Iberia - Lineas Aéreas de Espana

The cause of the accident was the unknown incursion of the DC-9 on the runway 01/19 when the Boeing 727 was on takeoff roll. The DC-9 was on the active runway because visibility conditions due to fog, by the zone where the aircraft was taxiing, impeded to the crew obtain sufficient visual references, to determine that, that was not the correct run that they should realize to reach the threshold of runway 01.

Wrong approach configuration on part of the flying crew who continued the approach above the glide and landed too far down the runway, reducing the braking distance available. The braking action was also limited because the runway surface was wet, which was considered as a contributing factor.

The captain did not recognize, and may have been unable to recognize, an increased rate of descent in time to arrest it before the aircraft struck the approach light piers. The increased rate of descent was induced by an encounter with a low-level wind shear at a critical point in the landing approach where he was transitioning from automatic flight control under instrument flight conditions to manual flight control with visual references. The captain's ability to detect and arrest the increased rate of descent was adversely affected by a lack of information as to the existence of the wind shear and the marginal visual cues available. The minimal DC-10 wheel clearance above the approach lights and the runway threshold afforded by the ILS glide slope made the response time critical and, under the circumstances, produced a situation wherein a pilot's ability to make a safe landing was greatly diminished.

The Clement Marot Plan, the military contingency system to replace the civil air traffic services units in the event of a strike, by the very reason of its exceptional nature implied the use of rigorous planning traffic limitation per sector on the basis of control capacity and particularly strict compliance with the special regulations of the RAC-7 plan. The assignment of the same flight level by the control to the two aircraft IB 504 and BX 400, due to arrive at Nantes at the same time, created a source of conflict. The solution chosen by Menhir to resolve the conflict was based on separation in time. This solution, because of the reduction in normal separation, necessitated either particularly precise navigation by the crew of BX 400 or complete radar coverage and, in both cases, trouble-free communication facilities, conditions which were not realized. The continuing progress of the flight was affected by delays attributable in part to the control, in part to the crew and also to difficulty in air/ground radio communications resulting in complete failure of the crew and the control to understand one another. At the critical juncture, the crew, unmindful of their exact position, commenced a turn in order to lose time, without having been able to obtain the agreement of the control, as a result of which the aircraft interesected the adjacent route. The unidentified aircraft whose return appeared on the radar scope of one of the Menhir sub-sectors was' not identified by Menhir control as BX 400.

Wrong approach configuration. The crew decided to land in a flapless configuration with a tailwind component at a too high speed, causing the aircraft to land too far down the runway. In such conditions, the airplane was unable to stop within the remaining distance.

It is believed that the accident was the result of a controlled flight into terrain due to a premature descent in limited visibility, causing the airplane to descent below the MDA.

Accident investigators failed to find the reason for the continued descent. A possible misreading of the altimeters was examined in detail. The aircraft was fitted with three-pointer altimeters with warnings to indicate when the aircraft was below 10,000ft. The aircraft descended continuously at a steady rate over a period of 13 1/2 minute and the pointers would have been in continuous motion throughout, increasing the likelihood of misreading. The cross hatching in this type of altimeter first appears in a window in the 10,000ft disc at an indicated altitude of 26,666ft and the edge of the cross hatching would have been visible within 2 minutes of the aircraft beginning its descent. At 10,000 feet the cross hatching completely fills the window and it remains filled as long as the aircraft is below 10,000ft. Thus the cross hatching would have been visible to the crew for a period of about 9 1/2 minutes before the aircraft passed through FL60. According to the investigation report, it is not difficult to read an indication of 6,000ft as 16,000ft with this type of altimeter if particular note is not made of the position of the 10,000ft pointer. Evidence against the possibility of a simple misreading of this sort is the message from the aircraft to ATC reporting passing FL145. indicating that at this time the crew knew that they were below 16,000ft. This evidence indicates that down to this level there had been no misreading or misinterpretation but it does not dispose of the possibility that the crew subsequently suffered a mental loss of sequence and transposed themselves in time and space back to some altitude above 16,000ft. This has been known to, happen in the past, the investigators noted. Nevertheless, there is no evidence to show that it happened on this occasion. In conclusion, the accident was due to the aircraft having continued to descend through its assigned flight level down to the ground. No reason could be established for the continued descent.

The accident resulted as a consequence of propeller overspeed on the left side, the cause of which could not be determined owing to the fact that the aircraft was not recovered.

The pilot-in-command should have proceeded in view of the adverse weather conditions prevailing at the airport, which he should have appreciated during his first fly-past. His partial view of the airport and the glow of lights must have induced him to make a new attempt with the consequences described above.