ARTOP flight 531 - ARTOP-Flug 531
On November 9, 1958, a crashed flying boat of the type Martin PBM Mariner on the ARTOP Flight 531 between Lisbon and Madeira from unexplained reason. Despite a four-day search, no debris or other traces of the machine or its 36 occupants were found.
Airplane and occupants
The Martin PBM-5 Mariner ( c / n : 45409) built in 1944 was originally delivered to the US Navy and had completed 2,240 flight hours by the time the accident occurred.  The Portuguese company Aero Topográfica ( ARTOP ) acquired the machine stored in San Diego in the summer of 1958 via the US Surex Trading Company from the aircraft dealer Spiders Aircraft Company .  The flying boat was Patrick Joseph Byrne, the technical director of the August 29, 1958 ARTOP American US, with the license plateN7824C transferred to Lisbon , where it arrived on September 4th. The Portuguese company Oficinas Gerais de Material Aeronáutico ( OGMA ) converted the former military aircraft in Lisbon for civilian use, including additional hull windows and a continuous cabin with modern interior fittings that offered space for 41 passengers. The flying boat, baptized with the name Porto Santo , had the Portuguese registration number CS-THB from October 9, 1958.  The conversion of the aircraft was completed on November 2. After several test flights, the national aviation authority DGAC issued an operating license for the Martin PBM-5 on November 8, 1958Mariner.
On November 9, 1958, the machine had an accident on the route to Madeira. There were 30 passengers and six crew members on board: two pilots, a flight engineer , an on-board navigator and two flight attendants . The flight captain in charge was the Briton Harry Frank Broadbent, who had set a number of Australian and international flight records in the 1930s. He had flown the route from Lisbon to Madeira for years for Aquila Airways until September 29, 1958 and had previously worked as a flight captain for Qantas , among others . Broadbent had a total of almost 11,000 hours of flight experience as a captain, 2,700 of them on flying boats.  
The Martin PBM was scheduled to take off at 7 a.m. local time from Lisbon's Cabo Ruivo airport for a scheduled flight to Funchal on Madeira. Because on the island of bad weather prevailed, the departure was postponed twice so that the flying boat until 12:23 from the Tagus took off. The pilots informed continental air traffic control at 12:30 p.m. that they were climbing abeam of Barcarena . At 12:40 p.m. they asked the air traffic controller to stop the flight to Madeira at a cruising altitude of 6,000 feet(1,828 meters) instead of 8,000 feet (2,438 meters) as planned. Appropriate approval has been given. The crew submitted a routine report at 12:47 p.m. and announced that they would leave the information area of the continental flight control zone at around 14:07 p.m. At 1:21 p.m., air traffic control in Lisbon received the short message “QUG Emergência” (meaning: “We are forced to land immediately, emergency.”). Subsequent attempts by the air traffic controller to call the pilots were unsuccessful. The emergency call was placed 58 minutes after take-off. Taking into account the course and the speed of the flying boat, the machine would have to be about 250 kilometers southwest of Lisbon and about 200 kilometers west of Cabo de São Vicente at this point(Position: 37 ° 12 'N 11 ° 16' W).  
Cause of accident
The search for the flying boat began the day after the accident at the last suspected position. In addition to Portuguese, Spanish, US American and British aircraft stationed in Gibraltar , merchant ships also took part in the search, which was discontinued on November 14, 1958 without any results. No traces of the machine or its occupants could be found. The cause of the accident remained unclear. 
The Portuguese commission of inquiry ruled out some possible accident scenarios. Due to the lack of debris, investigators believed it was unlikely that the machine would explode or break in flight. In the event of a one-sided engine failure, the pilots would have continued the flight at a low altitude and informed air traffic control of this. In the opinion of the commission, a simultaneous failure of both engines was the most plausible explanation, whereby the machine leaked or overturned while splashing in the Atlantic and then sank so quickly that no evacuation could take place. 
Patrick Joseph Byrne, who, during his forty years of service in the US Navy, had completed more than 22,000 flight hours, almost 4,000 of which on Mariner flying boats, and was the technical director of ARTOP , questioned the course of the accident. He stated that after both engines had failed, the pilots would have had about four minutes to land the plane on the sea surface while gliding . The crew would have had sufficient time to answer the calls from air traffic control and to send information about the type of incident or an exact position. The wave height prevailing in the sea areaPatrick J. Byrne regarded Byrne as uncritical of about two to two and a half meters, especially since Mariner flying boats had already dumped in considerably higher swell without their hulls having been damaged in the process. In his opinion, the machine crashed as a result of a sudden emergency situation or serious damage, so that the crew had neither the opportunity to send further messages nor to initiate a controlled splashdown.  For example, a structural failure, an explosion or a fire on board could be considered. Subsequently, among other things, the theory was expressed that a defect or deflagration in one of the gasoline- powered fan heaterscould have started a cabin fire. 
- ICAO Circular 59-AN/54, Aircraft Accident Digest No. 10, S. 235 ff.
- VOA Portugal, ARTOP - Aero Topográfica (in Portuguese), accessed on May 1, 2016
- Official accident report of the then Portuguese aviation authority DGAC, today GPIAA (in Portuguese)
- British Caledonian a tribute, Captain Harry Frank "Jim" Broadbent, 1910-1958 (in Englisch), abgerufen am 2. Mai 2016