Background
On the day of the accident, civilian air traffic controllers were on strike, and the air traffic control system had been taken over by military personnel. The Clement Marot Plan, a military contingency system, was in effect to replace civil air traffic services.
Accident Sequence
Iberia Flight 504, a DC-9, departed Palma de Majorca at 11:24 for a flight to London. At 12:19, the crew contacted Marina Control and reported at flight level (FL) 310, estimating arrival at Nantes VOR at 12:52. At 12:36, the crew was instructed to contact Menhir Control and descend to FL290.
At the same time, a Spantax Convair CV-990, flight BX400 also heading for London, was flying toward the Nantes VOR at the same altitude but on a different airway. The Convair crew was instructed to arrive at the VOR at 13:00. Because the Convair was already close to the VOR, the crew asked for confirmation. At 12:40, the controller told them to 'Stand by' and replied two minutes later. Due to the aircraft being at the ATC sector boundary, the crew could barely hear the Marina controller, who instructed them to contact Menhir control. The pilot erroneously thought he had to contact Menhir control when passing the Nantes VOR. To delay their arrival, the Spantax crew twice tried to request permission to perform a 360-degree turn. When they did not receive an answer from Marina, they initiated the turn without clearance.
While in overcast conditions, the Convair collided with the DC-9. The CV-990 lost an outboard portion of its left wing and managed to perform an emergency landing at Cognac-Châteaubernard Air Base (CNG). The DC-9 lost control and crashed. The wreckage was found in La Planche, about 25 km southeast of Nantes. All 68 occupants of the DC-9 were killed.
Probable Cause
The official probable cause states: 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 intersected 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.