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.

In Australia, mountain waves are commonly experienced over and to the lee of mountain ranges in the south-east of the continent. They often appear in the strong westerly wind flows on the east coast in late winter and early spring.

Mountain waves are a different phenomena to the mechanical turbulence found in the lee of mountain ranges and can exist as a smooth undulating airflow or may contain clear air turbulence in the form of breaking waves and 'rotors'. Mountain waves are defined as 'severe' when the associated downdrafts exceed 600 ft/min and/or severe turbulence is observed or forecast.

'Breaking waves' and 'rotors' associated with mountain waves are among the more hazardous phenomenon that pilots can experience. Understanding the dynamics of the wind is important in improving aviation safety.

Glider pilots learn to use these mountain waves to their advantage; typically to gain altitude. However, some aircraft have come to grief in those conditions. Encounters have been described as similar to hitting a wall. In 1966, clear air turbulence associated with a mountain wave ripped apart a BOAC Boeing 707 while it flew near Mt. Fuji in Japan. In 1968, a Fairchild F-27B lost parts of its wings and empennage, and in 1992 a Douglas DC-8 lost an engine and wingtip in mountain wave encounters.

Mountain waves are the result of flowing air being forced to rise up the windward side of a mountain barrier, then as a result of certain atmospheric conditions, sinking down the leeward side. This perturbation develops into a series of standing waves downstream from the barrier and may extend for hundreds of kilometres over clear areas of land and open water.

Mountain waves are likely to form when the following atmospheric conditions are present:
• the wind flow at around ridge height is nearly perpendicular to the ridge line and at least 25 kts
• the wind speed increases with height
• there is a stable layer at around ridge height.

If the wave amplitude is large enough, then the waves become unstable and break, similar to the breaking waves seen in the surf. Within these 'breaking waves', the atmospheric flow becomes turbulent.

The crests of the waves may be identified by the formation of lenticular clouds (lens-shaped), if the air is sufficiently moist. Mountain waves may extend into the stratosphere and become more pronounced as height increases. Some pilots have reported mountain waves at 60,000 feet. The vertical airflow component of a standing wave may exceed 8,000 ft/min.

Rotors or eddies can also be found embedded in mountain waves. Formation of rotors can also occur as a result of down slope winds. Their formation usually occurs where wind speeds change in a wave or where friction slows the wind near to the ground. Often these rotors will be experienced as gusts or windshear. Clouds may also form on the up-flow side of a rotor and dissipate on the down-flow side if the air is sufficiently moist.

Many dangers lie in the effects of mountain waves and associated turbulence on aircraft performance and control. In addition to generating turbulence that has demonstrated sufficient ferocity to significantly damage aircraft or lead to loss of aircraft control, the more prevailing danger to aircraft in the lower levels in Australia seems to be the effect on the climb rate of an aircraft. General aviation aircraft rarely have performance capability sufficient to enable the pilot to overcome the effects of a severe downdraft generated by a mountain wave or the turbulence or windshear generated by a rotor. In 1996, three people were fatally injured when a Cessna 206 encountered lee (mountain) waves. The investigation report concluded, "It is probable that the maximum climb performance of the aircraft was not capable of overcoming the strong downdrafts in the area at the time".

Crossing a mountain barrier into wind also reduces the groundspeed of an aircraft and has the effect of keeping the aircraft in the area of downdraft for longer, while an aircraft flying downwind on the upwind side of a mountain range is likely to initially encounter updrafts as it approaches rising ground. Rotors and turbulence may also affect low level flying operations near hills or trees. In 1999, a Kawasaki KH-4 hit the surface of a lake during spraying operations at 30 feet. The lack of sufficient height to overcome the effects of wind eddies and turbulence was a factor in the accident.

Research into 'braking waves' and 'rotors' or eddies continues but there is no doubt that pilots need to be aware of the phenomenon and take appropriate precautions. Although mountain wave activity is usually forecast reasonably well by the Bureau of Meteorology, many local factors may effect the formation of 'breaking waves' and 'rotors'. When planning a flight, a pilot should take note of the winds and the terrain to assess the likelihood of waves and rotors. There may be telltale signs in flight, including the disturbances on water or wheat fields and the formation of clouds, provided there is sufficient moisture for cloud to form.

Prudent flight planning may include allowing for the possibility of significant variations in the aircrafts altitude if updrafts and downdraughts are encountered. A margin of at least the height of the hill or mountain from the surface should be allowed, and consideration given to the need to adopt a manoeuvring airspeed appropriate to the circumstances. Ultimately, it may be preferable for pilots to consider diverting or not flying, rather than risk flying near or over mountainous terrain in strong wind conditions conducive to mountain waves containing 'breaking waves' and 'rotors'.

Illustration of mountain wave and associated turbulence

Further Reading
Bureau of Meteorology. (2007). Manual of Aviation Meteorology. Second Edition, pp 59, 60, 68. Airservices Australia.

Bureau of Air Safety Investigation Journal. (1991, September). Downslope winds are dangerous. BASI Journal, 9, pp 38-39.

Jorgensen, K. (undated). Mountain flying: A guide to helicopter flying in mountainous and high altitude areas. Westcourt, QLD: Cranford Publications.

Lester, P. F. (1993). Turbulence: A new perspective for pilots. Englewood, CO: Jeppesen Sanderson.

McCann, Donald W. (2006). Diagnosing and forecasting aircraft turbulence with steepening mountain waves. National Weather Digest, pp 77-92.

New Zealand Civil Aviation Authority (2006), Good Aviation Practice, Mountain Flying booklet.

Welch, John, F. (Ed.). (1995). Van Sickles modern airmanship (7th Ed). New York, NY: McGraw-Hill.

Woods, R. H., & Sweginnis, R. W. (1995). Aircraft accident investigation. Casper, WY: Endeavor Books.

Revised: 29 October 2009.

ATSB's air safety investigator, Mike Watson, in his unique style, discusses the insidious dangers of carburettor icing.

The aircraft was on short final for runway 29L when the pilot made a brief Mayday call. The aircraft was then observed to land in a car-yard, short of the runway. Both occupants managed to evacuate without injury.

The pilot later reported that the engine did not respond when an increase in RPM was required, as the aircraft was undershooting the approach. The aircraft subsequently collided with a fence, short of the runway.

Weather conditions at the time were conducive to severe carburettor icing at descent power. It is likely that carburettor icing occurred during the low power descent and precluded the engine accelerating above idle power on the final approach.

If I were to stuff a gag forcibly down your throat, you would not be able to get air into your lungs, and after quite a short time, your body would stop working. The same is true of aircraft engines: if I were to block their air intakes, they would also stop working.

The easiest way to block an engines air intake is to freeze water and simply choke the engine, so that it can no longer breathe.

Can this happen to my aircraft? Yes. Let us look at how water can find its way into the air intake when we least expect it. To do so, we need to examine how water is carried in the atmosphere and how it can choke a carburettor.

Water is dissolved in the air that both we and our engines breathe, in much the same way as sugar can be dissolved into a cup of tea. It is much easier to dissolve sugar into a hot cup of tea than a cold cuppa, and likewise it is easier to dissolve more water in warm air than into cold air. Water that has been dissolved into the atmosphere is actually a gas that you cannot see, and it is always present in the atmosphere.

Let us take a hot cup of tea, stir in as much sugar as we can, and then put the cup in the fridge. Once the tea has chilled, you will see that some of the sugar is no longer dissolved in the tea but has formed crystals of sugar in the cup.

In the same way, if you take a cup of warm, humid air, (lots of water dissolved in it), and cool it, you will see that some of the water that was dissolved in the air as a gas will change back into a liquid. Normally, this can be seen is as tiny droplets like those found in a cloud. Many clouds are formed in exactly this way: as humid air rises and cools, it cannot hold all its dissolved water, and some of the water condenses into a cumulus-type cloud.

How will this affect the engine in your aircraft? When air passes through the carburettor on the way to the engine, fuel is evaporated into the carburettor. This chills the air, in just the same way as evaporating water chills a swimmer leaving the ocean for the beach. If this chilled air was previously humid, then some of the water dissolved in the air will immediately change into cloud-type water droplets. If the chilling effect of the fuel was sufficient to cool the carburettor below freezing level, then when these water droplets hit the sides of the venturi (the part where the air passes through), or the throttle valve, the water droplets will freeze in place. This will start the process of choking the engine. Eventually, if the process is allowed to continue, it will no longer be able to breathe, and the engine will stop.

The problem will be more pronounced if the engine is operating at a low power setting. In this case, the airflow through the carburettor will be partially impeded by the throttle valve. This valve not only provides more area for the ice to form: it also increases the partial vacuum downstream of the valve, and that will cause a further chilling of the air and the water droplets.

It is interesting to note that although fuel does act as a refrigerant in a carburettor, it is also needed to keep the engine running. When your aircraft is flying in cruise, the engine should normally be leaned with the mixture control. If this is not done, then not only are you using more fuel than you need to, but you are also putting more refrigerant into to the carburettor airflow, thus increasing the likelihood of carburettor icing. This is yet another good reason for using correct procedures when controlling the engine!

Even at temperatures exceeding 25 degrees Celsius, air passing through a carburettor may form ice that can choke your engine. The more humid the air in which your aircraft is flying, the more likely it is that ice will form in the air-intake system.

Following a normal climb, the pilot dropped two parachutists over Hamilton Island. A power-off descent to circuit height followed. The pilot did not select carburettor heat during the descent. When on a long final approach, the pilot attempted to arrest a high descent rate with the use of engine power. The engine failed to respond. The pilot found that the aircraft was outside gliding range of the runway. Engine trouble checks failed to restore power to the engine. The aircraft was ditched in shallow water and after a successful escape from the cabin, the pilot was picked up by an island launch.

Bureau of Meteorology data showed that the relative humidity at ground level was 65 per cent. A carburettor icing-probability chart showed that serious icing at descent power was to be expected at such a humidity level.

How do I recognise the start of this problem? The best solution is to be on the lookout for carburettor icing at any time the air temperature is less than 30 degrees Celsius. If an engine is being choked by ice, then its power will be reduced. However, this is not always easy to detect in the early stages, particularly if the engine is operating at reduced power settings or if the air is humid.

Application of carburettor heat for a short time will melt any ice, and when the carburettor heat is turned off again, you will see an increase in engine power for the same throttle setting. If this happens, then apply the carburettor heat, and leave it on!

Textron Lycoming, the engine manufacturer, point out that a pilot should expect a delay of 30 seconds to several minutes while ice is melted after carburettor heat is applied. During this time, rough running and a further reduction in power can be expected. It is much better to experience a small reduction in power because of the application of carburettor heat, than to experience a large reduction in power because of the engine being throttled by ice!

If you are flying a carburetted engine with a constant speed propeller, such as a Cessna 180 or 182, then you will not detect the onset of carburettor icing by a change in RPM. The manifold air pressure (MAP) is measured between the carburettor and the engine air inlets, so if the inlet is being blocked by ice and the engine is still trying to suck-in air, there will be an increased vacuum in the inlet manifold. This can be seen as a decrease in the manifold air pressure indication, when there is no other good reason for it happening.

It's a bit like your lungs being the engine, your lips the carburettor, and your cheeks the manifold air pressure gauge. If you breathe normally through your mouth past your lips, the air pressure in your mouth is nearly the same as atmospheric pressure. As an analogy, think of when your friendly neighbourhood murderer sneaks up behind you, and puts his hand over your lips in an attempt to suffocate you. He is doing the same to you as the block of ice in the carburettor is to the engine. There will be a significant vacuum in your mouth, (sucking in of the cheeks) as you desperately try to suck in your last breath, like your aeroplanes engine desperately trying to suck in the air it needs past the ice blockage to the carburettor.

Carburettor icing can sneak up on you when you are cruising along. In my case, I've found that can happen as dusk approaches, and the air cools, making the atmosphere more humid. It's always worth carefully setting the correct power setting, and noting it, so that if the RPM or the MAP starts to slowly reduce, and there's no other good reason for it, like climbing, then you can immediately suspect icing and do something about it. It's best to keep an eye out for it.

Carburettor icing a contributing factor? How does ATSB know if carburettor icing is a contributory factor of an accident? This is often difficult to answer because ice melts, it leaves no evidence. It is usually a case of elimination: if the engine is OK, there is plenty of fuel and all the controls are in the right place, then the investigators will look at weather conditions at the time. All we can usually say is that there was no other good reason for a loss of power. It always seems a shame to come across such a case, where everything was working fine, only to find that an aircraft has been downed for such an easily preventable phenomenon.

During a test flight, on short final approach, the aircraft encountered windshear. The engine failed to respond to throttle application. The aircraft landed heavily, ran into a fence and overturned.

Post-accident inspection of the engine did not reveal any mechanical reason for the lack of response to throttle application. Information from the Bureau of Meteorology showed that conditions were conducive to the formation of serious carburettor icing at any power setting. The pilot thought that because carby heat was only applied for about 10 seconds, carburettor ice was the only reasonable explanation for the loss of power.

Am I likely to experience carburettor icing? Provided with this article is a chart that will help you to work out the likelihood of experiencing icing, based on information from your forecast. You will need to find the temperature and dewpoint, and these can be found in a meteorological aviation report (METAR), or a SPECI, or a TTF type forecast. Plot the dew point depression against the temperature on the chart, and you will see an indication of the likelihood of experiencing carburettor icing.

Remember, note the air temperature: the most severe icing will occur at temperatures up to around 20 degrees Celsius, and the severity will decrease slowly as the temperature increases. The other major factor is the humidity in the air. If the air feels muggy, it is humid; if perspiration does not dry rapidly off your body, it is humid; if a breeze does not cool you on a warm day, it is humid.

When you are flying, remember that the air gets cooler with an increase in altitude, and this can increase the humidity. If you are flying near clouds, then the air is likely to be humid, (the relative humidity in a cloud is normally 100 per cent).

If you aren't sure, check for carburettor icing by applying full carburettor heat for a short while, and checking for an increase in power after it is removed.

Prevent carburettor icing at the first indication, rather than leave it until the engine is choked by ice!

How do I obtain a METAR? Use NAIPS, or the pilot briefing page on the website www.airservicesaustralia.com/(Opens in a new tab/window), but you will need to arrange yourself a username and password first. Some aerodromes have an aerodrome weather information service (AWIS), which is available from ERSA.

A full listing of METARs is also available, by State or Territory from the Bureau of Meteorology on the internet at Aerodrome Weather Reports (METAR/SPECI) (bom.gov.au)(Opens in a new tab/window) and using the user ID and password provided on that page.

In 1991 ATSB's predecessor (BASI) published a research report titled Limitations of the See-and-Avoid Principle. This report concluded that 'the see-and-avoid principle in the absence of traffic alerts is subject to serious limitations'. Unalerted see and avoid has a 'limited place as a last resort means of traffic separation at low closing speeds' and is 'completely unsuitable as a primary traffic separation method for scheduled services'.

Nevertheless, operations in a number of types of airspace currently require the application of see-and-avoid techniques by the pilots of both visual flight rules and instrument flight rules aircraft operations. In areas such as mandatory broadcast zones, pilots should be assisted by radio calls from all other aircraft to provide an 'alerted' see-and-avoid environment. However, the final level of protection is provided by pilots being able to see potentially dangerous traffic in time to take avoidance action.

The report highlighted the fact that 'many of the limitations of see-and-avoid are associated with physical limits and human perception' and encouraged pilots to be 'made aware of the limitations of the see-and-avoid procedure, particularly the factors which can reduce a pilot's effective visual field'.

Each year ATSB investigates incidents where aircraft have come perilously close whilst operating in weather conditions well above the visual meteorological conditions minima. Some of these incidents occur in the circuit area, where pilots should have had an acute awareness of the position of all traffic at all times. Incidents also occur where aircraft were established in an en-route cruise. Given that there indeed is a lot of sky out there, there is often an understandable tendency during the cruise to be less assiduous in maintaining a lookout. The following paragraphs address the issue of detecting other aircraft during an en-route cruise by examining some of the problems of lookout or visual

The Avgas contamination event that happened over Christmas 1999 caught everyone by surprise. It had not been seriously considered as a potential hazard to aviation anywhere in the world, therefore the consequences had not been considered. The reasons behind why the fuel became contaminated were unexpected. Mike Watson, one of a team of transport safety investigators who had the task of sifting through an overwhelming amount of data and publishing the final report, gives some insight.

No one was hurt as a result of contaminated aviation fuel, and there were no accidents that could be attributed to a loss of power caused by fuel contamination. At the time of the crisis the fuel refiner responded immediately and recalled all Avgas that had been manufactured at the refinery, and CASA grounded all Avgas powered aircraft that could have been contaminated until it was known that they were safe to fly.

The chemical contaminant is now known to have been ethylene diamine. At the time of the event, there was a concerted effort to define what the contaminant was (concentration in the Avgas was low); how the contaminant had got there; and what the contaminants behaviour would be in an aircraft fuel system.

In the initial response a method to guarantee aircraft would be safe again was developed, and a testing process to detect ethylene diamine was also developed in a number of weeks. Components for the test kits were sourced from all over the world.

The ATSB's investigation looked at what had happened. It looked at what could have prevented it from happening and why it didn't. It also looked at lessons that could be learnt and applied to other aviation systems. This included what would have happened if a similar contamination event occurred in a large turbine-engine passenger aircraft operating with contaminated jet fuel.

The main defence against any safety-critical system failure in an airliner is to have backup, or redundant, systems for any system that is essential for safe flight. The problem with fuel storage and supply systems in an aircraft is that they simply don't have a redundant backup. If fuel is contaminated, the contaminant will be supplied to all an aircraft's engines at the same time and could make them all unreliable at the same time.

As the primary defence of a redundant system isn't available to protect against the safety critical problem of fuel quality, we could reasonably expect there to have been a number of fuel quality related accidents in the recent past; however, that was not so. This can only be attributed to a highly reliable system for manufacture and distribution of aviation fuels, with a well-managed quality control processes.

Despite this, it is clear that complacency on the part of any group that has a responsibility towards maintaining fuel quality, be they refiner, distributor, regulator or consumer, can have catastrophic consequences.

This Avgas contamination event must be seen as a clarion call to highlight an aspect of the system of safe aviation that is more vulnerable to abuse or neglect than most other safety critical aviation systems.

Avgas contamination investigation report released

The Australian Transport Safety Bureau (ATSB) released its report on the contaminated aviation gasoline (Avgas) investigation at a media conference on 30 March 2001. The investigation followed the grounding in January 2000 of thousands of piston engine aircraft across eastern Australia when a black gunk was found in fuel systems.

The investigation found that a very small amount of an anti-corrosion chemical that was not removed in Mobil's Avgas refining process in late 1999, and not detected by the usual tests, led to the safety problem.

The ATSB made 24 separate recommendations as a result of its investigation that included recommended safety actions for Mobil Oil Australia, US and UK fuel standards bodies, the Civil Aviation Safety Authority, and other Australian regulatory organisations.

ATSB Executive Director Kym Bills told the media that the scale of the Avgas contamination was an unprecedented event anywhere in the world and was unexpected in such a mature industry as fuel refining. As a result, it caught the refiner and regulators by surprise and also revealed deficiencies in international fuel standards.

The investigation found that a temporary variation in the production process at Mobil's Altona refinery in late 1999 involving problems with reduced caustic wash and increased acid carry over, led to an increased dosage of an alkaline anti-corrosion chemical by a contractor. This was not totally removed from the final Avgas. The normal tests for the quality of Avgas did not pick up the very small concentration of the chemical contaminant in the Avgas that was sufficient to react with brass in aircraft fuel systems and form a black gunk that clogged them.

Mr Bills said it was not the ATSB's role to ascribe blame to any party. The task was to uncover the facts including all of the significant contributory factors (including weaknesses in defences), and then to publish findings and recommendations in a report.

Accordingly, it was important that relevant parties learnt from the identified safety deficiencies and acted promptly on the 24 recommendations made to reduce the chances of a recurrence, either with Avgas or jet fuel.

On 3 July 2009, the Australian Transport Safety Bureau (ATSB) was notified that a SAAB Aircraft Company 340B (SAAB), registered VH-ZLW, had commenced its take-off roll along the runway 25 left edge lights at Sydney Kingsford Smith Airport, New South Wales. This was one of three occurrences over the previous 2 years that involved aircraft commencing take-off on the runway edge lighting.

In addition, within the previous 2 years the ATSB investigated two other occurrences involving pilot misidentification of runway alignment cues or lack of those cues during take-off. All five Australian misaligned take-off and landing occurrences involved aircraft with weights greater than 5,700kg and three of the six occurrences involved scheduled regular passenger transport (RPT) operations. The remaining two occurrences involved charter operations.

This research investigation examined each of these occurrences and relevant international occurrences to identify the common factors associated with misaligned take-off and landing occurrences.

After reviewing the Australian and international occurrences, eight common factors were identified that increased the risk of a misaligned take-off or landing occurrence. The factors included: distraction or divided attention of the flight crew; confusing runway layout; displaced threshold or intersection departure; poor visibility or weather; air traffic control clearance/s issued during runway entry; no runway centreline lighting; flight crew fatigue; and recessed runway edge lighting.

A significant proportion of all occurrences reported to the Australian Transport Safety Bureau (ATSB) involve aircraft striking wildlife, especially birds. This report provides aviation birdstrike and animal strike occurrence data for the period 1 January 2002 to 31 December 2009. It also describes the results of an ATSB survey of aerodromes concerning current wildlife control measures.

Reported birdstrikes have been generally increasing since 2002. In 2009, there were 1,340 birdstrikes reported to the ATSB. For high-capacity aircraft operations, reported birdstrikes have doubled from 2002 to 2009. However, taking into account an increase in aircraft movements, this increase is modest and is probably accounted for by a generally improving reporting culture within this time.

Birdstrikes have increased for the period of study in every Australian state and territory. Queensland, New South Wales, the Northern Territory and Western Australia have the highest birdstrike rates. The higher birdstrike numbers for Queensland and the Northern Territory may be related to bird populations within the tropics, while New South Wales has the highest number of major aerodrome aircraft movements in Australia.

Most birdstrikes occur within the confines of aerodromes (less than 5 km). Major and regional towered aerodromes had significantly higher rates of reported birdstrikes than General Aviation Airport Procedures (GAAP) aerodromes, and had considerably increasing rates from 2002 to 2009. GAAP aerodrome birdstrike rates do not appear to have changed.

Engine ingestion makes up 11 per cent of all birdstrike occurrences in high-capacity air transport for the 8- year period, and the highest number of damaging birdstrikes occurs in high-capacity air transport. Birdstrikes causing multiple parts damaged were not common throughout the period. General aviation had the highest proportion of damaging birdstrikes, with almost 24 per cent of birdstrikes causing damage. Aeroplane wings and helicopter rotor blades are the most commonly damaged aircraft components across all operational types, particularly in general aviation. There have been eight occurrences from the period of 2002-2009 that have resulted in serious aircraft damage, and four that have resulted in injury.

The most common types of birds struck by aircraft were lapwings/plovers, bats/flying foxes, galahs, and kites. Not surprisingly, larger birds were more likely to result in aircraft damage.

Animal strikes were relatively rare. High-capacity air transport had the highest average with 11.5 animal strikes per year, with general aviation having the second highest average with 9.3 animal strikes per year. The most common animals involved in strikes were hares/rabbits, kangaroos, wallabies, and foxes/dogs. Damaging strikes mostly involved kangaroos, wallabies and livestock.

Bird hazard control at aerodromes was found to be mostly related to the control of grass height (short or long) and growing specific plants or grass, and the daily or weekly use or auditory deterrents, especially car horns and shotguns.

Forty-four per cent of all accidents and over half of fatal accidents between 1999 and 2008 were attributed to private operations. These figures far surpassed the proportions for any other flying category, even though private operations contributed to less than 15 per cent of the hours flown in that decade.

This report aims to identify the factors contributing to fatal accidents in private operations and how these factors differed from non-fatal accidents. This was achieved through exploring common occurrence types (what happened), contributing factors (why the accident happened), contributing pilot errors, and aircraft and pilot characteristics.

Three occurrence types accounted for the majority of fatal accidents: collision with terrain (90%); loss of control (44%); and wirestrikes (12%). When all incidents and accidents are taken into account, the likelihood of being killed was about 36 per cent for a collision with terrain occurrence, 30 per cent for loss of control occurrences, and about 50 per cent for a wirestrike. For non-fatal accidents, there was greater variability in the common occurrence types - forced landings, hard landings, problems with the landing gear, and total power loss/ engine failure were also common.

Problems with pilots' assessing and planning were identified as contributing factors in about half of fatal accidents in private operations, and about a quarter involved problems with aircraft handling. Other contributing factors associated with fatal accidents to a smaller extent were visibility, turbulence, pilot motivation and attitude, spatial disorientation, and monitoring and checking. Non-fatal accidents were just as likely to involve aircraft handling problems, but had fewer contributing factors than fatal accidents.

Action errors and decision errors were both common to fatal accidents. Violations, while less frequently found, were mostly associated with fatal accidents.

In light of the contributing factors that were associated with fatal accidents in private operations, the report provides advice to pilots for improving the odds of a safe flight. Pilots are encouraged to make decisions before the flight, continually assess the flight conditions (particularly weather conditions), evaluate the effectiveness of their plans, set personal minimums, assess their fitness to fly, set passenger expectations by making safety the primary goal, and to seek local knowledge of the route and destination as part of their pre-flight planning. Also, becoming familiar with the aircraft's systems, controls and limitations may alleviate poor aircraft handling during non-normal flight conditions. Finally, pilots need to be vigilant about following rules and regulations that are in place - they are there to trap errors made before and during flight. Violating these regulations only removes these 'safety buffers'.

The ATSB receives around 15,000 notifications of aviation occurrences each year; 8,000 of which are accidents, serious incidents and incidents. It is from the information provided in these notifications that the ATSB makes a decision on whether or not to investigate. While further information is sought in some cases to assist in making those decisions, resource constraints dictate that a significant amount of professional judgement needs to be exercised.

There are times when more detailed information about the circumstances of the occurrence would have allowed the ATSB to make a more informed decision both about whether to investigate at all and, if so, what necessary resources were required (investigation level). In addition, further publicly available information on accidents and serious incidents would increase safety awareness in the industry and enable improved research activities and analysis of safety trends, leading to more targeted safety education.

To enable this, the Chief Commissioner has established a small team to manage and process these factual investigations, the Level 5 Investigation Team. The primary objective of the team is to undertake limited-scope fact-gathering investigations, which result in a short summary report. The summary report is a compilation of the information the ATSB has gathered, sourced from individuals or organisations involved in the occurrences, on the circumstances surrounding the occurrence and what safety action may have been taken or identified as a result of the occurrence.

The summary reports detailed herein were compiled from information provided to the ATSB by individuals or organisations involved in an accident or serious incident between the period 1 April 2010 and 30 June 2010.

The aviation industry has been slow to acknowledge the risks associated with ground operations. While most occurrences on airport aprons and taxiways do not have consequences in terms of loss of life, they are often associated with aircraft damage, delays to passengers and avoidable financial costs to industry. The focus of this report is to examine ground occurrences involving high-capacity aircraft operations.

This report examines occurrences involving ground operations and foreign object debris that occur at Australian airports which receive high-capacity aircraft. It uses occurrence and investigation data reported to the Australian Transport Safety Bureau to create a picture of ground occurrences. This picture begins when an aircraft is being prepared for takeoff and ends when passengers and crew have disembarked from the aircraft. It explores contributing factors associated with each type of occurrence, with the objective of providing some insight into what happened and why various events occurred. The key to preventing ground occurrences appears to revolve around ensuring effective communication between pilots, ground crews and air traffic services through a process of checks and balances.