The cause of the accident?
With the benefit of 20/20 hindsight, the accident was clearly due to "pilot error". The Captain had not prepared adequately for the approach in several respects and the First Officer had failed in his duty to monitor him. These pilots had crashed a perfectly serviceable aircraft, as a direct result of their poor Crew Resource Management and disregard of multiple elements of their Standard Operating procedures.
Despite the lack of some facilities - map display, ILS Glideslope and ILS DME - sufficient information to conduct the approach safely remained available, but the crew did not make full use of it. So were they alone the cause of the accident?
Situational awareness.
The First Officer’s situational awareness was also impaired by his increased workload, particularly the need to obtain the detailed information on handling the flap abnormality, and then to assist the captain by providing him with specific height information. His situational awareness was better - he was concerned that “something wasn’t right” earlier than the Captain - but the F/O was unable to translate his concern into effective action. His primary concern was to help the Captain as a good co-pilot should - unfortunately at the expense of monitoring the overall situation.
Failure to follow SOPs.
One conclusion that could be reached is simply that the accident was caused by the failure of the crew to follow their SOPs. For example the SOP required that a detailed briefing be carried out for the approach being used. The Captain briefed for R/W 35, but actually used R/W 17. Most aspects of the briefing would have been identical, but the specific approach plate was not.
The Captain’s attempt to address this was interrupted by ATC, and he did not return to it, perhaps because he was confident that his experienced F/O did not need detailed instructions on interpreting it. The SOPs also advised that “In operationally complex situations, for example serious aircraft deficiencies generating high workload, the Captain should give consideration to delegating the flying to the co-pilot, in order to maximise his/her ability to give attention to the full effect of such situations on the safety of the flight”.
CRM and workload
Clearly in this case the workload became excessive, but the Captain did not follow this advice on effective Crew Resource Management. What generated the excessive workload and the consequent CRM failure? The Captain, although very experienced, was by no means a “domineering” type of person. In his briefing he made it clear that he wanted the F/O to vocalise any concerns.
Subsequently, when the F/O queried the Captain’s interpretation of the altitude clearance, he supported obtaining clarification, and thanked the F/O for pointing out the error. Later, he thanked him for prompting over the need to switch from map to raw data. Likewise, the F/O was quite experienced, and had no difficulty in querying the Captain’s action regarding the altitude, or prompting him when he detected an omission. The interpersonal CRM attitudes of the crew would be hard to fault.
The flight was running late, and the crew recognised that it would be desirable to retrieve some of the lost time. Although they were fully expecting to be able to depart before the extended curfew, any additional delay would result in a significant problem with an enforced overnight stop and all the subsequent cost and other penalties.
Mental models and assumed conditions.
Although he had planned for a full instrument approach, however, the Captain’s mental model of the situation was biased by the fact that the forecast was for relatively benign weather (10 km.+, cloud 1500’ scattered, 4000’ broken, temporarily visibility 3500m in haze, 700’ broken), and the received ATIS seemed to confirm this. In addition, he could clearly see places many miles away, from cruise altitude. His expectation was obviously that he would be able to transition to visual flight considerably above the Decision Height for the approach.
This was further reinforced by the fact that the only weather information that was actually current was passed to him in response to his query about the wind. The ATIS information did not reflect the actual conditions in the approach area. The Captain was familiar with the airport and its surrounding terrain, but the F/O had never been there before. So the F/O could be expected to regard the Captain’s judgment on what was safe in this particular location as being superior to his own, and to want to learn from the Captain in this regard.
Despite these factors reducing the apparent need for a "bad weather" arrival, the Captain did formulate a complete plan for an instrument approach down to a mile from the runway, that would have ensured a safe arrival if followed through. If the weather had been significantly worse, the accident probably would not have happened.
A change of plan
The first point where the plan was amended was when the crew were advised of the opposite traffic, and realised it had not used the runway stated on the ATIS. This led to the Captain discovering that the wind was not exactly as reported. That in turn led him to realise that he could not only reduce the time pressure due to the late operation, but also make some economic savings for the airline. By making a straight in approach, not only would the flight time be reduced by several minutes, but he could avoid the lengthy taxiing back from the far end of the runway, and turn almost straight onto the terminal ramp after landing.
At a range of some 50 miles or more from the airport, a change of runway should be well within the crew and aircraft capability, especially in essentially good weather, but it did require prompt action to adjust the descent profile, which was taken. However, this decision reduced the available time to complete the operation, and increased the crew workload significantly.
A first snag.
The second significant change then came when the crew attempted to extend the flaps whilst turning in to join the 17 ILS. This relatively minor technical defect had two major effects: it increased the F/O’s workload significantly, as although the checklist items were very limited, they required concentration on the detail of a complex chart, with which he was not very familiar. It also meant that the aircraft’s speed had to be maintained considerably higher than was desirable, further compressing the time-scale and increasing the Captain’s workload.
A second snag.
The Captain's workload then took another step up when, because the speed was high, the autopilot was unable to execute a good capture of the ILS localiser. The need to hand-fly from this point dramatically reduced the Captain’s “spare” capacity. His familiarity with the area meant that the combination of “recognised landmarks” and apparently “good weather” allowed him to be satisfied that it was safe to continue, because he was confident that he would not only have full ILS instrument guidance, but the visual information would also improve rapidly as the aircraft descended.
"Spare capacity" is lost.
However, because he now had so little spare mental capacity he was unable to consider some of the other facts which had been of relatively little significance earlier, such as the promulgated unreliability of the 17 ILS glideslope, and unserviceability of the 17 ILS DME. The F/O meanwhile had no particular reason to question the Captain’s judgement, especially since the Captain had proven right in stating (from experience) that the runway would be long enough for the flap condition, when the F/O had had difficulty in extracting this data under pressure.
The F/O saw his main task as assisting the Captain as much as he could and in ensuring that the landing checks were properly completed as commanded. The crew recognised the desirability of finding some substitute vertical guidance. Since the ILS plate also contained DME data, it was obvious that this would be the thing to use, particularly since it was after all only needed as a temporary supplement to the visual cues - the Captain thought he knew where he was, and expected to become visual any time after 4000 feet.
The only DME data on display confirmed this mental model, and corresponded with the values which had been given in the initial briefing, although they were for the opposite runway and the source changed in that approach from the STN DME to the 35 ILS DME. Nevertheless, other factors somehow caused the F/O to have doubts, which he vocalised when the Radio altimeter audio came on: but these were not strong enough to convince him that his judgment was better than that of the Captain.
After all, the Captain knew the area, could see the ground, and was both flying the aircraft and in command. Very soon after, both pilots became seriously alarmed that things were not working out as they had anticipated - but by then, it was too late. The immediate cause of the accident was the Captain's failure to recognise that he was now in exactly the type of complex situation where his workload had risen, to the point where his situational awareness and judgment had become dangerously impaired.
A vicious circle.
This was however inevitable, since the reason for this failure was the workload itself. The company SOPs recommended that in such circumstances, he actually should abandon the basic SOP, and delegate the flying so as to better retain overall command judgment. But they did not tell him how to recognise that situation, or avoid being led into it. The example given in the SOPs implied that high workload would arise in the event of serious aircraft deficiencies, which is true, but not comprehensive - it can arise from a wide variety of unpredictable sources.
The accident cause:
The accident enquiry determined that the probable cause of this accident was the flight crew's continuation of an unstabilized approach and their failure to monitor the aircraft's altitude during the approach, which led to an inadvertent descent below the minimum approach altitude and subsequently into terrain.
Contributory factors were
- the captain's failure to brief properly for the changed runway;
- the Captain's failure to recognise that the combination of non-normal conditions and shortened approach had created a high workload situation which warranted positive action to allow more time for consideration;
- the flight crew's failure to properly verify the approach plate and radio aids to be used;
- the flight crew's expectation that they would break out of the clouds well before arriving at the MDA;
- the first officer's failure to effectively monitor the Captain's execution of the approach.