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Analysis

Summary

This occurrence is a CFIT accident resulting from inadvertent descent below the MDA on the final segment of a non-precision approach, fortunately without the catastrophic consequences normally associated with such events. The investigation was unable to conclusively determine why the aircraft descended below the MDA while in IMC, or why the descent continued until CFIT could no longer be avoided. However, the investigation identified a number of factors that influenced, or had the potential to influence, the development of the occurrence.

The pilot's workload during the approach was high, primarily due to hand flying of the instrument approach in IMC. A steeper than normal descent angle and higher than normal rate of descent, resulting from the delayed descent clearance from the ADC, added to that workload. While in IMC, with the absence of any alert at the MDA, the pilot relied on specific reference to the altimeter during his instrument scan to alert him to the proximity to the MDA and any descent below it. Although use of the altitude alerter on the final segment of the approach was not established, an alert from a unit configured with the MDA would not be expected until at least 300 ft below the MDA. As such, the altitude alerter was not effective as an immediate alert to descent below MDA.

Given the potential for downdraft activity from convective cells, the 'sinking feeling' experienced by the pilot and the 'falling sensation' experienced by the front seat passenger, indicate that the aircraft probably encountered a convective downdraft shortly before it impacted the surface. It was possible that the heavy rain also had an adverse effect on the sink rate of the aircraft. An absence of specific data prevented the investigation from quantifying the effect of the downdrafts and heavy rain on aircraft performance. Furthermore, the almost simultaneous application of power and the impact indicate that the go-around was initiated well below the MDA. It was therefore not possible to determine if the environmental forces were sufficient to overcome optimum aircraft performance.

In the context of high pilot workload and a high rate of descent probably compounded by downdraft activity, it is possible that, with an expectation of becoming visual before the MDA, the pilot may have become preoccupied with acquiring visual reference to the ground as he descended through 800 ft. As any increased scanning outside the cockpit reduces instrument scan time and therefore time to assimilate the significance of altitude information, the pilot may not have recognised that the descent was continuing unchecked. Had the pilot confirmed that the aircraft was maintaining level flight at the MDA prior to attempting visual acquisition, the risk of inadvertent descent may have been reduced.

The likely trajectory of the King Air over the restaurant indicates that the aircraft overflew the breakwater at sufficient height for any attached landing gear to clear it. Therefore, the left main landing gear was dislodged, and the right main landing gear was impacted, prior to the aircraft passing over the breakwater. Although the lack of impact marks on the tyres suggests impact with water, contact with an outlying reef could not be ruled out. Whatever the surface, the impact force probably had a similar effect to a 'bounced' landing, and helped impart sufficient upward momentum to the aircraft to allow it to clear the buildings on the breakwater as it overflew them in the go-around. Had the aircraft not bounced, it may not have had sufficient height to clear the breakwater, and as a result it could have impacted the buildings with catastrophic consequences. Additionally, had the aircraft not been in the go-around attitude and the nosewheel impacted the water, it may have resulted in the aircraft cartwheeling into the breakwater.

Failure of the landing gear and flaps to retract during the go-around distracted the pilot from the primary task of controlling the aircraft. When the pilot sensed the g-force produced by the inadvertent turn, the depicted attitude on the failed primary attitude indicator was not an accurate representation of the aircraft's attitude. This had the potential to induce loss of control at a low altitude. The pilot's prompt and effective response prevented uncontrolled flight.

CFIT in the approach and landing phases of flight is a well-known and well-researched risk for which there are formal assessment and prevention tools. The operator demonstrated an awareness of that risk, and a desire to treat that risk, by providing FSF CFIT training material to pilots and by conducting the CFIT checklist in February 2002. The operator reported that recommendations arising from the CFIT checklist were adopted by the aviation manager and check and training pilots. Notwithstanding, the investigation found that the ALAR task force recommendations had not been incorporated into the operator's standard operating procedures.

The absence of documented procedures regarding the use of the radio altimeter, flight director and autopilot in the conduct of GPS NPAs allowed for variability in pilot technique and did not provide assurance that the potential safety benefits of using that equipment would be optimised. Also absent was a definition of stabilised approaches and specification of associated go-around criteria in the operator's standard operating procedures; criteria considered by the ALAR task force to have the potential to reduce CFIT risk.

Notwithstanding, the pilot indicated that, had the radio altimeter been serviceable, he would have used it to alert him of proximity to the MDA. Given the demonstrated increased risk of approach-and-landing CFIT without an alert to proximity to MDA, the application of one or more defences to the conduct of the GPS NPA in IMC would have reduced the risk of CFIT. Possible defences included: limiting the rate of descent on final approach; nominating a higher MDA; utilising flight director or autopilot; and operating with two pilots.

Use of the flight director in conjunction with the altitude alerter after the final approach fix would have provided the pilot with an attitude indicator 'fly-up' command when the aircraft descended below MDA. As the primary reference in manual instrument flying is the attitude indicator, it is likely that the pilot would have noticed a 'fly-up' command earlier than recognition of the 'sinking feeling'. This may have prompted an earlier go-around or missed approach and avoided the surface contact.

Autopilots are commonly used to control an aircraft's track and descent profile during an instrument approach. Such use can reduce pilot workload and allows a pilot or crew to monitor the critical parameters of an approach. Had the pilot utilised the autopilot with the altitude capture function armed, after the final approach fix, it would have reduced the risk of inadvertent descent below the MDA.

Although the pilot was aware of stabilised approach criteria, such parameters were not defined in the operator's documentation, nor was a go-around policy specified for any exceedance of such parameters. With a lack of specific data, it is not known if there were any such exceedances during the approach. However, a specific missed approach policy increases the likelihood of a timely missed approach. Conduct of a holding pattern prior to commencement of the approach would have provided increased separation from traffic and probably allowed a more stabilised approach profile. Although the intent of the ATC approach clearance procedures was to prevent the imposition of altitude restrictions during an instrument approach in IMC, those instructions were not clearly defined in the MATS.

The aircraft was not fitted with a ground proximity warning system, nor was it required by regulation. The function of such systems is to prevent CFIT accidents. A ground proximity warning system may have provided the pilot with a more salient warning to enable him to take corrective action in time to avoid ground contact.

 
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