ARTOP Flight 531

On 9 November 1958, a Martin PBM Mariner flying boat operating ARTOP Flight 531 between Lisbon and Madeira crashed for unexplained reasons. Despite a four-day search, neither wreckage nor any other trace of the aircraft or its 36 occupants was found.

Flight

The Martin PBM was scheduled to depart at 7 o'clock local time from Lisbon-Cabo Ruivo Airport on the scheduled flight to Funchal on Madeira. Because of bad weather on the island, the departure was postponed twice, so that the flying boat did not lift off from the Tagus until 12:23. At 12:30 the pilots informed continental air traffic control that they were abeam Barcarena and climbing. At 12:40 they asked the controller for permission to conduct the flight to Madeira at a cruising altitude of 6,000 feet (1,828 metres) instead of the planned 8,000 feet (2,438 metres). A corresponding clearance was issued. At 12:47 the crew sent a routine report and announced that they expected to leave the information area of the continental control zone at about 14:07. At 13:21 air traffic control in Lisbon received the brief message “QUG Emergência” (roughly: “We are forced to land immediately, emergency.”). Subsequent attempts by the controller to call the pilots were unsuccessful. The distress call was made 58 minutes after take-off. Taking into account the course and speed of the flying boat, the aircraft must have been about 250 kilometres south-west of Lisbon and around 200 kilometres west of Cabo de São Vicente at that time (position: 37° 12' N 11° 16' W).

Cause of the accident

The search for the flying boat began on the day after the accident at the last presumed position. In addition to Portuguese, Spanish, US and British aircraft stationed in Gibraltar, merchant ships also took part in the search, which was called off without result on 14 November 1958. No traces of the aircraft or its occupants could be found. The cause of the accident remained unexplained.

The Portuguese commission of inquiry ruled out several possible accident scenarios. Because of the absence of wreckage, the investigators considered it unlikely that the aircraft had exploded or broken up in flight. In the event of a failure of one engine, the pilots would have continued the flight at low altitude and informed air traffic control of this. In the commission's view, a simultaneous failure of both engines was the most plausible explanation, in which case the aircraft must have sprung a leak or capsized when ditching in the Atlantic and then sunk so quickly that no evacuation could take place.

Patrick Joseph Byrne, who during his forty years of service in the US Navy had logged more than 22,000 flight hours, almost 4,000 of them on Mariner flying boats, and who was the technical director of ARTOP, doubted this sequence of events. He stated for the record that after a failure of both engines the pilots would have had about four minutes to glide the aircraft down onto the sea surface. The crew would thus have had sufficient time to answer the calls from air traffic control and to send information about the nature of the incident or an exact position. Patrick J. Byrne regarded the wave height of about two to two and a half metres prevailing in the sea area as non-critical, particularly since Mariner flying boats had already carried out landings on water in considerably heavier seas without their hulls being damaged. In his opinion, the aircraft crashed as a result of a sudden emergency or severe damage, so that the crew had neither the opportunity to send further messages nor to initiate a controlled ditching. Possibilities included, for example, a structural failure, an explosion or a fire on board. Subsequently, among other things, the theory was put forward that a defect or a deflagration in one of the petrol-powered fan heaters could have started a cabin fire.