Greater Wellington Regional Council

The following findings were identified:
- The pilot did not make the necessary left turn during the take-off roll to align with the strip centreline, so the aeroplane continued the take-off roll in a straight line.
- The right-rear undercarriage struck uneven ground with sufficient force to break the undercarriage mounting brackets and dislodge the undercarriage assembly from the aeroplane.
- The right main undercarriage subsequently struck the right flap, resulting in a partial dislocation of the flap. This very likely resulted in the aeroplane becoming uncontrollable.
- The pilot was about as likely as not to have been incapacitated early in the take-off roll.

The following findings were identified:
Findings are listed in order of development and not in order of priority.
- The crew was appropriately licensed and fit to conduct the flight.
- The captain was an experienced company line-training captain, familiar with the aircraft and route.
- The co-pilot while new to the Convair 580 was, nevertheless, an experienced pilot and had flown the route earlier in the week.
- The aircraft had a valid Certificate of Airworthiness and was recorded as being serviceable for the flight.
- The estimated aircraft weight and balance were within limits at the time of the accident.
- With a serviceable weather radar the weather was suitable for the flight to proceed.
- The captain was the flying pilot for the flight from Christchurch to Palmerston North.
- The flight proceeded normally until the aircraft levelled after passing Paraparaumu NDB.
- Why the aircraft was levelled at about 14 400 ft was not determined, but could have been because of increasing or expected turbulence.
- The weather conditions at around the time of the accident were extreme.
- The aircraft descended through an area of forecast severe icing, which was probably beyond the capabilities of the aircraft anti-icing system to prevent ice build-up on the wings and tailplane.
- The crew was probably aware of the presence of icing but might not have been aware of the likely speed and the extent of ice accretion.
- The rate of ice accretion might have left insufficient time for the crew to react and prevent the aircraft stalling.
- The transponder transmissions were impaired probably due to ice build-up on the aerials.
- The aircraft probably stalled because of a rapid build-up of ice, pitching the aircraft nose down and probably disorientating the crew. This could have resulted from a tailplane stall.
- Although the aircraft controls were probably still functional in the descent, a very steep nose down attitude, high speed and a potentially stalled tailplane, made recovery very unlikely.
- Under a combination of high airspeed and G loading, the aircraft started to break-up in midair, probably at about 7000 ft.
- Although there was no evidence to support the possibility of a mechanical failure or other catastrophic event contributing to the accident, given the level of destruction to ZK-KFU and that some sections of the aircraft were not recovered, these possibilities cannot be fully ruled out.
- The crew of AF 642 not being advised of the presence of a new SIGMET concerning severe icing should not have affected the pilotsí general awareness of the conditions being encountered.
- Had the crew been aware of the new SIGMET it might have caused them to be more alert to icing.
- Pilots awareness of the presence of potentially hazardous conditions would be increased if other pilots commonly sent AIREPs when such conditions were encountered.
- Operatorsí manuals, especially for IFR operators, might contain inadequate and misleading information for flight in adverse weather conditions.
- The search for the aircraft and pilots was competently handled in adverse conditions.
- The regular mandatory checks of the CVR failed to show that it was not recording on all channels.
- The lack of any intra cockpit voice recordings hampered and prolonged the investigation.
- The DFDR data and available CVR recordings provided limited but valuable information for the investigation.
- Had more modern and capable recorders been installed on ZK-KFU, significantly more factual information would have be available for the investigation, thus enhancing the investigation and increasing the likelihood of finding a confirmed accident cause, rather than a probable one.
- Had suitable ULB tracking equipment been available, the finding of the wreckage and recovery of the recorders would have been completed more promptly.
- The lack of tracking equipment could have resulted in the recorders not being found, and possibly even the wreckage not being found had it been in deeper water.

Conclusions:
- The pilot was appropriately licensed, experienced and fit to carry out the series of flights.
- The aeroplane had been operating normally up to the time of the accident.
- An undetected fatigue crack, or series of cracks, had been propagating in the forward area of the tail fin for some time.
- The cracks developed to a point where the remaining structure was unable to accommodate normal flight loads, and the fin separated from the aircraft.
- The departure of the fin probably resulted in some uncommanded yawing and pitching effects, with accompanying control feedback and unusual sounds.
- The rudder became lodged in the tailplane upper surface as the fin departed, with the potential to restrict elevator control.
- The extent and duration of any elevator control restriction could not be determined.
- The resultant impact with terrain may have been an attempt by the pilot to carry out an immediate forced landing, or may have been a result of limited control available to the pilot.
- The impact and subsequent fire were not survivable.
- The fatigue cracking in the fin originated from cuts in the skin, made when the leading-edge abrasion strip was being trimmed in situ.
- It was not determined when and by whom the cuts were made, however, measures have been taken to prevent a recurrence.

Captain's omission to abort the simulated takeoff in time. The following factors were considered as contributing:
- High ground speed,
- Overheated brakes.

Loss of control during takeoff following the tailwheel failure.