The crew of an aircraft awaiting take-off clearance reported that a departing Boeing 737-400 struck its tailskid during rotation. That information was passed to the Tower Controller, however there was a delay of almost 12 minutes in relaying the advice to the flight crew while the runway was inspected to confirm that a tail strike had actually occurred.

The crew of the 737-400 reported that they had felt a thump during the rotation but believed it was the sound of an oleo leg bottoming. Shortly after the tailskid contacted the runway surface, the aircraft's Master Warning System (MWS) drew the crew's attention to faults in the hydraulic and flight control systems. The initial part of the climb was therefore taken up with the Quick Reference Handbook (QRH) Non-Normal Checklists concerned with the hydraulics problem, in addition to normal departure procedures.

Shortly after takeoff, the Cabin Services Manager informed the pilot in command that the rear cabin crew had heard a bump during during the takeoff. However, the pilot in command believed this was related to the hydraulic problem. The Cabin Services Manager later passed on additional information describing the sound as a "metallic scraping noise". At that time the aircraft was climbing through flight level 320 and was fully pressurised. The possibility of a tail strike was further indicated to the flight crew when they were informed by air traffic control (ATC) that there had been some pavement damage to the runway from which they had departed. The QRH procedure for a tail strike on takeoff was to ensure that the aircraft was not pressurised in case of possible structural damage. However, the procedure was reliant upon the flight crew being immediately aware that a tail strike had occurred. It did not give guidance on a course of action when the tail strike was not recognised until after the aircraft was pressurised. As it was then only 25 minutes before the aircraft would be commencing descent (at which time the depressurisation process would commence) and the pressurisation system was operating normally, the PIC elected to continue the flight to Adelaide.

Maintenance personnel inspected the aircraft on arrival and confirmed a tail strike. No damage was sustained by the tailskid except removal of paint. The hydraulic systems were also checked but no fault could be found. During the subsequent departure from Adelaide there was a recurrence of the hydraulic fault. Indications of Hydraulic System "A" low pressure and "Flight Control" low pressure were a repetition of the faults experienced on departure from Melbourne. It was subsequently determined that a hydraulic reservoir T-piece was clogged where it metered the "A" system and was the cause of the pressure fluctuations. Analysis of the Flight Data Recorder (FDR) data determined that the MWS activated in response to low pressure in hydraulic system "A" while the landing gear was in its retraction cycle and was not connected to the tail strike event.

Boeing data (Airliner, Jul-Sep 1994) suggested that the most common factors in takeoff tail strike events are excessive rotation rate and early rotation. Information from the operator's Flight Crew Training Manual indicated that takeoff and initial climb performance depend on rotating at the correct airspeed and proper rate, to the rotation target attitude. Rotation should be smooth and at an average pitch rate of 3 degrees per second. A body attitude of 9 to 10 degrees would be achieved in approximately 2.5 to 3 seconds with all engines operating, with liftoff occurring at a pitch attitude of 9.1 degrees. When the rotation rate exceeded 3 degrees per second, the minimum tail clearance decreases, and may result in contact with the ground. The minimum tailskid clearance on a normal "flaps 5" takeoff should be approximately 58 cm and occurs after liftoff. This is a consequence of the aircraft geometry and the dynamic forces that are acting after rotation has been initiated.

Analysis of FDR data for the incident indicated that rotation commenced at the correct indicated airspeed of 144 kts. The recorded data also showed that the aircraft had been subject to a rapid rotation during the liftoff period. From nose wheel off the ground to the main wheels leaving the ground, the average rotation rate was 4.3 degrees per second.

The incident occurred on the first sector of the day, the pilot in command was the handling pilot and was very experienced on the Boeing 737-400. The takeoff was conducted from runway 27 with a surface wind of 150-180 degrees at 5-12 kts. The aircraft was therefore subject to approximately 10 kts of crosswind and up to 3 kts of tailwind during the takeoff roll. Analysis of FDR data confirmed that during the 7 seconds prior to rotation the wind effect on the aircraft changed from 3 kts of headwind to 3 kts of tailwind.

The Beech Super King Air aircraft was maintaining flight level (FL)250 on an aerial ambulance flight, when members of the medical crew advised the pilot that they noticed an unusual burning odour in the cabin, similar to that of a bakery. When the smell became stronger, the pilot elected to return the aircraft to the maintenance facility at Jandakot.

A short time later, in addition to the cooking odour, an odour similar to hot plastic and rubber was smelt by the crew. The medical crew elected to don therapeutic oxygen masks and advised the pilot that the smell had become more intense and that they had now gone onto oxygen. The flight nurse observed that the doctor's complexion had changed to ashen gray and he was leaning against the cabin bulkhead with his eyes closed. The nurse then assisted the doctor to don his oxygen mask. The nurse said that the doctor was in a confused mental state. The nurse only recognised the seriousness of the situation "when the doctor's speech became slurred and was running his words together in their sentences". The nurse stated that his own symptoms manifested very quickly and he "felt quite euphoric and light headed". The nurse also estimated that by the time he donned his own oxygen mask, he was rapidly approaching unconsciousness. The pilot donned his oxygen mask and, when he feared he may be sick in his mask, initiated an emergency descent to 10,000 ft. During the descent he depressurised the cabin in an attempt to clear the fumes.

The crew reported that the fumes in the cabin had caused them to experience nausea and confusion soon after they detected the smell. They remained on oxygen for the rest of the flight.

The pilot reported that while inbound he had asked Air Traffic Services (ATS) to repeat instructions on several occasions and this prompted ATS to ask him to confirm that operations were normal. He did not recognise any of the landmarks that he usually used to identify his correct track while on approach to Jandakot. He stated that on that occasion his vision was affected to the extent that he had great difficulty focussing outside the cockpit. The pilot said that following the emergency descent to 10,000 ft, he engaged the autopilot and changed the GPS to the coordinates for Jandakot. The aircraft autopilot then flew the GPS guided track to abeam Jandakot from where the pilot took control and conducted the landing. He could not recall anything about the approach and landing, and later had to ask the flight nurse if he had used any flap, because the flaps were in the retracted position after landing and he did not remember retracting them. After landing, the crew's symptoms did not significantly improve and they were taken to hospital for medical assessment.

Several months after the occurrence, some members of the crew reported to the ATSB that they were still suffering various residual effects including headaches, elevated blood pressure, reduced concentration levels and anxiety. They attributed the symptoms to their exposure to the fumes encountered on the occurrence flight.

A maintenance investigation following the occurrence discovered several air conditioning system defects. A bleed air pressure-reducing valve in the under-floor cabin area was found to be leaking hot bleed air onto an adjacent air conditioning duct. There was also evidence that the insulation was heat affected and had discoloured from the bleed air leak. A "spirap" type loom bundling plastic tie was also found in the vicinity, which had been melted. Samples of those heat-affected items were taken by the ATSB and forwarded to a laboratory for testing, to identify if heat application produced any emissions. The results of that testing and subsequent instrumented flight test showed that the bleed air leak did not reach a sufficiently high enough temperature to be considered a source of hazardous fumes in this incident.

In addition to the faulty reducing valve, two air conditioning refrigerant leaks were detected in the forward air conditioning evaporator refrigerant pipes. Airconditioning compressor lubricating oil was also observed on the evaporator and dripping from a fractured flared fitting onto the surrounding structure. The fitting was located on a pipe that delivered high pressure liquid refrigerant to the expansion valve and evaporator. When the system refrigerant was replenished during the maintenance investigation, it was noted that a considerable quantity of refrigerant was required to refill the system. The initial refrigerant loss was estimated as at least 1.1 pounds or approximately 0.5 kg. As the lubricating oil level could not be determined with any accuracy, maintenance personnel then decided to totally evacuate the system and replenish refrigerant and oil levels from empty. The nature of the leak, troubleshooting and repairs precluded an accurate measurement of the total oil and refrigerant quantities that had escaped and the initial estimate would have been the minimum system loss incurred from the leak. The pipes and fittings to the evaporator were located in a confined space to which proper access for some maintenance activities was very difficult.

The vapour cycle type air conditioning system in the aircraft used a new environment-friendly refrigerant HFC-134a instead of the ozone depleting refrigerant type R12 that had been in service for many years. The design of the vapour cycle systems was such that a significant amount of the oil lubricant for the air conditioning compressor was in solution with the refrigerant. The polyol ester-based lubricating oil had a particular odour when heated, which was similar to the smell detected by the crew.

The Material Safety Data Sheet (MSDS) for HFC-134a stated, "overexposure can cause central nervous system depression with dizziness, confusion, incoordination, drowsiness or unconsciousness. Irregular heartbeat with a strange sensation in the chest, `heart thumping', apprehension, light-headedness, feeling of fainting, dizziness, weakness, sometimes progressing to loss of consciousness and death. Suffocation if air is displaced by the refrigerant vapours".

The MSDS for the polyol ester-based lubricating oil that was used in conjunction with the refrigerant charge stated that inhalation may cause nasal respiratory irritation and dizziness. The operator's engineering manager also stated that the lubricating oil was known to produce valeric acid when heated which was known to cause dizziness and nausea. He stated that prolonged exposure could lead to unconsciousness. The MSDS sheet supported that. In addition to the refrigerant charge of 104 ounces or almost three kg of HFC-134a, the air conditioning system was also charged with 34 ounces or almost one kg of polyol ester oil, of which approximately 26 ounces or almost 75% was held in solution with the refrigerant gas/liquid.

In addition to the refrigerant and oil chemicals, the airframe manufacturer had a maintenance warning that when moisture entered the air conditioning system, it could cause the formation of hydrofluoric acid or hydrochloric acids. The MSDS data for those compounds gave warnings of eye irritations and burns when exposed to the mist or vapours.

The Programme for Alternative Flurocarbon Toxicity (PAFT) found in 1987 that the HFC-134a refrigerant exhibited a very low risk and was considered to be "practically non-toxic". The PAFT tests stated that at very high concentration levels (over 50%), exposure could sensitise the heart to adrenaline and that it could cause irregular heartbeat, even death.

A 1998 University of New South Wales (UNSW) study on the use of HFC-134a in the confined space of motor vehicles concluded that the refrigerant posed a considerable risk to vehicle occupants. It cited two scientific reports in the United States and four scientific papers in peer reviewed journals describing adverse effects expected from human inhalation as all being "scientifically consistent". The study recommended that fresh air vents be kept open at all times when using the air conditioning system and to introduce a pungent odour producing element into the systems to aid in the early detection of refrigerant leaks.

The United Sates Environmental Protection Agency (USEPA) and the US Navy Bureau of Medicine commissioned research to determine safe levels of exposure to HFC-134a. The studies concluded with statements of significantly different safe exposure values. The US Navy-determined levels of acceptable exposure were significantly lower than the values assessed by the USEPA. The US Navy research into the exposure of humans to HFC-134a was conducted in a confined area to simulate the confines of a submarine.

Although there was research data available on HFC-134a and polyol ester-based lubricants, at the time of preparing this report, the ATSB could not find any data on the effects of exposure to HFC-134a at air pressures less than sea level. Similarly, the ATSB could not find data on exposure to a mix or "cocktail" of the two agents HFC-134a and polyol ester lubricants at air pressures at sea level or less than sea level.

The owner/builder of the the Vans RV-6A aircraft was conducting a flight from Townsville to Toowoomba. The aircraft departed Townsville at 0846 local time. The pilot subsequently contacted Oakey Approach at 1324, and the aircraft was identified on radar at 3500 feet. The pilot was instructed to maintain that altitude. When the aircraft was about 26 nautical miles from Toowoomba, the pilot transmitted a mayday distress message stating that the aircraft's engine had failed. A short time later he reported that a propeller blade had failed. No further transmissions were heard from the aircraft. About 30 minutes later, a searching helicopter located the wreckage in a flat clear area amongst hilly, tree covered terrain.

The area apparently chosen by the pilot for an emergency landing was about 300 m long and relatively flat. There were trees under the likely approach path of the aircraft and rising ground at the far end. The surrounding hills were above the level of the landing area.

Examination at the accident site revealed that the aircraft struck the ground while banked about 90 degrees left, and descending at an angle of about 34 degrees. A 200 mm section of one propeller blade tip had broken off and could not be located at the accident site. Witness marks indicated that the propeller was rotating under power when the blades contacted the ground. Aside from the broken tip, the propeller blades were in good condition. No other faults were found that might have prevented the aircraft from operating normally.

During the impact sequence, the section of the fuselage forward of the pilot's seat was deflected upward relative to the rear fuselage. The cockpit sides had buckled outwards. That resulted in the pilot striking the instrument panel, even though his shoulder harness remained fastened.

Aircraft information

The pilot purchased the aircraft in 1996 and he first flew it in 1998. At the time of the accident, the aircraft had accumulated 383 hours time in service.

The pilot fitted a new engine and propeller to the aircraft during construction. The engine was modified to improve and balance the airflow through the valves of each cylinder to enhance engine performance. In an apparent further attempt to improve engine performance, the pilot replaced one magneto with an electronic ignition system that was capable of varying the ignition timing in response to changes in engine RPM and manifold pressure. That variation contrasted with the fixed timing ignition provided by the other "standard" magneto fitted to the engine.

The aircraft's wing was a constant chord, low aspect ratio wing. A characteristic of low aspect ratio wings is high induced drag at low speed. Unless the pilot controls the speed carefully, the effect on aircraft performance can be a rapid speed loss and a high rate of descent. If such a situation arose during an approach to land, the only means of regaining speed and arresting the rate of descent would be to increase engine power. If little or no engine power was available, the outcome could range from a heavy landing to a loss of control of the aircraft.

Examination of the failed propeller blade

A fatigue crack had initiated near the leading edge of the blade 216 mm from the blade tip. Crack growth had occurred as a result of alternating thrust loads, and had propagated along the thrust face (rear surface) of the blade. The characteristics of the crack indicated that it had grown under constant amplitude loading. There was no evidence of flight by flight striations. The propeller material was of the correct type and no damage or other reason for the crack to initiate was found.

All propellers are subject to alternating thrust loads during normal operation. Propellers are designed so that those loads will not exceed the design value, thus preventing the development of fatigue cracks during operation. The firing of each cylinder in a reciprocating engine produces torsional vibrations. That means that the crankshaft momentarily speeds up at each firing stroke, and then slows down again prior to the next firing stroke. The vibration leads to alternating thrust loads in the propeller.

Examination of the engine connecting rod big-end bearings revealed distress on the bearing surfaces. That distress was indicative of firing loads exceeding the designed capacity of the bearing lubrication. For optimum operation of spark ignition engines, the peak pressure developed by the combustion of the fuel air mixture should occur approximately 15 degrees after the crank has located past top centre.

Ignition timing was a critical factor, influencing engine power, fuel economy, and the operating condition of the engine. Timing depended on the rate of propagation of the flame front through the fuel-air mixture. Increased or advanced ignition timing resulted in increased combustion chamber pressures. Magneto timing was fixed and was optimised for the operating range of the engine. If the response of the electronic ignition system to reductions in manifold pressure created by part throttle opening was to advance the timing of ignition, that could increase cylinder head pressures and increase the magnitude of torsional vibration.

Pilot information

The pilot, aged 64, held a New Zealand Private Pilot Licence and a current Class 2 Medical Certificate. The medical certificate was subject to three restrictions; the pilot was required to wear spectacles, he was granted an exemption regarding his hearing standard, and he was subject to a restriction regarding a drug he was taking for tinnitus (a ringing or similar sensation in the ears, due to disease of the auditory nerve).

The post-mortem examination of the pilot revealed that he had previously suffered at least one myocardial infarction and had coronary artery disease. Specialist examination of the pilot's electrocardiogram traces over a number of years did not reveal any pre-existing signs of a heart problem. Specialist medical opinion was that the possibility of the pilot suffering a heart attack induced by high stress levels after the propeller failed could not be excluded.

According to the pilot's logbook, he had a total flying experience of 1,179.6 hours at the time of the accident, of which 1,109.9 hours were in command. He had 383.2 hours on the accident aircraft, all of which were in command. On the day before the accident, the pilot had flown the aircraft from Auckland, New Zealand to Townsville, Queensland. That flight took 13 hours.