The aircraft was being flown from Adelaide to Dubbo in accordance with instrument flight rules. At 1921 EST, the pilot reported over Mildura maintaining 9,000 ft, and estimating abeam Griffith at 2026 on a direct track to Dubbo.
The weather in the area was clear, with no restrictions to visibility, and scattered cloud at 30,000 ft. Sky conditions were dark, with no moon.
At 1958 the pilot made a routine frequency change, and 8 minutes later advised Melbourne Flight Service (FS) there had been a loss of vacuum and that he was returning to Mildura. In response to enquiries from FS the pilot advised that he had 'electric backups' and felt it safer to return to Mildura. He also confirmed his approach and landing would not be affected, and that his estimated time of arrival would be 2029. At 2007 FS asked the pilot for his approximate distance from Mildura. The pilot asked FS to repeat the request, but subsequently failed to reply. Further attempts to contact the pilot were unsuccessful. The last recorded radio transmission from the pilot was at 2007:49.
An uncertainty phase was declared after communication and ground checks failed to establish the location of the aircraft. A local resident reported seeing the lights of an aircraft shortly after 2000, and then hearing the sounds of an impact. The wreckage of the aircraft was subsequently found some hours later. The accident was non-survivable.
An examination of the wreckage indicated the aircraft had impacted the ground at high speed, in a steep nose-down attitude, consistent with loss of control. With the exception of the vacuum system, the aircraft was considered to have been capable of normal operation prior to impact.
The aircraft was equipped with an attitude indicator and a directional indicator, each reliant on air-driven gyroscopes. An electrically powered turn co-ordinator was also fitted. Examination of the attitude indicator showed evidence of a witness mark consistent with the gyro-rotor being stationary at impact. The turn coordinator gyro-rotor was recovered and showed evidence of rotation at the time of impact.
The engine-driven vacuum pump and drive coupling were recovered from the wreckage. The impact resulted in separation of the pump body from its base. Only a few large pieces of the rotor and vanes were recovered. The frangible drive shaft coupling had sheared at some time prior to impact. A specialist examination considered that either the carbon rotor, or one or more of its vanes, had failed, resulting in pump seizure and consequent shearing of the drive coupling.
An entry in the aircraft logbook indicated that the vacuum pump was installed in September 1991. From that date, until the last periodic inspection in August 1997, the pump had operated for some 1,248 hours. No evidence was found of vacuum pump replacement during that period.
The Mooney 20J Service and Maintenance Manual recommends that the schedule for the primary vacuum pump replacement be either on condition or at 500 hours, and at engine overhaul. The Civil Aviation Safety Authority provides no additional requirements regarding maintenance of the vacuum pump.
The pilot held a Private Pilot licence with a valid medical certificate. His command instrument rating had expired 3 days prior to the accident. Although no evidence could be found of the pilot having flown in instrument meteorological conditions in the previous 12 months, the pilot had conducted a night flight six weeks prior to the accident. A passenger on that flight reported they did not encounter cloud.
The circumstances of this accident were consistent with a loss of control by the pilot at night, resulting from inoperative attitude and directional indicators. The combination of a dark night, high level cloud, and limited ground lights, would have provided the pilot with few external visual cues. This would have required him to quickly modify his instrument scan to allow him to control the aircraft by sole reference to the remaining flight instruments. One of those instruments, the electric turn coordinator, was probably the 'electric backups' that the pilot referred to in his call to FS.
To achieve a desired flight performance, the aircraft is placed in a particular attitude, together with an appropriate power setting. Precise attitude information can either be gained by reference to the natural horizon, or to a gyro-stabilised attitude indicator, when external indications are either not available or are unreliable. The altimeter, air speed indicator and vertical speed indicator can, in combination, also provide limited attitude information.
The loss of vacuum to the air-driven gyroscopic flight instruments would have resulted in those instruments providing erroneous aircraft attitude and heading indications to the pilot. It is possible that the pilot did not mask the failed attitude and directional indicators. Consequently, the pilot may have inadvertently continued to respond, however briefly, to the erroneous indications from the failed instruments. His instrument scan proficiency, with the attitude indicator as the primary focus, would have been developed over several years. Such a scan could not easily have been modified to ignore the very powerful stimuli from erroneous attitude indications.
The pilot was dependent on alternative instruments for aircraft attitude information and it is likely that while attempting to control the aircraft as well as calculate flight time and distance to Mildura, the pilot became spatially disorientated and lost control.
- Vacuum pump failure during flight resulted in the loss of suction to the air-driven gyroscopic instruments.
- Erroneous attitude and heading indications from these inoperative flight instruments.
- Dark sky conditions with no discernible horizon.
- The pilot had limited recent experience flying in conditions of reduced external visual reference.
- Inability of the pilot to control the aircraft by sole reference to the remaining flight instruments.
Following a fatal accident involving a Cessna 310, the Bureau issued Interim Recommendation IR960059 on 21 October 1996. The recommendation stated "the Civil Aviation Safety Authority (CASA) ensure appropriate maintenance policies are developed for all general aviation aircraft pneumatic vacuum system components".
In its response to this recommendation, CASA undertook to prepare an article for inclusion in the journal Flight Safety Australia. This article, titled "The Silent Emergency", was published in the March 1998 issue and is an extract from the United States General Aviation Maintenance Administration's and Federal Aviation Administration's accident prevention program. The article deals with the hazards of flying in conditions of reduced visibility and encourages aircraft owners to consider the installation of a backup or standby pneumatic system for gyroscopic instruments.
CASA also undertook to prepare and issue an Airworthiness Advisory Circular (AAC 1-97). This AAC, issued on 21 May 1998, is titled "Functional Testing Aircraft Vacuum/Pressure Systems". The AAC highlights the manufacturer's recommended maintenance requirements for vacuum manifold systems.
AAC 1-98 titled "Dry Vacuum Pumps" has also been released. This AAC explains why dry vacuum pumps fail and outlines a pump replacement checklist. AAC 1-87 titled "Gyro Failures and How to Identify Early Failures" was also issued some time ago.
The Bureau identified a similar safety deficiency following this accident and will be investigating further aspects of vacuum pump maintenance.
|Date:||03 October 1997||Investigation status:||Completed|
|Time:||2008 hours EST|
|Location:||113 km NNE Balranald, Aero.|
|State:||New South Wales||Occurrence type:||Collision with terrain|
|Release date:||01 November 1998||Occurrence category:||Accident|
|Report status:||Final||Highest injury level:||Fatal|
|Aircraft manufacturer||Mooney Aircraft Corp|
|Type of operation||Private|
|Damage to aircraft||Destroyed|
|Departure point||Adelaide, SA|
|Departure time||1757 hours EST|
|Role||Class of licence||Hours on type||Hours total|