Keeping Your Distance

Melbourne AirporWith 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.

Type: Educational Fact Sheet
Author(s): Sarah-Jane Crosby
Publication date: 12 October 2001
Last update 07 April 2014