BASI recommends that Airservices Australia modify the GPS
Arrival procedure for Wynyard so that the track does not require
aircraft to fly directly over the steeply rising terrain of Table
Cape.
SUBJECT
GPS Arrival - Wynyard (Tasmania)
BACKGROUND
A report was received by BASI through the "INDICATE" safety program
[see note below] that aircraft equipped with Ground Proximity
Warning Systems [GPWS] experience nuisance warnings on the approach
to Wynyard [Tasmania] when using the GPS arrival procedure.
These warnings occur on the inbound track, from position CAMUS, of
158 degrees which takes aircraft directly over the steeply rising
terrain of Table Cape. The rapid change in elevation from sea-level
to 716 feet AMSL can result in a "Terrain Terrain" warning from the
GPWS while aircraft are at or approaching the Minimum Descent
Altitude [MDA] of 1000 feet with the altimeter set on actual QNH.
In Instrument Meteorological Conditions the warning requires the
crew to respond immediately by initiating an overshoot procedure
[refer to attachment 1].
It is reported that this warning can occur even with aircraft in
the landing configuration and at reduced airspeeds.
Note: The "INDICATE" safety program is a proactive system for
monitoring airline safety performance.
SAFETY DEFICIENCY
The GPS arrival procedure for Wynyard [Tasmania] requires aircraft
to track directly over Table Cape. This can result in nuisance GPWS
warnings that either distract crews from their duties at a time of
high workload or result in crews ignoring these warnings
altogether.
ANALYSIS
In addition to the report through "INDICATE", another operator
advised that GPWS equipped aircraft had experienced similar
warnings when conducting the decommissioned Australian DME arrival
procedure, which had a similar track and profile.
The safety value of GPWS is dependent upon the reaction of crew to
any warning from the system. The standard response to a GPWS
warning requires the crew to immediately initiate a climb. The
existence of nuisance warnings will diminish the effectiveness of
GPWS as it may result in crews ignoring warnings.
If the GPS Arrival track were displaced by 15 to 20 degrees to
avoid directly overflying Table Cape, the terrain profile would not
trigger unnecessary GPWS warnings.
BASI recommends that Airservices Australia modify the GPS
Arrival procedure for Wynyard so that the track does not require
aircraft to fly directly over the steeply rising terrain of Table
Cape.
I refer to your letter R980002 (B97/131) dated 13 January 1998
and the recommendation contained therein, that the GPS Arrival
procedure from position CAMUS to Wynyard be amended so that the
inbound track does not require aircraft to fly directly over the
steeply rising terrain of Table Cape.
Your proposal has been examined and indeed discussed with the
operator concerned, and it is agreed that your proposal is the only
viable option to overcome the GPWS alert problem on this instrument
approach. Accordingly, this approach will be redesigned into a
sector arrival thereby allowing GPWS equipped aircraft to track to
the east of Table Cape, thereby overcoming the unnecessary GPWS
warnings on this approach. The revised procedure will be published
in DAP-E AL 63, which becomes effective on 26 May 1998. An advanced
copy of the revised procedure will be made available to the CASA
Melbourne District Office as soon as it becomes available and they
have agreed to issue it to the affected operators for early
implementation.