SUBJECT - INSTRUMENT APPROACH DESIGN
INTRODUCTION - REGIONAL AIRLINES SAFETY STUDY
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
(a) identify safety deficiencies affecting regional airline
operations in Australia; and
(b) identify means of reducing the safety impact of these
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
(b) discussions with Australian regional airline employees and
(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.
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.
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
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.