Air Safety, DAFS RAAF

ACCIDENT REPORT: Caribou A4-264, Camden Aerodrome July 4, 1986

On 4 July 1986 Caribou A4-264 crashed at Camden Aerodrome while attempting to overshoot from a 40° flap STOL approach. The aircraft had been engaged in Army static line para drop sorties. The only injury from the crew of three and two Army dispatchers was one dispatcher who received minor head injuries. The aircraft suffered Category 5 damage.

History of Flight

This flight was the tenth in a series of uneventful paratrooping sorties and had been flown by the copilot from the left seat. On completion of the tenth drop the captain took control and briefed a 40° flap STOL landing onto runway 06. After completing the descent and before landing checks, the Caribou joined on a right base for runway 06 at about 1300ft. Power was reduced to 15 to 20 inches MAP, 30° flap set and the aircraft rolled out on finals at four and five hundred feet. The glide path was slightly steeper than normal. Forty degrees flap was selected at about 200ft AGL and 70 to 72 knots, aiming for a threshold speed of 62 knots. The captain realized the aircraft was drifting right of the centreline and attempted to correct with a left turn. Caution was used in applying bank due to the known poor handling characteristics of the Caribou with full flap extended. However, the aircraft continued to drift, resulting in a 20 to 30 degree intercept with the runway centreline being established. At about 100ft AGL and very close to the threshold the captain initiated a go-around.

Accident Sequence

A power setting of 50″ MAP was selected but the aircraft experienced a marked sink rate. A climb could not be established and the sink continued. Full available power was then selected by advancing both throttles to the forward gate. At about this time the right wing dropped to about 30° of bank. Full left aileron failed to raise the wing and the aircraft continued in this attitude for a short time. When impact was inevitable the captain selected both mixtures to idle cut-off.

The aircraft hit the ground right wing tip first at a low airspeed and 30° of bank. The right propeller then struck the ground; the nose gear failed; the aircraft slid through the perimeter fence; the left main gear failed; the left propeller struck the ground; and the aircraft pivoted to a stop heading 290°. The five occupants evacuated safely.

Investigation

The investigation of the aircraft was limited to establishing that it was serviceable and airworthy before impact. All damage was attributed to the post-impact sequence of events. The aircraft serviceability, attitude and performance at ground impact, as deduced from wreckage analysis, was consistent with crew and eye witness testimony.

Maintenance documentation indicated that the Caribou was serviceable. A post-R3 servicing had recently been completed and the aircraft rigged by the contractor and ‘fine-tune’ re-rigged by No38 Squadron. The accident captain flew part of the post-R3 test flight and the accident crew were satisfied with flying characteristics and serviceability.

Crew

The crew was fully qualified. The captain was an experienced Category B right hand seat pilot used as above average. The co-pilot, less experienced, was preparing to upgrade to captain. The flight engineer had recently graduated. The captain conducted a comprehensive briefing before departing on task. STOL currency and flying experience were more than adequate following two days of air drops at Camden.

The crew were all A,G,Z, (extremely medically fit) with no contributing adverse medical conditions. The only psychological factor was the copilot’s awareness of the difference in experience between himself and the captain. This led him to avoid stating any uneasiness during the approach.

Crashworthy Aspects

The mixtures being selected to cut off may have contributed to the absence of fire during the crash sequence. The forward section of the fuselage carries hydraulic and fuel lines and contains the cockpit heater and, normally, oxygen bottles. Damage was considerable to this area and, if oxygen bottles had been fitted, the danger of a fire or explosion would have been high. Although the G-forces were light, the flight compartment bottom hatch was obstructed and damaged by the nosewheel. All other exits functioned normally as did all seats and harnesses.

As a result of impact forces, the left inboard flap was driven inwards through the left wall of the fuselage, crossed the cabin and was arrested by the static line only 0. 3 metres from the right wall. It narrowly missed one dispatcher and gave the other dispatcher a light blow on the head.

The flight engineer had adopted the crash position sitting unrestrained on the lower escape hatch doors with his back against the front bulkhead. He was very lucky to escape injury as the doors buckled on impact.

During the crash sequence those on board were in considerable danger from unrestrained items. A hot water urn stowed on the flight compartment radio rack behind the access ladder moved into the area above the bottom hatch. The cargo loading strut was loose in the cargo compartment under a passenger seat. Heavy tie-down chains were released from the upper stowage compartments when velcro lid restraints failed.

Environment

Evidence indicates that very close to the time of the accident a rapid, low level wind change occurred from 360° at 5-10 knots to about 310° at 20 knots. Experienced local light aircraft pilots attested to severe turbulence near the threshold of runway 06 with north westerly winds in excess of 15 knots. Accentuating the turbulence are a ridgeline, a river valley and a dense line of 50′ high trees, all upwind of the threshold.

Flight Trials

Flight trials were requested when the investigation indicated that aircraft handling or performance may have contributed to the accident sequence. The Flight Manual was limited in its description _ of handling qualities in the 40° STOL configuration and, in particular, there was little information available on approach, overshoot and initial climb performance. Flight Manual information was not available to evaluate what effect 30° angle of bank would have on go-around, height loss and initial rate of climb.

The flight trials in STOL configuration did not reveal any unacceptable characteristics. The trial Caribou exhibited an apparent lateral static instability with a tendency to gradually roll in the opposite direction to an applied rudder input with ailerons neutral. The aircraft was spirally divergent but the divergence was easily overcome with aileron. A slight tendency to roll right during the application of full power was also noted. All these characteristics were easily overcome with aileron and the first two were not evident to the pilot during the controlled flight.

Lateral control was adequate in the trials. However, it was that coordinated rudder (eg left rudder, left aileron) the aileron deflection and force required to raise the wing in a 30° bank. This finding is consistent with the Flight Manual Warnings.

Conclusions

The aircraft probably experienced a sudden change in wind velocity about the time 40° flap was selected. The crew concentrated on regaining the centreline and that, coupled with the copilot’s resistance to communicate, resulted in a late overshoot decision. The overshoot was initiated into a problem area of severe turbulence which resulted in a sink rate and sustained right wing drop. Preoccupation with the tree line power application and the attempt to raise the. right wing may have distracted the captain from selecting the optimum pitch attitude. It is possible that left rudder was instinctively applied, to the detriment of roll recovery.

The accident was caused because the captain continued with the approach while significantly displaced from the centreline at or below 200′ AGL. The major contributing cause was the unexpected wind change, which resulted in shear and turbulence.

The included an insertion in the Flight Manual detailing the Caribou handling characteristics with 40° flap and direction on when to initiate a go-around. FLIP lnforma tion Publications should include a note warning of significant local wind effects at Camden which may be in variance with ATIS Information. Further recommendations included better restraint for the flight engineer and better stowage for the various items that must be carried and, finally, relocation of the oxygen bottles should be considered.