- Shortly after departure from Port Keats aerodrome, the pilot lost control of the aircraft for reasons unknown.
- Aerodynamic loading of the left wing in excess of the ultimate load limit occurred, resulting in an inflight breakup of the airframe.
The investigation was unable to determine the circumstances that led to the loss of control and subsequent inflight break-up of the aircraft.
Structural failure
The wing separation from the aircraft was consistent with aerodynamic loads in excess of the aircraft structural limit during a high-speed or unusual attitude recovery manoeuvre. Those loads could have been imposed by either excessively high speed during the manoeuvre or a control input sufficiently abrupt to generate loads in excess of the wing load bearing capacity. In symmetrical flight conditions, both wings would have experienced identical flight loads and have failed in a symmetrical manner. Since only the left wing separated, it suggested that the wing load was not symmetrical or that the separation was triggered by sudden onset of aerodynamic load on the left wing only, such as would occur when aileron is applied to roll an aircraft.
Pilot distraction
Although the pilot was qualified to operate aircraft under Instrument Meteorological Conditions, his recent experience was limited and the night was dark. There would have been no visible horizon as the aircraft tracked over the sparsely populated countryside on a nearly moonless night. Under those conditions the pilot would have been required to monitor and control the attitude of the aircraft solely by reference to the flight instruments.
The possibility existed that during the climb, accomplished without the use of the autopilot, some unidentified fault or unexpected event diverted the pilot's attention from hand flying the aircraft. In those circumstances it would not have taken long for a spiral to develop and for the aircraft to rapidly increase airspeed and lose altitude. The low engine power at impact and the extended landing gear, support the theory that the pilot may have been attempting recovery from a spiral manoeuvre when the structural failure occurred.
Physiological factors
Although the pilot was well rested and of normal behaviour on the day, he had nevertheless been awake in excess of 15 hours at the time of the accident. Fatigue cannot be eliminated as a factor but there was insufficient evidence to draw any conclusions as to its significance. Given that an autopsy was not able to be conducted, the investigation was unable to exclude the possibility of pilot impairment or incapacitation.
History of the flight
The pilot had submitted a flight plan nominating a charter
category, single pilot, Instrument Flight Rules flight, from Darwin
to Port Keats and return. The Piper Aerostar 600A aircraft, with 6
Passengers on board, departed Darwin at 2014 Central Standard Time
and arrived at Port Keats at 2106 hours after an uneventful flight.
The passengers disembarked at Port Keats and the pilot prepared to
return to Darwin alone. At 2119 hours the pilot reported taxying
for runway 34 to Brisbane Flight Service. That was the last radio
contact with the aircraft. Witnesses noted nothing unusual as the
aircraft taxied and then took off from runway 34. As a departure
report was not received, a distress phase was declared and
subsequently a search was instigated.
The following morning a number of major structural components of
the aircraft, including the outer left wing, were located at a
position 24 km north-east of Port Keats aerodrome and close to the
aircraft's flight planned track. The main portion of wreckage was
found four days later, destroyed by ground impact. The impact
crater was located a considerable distance from the previously
located structural components and indicated that an inflight
breakup had occurred. The accident was not survivable.
Wreckage examination
A portion of the left wing, part of the left aileron, the cargo
door, a 1.3 metre section of the left horizontal stabiliser and the
right landing gear door skin were found remote from the main impact
area. The section of left wing had separated outboard of the engine
nacelle. All fracture surfaces and tear lines on the wing section
and the aileron were consistent with overload failure. No evidence
of pre-existing deficiency such as fatigue fracture or corrosion
was found.
The main body of the aircraft impacted the ground in a steep,
nose down, approximately wings level attitude with substantial
forward velocity on a generally southeasterly heading. Damage to
the structure was extreme with very little fuselage intact and
severe compression crush damage to the fuselage and wing
structures. There was no evidence of fire in the aircraft or its
components.
All control surfaces were found. Examination of the flight
control continuity was not possible due to the extent of the
aircraft damage. The flaps were found in the stowed position on
both wings. All landing gear components were found outside the
impact crater, indicating that the landing gear may have been
extended at the time of impact. Recovery of the right main landing
gear door skin remote from the main wreckage area was consistent
with the gear being extended in flight. The engines and propellers
remained attached to the airframe and were found at a depth of 1.5
metres and exhibited indications of low power settings at impact.
No pre-existing damage to the engines was evident. The
engine-driven vacuum pumps were examined and there was no evidence
of abnormal operation or failure of the pumps prior to impact.
Damage to the cockpit, flight and engine controls, instruments
and switches was extensive. It was not possible to determine the
selected position of controls and switches. Both attitude
indicators were recovered and examination determined that they had
both been operating at impact and indicated the same steep pitch
attitude of 65-70 degrees nose down. That pitch attitude
corresponded with observations at the accident site where the
inclination between the crater and the damaged foliage was measured
at approximately 60 degrees. Rotational scoring on the gyro
mechanisms and similarities in indicated pitch attitudes suggested
that the attitude indicators were most likely operating normally at
impact.
The aircraft had a valid Maintenance Release and there was no
maintenance outstanding at the time of the accident. The
investigation found no evidence to suggest that the aircraft was
not serviceable at the time of the accident.
Reconstruction of the flight
Calculations based on the aircraft's performance charts
determined that if the flight had progressed normally to a point
overhead the general location of the wreckage, the aircraft should
have been close to its flight planned cruise altitude of 10,000 ft.
Trajectory analysis of the wreckage distribution determined that
the aircraft broke up between 2,700 and 4,000 ft.
The aircraft was equipped with a serviceable autopilot. However,
a colleague who had flown with the pilot reported that the pilot's
routine was to leave the autopilot disengaged until the aircraft
was established in the cruise.
Pilot information
The pilot was aged 68 and the holder of a Commercial Pilot
Licence endorsed for the Piper Aerostar 600A aircraft. The licence
contained endorsement limitations that vision correction was
required and the licence was not valid for operations requiring an
Airline Transport Pilot Licence. The pilot held a valid Command
Instrument Rating and had passed a renewal test on 25 July 2000. He
had a total flying experience in excess of 15,000 hours of which
122 were in the Aerostar. Colleagues commented on his very
professional and careful approach to flying, describing him as
meticulous when it came to safety. His total flight time in the
previous 30 days was 13.1 hours of which 3.5 hours were at night
(including the accident flight). His most recent flight prior to
the day of the accident was 2 days earlier in a Piper PA-31
aircraft. It had been 16 days since he had flown the Aerostar. The
pilot held a Class 1 Medical Certificate valid until 13 October
2000. His last flight crew medical examination on 13 October 1999
revealed no deficiencies other than the continuing requirement to
wear prescription lenses.
Examination of the pilot's medical history revealed that in 1990
the pilot's risk factors for ischaemic heart disease required that
he undertake additional tests and monitoring. He was also placed on
medication for a raised serum lipid (high Cholesterol). Specialist
medical assessment found that while that condition was not
considered to be critical, the combination of age, raised serum
lipid and abnormal Stress Electrocardiogram increased the
possibility of coronary ischaemia developing with time. No autopsy
was able to be conducted.
Witnesses described the pilot as apparently healthy and in good
spirits on the day of the accident. The pilot was semi-retired and
he generally had a relaxed lifestyle. Enquiries revealed that his
normal sleep pattern was approximately 10:00 pm to 6:00 am and that
he slept well. In the 2 weeks prior to the accident most of his
time was spent renovating the boat on which he resided. He showed
no signs of fatigue, illness or injury. At the time of the accident
he had been on duty for approximately 4 hours.
Meteorological information, terrain features and visual
clues
An analysis of the meteorological conditions in the vicinity of
the crash site at 2125 hours indicated that there was no
significant weather. Light wind and clear skies were observed and
there was no indication of turbulence. One of the passengers on the
outward journey remarked that that it was a smooth flight. Moon
data for that evening indicated a waxing crescent with
approximately 20 percent of the moon's visible disc illuminated. At
the time of the accident the moon was 13.2 degrees above the
horizon at an azimuth of 265.3 degrees. The aircraft's flight
planned track was 037 degrees and therefore flying away from the
moon. Local witnesses reported the night as being very dark.
The area directly north of Port Keats aerodrome had few
prominent features apart from a small settlement that was vacant at
the time of the accident and was therefore probably unlit. During
the climb to cruise the natural horizon would have been obscured or
nonexistent. The view outside the cockpit in the prevailing
conditions of good visibility (in the meteorological sense) would
have been close to total darkness.
Manoeuvring limits
The operating limitations section in the Flight Manual for the
aircraft specified a maximum manoeuvring speed of 162 kts Indicated
Air Speed (IAS). At that speed, the aircraft's structure was
designed to withstand full symmetrical deflection of the flight
controls. That maximum safe manoeuvring speed decreased as the
aeroplane gross weight was reduced. The manoeuvring speed at the
estimated weight of the aircraft at the time of the accident was
calculated to have been 148 kts IAS. Allowing for a design safety
factor of 1.5, the ultimate load limit would have been exceeded at
181 kts IAS with a full, abrupt movement of the controls. The
normal cruise speed of the Aerostar was approximately 170 knots IAS
and 200 knots IAS was easily attainable in a cruise descent. Abrupt
control movements at any given airspeed can cause peak loads
significantly higher than those produced by steady inputs. A rapid
and large application of aileron control input at high speed in a
dive could produce torsional loading in a wing in excess of the
design strength. That loading could result in wing structural
failure consistent with the failure observed to the left wing.
Spiral instability
The Aerostar exhibits weak spiral stability. If, for example,
the pilot becomes distracted while hand flying the aircraft on
instruments, the aircraft may start to turn. The initial turning
tendency is usually small and difficult to detect, especially if
the pilot is not concentrating on instrument scanning. The nose of
the aircraft will drop, a spiral descent commences and the speed
will rapidly increase. Although the onset of a spiral dive is
usually insidious, with physiological clues not strong enough to
alert a distracted pilot, a well-developed spiral can rapidly
develop. Airspeed can quickly increase to the point where there is
much greater potential of exceeding the design G-loads. When denied
strong visual cues, inadvertent entry to a spiral dive can happen
even to highly experienced pilots. A pilot, confronted with a
high-speed spiral, is trained to slow the buildup of airspeed, roll
the aircraft to a wings-level attitude and then recover from the
dive. The first step would therefore be to ensure the throttles are
closed. Additional slowing, in aircraft like the Aerostar, can be
achieved by lowering the landing gear and moving the propeller
controls to a high RPM setting.
Human Physiology
Spatial disorientation is a situation in which a pilot fails to
sense correctly the position, motion or attitude of the aircraft.
It results from conflicting information from the pilot's senses,
primarily those of vision and balance. Alternatively, where there
are insufficient visual cues, the information from the sense of
balance is all that is available to determine orientation. The
sense of balance is extremely unreliable and, depending on the
circumstances of the flight, may provide erroneous information to
the pilot. If there is no visual means with which to crosscheck the
information from the balance senses, the pilot may be unaware that
it is in error. A pilot's perception of the aircraft's orientation
in space may thus be incorrect, and the pilot will not be aware
that this is so.
Fatigue can result in a number of potentially significant
performance decrements including increased reaction time, lowered
arousal, increased susceptibility to distraction, poor self
monitoring, and reversion to previously well learned skills.