Recommendation R20070008

Recommendation issued to: Civil Aviation Safety Authority

Recommendation details
Output No: R20070008
Date issued: 04 April 2007
Safety action status: Closed - Action Taken
Background: Why this Recommendation was developed

Output text

Safety Issue

Based on the available evidence, the Lockhart River Runway 12 RNAV (GNSS) approach design resulted in mode 2A ground proximity warning system alerts and warnings when flown on the recommended profile or at the segment minimum safe altitudes.

This safety issue was not listed in the draft report but was identified during assessment of comments on the draft report. CASA was formally advised of this safety issue on 20 March 2007.

Safety Recommendation

The Australian Transport Safety Bureau recommends that the Civil Aviation Safety Authority address this safety issue.

Initial response
Date issued: 01 June 2007
Response from: Civil Aviation Safety Authority
Action status: Monitor
Response text:

On Tuesday 22 May 2007 CASA conducted approaches to the Lockhart River aerodrome in a Fairchild Metroliner aircraft using the Ground Proximity Warning System GPWS).

The test flights confirmed that the GPWS would not give terrain warnings during an instrument approach when the aircraft was on the correct profile, on track and within the speed range specified for the approach. This evidence calls into question the validity of the material cited by the ATSB regarding mode 2A ground proximity warnings.

CASA believes that an instrument approach not flown in accordance with the published approach will potentially cause GPWS warnings, which if in instrument metrological conditions, should be heeded.

CASA will examine this issue further. As stated above, a draft AC covering approach validaton procedures has been developed.

ATSB response:

Evidence in support of this recommendation at the time of the issue of the final report on 4 April 2007 included that Honeywell had conducted Lockhart River Runway 12 RNAV (GNSS) approach simulations (using groundspeeds typical of a Category B and C aircraft) for the constant angle approach along the recommended 3.49 degree profile and a step-down approach along the segment minimum safe altitudes (see page 68 and Appendix C of the final report). The simulations indicated that mode 2A alerts and warnings should be generated during both the constant angle and step-down approaches at both speeds when in the approach flap configuration. These alerts and warnings occurred in the vicinity of South Pap.

The final report also included information on reports received by the ATSB following the accident involving VH-TFU from the pilots of two aircraft, that they could not conduct the Lockhart River Runway 12 RNAV approach without the GPWS announcing 'terrain terrain pull up pull up'. This was reported to occur in both aircraft types (one was a Category B performance aircraft and the other Category C). The occurrence was always after passing LHRWF inbound and the pilots reported that the warnings had occurred while the aircraft were on the published constant angle approach path with the autopilot coupled to the flight management system, in the approach configuration, and within the appropriate approach speeds for the aircraft category.

The report also noted the report from a pilot of another operator who recalled conducting a runway 12 RNAV (GNSS) approach soon after the procedure was published. He stated that the approach was flown with the autopilot coupled to the flight management system, which had calculated a constant angle approach path. The pilot reported that the GPWS did not generate any alerts or warnings. The ATSB was unable to confirm the aircraft configuration or the calculated constant angle approach used on that occasion.

The ATSB has subsequently received a further report of EGPWS warnings and alerts being activated when flying the normal approach profile for the Lockhart River Runway 12 RNAV (GNSS) approach (as advised to CASA by e-mail on 31 May 2007). Specifically, on 29 May 2007, the crew of a Super King Air 350 was on the approach when the EGPWS system generated both a 'terrain terrain' alert and 'pull up' warning when the aircraft was in the vicinity of South Pap. The crew reported that the published constant angle approach was being flown with the autopilot engaged in a flight management system mode which was providing vertical guidance with the crew cross-checking the altitudes with the approach chart. The crew conducted a go-around and missed approach manoeuvre and attempted a second approach. During the second approach, flying the same constant angle approach, the same alerts and warnings were heard.

Given the evidence gathered during the investigation, the subsequent report from the crew of the Super King Air 350, and the results of the test flights conducted by CASA on 22 May 2007, the ATSB has initiated an investigation into the conduct of Lockhart River runway 12 RNAV (GNSS) approaches as a transport safety matter, as defined in Section 23(2) of the Transport Safety Investigation Act 2003. As part of that process, the ATSB would appreciate a copy of the Flight Data Recorder (FDR) data relating the test flights conducted by CASA on 22 May 2007 (Section 32 Notice to be provided separately as per normal procedure through CASA Corporate Relations).

Noting that CASA is also examining this issue further, the ATSB has classified the status of this recommendation as 'Monitor'.

Further correspondence
Date issued: 30 September 2008
Response from: Civil Aviation Safety Authority
Response status: Closed - Partially Accepted
Response text:

On 11 August 2008, CASA and Airservices staff flew the Runway 12 RNAV GNSS approach with an aircraft fitted with a Garmin 430w GPS and TAWS-B. The first approach was flown using the 3.49 degree profile on autopilot. No terrain or 'pull-up' warnings were received. After flying the approach on profile, two lower than profile approaches were flown, below the cloud base (around 2500') along the laterally guided flight path. On each occasion the terrain warning and pull up functions of the TAWS sounded well before the significant ridges. It was also noted by the operating pilot that the approach was no more difficult to fly than any other he had experienced.

As previously advised, the issue of Mode 2A warnings from TAWS A is recognised and has been experienced on approaches other than LHR; specifically Cairns VOR and Cooktown - NDB. That is a -characteristic of the TAWS, aircraft configuration and terrain:- It is an unavoidable issue if the aircraft is not correctly configured with steep terrain on the approach. There is nothing CASA can do to change this. With more experience of TAWS A CASA will develop relevant educational information for pilots.

I have also attached, for your information, a more detailed report with recommendations for further CASA action. I understand that these outcomes have been communicated to your staff and that it has been agreed that TAWS A warnings could be experienced on a range of approaches besides RNAV GNSS depending on the aircraft configuration and the nature of terrain in the vicinity and was affected by any pilot decision to not fly the optimum profile as published. This is not a circumstance unique to Lockhart River or RNAV (GNSS) approaches.



  1. The current LHR RNAV (GNSS) approach remain in place until a redesign is undertaken that includes the Baro-VNAV vertical approach profile.
  2. That APV approaches be initiated as soon as practicably in Australia commencing with Baro-VNAV redesigns for existing RNAV (GNSS) approaches.
ATSB comment:

The ATSB is concerned that the flight test did not provide a true validation test as the TAWS Class B (TAWS-B) as fitted to the test aircraft is a reduced capability system aimed at reducing the cost of the equipment for use in general aviation. The primary difference between TAWS-A and TAWS-B is that TAWS-B does not include the basic GPWS components, which are dependent upon a height input from a radio altimeter. As such, it is our understanding that the aircraft was not appropriately equipped to conduct flying to validate (or otherwise) the activation of the ground proximity warning system mode 2A warnings that was the subject of ATSB recommendations R20070005 and R20070008.

The implication of the second paragraph above is that the TAWS/GPWS warnings are due to incorrect operation of the aircraft in that they are 'incorrectly configured'. It is the ATSBs understanding that for many aircraft (particularly those in the general aviation sector), that it is not normal for the aircraft to be configured with full landing flap at the final approach fix and that the flap may not be fully deployed until the aircraft becomes visual.

The ATSB also makes the following comments in relation to the recommendations contained in the report attached to your letter.

Recommendation 1: As noted in Air Transport Communication (ATcom) issue 2, dated Friday 18 July 2008, APV approaches are 'some time away' and it is our understanding that unless the APV solution in Australia is compatible with overseas systems (for example, WAAS) the expense and complexity of such systems will result in them being feasible for the higher end of the market. The majority of general aviation will still be dependant upon existing approach designs, primarily RNAV (GNSS).

There may be an interim solution by redesigning the approach to the new version of ICAO PANS-OPS that could result in a more closely runway aligned approach, providing lateral clearance from the South Pap Terrain feature. The ATSB supports an assessment of a redesigned RNAV (GNSS) non-precision approach using the new ICAO standards as an interim measure.

Recommendation 2: The ATSB agrees with this recommendation, with the reservation regarding the feasibility and availability of the equipment required for APV approaches discussed above.

The ATSB remains willing to assist CASA in any way possible to resolve any outstanding issues with our collective aim to improve safety.

The ATSB is continuing the Safety Issue investigation into the conduct of Lockhart River runway 12 RNAV (GNSS) approaches as a transport safety matter.

Further correspondence
Date issued: 18 September 2009
Response from: Airservices Australia
Response status: Closed - Accepted
Response text:

On 18 September 2009, Airservices Australia advised the ATSB that the Lockhart River runway 12 RNAV (GNSS) non-precision approach had been redesigned and flight validated. The flight validation was conducted with a TAWS-A equipped aircraft and the 'the procedure did not trigger terrain warnings'. The new approach will be published in the Aeronautical Information Publication amendment effective 19 November 2009.

ATSB comment:

The ATSB recognises that the revised Lockhart River runway 12 RNAV (GNSS) non-precision approach has now met the intention of the safety recommendation and has closed this safety recommendation - Action taken.

Last update 05 April 2012