AT76, vicinity Moranbah Queensland Australia, 2013
AT76, vicinity Moranbah Queensland Australia, 2013
On 15 May 2013, an ATR 72-600 on a visual approach to Moranbah descended sufficiently low in order to avoid entering cloud that a number of TAWS Warnings were activated. All were a consequence of the descent to below 500 feet agl at a high rate of descent which appeared not to have been appreciated by the flight crew.
On 15 May 2013, the crew of an ATR 72-600 (VH-FVR) being operated by Virgin Australia Regional Airlines on a scheduled passenger flight from Brisbane to Moranbah continued their day Visual Meteorological Conditions (VMC) descent in the circuit in Class 'G' airspace in order to remain clear of cloud and so complete a visual approach to land but in doing so came into sufficient proximity with terrain for a number of GPWS/TAWS alerts/warnings to be triggered.
An Investigation was carried out by the Australian Transport Safety Bureau (ATSB). It was established that the aircraft commander had been promoted to the rank of Captain four days earlier and had since flown 3.5 hours in command on the ATR72. He had about 4530 flying hours of which about 1750 were on the ATR72. The First Officer had about 2880 flying hours of which just over 600 were on the ATR72. This crew pairing was within Company requirements for new Captains.
It was noted that the Moranbah TAF available to and reviewed by the crew prior to departure from Brisbane at 0532 local time indicated that from 0600, Cloud was expected to be FEW at 1000 feet and BKN at 4500 feet with visibility 10 km. A 30% probability of fog and a visibility of 800 metres was also included in the TAF. A SPECI present weather report issued at 0700 gave the actual visibility as 1000 metres.
The airport had a non-precision approach procedure available based on the MRB Non-Directional Beacon located on the aerodrome and valid for runway 16. The other available approaches were Area Navigation Systems procedures which the crew were not approved to conduct.
It was established that the Captain had been PF for the flight and that, in view of the expected weather conditions at Moranbah, the initial intention had been to make an NDB approach. However, when ATC advice that another faster aircraft was expected to arrive at Moranbah two minutes before them was subsequently confirmed by the crew of that aircraft who also announced their intention to make a visual approach to 16 and join downwind to do so, the ATR crew changed their own plan to also make a visual approach. Subsequently, the other aircraft advised that they would slow down so that the ATR72 could be No 1. The latter crew stated that at about this time, they obtained the latest automatic weather report for Moranbah which confirmed that a visual approach was still appropriate for their arrival.
Once below an overcast 6,000 feet, the crew could see the Moranbah township and the area surrounding the airport and noted that there was "some low cloud and patches of fog around the runway 34 threshold". With the AP engaged, the aircraft began to level at the pre-selected altitude of 2300 feet - set because the airport elevation made it approximately equivalent to 1500 feet aal which was the standard circuit height for the ATR72.
It quickly became apparent that if level flight was continued, cloud would be entered. Without any discussion with the First Officer, the Captain "decided and announced that they were disconnecting the autopilot and continuing the descent in an attempt to remain clear of the cloud". He reported that "they both believed the aircraft would only need to descend 200-400 ft to be clear of the cloud layer". He added that during the rest of the flight, the runway and the ground below remained in sight although "visibility along the flight path was reduced because of the cloud". The First Officer recalled having sight of cloud that was "at or below 1500 feet and sloped in appearance with underlying fog". He stated that his workload both generally as the downwind leg was being flown and because of the sudden and for him unexpected descent, he "gave less attention to the outside conditions". Although he was not consulted about the decision to descend to remain clear of cloud, he advised the Investigation that since he had understood what the Captain was trying to achieve, he had no concern about the safety of the aircraft or decision to descend and "believed that communicating any in-flight observations to the Captain was unnecessary and would only have added to their existing workload".
Both pilots subsequently reported observing an initial rate of descent of around 700 fpm which was consistent with the recorded vertical speed, but neither noticed a subsequent increase in this rate to almost 2000 fpm and this, together with the approach to around 500 feet agl, led to a series of TAWS activations - first a 'Too Low Terrain' Alert, then a 'Terrain Ahead' Warning and finally a 'Too Low Gear' Alert - the latter because the aircraft was still clean but below 500 feet agl. All three activations occurred within 12 seconds of the aircraft being levelled at about 440 feet agl and it was noted that the recorded average vertical speed during the descent from circuit height had been 1750 fpm.
The cloud base above at the lowest point in the circuit was reported by the crew as having been "about 500 feet agl". They stated that they had remained clear of the cloud with the ground and runway in sight at all times and that since they "were aware that no obstacles existed along the downwind leg of the circuit and...knew why the TAWS...(had) activated", they considered that they were permitted to acknowledge and ignore the TAWS and continue the approach. The downwind leg was continued with a climbed to about 870 feet agl whilst beginning to configure the aircraft for landing by selecting flap 15 and the landing gear down. The crew stated that they had felt it "unnecessary" to climb the aircraft back to the normal circuit height of 1500 feet agl since "they would only have had to descend the aircraft again as they approached the base leg position".
During the gradual turn onto and during base leg, a bank angle of 38° was recorded (not explained) and then, shortly after flap 35 had been selected, a number of TAWS 'Don't Sink' Alerts were recorded. The crew told the Investigation that as the aircraft had not been descending at the time, the alert was therefore considered erroneous and ignored. The Investigation found that these had been Mode 3 Alerts, which are provided for the take off climb case and which had been triggered after a climb in clean configuration was commenced from 440 feet agl and not continued until at least 1500 feet agl before beginning descent again.
The aircraft subsequently turned onto finals where the Captain stated that the aircraft had been "stabilised by 500 feet agl" with a normal landing on runway 15 following.
The crew stated that their descent below the standard visual circuit altitude of 1500 feet agl had not been contrary to Company SOP since it was permitted if necessary "due stress of weather". The Captain further stated that "according to the regulations, the aircraft could be flown to an altitude of 500 feet agl which, although below the standard circuit altitude, was considered a ‘safe’ altitude".
The Investigation reviewed the evidence available on the conduct of the approach and made a number of observations. These included that:
- As the aircraft was not in the process of actually landing on runway 16 during the initial descent to below 500 feet agl and other valid approach options were available, standard operating procedures for the conduct of visual flight patterns contained in the Company ATR72 FCTM which indicated that the required circuit height was 1500 feet agl applied.
- The spontaneity of the descent below 1500 feet agl cancelled any opportunity for the crew to consider other approach options. These included the originally-planned NDB approach, for which the aircraft was still appropriately configured. Similarly, while the continuation of the visual approach was valid given the Captain had the airport in sight, the lack of associated descent planning meant that an altitude limit was not established for the descent. This negated any opportunity for the crew to recognise that the rate of descent required to avoid the cloud was greater than either anticipated, recognition of which may have prompted them to discontinue the descent and consider a different approach.
- The descent led to the onset of an undesired aircraft state, in that the aircraft was lower than desired without being correctly configured and with a high rate of descent.
- The crew perceived that the SOP requirement to maintain 1500 feet in the circuit was guidance rather than a requirement because information contained in the State AIP allowed them to descend lower. This misunderstanding that the 1500 feet agl was not a hard circuit altitude limit may have negated one of the intended triggers by the operator for crews to initiate a go-around.
The Investigation also found that although the rate of TAWS/GPWS activations known to Virgin Australia Regional Airlines was similar to the industry average, the reporting rate for such events (the reporting of which is a regulatory requirement) was well below the industry standard.
The Investigation formally identified three Contributing Factors for the event as follows:
- Approaching the circuit, the Captain assessed that a descent below the standard circuit height was necessary to avoid cloud, but did not communicate this to the First Officer in a timely manner, thereby preventing identification of a descent limit or appropriate approach alternatives.
- Due to the crew’s focus on avoiding the cloud, the high rate of descent at a lower than normal altitude was not identified and corrected by the crew in the short time available, resulting in the terrain awareness warning system 'Terrain Ahead' and ‘Too Low Terrain’ alerts.
- Despite briefing the intent to conduct a visual approach, descent in visual conditions was not assured and the crew did not discontinue the approach. This resulted in an undesired aircraft state and subsequent terrain awareness warning system alerts.
One other 'Factor that Increased Risk' was also identified:
- There was a significant underreporting by Virgin Australia Regional Airlines Pty Ltd of ATR72 terrain awareness warning system-related occurrences. [Safety Issue]
Safety Action taken as a result of the occurrence by Virgin Australia Regional Airlines was noted as having included:
- the issue of a Flight Crew Memo that occurrence reports must be submitted for TAWS/GPWS events and the addition of this requirement to the Flight Operations Policy and Procedures Manual
- the development of internal process to ensure that all TAWS/GPWS occurrence reports are reviewed by the safety department and passed to the ATSB as applicable.
The Final Report was published on 12 March 2015. No Safety Recommendations were made.
- Approach and Landing Risks
- CFIT Precursors and Defences
- Controlled Flight Into Terrain (CFIT)
- Cross-checking Process
- Response to a "PULL UP" Warning
- Terrain Awareness
- Safety Occurrence Reporting
- Flying a Visual Approach
- Monitored Approach
- Classification of Airspace
- Terrain Avoidance and Warning System (TAWS)
- Adherence to SOPs (OGHFA BN)