Safety Advisory Notice SAN19980116

Safety Advisory Notice issued to: Airservices Australia

Recommendation details
Output No: SAN19980116
Date issued: 31 July 1998
Safety action status: Closed



Between October 1995 and July 1997 the Bureau of Air Safety Investigation undertook a study of the safety of Australian regional airlines. The objectives of this BASI safety study were to:

(a) identify safety deficiencies affecting regional airline operations in Australia; and
(b) identify means of reducing the safety impact of these deficiencies.

For the purposes of the survey, regional airlines were grouped according to the number of passenger seats fitted to the largest aircraft operated by that airline in January 1997. The groups are defined as follows:

(a) group 1: 1-9 seats;
(b) group 2: 10-19 seats; and
(c) group 3: more than 20 seats.

The study involved analysing data obtained from:

(a) responses to a survey of Australian regional airline employees;
(b) discussions with Australian regional airline employees and managers;
(c) air safety occurrence reports involving regional airlines over a 10-year period (1986-1995) from the BASI database.

This Safety Advisory Notice addresses one of the safety deficiencies identified as a result of this study.


Although meeting all design criteria, some pilots consider that the intercept of the localiser from the 9NM arc (using distance measuring equipment) for the runway 21 instrument landing system (ILS) approach at Perth, is difficult to fly under some conditions and may lead to unstabilised approaches.

Survey Results

The ILS approach for runway 21 at Perth contains a procedure for intercepting the localiser from the 9NM arc (using distance measuring equipment). Four of the survey respondents commented that attempting to intercept the localiser from this arc is difficult, and may result in the aircraft flying well above glidepath before being established on the localiser. They stated that flying a stabilised approach from this position is difficult.

"[When flying the runway 21 ILS approach at Perth from the 9 DME arc, if we start turning] inbound at the [lead bearing from Caversham locater], the localiser will not be captured. As we cannot begin descent until established on the localiser, it is easy to get hung up at 2000 feet, particularly if a strong sea breeze is blowing, which in winter is common. Move the arc out to a minimum of 10 miles."

- Pilot, respondent 523.

Factual Information

A stabilised approach is a recognised method of conducting a final approach and landing so that flight crew can minimise the hazards associated with this critical stage of flight. The approach is flown so that certain flight parameters, such as rate of descent and pitch attitude, do not exceed defined safe limits. As the number of variables that need monitoring is reduced to a minimum, the crew can maintain complete awareness of the situation and environment that the aircraft is operating in. The stabilised approach also enables the crew to conduct standard pre-landing checks, thus ensuring that the aircraft is correctly configured for a successful landing. Any situation that leads to aircraft flying unstabilised approaches is a hazard as it can lead to an aircraft being incorrectly configured for landing or, in some cases, to the crew losing control of the aircraft.

As part of the approval procedure, all Australian instrument approaches are subject to flight validation prior to being published in the Departure and Approach Procedures handbook. Flight validations are normally carried out by the calibration aircraft currently contracted by Airservices Australia.


Survey respondents were concerned that any delay in commencement of the approach due to difficulties in establishing the aircraft on the localiser required unacceptably high rates of descent in order to correctly establish the aircraft on the glideslope. This difficulty appears to be common in certain, but well known, local conditions and increases the risk of an unstabilised approach.

Despite the calibration aircraft conducting a flight validation of all approach procedures before they are approved, this testing will not normally cover all seasonal variations in local conditions. Subsequently, difficulties in flying an approach, such as those described in the above example, may not be known to Airservices at the time the approach is published. The response to the Regional Airlines survey indicates that the runway 21 ILS approach at Perth is subject to seasonal variations which can make it difficult to fly. Therefore, a review of this approach should be conducted in order to fully assess the impact on safety of the strong sea breezes which are common in Perth during winter.

Output text

Airservices Australia should note the safety deficiency identified in this document and take appropriate action.

Initial response
Date issued: 24 August 1998
Response from: AirServices Australia
Response text:

I refer to your letter SAN980116 (B98/092) dated 30 July 1998, in which you raised the problem being experienced by some aircraft joining the Perth Runway 21 ILS or ILS/DME instrument approach procedure off a 9 DME arc.

As you rightly point out in your Safety Advisory Notice, the procedure in question meets all of the procedure design criteria. Whilst under PANS OPS a 7 DME arc may be used, the norm for Australian procedures is a 10 DME arc. However at Perth, due to the requirement to contain the procedure within the Perth CTR, a 9 DME arc had to be settled on. A 10 DME arc will infringe the RAAF Pearce military control zone; a situation which is unacceptable to the military.

A copy of the Safety Advisory Notice has been forwarded to our Perth ATS Centre. They will raise this issue within the appropriate local airspace consultative forum. As soon as additional airspace for the Perth CTR becomes available or alternative air traffic management procedures with the military are agreed to, the procedure will be redesigned with the aim of moving the approach arcs to 10 DME.

Last update 01 April 2011