Published: 7 December 2017
Sequence of events
On 23 October 2017, two pilots from Air Frontier were operating a Cessna C210L aircraft, registered VH-HWY (HWY), on a charter flight from Darwin Airport to Elcho Island, Northern Territory. The pilot in the left seat was the pilot in command, under supervision of the right seat pilot and the flight was operating under the visual flight rules (VFR). The pilots had submitted a flight plan to track via ‘VFR route No. 2’ to Castle Point (Figure 1) and then direct to Elcho Island.
Source: Airservices annotated by ATSB
The aircraft took off from runway 29 at about 1307 Central Standard Time. Recorded air traffic control (ATC) data showed that as the aircraft climbed through 700 ft, the pilot contacted ATC and was cleared to climb to 7,500 ft and turn onto a heading of 320° (items 1 and 2 in Figure 2). About 5 minutes later, the controller cleared the aircraft to turn right onto a heading of 100° (item 3 in Figure 2).
As the aircraft tracked east and passed through 6,100 ft on climb, the pilot requested clearance to divert 5 miles left or right of track due to weather (item 4 in Figure 2) and to climb to 9,500 ft. Air traffic control advised that left of track was unavailable due to the nearby active restricted airspace (Figure 1) and so was cleared to divert up to 5 NM right of track and to climb to 9,500 ft. Just over 2 minutes later, the controller cleared the aircraft to operate up to 10 NM right of track (item 6 on Figure 2).
At 1325, the aircraft turned north-east and continued to climb for another 4 minutes, to about 10,000 ft. At 1329, the controller recalled seeing the aircraft turn abruptly to the south west. The controller asked the pilot if they required alternate tracking (item 8 on Figure 2). The pilot replied ‘affirm’ and the controller cleared the aircraft to deviate up to 20 NM right of route. The aircraft continued to track south west.
Source: RAAF radar data overlaid on Google earth, annotated by ATSB
At 1332, the aircraft’s recorded groundspeed increased from 130 kt to 150 kt. Air traffic control radar recorded the aircraft descending and climbing between 9,600 ft and 10,100 ft (see the section titled Recorded data). At 1332:20 while at 10,100 ft and a recorded groundspeed of 100 kt, the aircraft’s altitude (radar mode ‘C’) disappeared from the radar display (item 9 in Figure 2). The controllers immediately assessed the absence of this line as abnormal.
About 10 seconds later, three short transmissions were recorded, resulting from separate ‘push-to-talk’ activations, likely from the aircraft’s radio. At 1332:45, the aircraft’s altitude (mode C) briefly reappeared, recording the aircraft at 5,100 ft and 70 kt groundspeed, and 15 seconds later the controllers reported that the aircraft disappeared from the radar screen. The controllers attempted to make radio contact with the pilot, but were unsuccessful.
Witnesses in the vicinity of Howard Springs (Figure 1) saw the aircraft descend rapidly in a relatively flat attitude with a portion of each wing missing. The main fuselage was found less than 1 NM from the last recorded radar position and both aircraft wings were located about 700 m south-east of that site.
Both pilots were fatally injured and the aircraft was destroyed.
Weather and environmental information
On the day of the occurrence, the environment was typical of the Northern Territory early wet season or ‘build‑up’, with unstable conditions, and showers and storms expected.
A thunderstorm to the north of Darwin, combined with the north-west sea breeze, triggered a convective cell to develop rapidly between 1300 and 1330 between Howard Springs and Koolpinyah (19 km to the north east). Based on the cloud top temperature, the top of the cell was around 6,000–7,000 ft at 1300, 9,000 ft at 1320, 13,000–14,000 ft at 1330, and around 14,000 ft at 1340. The developing cumulus clouds may have produced strong updrafts or downdrafts.
The air traffic controller and supervisor reported that their observations of the weather radar, using the Bureau of Meteorology internet website, indicated a cell (painted yellow, indicating rain) but not one that was indicative of a thunderstorm.
Witnesses reported seeing a large cumulus cell form over the Howard Springs area, which they described as a regular occurrence in the build-up season in Darwin. Some reported that the cloud went ‘very black’ at the time of the accident, and that starting about 10 minutes after the accident, it rained heavily for about an hour.
The aircraft was not equipped with a flight data recorder or cockpit voice recorder, nor was it required to be.
The aircraft’s altitude and groundspeed were recorded by the Darwin ATC radar for the last 6 minutes of the flight (Figure 3).
The aircraft’s airspeed was not recorded. The forecast wind at 10,000 ft was 10 kt from 190°, so the aircraft’s airspeed may have been up to 10 kt higher than the recorded groundspeed in the last few minutes of the flight. However, the actual airspeed cannot be accurately determined, given the likelihood of wind shear and turbulence in the air mass.
The manoeuvring speed was specified by the aircraft’s manufacturer as 118 kt at the aircraft’s maximum take-off weight, shown as a dotted line in Figure 3. At airspeeds above the manoeuvring speed, control inputs or turbulence may produce wing loading that can damage the aircraft’s structure. At airspeeds above about 145 kt, this loading can result in failure of the aircraft structure.
The graph shows that shortly after the aircraft climbed to 10,000 ft, the aircraft’s groundspeed exceeded the manoeuvring speed. The groundspeed remained above the manoeuvring speed, increasing to a maximum of 150 kt in the final minute of the flight. During the same timeframe, the aircraft’s recorded altitude varied between 9,700 ft and 10,000 ft, above the cleared altitude of 9,500 ft.
Source: RAAF radar data analysed by ATSB
The Cessna Aircraft Company 210L is a six-seat, high cantilever wing, single-engine aircraft equipped with retractable tricycle landing gear and was designed for general utility purposes. The aircraft was powered by a Teledyne Continental IO-550P engine.
HWY was manufactured in the United States in 1974 and was first registered in Australia in 1988. The aircraft was operated in the charter category.
In 2012 the Federal Aviation Administration (FAA) published Airworthiness Directive (AD) 2012‑10-04 Wing main spar lower cap inspection. This AD was applicable to HWY and required an inspection of the left and right wing lower main spar caps for cracks. Aircraft technical documentation identified this AD was completed in June 2012 with no defects found. During scheduled maintenance completed in March 2016, the wing main spar carry through was replaced with a serviceable item due to corrosion. HWY was then operated by Air Frontier and maintained under an approved system of maintenance from March 2017.
A periodic inspection of the aircraft was completed on 26 September 2017 and a new maintenance release was issued, which was still current at the time of the occurrence. In addition, a scheduled 50 hourly inspection was completed on 23 October 2017. The maintenance release was current at the time of the occurrence and it was reported there were no concerns with aircraft serviceability prior to departure from Darwin Airport. In addition, the pilots did not advise ATC of any aircraft-related issues.
Wreckage and impact information
Examination of the aircraft wreckage indicated that the aircraft impacted terrain from a vertical descent, right side slightly down, in an almost level attitude. The wings were located about 24 m apart and about 740 m south-southeast of the fuselage, consistent with an in-flight breakup (Figure 4). There was no evidence of fire. Various aircraft components were located between the fuselage and an area about 70 m beyond the wings, over 810 m in total.
Source: Google Earth, modified by ATSB
Both wings had separated between 0.5 and 1.5 m outboard from the wing-to-fuselage attachment. The wing spars had fractured in over-stress, and exhibited bending deformation consistent with forces acting upwards and rearwards on the wings. Examination of the wings showed no evidence of pre-existing defects.
On-site examination of the severely impact-damaged fuselage (Figure 5), engine and propeller did not identify any pre‑existing faults or anomalies with the aircraft that could have contributed to the accident. However, a number of aircraft components were retained for further examination and testing. The propeller did not exhibit any evidence of rotation at impact, consistent with fuel exhaustion resulting from the ruptured integral wing-fuel tanks.
Source: Northern Territory Police and ATSB, modified by ATSB
Both pilots were secured in their seats prior to impact. Notwithstanding the severe disruption to the airframe, examination identified both pilot seats were about mid-travel with one locator pin on each seat still engaged in the seat rails.
ATSB investigation AO-2011-160 involved a Cessna 210M aircraft, VH-WBZ, which broke up in flight. Although the precise circumstances were not known, a combination of aircraft airspeed with turbulence and/or control inputs generated stresses that exceeded the design limits of the aircraft structure.
The United States National Transportation Safety Board investigated seven in-flight breakups of Cessna 210 aircraft since 2000. All those occurrences involved flight into thunderstorms or associated turbulence, a loss of control following inadvertent flight into instrument meteorological conditions or a combination of both.
The investigation is continuing and will include examination of the following:
- recovered components and available electronic data
- aircraft and site survey data collected
- further interviews with a number of witnesses and involved parties
- weather conditions and its effect on the flight
- pilot qualifications and experience
- the aircraft’s maintenance and operational records
- the operator’s training and professional development programs
- previous research and similar occurrences.
The information contained in this web update is released in accordance with section 25 of the Transport Safety Investigation Act 2003 and is derived from the initial investigation of the occurrence. Readers are cautioned that new evidence will become available as the investigation progresses that will enhance the ATSB's understanding of the accident as outlined in this web update. As such, no analysis or findings are included in this update.
- Visual flight rules (VFR): a set of regulations that permit a pilot to operate an aircraft only in weather conditions generally clear enough to allow the pilot to see where the aircraft is going.
- Central Standard Time (CST): Coordinated Universal Time (UTC) + 9.5 hours.
- Where ‘the pilot’ is referenced in the sequence of events, the ATSB has not yet established whether it was the pilot in the left or the right seat making the radio calls.