What happened
On 15 April 2016, the instructor and student of a Pacific Aerospace CT/4B aircraft, registered VH-YCO (YCO), conducted a dual (military) training flight under the instrument flight rules.[1] The aircraft departed from Tamworth Airport, and tracked to Narrabri Airport, before commencing the area navigation (RNAV) runway 11 approach to Gunnedah Airport, all in New South Wales.
Prior to commencing the approach, the instructor of YCO contacted the military radar controller (see Military radar control), who advised that they were not aware of any other aircraft in the area. The instructor reported that they broadcast on the Gunnedah common traffic advisory frequency (CTAF), when 18 NM from the aerodrome, advising that they were going to conduct the RNAV runway 11 approach, and stating their estimated time at the airport (Figure 1). The crew reported hearing a voice-back response from the aerodrome frequency response unit,[2] indicating that no one else had transmitted on the CTAF in the previous 5 minutes, and no response from any other aircraft on the CTAF.
At that time, an instructor and student pilot of a Piper PA-28-181 aircraft, registered VH-WJZ (WJZ), were conducting a local dual training flight from Gunnedah Airport. The instructor reported that they were broadcasting on and monitoring the CTAF.
At about 1450 Eastern Standard Time (EST), YCO was 13 NM north-west of Gunnedah on the RNAV approach for runway 11, and at 4,500 ft above mean sea level (AMSL), when they were alerted by the aircraft’s traffic collision avoidance device[3] of another aircraft. The device indicated that the other aircraft was 200 ft below them and 2 NM away. The instructor looked south and within 20 seconds sighted WJZ at the same level. The instructor took control of the aircraft from the student, and conducted a 60° angle of bank turn to the left to avoid WJZ.
The instructor of WJZ sighted YCO when about 13 NM north-west of Gunnedah aerodrome at about 4,000 ft AMSL. YCO was then to their north in their 2 o’clock[4] position. The instructor of WJZ conducted a left turn and reported sighting YCO commence a left. The aircraft passed at the same level about 150 to 200 m horizontally apart.
The instructors of the two aircraft subsequently communicated on the CTAF. The instructor of WJZ reported that they had not heard any relevant calls on the CTAF leading up to the incident.
Figure 1: Recorded track of VH-YCO, approximate track of VH-WJZ, and approximate location of near collision
Source: Instructor of VH-YCO – annotated by ATSB
Military radar control
The ADF established and operated a radar system in the vicinity of Tamworth to provide directed traffic information (DTI) to pilots operating CT4B aircraft in the Tamworth training areas. This was an interim measure to treat the risk of separation breakdown in the Tamworth training areas while those training areas and associated procedures underwent redesign and other systems were established. The DTI was provided on a discrete frequency monitored only by ADF aircraft. Directed traffic information was an advisory only service and controllers did not provide control or direction to pilots, but gave information aimed at increasing their situational awareness.
The military controller did not report any conflicting traffic before or during the incident.
Traffic collision avoidance device
YCO was fitted with a traffic collision avoidance device (TCAD), which warns of the presence of threat aircraft if the other aircraft is fitted with a functioning transponder that is being interrogated by a radar transmitter. The TCAD detects Secondary Surveillance Radar (SSR) transponders in aircraft within a certain proximity.
The TCAD displays threats detected within a predetermined volume of airspace known as a shield. The shield setting for the TCAD during the incident flight was +/- 1,000 ft in altitude and 2 NM. When a detected aircraft enters the pre-set shield, the pilot is alerted via aural and visual indications.
The TCAD system will not detect a threat aircraft that is not equipped with a transponder, the transponder is inoperative, or the transponder is operating but not being interrogated by either an SSR or a TCAS fitted aircraft. The TCAD is designed as an aid to situational awareness and should not be relied on for traffic separation.
In this incident, the TCAD identified WJZ as a threat and alerted the pilots of YCO.
Pilot comments
Instructor of VH-WJZ
The instructor of WJZ commented that the other aircraft was conducting military training and they have a radar in the Gunnedah training area, to provide them with traffic warnings. The military radar service did not identify any conflicting traffic, however, YCO’s TCAD identified WJZ, which indicated that WJZ’s transponder was functioning.
The instructor reported that there was some distortion in the broadcasts from YCO heard after the incident.[5]
The instructor further commented that in future, they would broadcast their position in the training area every 15 minutes; even if there were no broadcasts to indicate there may be nearby aircraft.
Instructor of VH-YCO
The instructor of YCO commented that if pilots of aircraft conducting instrument approaches broadcast their aircraft’s position with reference to a compass, this may assist visual flight rules’ pilots to assess whether there could be a conflict.
Safety action
Whether or not the ATSB identifies safety issues in the course of an investigation, relevant organisations may initiate safety action in order to reduce their safety risk. The ATSB has been advised of the following safety action in response to this occurrence.
Operator of VH-WJZ
As a result of this occurrence, the operator of VH-WJZ has advised the ATSB that they are taking the following safety actions:
Airspace procedure
As the instrument approach lies within the Gunnedah training area, student pilots are required to remain below 3,000 ft when within 5° of the approach path. Additional risk assessments were conducted at the other company flying school locations and a similar hazard was found at Scone, where similar de-conflicting provisions have been made.
Safety message
The ATSB report Limitations of the See-and-Avoid Principle outlines the major factors that limit the effectiveness of un-alerted see-and-avoid. In this occurrence, un-alerted see-and-avoid did lead to the instructor of one aircraft sighting the other. However, insufficient communication between pilots operating in the same area is the most common cause of safety incidents outside controlled airspace and near non-controlled aerodromes. A broadcast that does not provide a clear understanding of the location of an aircraft, or the intentions of the pilot, is often ineffective in directing other pilots where to focus their lookout.
Aviation Short Investigations Bulletin - Issue 51
Purpose of safety investigationsThe objective of a safety investigation is to enhance transport safety. This is done through:
It is not a function of the ATSB to apportion blame or provide a means for determining liability. At the same time, an investigation report must include factual material of sufficient weight to support the analysis and findings. At all times the ATSB endeavours to balance the use of material that could imply adverse comment with the need to properly explain what happened, and why, in a fair and unbiased manner. The ATSB does not investigate for the purpose of taking administrative, regulatory or criminal action. TerminologyAn explanation of terminology used in ATSB investigation reports is available here. This includes terms such as occurrence, contributing factor, other factor that increased risk, and safety issue. Publishing informationReleased in accordance with section 25 of the Transport Safety Investigation Act 2003 Published by: Australian Transport Safety Bureau © Commonwealth of Australia 2016
Ownership of intellectual property rights in this publication Unless otherwise noted, copyright (and any other intellectual property rights, if any) in this report publication is owned by the Commonwealth of Australia. Creative Commons licence With the exception of the Coat of Arms, ATSB logo, and photos and graphics in which a third party holds copyright, this publication is licensed under a Creative Commons Attribution 3.0 Australia licence. Creative Commons Attribution 3.0 Australia Licence is a standard form licence agreement that allows you to copy, distribute, transmit and adapt this publication provided that you attribute the work. The ATSB’s preference is that you attribute this publication (and any material sourced from it) using the following wording: Source: Australian Transport Safety Bureau Copyright in material obtained from other agencies, private individuals or organisations, belongs to those agencies, individuals or organisations. Where you wish to use their material, you will need to contact them directly. |
__________
- Instrument flight rules permit an aircraft to operate in instrument meteorological conditions (IMC), which have much lower weather minimums than visual flight rules. Procedures and training are significantly more complex as a pilot must demonstrate competency in IMC conditions, while controlling the aircraft solely by reference to instruments. IFR-capable aircraft have greater equipment and maintenance requirements.
- An aerodrome frequency response unit (AFRU) assists in indicating selection of the correct VHF frequency at non-towered aerodromes by automatically responding with either a pre-recorded voice message if no transmission has been received in the last five minutes or otherwise a ‘beep-back’, on the CTAF.
- Traffic collision avoidance device is an aircraft collision avoidance system. It monitors the airspace around an aircraft for other aircraft equipped with a corresponding active transponder and gives warning of possible collision risks.
- The clock code is used to denote the direction of an aircraft or surface feature relative to the current heading of the observer’s aircraft, expressed in terms of position on an analogue clock face. Twelve o’clock is ahead while an aircraft observed abeam to the left would be said to be at 9 o’clock.
- The instructor of YCO responded to this comment that they had not had any issues with their radios during the flight.