B737. The ADC was unable to provide visual separation between the B737 and the Hercules before radar and vertical separation were infringed.

The CBE controller reported that his workload was very high at the time of the occurrence. He did not provide either crew with a safety alert in respect of the other aircraft, as required by MATS, despite the distance between the two aircraft being less than that prescribed by the applicable separation standards.

Both aircraft were equipped with a traffic alert and collision avoidance system (TCAS). The crew of the Hercules did not receive either a traffic advisory (TA) or a resolution advisory (RA) on the TCAS. The crew of the B737 did not report receiving a TA or a RA either during, or after, the occurrence.

Shortly after departing Melbourne en route to Port Vila, Vanuatu, the crew of a Boeing 737 (B737) aircraft received advice from Air Traffic Control (ATC) that a following aircraft had observed rubber and debris on the runway. As nothing unusual had been noticed during take-off, the flight crew of the B737 considered that the debris was possibly from the BAe 146 ahead of them and asked ATC if the debris could be identified. Assessment by ground engineers confirmed that the debris was a section of the left main landing gear door and tyre tread from a B737.

On receiving that information the crew checked their flight controls for any signs of restriction or abnormal handling. As the left wing trailing edge was checked for damage through the cabin window by one of the cabin crew, a passenger advised her that a bang had been heard from under the wing during take-off. No damage was observed but the flight crew decided to divert the aircraft to Sydney. Approaching Sydney, the crew contacted ATC and requested a gear-down fly-past to assess the condition of the landing gear.

The fly-past confirmed that part of the left main landing gear door was missing but the wheels appeared to be intact. An emergency was then declared and the flight crew consulted the operator's emergency procedures manual. After burning off excess fuel, the cabin crew and passengers were briefed for the emergency. The flight crew then commenced their approach with the cabin crew and passengers in the brace position and an uneventful landing was carried out.

The aircraft was stopped on the taxiway and inspected by ground engineers where it was found that the left outboard tyre had lost its tread but was still inflated, although at a lower than normal pressure. The aircraft was then cleared to taxi under its own power to the terminal and the passengers disembarked normally. Both left main wheels were deflated and replaced. Further inspection of the aircraft revealed damage to the left main gear mid door, the left inboard trailing edge flaps and the underwing false structure. Temporary repairs were carried out to the aircraft and it was flown, without passengers, to a maintenance facility in Melbourne for further repairs.

The investigation revealed that there had been impact damage, marked with black rubber, to the flaps and underwing, and ductile overload damage to the landing gear door tie rod and underwing panel support links. The left outboard main wheel and tyre were sent to overhaul and retread facilities for examination and testing.

An examination of the wheel and tyre showed evidence of air leakage from the bead area and on the tyre surface. After a pressure retention check, the wheel was disassembled and the hub-halves examined with no evidence of damage to the hub seal or mating surfaces found. The tyre liner was found to be intact with no air leakage apparent and there were no signs of delamination of the carcass inner plies. An inspection of the separated surfaces revealed areas of shiny polished rubber. That indicated that there may have been localised ply movement and separation prior to failure.

A blueing of the rubber around the tyre's shoulders and on the separated sections of tread was also noticeable. That indicated that the tyre had been overheating. Factors contributing to overheating can be, low tyre pressures, high taxi speeds, long taxiing or heavy landings. There was no evidence of cuts or foreign object damage on the tyre or tread segments.

The aircraft normally operated with moderate to high payloads and at airports with long taxiways. Both those factors would have affected the temperature of the tyre and increased the importance of maintaining correct tyre pressures. The company's standard operating procedures specified a maximum taxi speed of 30 knots, which was within the tyre manufacturer's limit. An inspection of the operator's maintenance records revealed that the tyres were checked for correct pressure daily before the first flight and every 30 flying hours on an ancillary check. However, the pressures were not recorded if they were found to be within the serviceable limits.

The failed tyre had been fitted to the aircraft for 4 weeks and during that time only one occurrence of lower than normal tyre pressure had been recorded. It was established that the tyre was at its fourth (R4) retread, with that tyre type permitted to have up to seven (R7) retreads. The failed tyre's service history, and the retread and wheel assembly processes were assessed but nothing was found that may have contributed to the tyre failure.

A Boeing 767-336 (B767) was en route from Sydney to Melbourne and was maintaining flight level (FL) 350. A Boeing 737-800 (B737) was en route from Melbourne to Nadi, Fiji, and was also maintaining FL350. The aircraft were on segregated routes that provided lateral separation until the crew of the B737 was provided with track shortening. That decision placed the two aircraft on conflicting flight paths at the same level. The Eildon Weir/Benalla (ELW/BLA) sector controllers saw the impending conflict and alerted the Wollongong/Jervis (WOL/JVS) controller. Both controllers then issued traffic information and instructions to the crews for avoiding action. Both crews received Traffic Alert and Collision Avoidance System (TCAS) Resolution Advisories (RA) and the controllers received a Short Term Conflict Alert (STCA) from The Australian Advanced Air Traffic System (TAAATS). The aircraft passed within 4.8 NM laterally and 800 ft vertically. The required separation standard was either 5 NM laterally or 2,000 ft vertically. There was an infringement of separation standards.

The B737 crew had flight planned, and had been initially cleared to, FL330. That was a standard level for the direction of flight. Levels were assigned depending on a number of factors including the track of the aircraft in accordance with the Table of Cruising levels in the Manual of Air Traffic Services (MATS). Aircraft operating at altitudes and flight levels in accordance with the table were considered to be at standard levels, and those operating at altitudes and levels not in accordance with the tables were considered to be at non-standard levels. The crew of the B737 subsequently requested, and was assigned, non-standard FL350. At the time of the level change, both the crew of the B767 and the crew of the B737 had been issued airways clearances on one-way, designated air routes that formed a segregated race-track pattern between Sydney and Melbourne.

The controller responsible for the ELW/BLA sectors was instructing a student training on the ELW sector. Both were concentrating on the sequencing and separation of two jet aircraft in a step descent into Melbourne. A step descent allowed aircraft to simultaneously descend to vertically separated levels provided that the higher aircraft was progressively assigned levels that provided vertical separation with the lower aircraft. The step descent was occurring in the bottom left quadrant of the controller's Air Situation Display (ASD). The instructor was positioned behind and to the left of the student so that he could readily view the ELW sector and the traffic on the left side of the ASD. The B767 and the B737 were displayed in the top right quadrant of the ASD. Once the step descent had been established, the instructor noticed that the B737 was about to conflict with the B767. By that time the B737 was within the WOL/JVS combined sector and the instructor advised the WOL/JVS sector controller that he would be turning the B737. The student instructed the crew of the B737 to turn right to avoid the opposite direction B767.

The B737 was within the WOL/JVS control area, but under the jurisdiction (and control) of the ELW/BLA controller. The Australian Advanced Air Traffic System Human Machine Interface (HMI) used different coloured tracks and labels to aid situational awareness. The track and label colour of the B737 was green to the ELW/BLA controller, and blue to the WOL/JVS controller. The B737 crew should have been transferred to the WOL/JVS sector prior to crossing the boundary between those sectors and the ELW/BLA sectors. Had the transfer been made the track label and symbol would have been green on the WOL/JVS ASD.

The WOL/JVS controller initiated the amended route clearance for the B737 at FL350, and verbally coordinated the change with the ELW/BLA controller. At that time, the southbound B767 was on climb to FL280. The crew of the B767 contacted the WOL/JVS controller and was assigned FL350. The amended route for the B737 was direct to ALLOC, a waypoint located 77NM east-north-east of Sydney.

The workload on the WOL/JVS sectors at the time of the occurrence was reported by the controller to be low. The controller was providing a directed traffic information (DTI) service to military helicopters operating under visual flight rules (VFR) in class "G" airspace. The provision of DTI to VFR aircraft was available on request and subject to controller workload.

The WOL/JVS controller reported that the ASD at that console had recently "been faulted" and found to be outside the acceptable parameters for contrast. Subsequent to the occurrence, the screen was again checked and was found to be below acceptable contrast parameters. The controller reported difficulty distinguishing the blue track colours from the grey background of the screen; they appeared faded. A track was blue to indicate that the track was about to become the responsibility of that controller. Tracks will appear (in this case) blue to only one console at a time. The same track symbol and label was a different colour to all other controllers to indicate the relevance of that track to each control position.

The WOL/JVS controller stated that a smaller information screen that was open on the WOL/JVS ASD initially obscured the track symbol and label of the B737. He had used the route function available in TAAATS to determine the cleared route of the B737. The controller also stated that he normally used the text message box in TAAATS as a visual reminder of potential conflicts. He did not use the text box on this occasion. He also indicated that he would have preferred using strips because they better facilitated the acquisition and maintenance of the traffic picture.

The ELW/BLA instructor commented that the angle of view from behind and to one side of a student made it difficult to see some areas of the ASD and to monitor keyboard entries.

A Boeing 737-376 aircraft was being taxied for a departure from runway 20 at Christchurch on a scheduled service to Auckland. The Automatic Terminal Information System indicated that the surface wind was from 200 degrees at 10-15 kts, the temperature was 6 degrees Celsius and the dewpoint was 5 degrees Celsius. The visibility was 10 km reducing to 3,000 metres in rain and drizzle. During the taxi, the crew observed that the weather was fine and that the taxiway was wet after recent rain.

The route to the threshold of runway 20 included a 1,600-metre straight section of the main taxiway which ran parallel to runway 02/20, and which had a slight downhill grade. Analysis of flight data recorder (FDR) data revealed that the aircraft was on that section of the main taxiway for 2.3 minutes and that the speed of the aircraft gradually increased from 11 kts until it reached a maximum of 29 kts. The operator's flight crew training manual (FCTM) contained the following information regarding taxi procedures:

"To the pilot, the airplane appears to be moving slower than it actually is due to the flight deck height above the ground. Consequently, the tendency is to taxi faster than desired.

"Taxi speed should be closely monitored during taxi out, particularly when the active runway is some distance from the departure gate.

"Avoid taxi speeds greater than 30 kt on long taxi routes.

"Allow for decreased braking effectiveness on slick surfaces."

Although it was not included in the most recent edition of the FCTM, previous text stated that when approaching a turn, the aircraft should be slowed to the appropriate speed for the conditions. On a dry surface, approximately 8 kt to 12 kt was recommended. The aircraft manufacturer inadvertently removed that maximum recommended cornering taxi speed, when the Boeing 737 FCTM was extensively rewritten.

At the end of the main taxiway, the route to the threshold of runway 20 required that the aircraft be steered to the left, through approximately 60 degrees, onto taxiway "alpha". The pilot in command began braking the aircraft before reaching that turn point, but the aircraft had only slowed to 25 kt when the turn was commenced.

During the turn the nosewheel started "scrubbing", indicating excessive steering angle and/or taxi speed for the surface condition. Lacking traction, the nosewheel moved sideways and directional control was lost. That was followed by loss of traction on the main gear. As the aircraft moved sideways, the right main gear slid off the paved surface and onto the grassed area adjacent to the taxiway. The aircraft came to a halt with the right main gear settled slightly in the soil.

A maintenance engineer attended the aircraft, assessed that there was no obvious damage and then marshalled the aircraft back onto the taxiway. The aircraft was then taxied back to the terminal where a detailed inspection of the landing gear, the brakes and the engines was carried out. That inspection did not reveal any damage and the aircraft was released back into service.

The technical crew of the Boeing 737 consisted of a pilot in command and a co-pilot; both of whom were very experienced on the aircraft type. The airline operator had only recently commenced operations from this airport; the crew's local experience was therefore limited. The Boeing 737 nose wheel steering system was only controllable from the left control seat. Therefore, the pilot in command always taxied the aircraft. Either pilot could have used the ground speed readout on the pilots' electronic attitude director indicator (EADI) to monitor the taxi speed. The airline's normal procedure was for the co-pilot to bring to the attention of the pilot in command, any unsafe speed. During the airline operator's subsequent investigation of the incident, both pilots expressed surprise at the high speed recorded by the FDR during the turn.

The taxiway in question was treated during February 2001 with a surface enrichment treatment that resulted in making the surface more slippery. A notice to airmen (Notam) was issued on 20 March 2001 that advised poor braking action was possible when the surface was wet. That Notam was subsequently cancelled on 27 May 2001, when it was considered that the taxiway surface had weathered sufficiently for friction levels to return to normal.

During August 2001, a different airline operator reported another two minor skid events at that same location. Neither of those incidents resulted in the aircraft leaving the sealed area. As a precautionary measure, the airport operator re-issued the Notam that warned of the possibility of poor braking action. The taxiway surface was subsequently roughened by water blasting. That action removed the smooth bitumen top layer and exposed the aggregate for a better friction surface.

The Cessna Aircraft Company 402C (Cessna) was established on final for a straight-in approach to runway 15 at Cairns. The pilot had been assigned a descent to 3,000 ft, due to a de Havilland Canada Dash 8 (Dash 8) that was on final approach ahead of the Cessna and was at or below 2,000 ft. The assigned altitude ensured that the 1,000 ft vertical separation standard was maintained. The Approach One controller did not notice the Cessna descend through 3,000 ft or that the ground speed of the Dash 8 had reduced such that the spacing between the aircraft was less than the required 3 NM radar separation standard. There was an infringement of separation standards. The weather was reported to be fine with a clear sky.

Cairns approach control was managed by two control positions; Approach One and Approach Two. The Approach One area of responsibility was over the sea and included the final approach for runway 15 and all departures. Approach Two was responsible for the area over land.

The Approach One controller had accepted responsibility for the position about 10 minutes before the occurrence. The controller had been the Flow controller for the afternoon and was due to finish his shift at 1830 Eastern Standard Time. Prior to leaving, he offered to take over Approach One to enable another controller to take a short break. A handover/takeover was conducted at the Approach One position and he was aware of the six aircraft in the arrival sequence; having established the sequence himself while in the Flow position. He was also aware of another four aircraft taxiing for departure.

The initial aircraft in the arrival sequence were a Twin Otter from Mackay, the Dash 8, the Cessna and then an Embraer Bandeirante from the west. The Approach One controller was responsible for the Twin Otter, Dash 8 and the Cessna, while the Bandeirante was under the control of the Approach Two controller.

The Twin Otter crew had been assigned a visual approach for left base to runway 15 but subsequently requested an instrument landing system (ILS) approach. Approval of the ILS would entail additional track miles and reduce the spacing between the Twin Otter and the Dash 8. The controller believed sufficient spacing for separation and sequencing would be maintained with some minor track adjustments. He instructed the Dash 8 crew to turn left heading 330 degrees and to descend to 3,000 ft to position the aircraft east of the coast and 6 miles behind the Twin Otter. About two minutes later the controller instructed the Dash 8 crew to descend to 2,000 ft and the pilot of the Cessna to descend to "3000 (ft) visual". Those instructions were appropriately acknowledged by the crews. As the Cessna was from the north, and effectively on a long final, the controller issued the instructions to establish the vertical separation standard between the aircraft.

The Approach One controller instructed the Dash 8 crew to turn left heading 250 degrees and to descend to 1,500 ft. The crew was aware of aircraft arriving from the north. As the Dash 8 approached the runway 15 extended centreline, the crew thought the approach controller may have forgotten them and they discussed whether or not to contact the aerodrome controller direct. The crew could see the terrain to the west and reduced the aircraft's speed to reduce the rate of closure with the terrain. Shortly after, the Approach One controller instructed them to make a visual approach. That required them to turn the Dash 8 approximately 140 degrees to the left and then turn back to the right to establish the aircraft on final for runway 15. The crew did not report, nor were they required to report, their reduced speed. The controller could monitor aircraft ground speeds via the air situation display radar track labels.

The pilot of the Cessna had been assigned a descent to 3,000 ft and was instructed to maintain the best approach speed. He had been given traffic information on the Dash 8 and was expecting to be instructed to sight and follow that aircraft. Use of the sight and follow procedure by air traffic control (ATC) was common and transferred separation responsibility from ATC to a pilot. The Cessna pilot could see the Dash 8 ahead on final and continued descent below 3,000 ft without a clearance. He later stated that he only became aware that he had not complied with his clearance after he transferred to the aerodrome control frequency. The pilot also said that he believed there was no risk of collision.

The Approach One controller was an experienced controller and was also a team leader. He was on the second shift of a four-day cycle and had commenced the shift at 1100. He had worked the same shift the previous day. The controller had two days off duty before commencing this shift cycle. On the day of the occurrence he had visited a relative in hospital at 0800 and then returned home to do some paperwork before going to work. The relative had entered hospital about a month previously and the controller had visited regularly over that period. The controller was aware of the need to not allow the relative's illness to impact on his work. He believed he had achieved a balance between work and non-work commitments.

The controller spoke briefly with the Approach Two controller and gained the impression that the Cessna was under the control of the Approach Two controller. Consequently, as he saw the distance reducing between the Dash 8 and the Cessna he was waiting for a response from the Approach Two controller despite the fact that both pilots were operating on the Approach One radio frequency.

The Aerodrome controller was concerned with the reduction in spacing between the aircraft, and asked the Approach One controller to slow the Cessna to provide sufficient time for the Dash 8 to vacate the runway. That prompted the controller to ask the pilot of the Cessna if he could see the Dash 8. The pilot replied that he could, and was then instructed to contact the Aerodrome controller, who instructed the pilot to make an orbit, as there was insufficient spacing to ensure that the runway standard would be met.