The Agusta/Bell 47G-2A1 helicopter departed from Maroochydore airport at about 1420 Eastern Standard Time (EST) on a solo navigation exercise. The pilot intended to track via Somerset Dam, Kenilworth, Nambour and return to Maroochydore under the Visual Flight Rules (VFR), with an expected enroute flight time of about 1.9 hours. A flight plan was not submitted to Airservices Australia by the pilot. However, a flight notification form was retained by the company for search and rescue (SAR) purposes. Shortly before take-off, the pilot was cleared by the Maroochydore Tower controller to track direct to Somerset Dam via The Big Pineapple initially at 1,500 ft above mean sea level (AMSL).
Air Traffic Services (ATS) primary radar intermittently tracked the helicopter at a position 7 NM northeast of the accident location about 40 minutes after departure. The primary "paint" ceased about that time and location. A witness reported seeing the helicopter near the northern side of Mount Archer at about 1515 EST and flying in a manner consistent with the pilot experiencing controllability difficulties. A subsequent aerial search located the wreckage at a position about 1 NM right of the direct track from Maroochydore to Somerset Dam and on the north-north-eastern slope of Mount Archer. The helicopter sustained severe impact damage. The pilot received fatal injuries.
Some notes containing pre-flight navigation planning calculations and small pieces of the perspex cockpit bubble were found several hundred metres before the accident site. The notes contained navigation calculations that did not take into account the forecast enroute winds. Personnel at the flight training school did not recall discussing at length the forecast weather conditions with the pilot and, in particular, they did not recall briefing the pilot about the forecast mountain waves prior to the navigation exercise. The personnel at the flight training school also reported that helicopter pilots had been flying throughout the day in the Maroochydore region without experiencing any controllability difficulties induced by the forecast and actual strong winds.
The ATSB investigation team did not attend the accident site but viewed video footage and police photographs of the wreckage. The video footage had been recorded by the search and rescue helicopter crew at the time the wreckage was located. Damage to the helicopter structure was extensive and the tail boom was severed. According to the search and rescue helicopter crew, the helicopter's emergency locator transmitter (ELT) did not activate. The pilot also carried a portable ELT but it was damaged during the impact and did not activate.
The pilot held a Student Pilot Licence and a Restricted Private Pilot (Aeroplane) Licence. At the time of the accident the pilot had accumulated a total of 72.5 flying hours in helicopters, including 21.5 hours on Bell 47G helicopters. The pilot's aeroplane flight time records were not available to the investigation.
At the time the accident report was compiled, the pilot toxicology and autopsy results were not available. Consequently, the investigation was unable to comment on whether the pilot's performance was adversely affected by any pre-existing physiological condition.
There were no known maintenance deficiencies and the helicopter was considered capable of normal flight prior to the accident.
A Bureau of Meteorology (BOM) area forecast, issued at 1338 EST on the day of the accident, indicated isolated severe turbulence and mountain waves below 9,000 ft. The BOM examination of the available data indicated that the wind between 1,000 ft and 5,000 ft above ground level (AGL) in the Mount Archer area was constant with height at about 250 degrees True in the range 25 to 30 kts. The surface wind speed was estimated to be around 15 to 20 kts with frequent gusts in the range 25 to 30 kts. The relative orientation of the ridge and wind direction were conducive to mountain waves and possible rotor effects (see Attachment A) to the northeast of Mount Archer. The helicopter impacted terrain on the north-north-eastern slope of Mount Archer. The search and rescue helicopter pilot's report of actual meteorological conditions in the vicinity of the accident site was consistent with the BOM forecast.
Initial video and photographic evidence indicated that the helicopter probably encountered severe turbulence from mountain waves or rotors in flight while approaching the lee of Mount Archer. The evidence suggested that the main rotor blades may have severed the tailboom approximately 1 m forward of the tail rotor assembly. This accident signature is consistent with excessive blade flapping. The evidence indicated that a divergence of the main rotor blade from its normal plane of rotation probably occurred as a result of severe turbulence generated by mountain wave or rotor activity, and a main rotor blade contact with the tailboom and cockpit area ensued, resulting in a loss of control of the helicopter.
It is also possible that the collective lever friction may have been overcome by the severe turbulence that caused the non-powered collective lever to suddenly drop. The collective lever drop would have induced a sudden nose down attitude and this may have caught the pilot by surprise. The pilot may have instinctively and rapidly applied aft cyclic to correct the aircraft's attitude. The rapid application of aft cyclic in this situation may have been sufficient to induce main rotor blade contact with the tailboom.
A further discussion of mountain wave phenomena is provided in Attachment A.
Mountain wave turbulence
Aviators need to be always aware of the wind and to seek to understand its potential effects and read the environment to appreciate and anticipate its effects on aircraft.
Wind effects around mountains and large features are the result of an interaction between the features, solar heating or cooling, mechanical turbulence caused by obstacles such as trees, and the ambient wind. The effects can be felt as anabatic and katabatic winds (resulting from solar heating and cooling), mountain waves, and rotors or eddies. Mountain waves and rotors are among the more hazardous phenomena aircraft can experience and understanding the dynamics of the wind is important to improving aviation safety.
Encounters with mountain waves can be sudden and catastrophic. Although glider pilots learn to use these mountain waves to their advantage, other aircraft have come to grief. Encounters have been described as similar to hitting a wall. In 1966, 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. In Australia, mountain waves are commonly experienced over and to the lee of mountain ranges in the southeast of the continent. They also often appear in the strong westerly wind flows Australia's east coast experiences in late winter and early spring.
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 `bounce' forms a series of standing waves downstream from the barrier and may extend for hundreds of kilometres; being felt over clear areas of land and open water. Formation of the mountain waves relies on several conditions. The atmosphere is usually stable and an inversion may exist. The wind needs to be blowing almost constantly within 30 degrees of perpendicular to the barrier at a minimum speed of about 20 to 25 knots at the ridgeline. Wind speed needs to also increase uniformly with height and remain in the same direction. Wave `crests' can be upwind or downwind from the range and their amplitude seems to vary with the vertical stability of the flow. The crests of the waves may, (depending on the air having sufficient moisture content), be identified by the formation of lens-shaped or lenticular clouds. Mountain waves may extend into the stratosphere and become more pronounced as height increases with U2 pilots reportedly experiencing mountain waves at 60,000 feet. The vertical airflow component of a standing wave may exceed 8,000 feet per minute.
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 within a rotor.
Many dangers lie in the effects of mountain waves and rotors 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 an aircraft's climb rate. 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 that, "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 barrier into wind also means that an aircraft's groundspeed would be reduced, remaining in an area of downdraft for longer. Flying downwind would likely put the aircraft in updraft as it approached rising ground. Rotors and turbulence may also affect low level flying operations near hills or even 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 implicated as a factor involved in the accident.
Research into mountain 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 normally forecast, many local factors may effect the formation of rotors and eddies. When planning a flight, the pilot needs to 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 formation of clouds (provided there is sufficient humidity to provide for cloud formation) and disturbances on water or wheat fields. Some considerations include allowing for the possibility of significant variations in the aircraft's altitude if up and downdraughts are encountered. A margin of at least the height of the hill or mountain from the surface should be allowed. 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 and rotors.
Bureau of Meteorology. (1988). Manual of meteorology part 2: Aviation meteorology. Canberra, ACT: Australian Government Publishing Service.
Bureau of Meteorology. (1991, September). Downslope winds are dangerous. BASI Journal, 9, 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.
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.
|Date:||29 August 2001||Investigation status:||Completed|
|Time:||1515 hours EST|
|Location:||Mount Archer||Investigation type:||Occurrence Investigation|
|State:||Queensland||Occurrence type:||Collision with terrain|
|Release date:||20 December 2001||Occurrence class:||Operational|
|Report status:||Final||Occurrence category:||Accident|
|Highest injury level:||Fatal|
|Aircraft manufacturer||Agusta, S.p.A, Construzioni Aeronautiche|
|Type of operation||Flying Training|
|Damage to aircraft||Destroyed|
|Departure point||Maroochydore, QLD|
|Departure time||1420 hours EST|
|Role||Class of licence||Hours on type||Hours total|