What happened
On the day of the accident, Iberia Flight 504, an Iberia DC-9, departed from Palma de Majorca with a destination of London. At approximately 12:36, while cruising at FL310, the crew received instructions to descend to FL290 and contact Menhir Control. Simultaneously, a Spantax Convair CV-990, operating as flight BX400 and also bound for London, was approaching the Nantes VOR at the same altitude on a separate airway.
To manage traffic, controllers instructed the Spantax crew to delay their arrival at the VOR until 13:00. Due to their proximity to the waypoint, the Spantax crew sought clarification of these instructions. During this period, communication difficulties arose because the aircraft was positioned at an ATC sector boundary, making it difficult for the crew to hear the Marina controller. The pilot of the Convair CV-990 mistakenly believed they were required to contact Menhir Control only upon passing the Nantes VOR.
In an attempt to delay their arrival and adhere to the new timing, the Spantax crew requested permission to perform a 360-degree turn. After receiving no response from the controller, the crew initiated the maneuver without authorization. While flying through overcast conditions, the Convair CV-990 collided with the DC-9. The impact caused the Convair to lose part of its left outboard wing, though it successfully performed an emergency landing at Cognac-Châteaubernard Air Base. The DC-9 lost control and crashed near La Planche, approximately 25 km southeast of Nantes. The accident resulted in 68 fatalities.
Findings
At the time of the incident, military personnel were managing air traffic services under a contingency plan due to a strike by civilian controllers. This arrangement involved strict traffic limitations and reduced separation standards. The investigation determined that assigning the same flight level to both aircraft created a conflict. While controllers attempted to resolve this through temporal separation, the lack of adequate radar coverage and poor radio communication prevented effective management. Ultimately, the Spantax crew began an unauthorized turn to lose time without knowing their exact position, causing them to intersect the path of the Iberia aircraft.