With the
development of airborne collision avoidance systems (ACAS) and
their fitment in aircraft since the mid-nineties it has become
possible for pilots to know if their aircraft is on a collision
course with another.
When an ACAS warning is received the pilot or crew has time to
take avoiding action. Some of the systems fitted in aircraft today
will advise what to do - climb or descend away from a conflicting
aircraft. Future developments will also give turn advice.
The effectiveness of ACAS is totally dependent on the presence
of an operating Mode C or Mode S (altitude encoding) transponder in
the intruding aircraft.
ACAS can be active or passive
The two most common ACAS systems are:
- TCAS: The Traffic Alert and Collision
Avoidances System (TCAS), depending on its level of sophistication,
can give three levels of warning. Traffic information, where it can
'see' traffic; a traffic advisory where aural or visual warnings
will alert to the possibility of conflict; and a resolution
advisory, an aural alarm which will alert to impact in 20
seconds. - TCAD: A Traffic and Collision Alert Device
(TCAD) is a passive system that requires a third party to provide
the response from transponders. It will identify a target aircraft
if it has a transponder, which is turned on, and if an independent
activator, such as a ground-based radar or an airborne active
system like TCAS, has activated that transponder.
When it works well
In the following incident outside controlled airspace, the
system worked and two aircraft avoided further miss-hap. It is also
a good example of the enhanced 'safety net' because air traffic
services did not fully appreciate the unusually busy and complex
traffic disposition. On 27 January 1999 an Instrument Flight Rules
(IFR) Airtrainer departed Moruya for Tamworth climbing to 6,000ft
and estimated Bindook at 1842. An IFR Aerostar departed Young for
Bankstown on climb to 7,000ft.
Neither aircraft was provided with traffic information when the
Airtrainer elected to climb to 7,000ft which would put it into
conflict with the Aerostar north of Bindook.
The crew of the Airtrainer subsequently reported having passed
Bindook at 1843 leaving 6,000ft for 7,000ft. The pilot of the
Aerostar reported passing abeam Bindook at 1845 maintaining
7,000ft.
The Airtrainer crew, by this time in Instrument Meteorological
Conditions, had a TCAD alert, which indicated an impending conflict
with another aircraft at 7,000ft about 10NM north of Bindook.
The pilot initiated a rapid descent to 5,500ft and turned away
from the unknown traffic. The ATSB's investigation and radar
analysis determined that at the time of the TCAD alert the aircraft
were within 3NM of each other and closing with only 100ft vertical
separation.
In another example, the incident again highlights the advantage
to pilots of increased situational awareness while outside
controlled airspace. On 5 May 1999 while on descent to Proserpine
in class G airspace, the crew of a BAe 146 received a TCAS traffic
advisory on a slower aircraft below and ahead of them. Although
transmissions were made they were unable to make radio contact with
the aircraft. The crew used the TCAS information to take
appropriate avoiding action.
When things go wrong
Near Port Hedland on 23 November 1999 a Visual Flight Rules
(VFR) aircraft outside controlled airspace passed a Twin Otter
within an estimated 20 - 50 feet. The pilot chose to cruise the
Cessna 310 at an IFR level in class G airspace, and not being
subject to a directed traffic information service, no-one knew he
was there.
There was no time for the Twin Otter crew to take evasive
action. While maintaining the same IFR cruising level they only saw
the other aircraft when it passed them travelling in the opposite
direction. The aircraft was not fitted with an ACAS.
A similar incident took place inside controlled airspace. On 25
May 1999 a Boeing 737 inbound in cloud to Hamilton Island was
conducting a VOR/DME instrument approach while a Cessna 182 was on
climb from Shute Harbour for a parachute drop at 10,000ft. When the
B737 was established on final and visual, the pilot and
parachutists in the C182 sighted it in a left banking turn in their
two o'clock position at the same level with less than 100m of
lateral separation.
According to the ATSB report the B737 crew were unaware of the
near mid-air collision with the Cessna. 'The TCAS did not alert
them because the Cessna's transponder was turned off. According to
the Aeronautical Publication (AIP) Australian ENR 1.6-8, the pilot
of the Cessna was required to have activated the transponder on the
selected code 1200,' the report said.
The message then, is pilots need to turn their transponders on
in whatever airspace they are flying.
Increase in proximity warnings
Between 1 January 1993 and 19 September 1994, 47 near misses
were reported to the ATSB where two or more Regular Public
Transport (RPT) aircraft were involved.
TCAS Resolution Advisory occurrences
Between 19 September 1994 and 25 May 1995 there were 47
occurrences in controlled airspace where an infringement of
separation standards involving aircraft not equipped with TCAS
occurred. In addition there were 10 cases where TCAS was fitted and
had activated and assisted crews in their decision-making.
In the same period 29 occurrences were reported outside
controlled airspace where an ACAS was not fitted, and in the ATSB's
opinion, an ACAS could have assisted in situations where aircraft
came into conflict.
Air Safety Interim Recommendation IR19950117 of 4 May 1995 said,
'The fitment of a TCAD in some general aviation aircraft had led to
three alert situations outside controlled airspace.
'In two of them, the other aircraft was sighted and avoiding
action was taken. In all three cases the installation of the TCAD
improved the options of the pilots and gave them timely advice for
avoiding a potential near-miss.'
Since 1995 the ATSB has received almost two thousand reports of
events where the proximity to another aircraft was considered to be
a hazard. It has investigated more than 350 occurrences in all
classifications of airspace where it considers that ACAS (or would
have if fitted in the aircraft) significantly improved situational
awareness for flight crews.
By the year 2000 occurrences where an ACAS would improve
situational awareness outside controlled airspace had increased to
40 compared to 60 inside controlled airspace. In 1997 there were
only two reported ACAS occurrences outside controlled airspace. The
increase is mostly due to the fitment of ACAS in aircraft that were
previously not equipped.
"The result is that crews of aircraft today have the ability to
'see' other aircraft. Consequently occurrences reported today were
generally not known about prior to 1997. It also shows that a
potential collision has always existed outside controlled
airspace," said Bernie Rodgers, one of the ATSB Senior Transport
Safety Investigators tasked with analysing air safety incident
reports.
VCAs: a reality
Violations of controlled airspace (aircraft entering controlled
airspace without a clearance) continue to occur in significant
numbers every year. Sometimes air traffic control has been
initially unaware of it.
Violations of controlled airspace 1993-2000
"An aircraft that inadvertently enters controlled airspace with
its transponder on is more likely to avoid conflicting with a fully
loaded passenger aircraft, which is fitted with ACAS. The frequency
of VCAs has not diminished so it is reasonable to assume they will
continue," said Mr Rodgers.
ACAS has already proven its worth in a VCA situation as the crew
of a Boeing 737 discovered. When on approach to Melbourne at
3,000ft on 26 July 1999 they received a TCAS traffic advisory on an
aircraft that had infringed controlled airspace at 2,300ft. The air
traffic controller had not noticed the intrusion.
The crew were able to use the information provided by TCAS to
sight the aircraft and maintain visual separation until they were
clear.
The future
It is apparent given the increasing numbers of reported
conflicts since the mid-nineties that near misses both inside and
outside controlled airspace do occur and more often than previously
thought.
This reinforces the earlier Safety Advisory Notice (SAN 941261)
issued on 30 September 1994 to the former Civil Aviation Authority
suggesting a timetable be introduced to mandate the fitment and use
of ACAS equipment.
On September 1997 Australia was party to a regional agreement
that ACAS would be fitted to all turbine-powered aircraft above
15,000kgs maximum takeoff weight with more than 30 passenger seats
effective from 1 January 2000.
The International Civil Aviation Organisation (ICAO) has set the
following time frame for introduction of ACAS 11 to aircraft
engaged in international operations:
- 1 January 2003 for all turbine-powered aircraft with a maximum
certified takeoff mass in excess of 15,000kgs and more than 30
passenger seats; and - from 1 January 2005 for all aircraft in excess of 5,700kgs
takeoff mass and more than 19 passenger seats.
The ATSB will continue to monitor developments as the Civil
Aviation Safety Authority considers what further actions are
necessary to increase the effectiveness of ACAS in Australian
airspace.
Will you make sure that your transponder is turned on next time
you fly?
Melbourne Airport photograph by Leigh Atkinson, courtesy of
Airservices Australia, used in head
photo-illustration.