ATSB INVESTIGATION REPORT: Cessna 172N collision with water June 21 2010

The ATSB has released its final report on a fatal accident involving a Cessna 172N that crashed into Woolcunda Lake in NSW in mid-2010.

On the morning of June 21, 2010 the owner/pilot of a Cessna 172N, registered VH-UFN, departed from a private airstrip near Woolcunda Lake, NSW for a flight over his property. The aircraft did not return and was reported missing that afternoon. A search was initiated and two days later the aircraft was located in Woolcunda Lake.

The investigation identified that the aircraft impacted the lake in a left banking turn. Irrespective of the reason for being at low level, it is probable that the pilot misjudged the height of the aircraft above the lake’s surface, or was distracted at a height from which the pilot was unable to recover before impacting the water.

This investigation provides a timely reminder of the risk of flight over expanses of flat, featureless terrain or water. To better equip yourself to ensure this sort on incident doesn’t happen to you, be sure to read the ATSB’s Avoidable Accidents No. 1: Low-level flying report, which you can find by clicking here.

CAPTION: The lower wing surfaces show the hydraulic buckling damage. (ATSB)

History of the flight
At about 0915 Eastern Standard Time on June 21 the owner/pilot of VH-UFN departed on a private flight from an airstrip on the pilot’s property near Woolcunda Lake, NSW. The flight was to search for sheep that may have been missed from the previous day’s muster in a paddock to the north of the airstrip. The pilot had planned to return around midday.

A witness reported hearing the aircraft operating in a paddock to the north of the property’s homestead and, at about 1000, hearing the aircraft depart that paddock towards the south. When the aircraft did not return by midday, a local search was initiated. That search was unsuccessful and the aircraft was reported missing to authorities at about 1400.

The aircraft was located in the southern part of the lake at about 1030 on June 23. The aircraft was seriously damaged. Shortly after, the fatally injured pilot was located in the lake about 80 m north-west of the wreckage.

Pilot information
The pilot was issued an Unrestricted Private Pilot Licence (Aeroplanes) in 1978 and, at the time of the accident, held a PPL-A. The pilot was endorsed on single-engine aircraft of less than 5700 kg MTOW. He did not hold any other aircraft endorsements or permissions.

He had accumulated 2604 flight hours, of which 2571 were in command. It was reported that, of those flight hours, the majority were flown in VH-UFN. The pilot completed an aeroplane flight review on October 2, 2009. He held a valid Class 2 Medical Certificate with requirements that he wear distance correction spectacles when flying and have reading correction spectacles available. The pilot’s spectacles were recovered from the wreckage.

Between December 2009 and June 2010, the aircraft was in Broken Hill undergoing an engine and propeller overhaul. The pilot did not log any flights during that period. On 18 June 2010, the aircraft was delivered to the pilot’s residence following the overhaul. The owner/pilot flew the delivery pilot back to the maintenance facility that day and then flew home. The pilot conducted two more flights in VH-UFN before the accident flight, one on June 19 and the other on June 20.

Aircraft information
The aircraft was manufactured in the US in 1978 and registered in Australia in 1979. The pilot acquired the aircraft in 1983 and its maintenance release was valid until May 21, 2011.

The aircraft was fully refuelled in Broken Hill before the delivery flight on June 18. The accident occurred 5.3 hours from that last known refuelling. Using the known flight times for the subsequent flights, assumed flight profiles that were based on demonstrated pilot behaviour and fuel burns from the Pilot’s Operating Handbook (POH), between zero and 40 L of fuel was estimated on board the aircraft at the commencement of the mustering flight. The range of possible fuel remaining related to a 20 per cent error margin applied to the projected fuel burn.

Meteorological information
The nearest available Bureau of Meteorology surface observation sites to the accident location were at Broken Hill and Mildura, Victoria. The aerodrome weather reports for Broken Hill and Mildura Airports from 1000 to 1800 that day indicated that the winds were light and trending from the south to south-east, that visibility was greater than 10 km and that any cloud was at or higher than 3000ft AGL. The investigation concluded that the weather that day was not a factor.

Wreckage information
The wreckage was located in deep water towards the centreline of the lake. Initial inspection of the aircraft by police divers identified that, as a result of the impact with water, it had broken into three separate pieces as follows:

• Engine section: The engine was located in its mounts and affixed to the firewall. The firewall also formed the forward bulkhead of the cabin and had the aircraft’s instrument panel still attached. The firewall had detached from the fuselage at the forward cabin pillars, but remained connected to the cabin by the control yoke flight control cables.

• Cabin section of the fuselage/wing: The cabin’s forward bulkhead was detached from the cabin. The rudder pedals and associated nosewheel steering assembly remained attached to the cabin section of the fuselage. The forward and rear windshields were destroyed by the impact and pieces of the forward windshield were located in the rear of the cabin. The pilot’s cabin door was latched and jammed closed, while the passenger door was unlatched.

• Tailplane section: The tailplane and a section of the fuselage immediately aft of the cabin had detached, but also remained connected to the fuselage by flight control cables.

CAPTION: VH-UFN's engine, propeller and firewall assembly and instrument panel. (ATSB)

All structural components, except for the right wing aileron (which was not recovered) were contained in the wreckage. A small section of the right aileron and hinge remained attached to the wing. The damage associated with that section of the aileron was consistent with the aileron being torn from the wing by impact forces. Control cable continuity was established for all flight and engine controls.

During the recovery of the aircraft a fuel slick was evident on the surface of the lake. The wing fuel tanks were drained post recovery. They contained a significant amount of water; however, a small amount of fuel was recovered.

The leading edge of the left wingtip had been flattened and that wingtip’s fairing torn from the wing. This type of damage was isolated to the left wing only. The lower surfaces of both wings displayed significant water impact damage (hereafter referred to as hydraulic damage), which was particularly prevalent at the rear sections of each wing. The forward section of the left wing structure just outboard of the wing strut had been ruptured during the impact sequence. The upper wing surface immediately adjacent to the rupture was scored, with evidence of paint transference consistent with the orange engine cowl.
 
The aircraft’s nosewheel was detached from the wreckage and recovered with other flotsam on the lake shorefront to the north of the wreckage. The nosewheel mounting collar displayed evidence of over-stress failure in the longitudinal direction. The aircraft’s propeller showed evidence of rotation during the impact sequence. It could not be determined whether the propeller was producing thrust at that time. The tailplane section exhibited a clean tear of the fuselage skin on the left side, with tearing and buckling on the right side.

A number of aircraft components were removed from the wreckage for later examination. This examination determined that, at the time of impact, the aircraft’s flaps were retracted and that the fuel selector was in the BOTH position. The airspeed indicator was recovered intact and undamaged. It displayed a possible witness mark consistent with contact between the airspeed pointer and the instrument face as a result of impact forces. That contact mark was at 87 kts on the airspeed scale.

There was no evidence of mechanical failure of the aircraft’s engine or associated systems. There was no evidence of a birdstrike.

Survival aspects
The pilot normally carried a portable emergency locator beacon and a SPOT Satellite Messenger. The SPOT and ELB were recovered from the flotsam that washed up on the northern shore of the lake. Neither had been activated during or after the accident. The separation of the aircraft’s nose structure opened up the front of the cockpit and cabin area to the impact force of the water. Both front seats were forcibly detached from their mounting rails and driven rearward into the cabin area during the impact sequence.
The pilot’s seatbelt buckle was not fastened.

CAPTION: VH-UFN's left wing with its flattened leading edge and a rupture. (ATSB)

Analysis
The initial contact of the left wingtip with the water yawed the aircraft to the left, such that the subsequent impact was almost front on in a level or slightly nose-low attitude. That yaw and the aircraft’s forward momentum combined to detach the aircraft’s engine section from the fuselage, and resulted in the engine section impacting the left wing.

Two scenarios were considered that might explain the aircraft being so low that the wingtip contacted the surface of the water. Either the pilot ditched the aircraft in response to medical, mechanical or fuel related difficulties or, during intentional low-level flight, the wingtip contacted the surface of the lake.

Pilot-initiated ditching: The post-mortem examination did not identify any medical condition that may have resulted in pilot incapacitation, or that would have necessitated an urgent landing. Further, it would be unlikely that the pilot would have ditched the aircraft for a medical emergency when there was a landing strip, with which the pilot was familiar, about 3.5km away. The investigation discounted a medical event as a factor.

There was no evidence of a failure of the aircraft’s engine or other systems that would have caused the pilot to ditch the aircraft. While the right aileron was unable to be located, it probably detached from the wing as a result of the considerable hydraulic force applied to the rear of the wings during the impact sequence. The investigation concluded that aircraft/engine mechanical failure was not a factor.

Due to the unknown fuel state of the aircraft at commencement of the mustering flight, fuel starvation/exhaustion could not be positively excluded as a factor. However, the ready availability of drum fuel stock at the pilot’s residence and his reported meticulous care of the aircraft, suggested that he probably would have refuelled the aircraft prior to the mustering flight. That reduced the likelihood that fuel starvation/exhaustion was a factor.

A number of cockpit controls and the aircraft’s configuration were inconsistent with the aircraft manufacturer’s requirements for a ditching with or without engine power. In addition, the witness mark on the airspeed indicator at 87 kts indicated a speed well in excess of the recommended 55 to 65 kts ditching speed. Based on that excess speed, the aircraft’s angle of bank of at least 25° on contact with the water and the cockpit controls and aircraft configuration, the investigation concluded that the ditching was not deliberate.

The location of the wreckage was about 250m equidistant from the eastern and western shorelines. The aircraft’s glide potential was such that, had it been at or above 100 ft above the surface of the lake when an event occurred that necessitated an immediate ditching, the pilot could have ditched adjacent to the shoreline.

The apparent lack of any initiating cause that would lead to a ditching, the inconsistency of the flight profile with a ditching, and the location of the wreckage, all weighed considerably towards a conclusion that the aircraft was probably not ditched deliberately.

Low flying: Low-level flight over the lake that day could not be discounted. The return flight to Broken Hill below 500 ft AGL and the pilot’s implied lower limit for operations that was based on the 35 m (115 ft) high powerline indicated that the pilot probably engaged in routine flight below that required under Civil Aviation Regulation 157, and possibly down to about 100 ft AGL, at least while operating over his property.

Given the calm conditions on the day, it is probable that the lake’s surface was glassy, which had the potential to adversely impact on the pilot’s depth and height perception over the water. However, the numerous other visual cues available, such as dead trees and the shoreline, should have enabled visual height perception by the pilot, provided the pilot’s focus was not concentrated on the lake’s surface close to the aircraft.

If the pilot’s attention was focussed close to the aircraft, a shallow descent could have developed without the pilot being aware of any height loss until contact with the lake’s surface was unavoidable. While the aircraft’s attitude when the wing contacted the lake’s surface adds weight to this scenario, there was insufficient evidence to rule an unintentional descent into the water in or out as a contributing factor.

Irrespective of the reason for being at low level, it is probable that the pilot misjudged the height of the aircraft above the lake’s surface, or was distracted at a height from which the pilot was unable to recover the aircraft before impacting the water. The investigation did not identify any organisational or systemic issues that might adversely affect the future safety of aviation operations.

CAPTION: The upper surface of the left wing, showing the ruptured section and adjacent scoring and paint transference. (ATSB)

Findings
From the evidence available, it was found that while flying at low height, the pilot either misjudged the height of the aircraft above the lake’s surface or was distracted. This was found to be a contributing safety factor, but should not be read as apportioning blame or liability to any particular organisation or individual.

Sources of information
The sources of information during the investigation included:
- CASA
- Bureau of Meteorology
- Geoscience Australia
- United States Federal Aviation Administration
- NSW Coroner
- NSW Police Service