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B737, Mildura VIC Australia, 2013
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|On 18 June 2013, a Boeing 737-800 crew en route to Adelaide learned that un-forecast below-minima weather had developed there and decided to divert to their designated alternate, Mildura, approximately 220nm away where both the weather report and forecast were much better. However, on arrival at Mildura, an un-forecast rapid deterioration to thick fog had occurred with insufficient fuel to divert elsewhere. The only available approach was flown to a successful landing achieved after exceeding the minimum altitude by 240 feet to gain sight of the runway. An observation immediately afterwards gave visibility 900 metres in fog with cloudbase 100 feet.|
|Actual or Potential
|Controlled Flight Into Terrain (CFIT), Weather|
|Type of Flight||Public Transport (Passenger)|
|Intended Destination||Adelaide International Airport|
|Actual Destination||Mildura Airport|
|Take off Commenced||Yes|
|Location - Airport|
|Tag(s)||Non Precision Approach,|
|Tag(s)||No Visual Reference,|
IFR flight plan
|Damage or injury||No|
|Causal Factor Group(s)|
|Group(s)||Air Traffic Management|
|Group(s)||Air Traffic Management|
On 18 June 2013, a Boeing 737-800 (VH-VYK) being operated by Qantas Airways on a scheduled domestic passenger flight from Sydney to Adelaide as Qantas 735 with 152 persons on board was unable to land at its destination due to a deterioration in the weather there. Upon diversion to its designated alternate, Mildura, the weather had deteriorated but there was insufficient fuel to go anywhere else and so the only available (non precision) approach was continued below the MDA by 240 feet until the runway was visually acquired in day IMC on emerging from cloud at a reported 150 feet agl.
A detailed combined Investigation was carried out by the Australian Transport Safety Bureau (ATSB) into this event and a similar below-minima landing event involving a Virgin Australia Airlines Boeing 737-800 which had been inbound to Adelaide from Brisbane as Velocity 1384 with a very similar ETA the same day and had also diverted to Mildura. This aircraft had only been able to land at Mildura 28 minutes after the Qantas aircraft after continuing the second of two approaches below minima, by which time the observed visibility had dropped to 200 metres in fog. Data from the FDR and CVR from both aircraft were downloaded to assist the Investigation.
It was noted that the Virgin Australia Captain had accumulated 19,966 total flying hours which included 9,111 hours on type and their First Officer had accumulated 7,100 total flying hours which included 1,120 hours on type. The Qantas Captain had accumulated 17,069 total flying hours which included 7,590 hours on type and their First Officer had accumulated 7,714 total flying hours which included 3,982 hours on type.
It was noted that Mildura was a Non-Towered airport on which communications, including those with the ACC frequency covering arrivals and departures from and to the airways system were conducted on the common traffic advisory frequency (CTAF). It was also noted that at the time of the investigated event, major international aerodromes in Australia, which included Adelaide, differed from ICAO/WMO standards for forecasts in that both TAFs and Trend (TTF) Forecasts were issued with the latter having a higher update priority than TAFs. This difference would have been widely understood to Operators based in Australia such as those which operated the two aircraft which breached landing minima at Mildura. A Bureau of Meteorology (BoM) Review referenced by the Investigation and completed since this event has now led to a decision to cease issuing TTFs and simply issue and update TAFs using an ICAO-compliant procedure.
It was also noted that the lowest minima IAP at Adelaide was a Cat 1 ILS and that the lowest minima IAP at Mildura, the Alternate filed for both flights, was an RNAV GNSS non precision procedure with an MDH of 393 feet provided an aerodrome QNH was available which it was. Mildura was approximately 220 nm east of Adelaide and both aircraft would pass close to its overhead en route to their destination. It was found that the Virgin Australia 737 had departed Brisbane with 940kg of "Additional Fuel" on board and that the Qantas 737 had departed Sydney with 1,300kg of "Additional Fuel" on board. The Qantas Captain reported that he "routinely uploaded additional fuel for contingencies" but that the upload made that day had not been in response to forecast conditions.
Shortly after the Virgin Australia aircraft had left Brisbane but almost half an hour before the Qantas aircraft had left Sydney, the previously-issued Adelaide TAF was updated to add a PROB30 of fog developing. One hour later, a TTF for Adelaide which showed that fog had reduced visibility there but was expected to clear before the ETAs of both aircraft. The Qantas crew were aware of the changes contained in the TTF and continued towards destination in the knowledge that they had sufficient fuel to hold for about 45 minutes should the fog last longer than forecast and still land with required fuel reserves. The Virgin Australia crew were not aware of the changes to the forecast and first learned of the presence of fog at their destination when they changed to the en route sector immediately prior to the Adelaide TMA 35 minutes prior to their ETA.
The Qantas crew elected to take up the hold at a waypoint which was 48 nm from Adelaide rather than continue their descent. The Virgin Australia crew had already commenced their descent and obtained further weather information from the Adelaide TWR controller who advised a reported 150 metres visibility in fog with broken cloud on the surface and a TTF giving a clearance beginning at 0930. Based on this, they decided - fifteen minutes prior to their ETA - to divert to Mildura where the METAR was giving visibility >10km with the lowest cloud BKN at 3,900 feet and the just-issued Mildura TAF was giving no change in visibility and just scattered cloud at 3,000 feet and at 5,000 feet with a 1° temperature/dew point split. On noting the Virgin Australia decision to divert and after re-checking the Adelaide and Mildura weather reports and forecasts, the Qantas crew also decided to divert to Mildura.
Whilst both aircraft were still en route to Mildura, an aircraft which had just taken off from there advised that contrary to the recently issued forecast, the weather conditions there were deteriorating with the cloudbase now around 400 feet, only just above the lowest IAP minima there. The Aerodrome Weather Information System (AWIS) at Mildura was inoperative and so the first weather report received was from the en route sector Controller as both aircraft were approaching Mildura a few minutes later, which was a cloudbase of 200 feet and visibility 2,100 metres in mist with equal temperature/dew point.
The crews of the two aircraft were in contact with each other on the Mildura CTAF and after hearing that a Qantaslink DHC8 had gone around from its approach without seeing anything at MDH, it was agreed by the Virgin Australia crew that Qantas could begin an approach first "due fuel". This resulted in a successful landing which involved agreeing to continue the runway 27 RNAV (GNSS) non-precision approach to a modified MDA of 360 feet QNH - 193 feet aal - and then continuing descent beyond that until the runway became visible at 150 feet aal. Fuel reserves were not used. A SPECI weather observation promulgated two minutes after this landing gave the visibility as 900 metres in fog and the lowest cloud as overcast at 100 feet.
A few minutes later, the en route Sector Controller broadcast the first updated TAF since the one issued as the two aircraft had been approaching Adelaide which had had no mention of any risk of low cloud or poor visibility. This new forecast gave a 3,000metre visibility, scattered cloud at 300 feet and an improvement in both as imminent. It also included a PROB 30 of visibility 500 metres in fog for the next two hours.
The Virgin Australia crew began the runway 27 RNAV (GNSS) non-precision approach soon after this based on having sufficient remaining fuel to carry out a go around and a further approach if necessary. When they were not visual with the runway at 132 feet agl - 261 feet below the MDA for the approach - a go around was initiated. The First Officer reported that "as they commenced the missed approach, it was possible to confirm that they were aligned with the runway by looking directly down". ATC "initiated a distress phase" as the aircraft was positioned for a second approach. The Cabin Crew were briefed to prepare for an emergency landing and instructed passengers to brace accordingly. After again significantly exceeding the approach minima, with no documented prior discussion about a lower minimum transition to flight by visual reference, the aircraft made a successful landing 10 minutes after initiating the go around. The SPECI weather observation promulgated three minutes prior to touchdown gave the visibility as 200 metres in fog with the base of the overcast 100 feet. Fuel remaining after the flight was below minimum reserves with 594 kg in the No. 2 tank and 18 kg in the No. 1 tank with the crossfeed valve open. The imbalance - which with the crossfeed valve open would have not caused any interruption to the fuel supply to the no. 1 engine had its usual tank run dry - was found to have been a consequence of the crew opening the cross feed valve (prior to the display of 'LO FUEL QTY' messages for both main tanks) from memory without the usual subsequent check against the written QRH procedures for low fuel and fuel imbalance. This was considered both understandable, given the pressure of other priorities, as well as beneficial, since actioning the fuel imbalance drill with low fuel would have directed the closure of the crossfeed to protect against a possible causal fuel leak, for which there had been no evidence but introduced the potential for the No. 1 engine to be starved of fuel.
It was noted that neither aircraft formally declared any urgency or emergency status based on their fuel status or their prior intention to substantially breach the applicable approach minima but based on their air traffic communications, the appropriate precautionary measures were implemented at Mildura.
The 50 minute delay in updating the significantly inaccurate Mildura TAF which was the basis for the diversion decisions made by the crews of both aircraft was found to have been a consequence of priorities in place in the aviation forecasting section of the Bureau of Meteorology (BoM). The updated forecast was also not issued until after one of the aircraft had already landed and not valid until the other was on final approach. It was also not updated until after the Mildura Airport Reporting Officer telephoned Melbourne ACC to "query why aircraft were diverting to Mildura given the conditions" and the systems supervisor there had, in turn, "contacted the BoM to advise them of the unexpected deterioration and that the current Mildura TAF did not reflect the severity of the conditions".
The Investigation noted the fact that the significant breach of approach minima had, of necessity, involved a non-precision approach but considered that had the Mildura conditions been correctly forecasted, the only other diversion option open to the two aircraft would have been Woomera, an airport much further from Adelaide than Mildura which was not considered by either crew as "there were no company facilities at that airport, nor was it a routine destination for either airline". In the light of that, the observation was made that although Adelaide only had a Cat 1 ILS approach, "it had demonstrated the ability to support emergency landings below CAT I meteorological conditions during practice approaches" and "both aircraft had the necessary equipment to conduct at least a CAT II (fail passive) autoland at Adelaide and both flight crews were trained and approved in Cat II/III autoland procedures" although at the time "only Qantas had the appropriate CASA approval to conduct Boeing 737 autoland operations".
It was noted that neither below-minima approach had breached the prevailing regulatory requirements which, although normally requiring compliance with published IAP minima, included discretion to continue below them if an emergency arises such that "the interests of safety make it necessary for an aircraft to land at an aerodrome".
In addition to an extremely detailed analysis of the provision and distribution of airport weather information issued by the BoM and the prioritisation of its aviation forecasting tasks, the Investigation also looked at the differences between Australian practices in the provision of weather information as part of Flight Information Service (FIS) and corresponding practices in other States.
In support of the Investigation and to "better quantify the risk of un-forecast weather deterioration", the ATSB also initiated "a research investigation into the reliability of aviation weather forecasts at Adelaide and Mildura Airports between 2009 and 2013" which although since published had not been finalised by the time the Investigation was completed. Nevertheless, it had already been established that "exposure to weather conditions below the landing minima at Mildura was found to be very unlikely during the period studied". Specifically, between 2009 and 2013, weather conditions at Mildura were below landing minima 1% of the time and below alternate minima about 2.6% of the time and that "the forecasting process at Mildura Airport appeared to be conservative, with a high false alarm rate". However, "significant fluctuations in forecast accuracy and the resulting risk to safety were observed over time" and these were being assessed in the research against aspects such as "aircraft holding time and the number of traffic movements" in order to "determine the operational effect and risk of any inaccuracy."
Similar (Australian) occurrences to the one under investigation where un-forecast weather deterioration and limited remaining fuel had obliged crews to land below minima were identified as having occurred in 1999 - an Airbus A320 at Adelaide, in 2004 - an Airbus A330 at Sydney in 2004, an Airbus A330 at Perth in 2006 and a Boeing 717 at Perth in 2012, all of which had involved continuing precision approaches below the promulgated DA.
The Conclusions of the Investigation included the following:
- Both crews were compelled to land at Mildura in conditions below the minima permitted for landing, with Velocity 1384 also landing below their required fuel reserves.
- Given the intent of (all) the supporting systems, the actions of individuals were predominantly reasonable for the information the individuals had at the time.
- This occurrence has highlighted the effect of various factors coming together to create and influence a rare event.
The Findings of the Investigation were formally documented were as follows:
- The meteorological conditions at Adelaide Airport deteriorated below the landing minima while Velocity 1384 and Qantas 735 were en route to Adelaide.
- The inaccuracy of the forecast clearance of the fog at Adelaide Airport compelled the flight crews of Velocity 1384 and Qantas 735 to either conduct an emergency landing at Adelaide or divert to Mildura Airport.
- The actual weather conditions encountered by the flight crews of Velocity 1384 and Qantas 735 on arrival at Mildura were below landing minima and significantly worse than the aerodrome forecast and weather reports used by both flight crews to assess its suitability as an alternate destination to Adelaide.
- On arrival at Mildura, Velocity 1384 and Qantas 735 had insufficient fuel to divert to any other airport and were committed to a landing in conditions below their landing minima.
Other Factors that increased risk:
- The flight crew of Velocity 1384 did not obtain updated weather information for Adelaide while en route and were therefore unaware of the weather deterioration affecting the airport, limiting the options and time available to plan a diversion to an alternate destination airport.
- The flight crews of Velocity 1384 and Qantas 735 gave precedence to the aerodrome weather reports at Mildura over the aerodrome forecast when deciding to divert.
- Despite the Bureau of Meteorology (BoM) knowing of the deteriorating weather at Mildura from other sources, by not passing on the in-flight weather report of deteriorating weather from the departing air ambulance pilot, the controller removed an important source of information for use by the BoM.
- The in-flight weather report given by the air ambulance pilot was not passed to the flight crews of Velocity 1384 and Qantas 735 by the controller when they changed frequency in-bound to Mildura Airport, removing an important source of information for flight crew planning and decision making.
- The automatic broadcast services did not have the capacity to recognise and actively disseminate special weather reports (SPECI) to pilots, thus not meeting the intent of the SPECI alerting function provided by controller-initiated Flight Information Service. [Safety Issue]
- For many non-major airports in Australia, flight crews of arriving aircraft can access current weather information using an Automatic Weather Information Service via very high frequency radio, which has range limitations. Where this service is available, air traffic services will generally not alert pilots to significant deteriorations in current weather conditions at such airports, increasing the risk of flight crew not being aware of the changes at an appropriate time to support their decision making. [Safety Issue]
- The flight crews of Velocity 1384 and Qantas 735 planned for, and uploaded, sufficient fuel for the forecast conditions at Adelaide Airport in accordance with the respective operator's fuel policies.
- The flight crew of Qantas 735 proactively sought weather information for various airports soon after departing Sydney.
- The meteorological information obtained from an Aerodrome Weather Information Service (AWIS) is operationally equivalent to that provided in routine (METAR)/special weather (SPECI) reports. However, as the AWIS broadcast doesn't contain the label 'SPECI', pilots are required to recognise and interpret its operational significance.
- Critical to the assurance of safe flight, all elements of the aviation system including weather services, air traffic services, aircraft operators and flight crews need to have a clear understanding of their roles and responsibilities in that system.
In respect of the two Safety Issues identified by the Investigation, the ATSB reviewed the responses of the ANSP Airservices Australia. It was concluded that action taken or intended in respect of the "Limited provision of flight information service for some non major airports" was acceptable and the issue would be monitored until the outstanding actions were completed. In respect of the "Alerting function of Special Weather Reports (SPECI) not being met by the current Automatic Broadcasting Service", the intention of ANSP Airservices Australia to work with the Bureau of Meteorology to establish a solution was noted but the ATSB was concerned that Airservices had not taken responsibility for the resolution of this Safety Issue and "in addition, the indefinite nature of the proposed activity does not provide a high degree of confidence that the safety issue will be adequately addressed".
As a result, the following Safety Recommendation was issued:
- that Airservices Australia as the Safety Issue owner works in collaboration with the Bureau of Meteorology to instigate a system change to reinstate the alerting function of SPECI reports currently not available through an automatic broadcast service. [AO-2013-100-SR-057]
Other relevant Safety Action already taken or intended in relation to the circumstances of the investigated event and known to the Investigation prior to its completion by both the Bureau of Meteorology and the Virgin Australia Airlines was also noted.
The Final Report was released on 17 January 2017.
- Weather Forecast
- Weather Observations at Aerodromes
- Alternate Aerodrome
- Aerodrome Operating Minima (AOM)
- Fuel Management
- Fuel - Diversion to Weather Alternate
- Fuel - In-Flight Management (Abnormal Operations)
- [The effect of Australian aviation weather forecasts on aircraft operations: Adelaide and Mildura Airports, Australia, ATSB Australia, Jily 2017.