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Output Number
R20060021
Approval Date
Organisation
Airservices Australia
Published Date Time
Recommendation type
Recommendation
Status
Closed - Action Taken
Mode
Aviation
Date released

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.

Organisation Response
Date Received
Organisation
Airservices Australia
Response Text

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.

ATSB Response

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.

ATSB Response date
Date Received
Organisation
Airservices Australia
Response Status
Monitor
Response Text

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.

ATSB Response

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.

ATSB Response date