Safety issue: Sub-optimal RNAV (GNSS) approach design

The 21.5% of Australian area navigation global navigation satellite system (RNAV (GNSS)) approaches deviates from the optimum design parameters (short and irregular segments less than 5 NM and/or multiple steps within segments, and/or multiple minimum segment altitude steps) particularly approaches in the vicinity of high terrain. This was identified as a major concern by many pilots. A review to determine whether designs closer to the optimum approach profile could be developed, within the ICAO Pans-Ops limitations, was considered appropriate.

Safety Recommendation

The Australian Transport Safety Bureau recommends that Airservices Australia address this safety issue.

Approach procedures in areas of high terrain can be more complex. The example shown, Merimbula 03 RNAV (GNSS) procedure, has one 'non-ideal' segment length, the final approach segment of 7nm.  By inspection and well inside the capabilities of pilots to calculate, the distance is 17nm (5+5+7) from the initial approach fix to the MAPt (clearly shown below the profile view on the chart - see Fig 1). 

There is no other approach to this runway end. Using existing navigation aids to this runway end the PANS-OPS criteria would only allow an approach that was of no operational benefit. An RNAV approach design closer to the optimum, in this instance changing one segment, would raise the minima.

The task of designers is to balance the complexity of the design against operationally acceptable minima.  The complexity is limited by the criteria in ICAO PANS-OPS Vol II, and the Civil Aviation Safety Regulations and associated Manual of Standards Part 173 that describes the design criteria to which Airservices must adhere.

A review of procedures to give standard segments lengths would raise minima and then the question of operational acceptability would be raised.

This recommendation is not accepted.

The ATSB notes the many factors that influence the design of RNAV (GNSS) approaches, including the safety benefit of aligning the approach for a straight-in approach to the runway, and the need to adjust segment lengths on some approaches to provide adequate terrain clearance.  It is not the ATSB's intention that instrument approaches be altered to reduce any of the safety margins inherent in their design.  

The ATSB recognises that higher minimas would be a consequence standardising segment lengths, however, the results of our study suggest that non-standard segment lengths impose additional challenges to pilots' situational awareness and increase their workload. Even simple arithmetic calculations can be challenging under high workload conditions or in stressful circumstances. Where only a relatively small increase in the approach minima would result, we believe there would be strong arguments to accept this trade-off.

Our recommendation to review the 21.5 per cent of approaches with non-standard segment lengths was with the intention that Airservices might identify whether some of these approaches could be standardised with acceptable increases in approach minima.  Airservices Australia might consider the merits of a study to determine whether any of these sub-optimal approaches might be redesigned with only a marginal increase in approach minima.  Such a review might occur as part of the routine review of approaches undertaken by Airservices Australia.

Regarding RNAV (GNSS) procedures in general, Airservices conducted an internal assessment of procedures in 2006 and has set up a review programme, starting this Spring, to look at those aerodromes with offset approaches and higher than optimum descent gradients to see what could be achieved more toward the optimum, especially as the RNAV (GNSS) criteria has recently changed with the latest amendment to PANS-OPS, 15 March 07.

In addition to this review, Airservices will consult with CASA to explore whether the parameters procedure designers use (including segment lengths) can be further optimised and to consider pilot workload aspects of the design.

The ATSB notes the response from Airservices Australia and will monitor progress. The ATSB will review the status of this recommendation in approximately 6 months.