Compliance Impossible: The 1973 Nantes Midair Collision

On the 5th of March 1973, two Spanish airliners headed for London collided in midair over northwestern France. A Spantax Convair 990 Coronado crashed into an Iberia DC-9 at 29,000 feet, sending the crippled DC-9 into a terrifying death spiral over the village of La Planche. The pilots of the damaged Coronado eventually managed to regain control and landed safely at a French air base, saving the lives of their 107 passengers and crew, but the occupants of the DC-9 were not so lucky: of the 68 on board, none survived. Investigators had one important lead right off the bat. The collision occurred while French air traffic controllers were on strike, collectively refusing to work until the authorities met their demands for pension reform. As a result, France’s ATC network had been switched to a hastily created backup system run by the military. It was in this environment that a series of escalating miscommunications put the two planes on a collision course and left the Coronado unable to contact ATC. Its pilots faced a critical decision: arrive at an intersection too early, or turn back without permission? Their choice would highlight important lessons about the responsibilities of both flight crews and controllers during those rare moments when the system seems to be on the brink of a total meltdown.

In February 1973, the French Civilian Air Controllers Association was engaged in a bitter dispute over controllers’ pensions. With the government stonewalling their efforts to win improved retirement benefits, the union took drastic action, ordering a nationwide air traffic controllers’ strike on the 20th of February. Under a 1964 law, it was illegal for controllers in France to go on strike, but the union opted to do so anyway, adding the right to strike to its list of demands. After the strike failed to resolve quickly, on the 24th of February the French government launched a contingency operation called the Clément Marot Plan, which shifted the air traffic control network into the hands the military. Military controllers at military airports took over command of all civilian airspace in France.

A Spantax Convair 990 Coronado similar to the one involved in the accident. Image source: Eduard Marmet

By the 5th of March, one week after the activation of the Clément Marot Plan, the dispute remained unresolved. It was on this day that the Spanish airline Spantax prepared to operate a charter flight carrying a group of 99 vacationers from Madrid back home to London. The aircraft commissioned for the flight was a Convair 990 Coronado, a four-engine jet briefly produced by US manufacturer Convair between 1961 and 1963. The Coronado, a stretched version of the earlier Convair 880, was intended to be faster than its rivals, the Douglas DC-8 and the Boeing 707, but it was inefficient and carried fewer passengers. As it turned out, airlines didn’t want faster planes — they wanted planes that had lower operating costs and carried more passengers. As a result, the Convair 990 Coronado was a commercial flop; only 37 were built, and major airlines quickly dumped them after wising up to their subpar performance. Spantax subsequently picked up several second-hand Coronados at bargain basement prices, and it was one of these that arrived at Madrid’s Barajas Airport to pick up the tour group on its return journey to London. In command of the flight, designated flight 400, were Captain José Antonio Arenas Rodriguez, First Officer Esteban Saavedra Martinez, and Flight Engineer José Maria Gonzalez Zaraus Navas, all of whom had thousands of hours of experience.

At 1:01 p.m., Spantax flight 400 departed Madrid with 99 passengers and 8 crew on board. 23 minutes later, Captain Arenas Rodriguez made contact with the regional air traffic control sector in Western France. French airspace was divided into several regional sectors, each of which was managed from a separate control center on the ground. The majority of France’s west coast, from the Spanish border until some distance south of Nantes, fell under the Marina Sector, which under the Clément Marot Plan was managed from a facility on the French air force base in the town of Mont-de-Marsan. The Marina Sector was in turn divided into several sub-sectors, each of which contained one or two designated airways. One controller managed each sub-sector on a different radio frequency. Spantax flight 400 planned to cross the Marina Sector northbound on the airway designated W-187, which stretched north-northeast from Bilbao, Spain to the French city of Nantes. Flight 400 initially contacted the center on the wrong frequency, but then switched to the correct one and reported that they were at 26,000 feet, or flight level 260, and would arrive over Nantes at 1:52. (Hereinafter, flight levels will be used instead of exact altitudes in feet.)

A map of the relevant area of the Marina Air Traffic Control Sector. (Own work)

Also in the Marina Sector that afternoon was Iberia flight 504, a Douglas DC-9 operating a scheduled passenger flight from Palma de Mallorca to London’s Heathrow Airport. On board were 61 passengers and 7 crew; a further 39 passengers who were part of a British tour group arrived at the airport half an hour late and missed the flight. Flight 504 had left Palma de Mallorca at 12:24 and entered the Marina Sector at 1:19, where it was routed northbound onto the W-132 airway. This airway ran north-northwest from the Spanish city of Reus in Catalonia and, like the almost parallel W-187 airway, terminated in Nantes; however, the two airways were in different sub-sectors and were managed by different military controllers working out of the same facility in Mont-de-Marsan. Flight 504 reported that it was at 34,000 feet and would also arrive over Nantes at 1:52.

An Iberia Douglas DC-9 similar to the one involved in the accident. Image source: Aero Icarus (Wikimedia)

At 1:30, the controller on the W-132 airway ordered Iberia flight 504 to descend and maintain flight level 290. Two minutes later, the controller on the W-187 airway asked Spantax flight 400 to climb to that same altitude. Because both flights expected to arrive over Nantes at 1:52, this put them on a collision course; however, the controllers at Mont-de-Marsan apparently never discussed this, and there was no communication whatsoever between them regarding these flights, even though the controllers were sitting just a few meters from each other.

At 1:36, the controller on W-132 handed over flight 504 to the next air traffic control sector, called Menhir, which was based out of Brest. This was much earlier than usual because Marina control was talking to flight 504 on a backup frequency that didn’t have enough range to reach the actual boundary between the Marina and Menhir sectors. Now only Spantax flight 400 was under the control of the Marina Sector.

How the Menhir controller made a critical misunderstanding when telling the Marina controller where he wanted the Coronado. (Own work)

One minute later, the Marina controller on W-187 informed Menhir by telephone that flight 400 was at flight level 290 and would arrive over Nantes at 1:52. The Menhir controller replied that this wouldn’t work — the flight could not arrive at Nantes before 2:00, or it would be in conflict with other aircraft. Nor could he simply ask it to change altitude, or so he believed. On the W-187 route, planes were allowed to occupy flight levels 200, 210, 220, 290, 300, and 340; most of these were indeed full, but flight level 300 was empty. Normal air traffic control rules stipulated that crossing aircraft must be kept 1,000 feet apart vertically when at or below 30,000 feet, and at least 2,000 feet apart when above this altitude; therefore, he could have assigned flight 400 to flight level 300, and it would then pass 1,000 feet over the Iberia DC-9, which was at flight level 290. But under the rules used by the French Air Force, 1,000 feet of vertical separation was only allowed at altitudes below 29,000 feet; at or above this level, regulations required 2,000 feet of separation. Unaware of the difference in the civilian and military rules, he mistakenly believed that he could not assign Spantax flight 400 to flight level 300; instead, he asked that it stay at 290 but delay its arrival over Nantes.

At 1:40, the Marina controller told flight 400 that it must not arrive over Nantes before 2:00. The pilots repeated the instruction, then asked if their readback was correct; the controller replied, “Stand by.” “Stand by” means “wait for further instructions,” and this is how the pilots interpreted it. But the controller, apparently unfamiliar with the standard terminology, had simply intended to confirm that the pilot’s readback of the instruction was correct. Now flight 400 was waiting for a follow-up that the controller had no intention of providing. Finally, at 1:43, the pilots concluded that no follow-up was forthcoming and acknowledged the instruction. Meanwhile, Iberia flight 504 made contact with the Menhir Sector and informed the controller there that they now expected to arrive over Nantes at 1:54, two minutes behind the Spantax Coronado.

Spantax flight 400 is given an order with which it is impossible to comply. (Own work)

With their expected arrival time over Nantes less than ten minutes out, the Spantax pilots somehow needed to add an additional eight minutes of flying time if they were to arrive there at 2:00. A simple calculation should have shown that it would be impossible to slow down enough to turn a nine-minute flight into a 17-minute flight, but at 1:44, the pilots began to slow down anyway.

By now, flight 400 had actually crossed over the sector boundary and entered Menhir, putting it beyond radar range of the Marina control center in Mont-de-Marsan. But the Marina controller still hadn’t given the pilots the radio frequency to contact the Menhir Sector. This far from Mont-de-Marsan, the radio connection weakened rapidly, a problem that was only exacerbated by an unusually high level of interference in the signal from the nearest repeater in Cognac. When the pilots attempted to inform Marina at 1:44 that they were slowing down, the message was rendered totally unintelligible. Nevertheless, the Marina controller told the Menhir controller that flight 400 would arrive over Nantes at 2:00. He then told flight 400 to contact Menhir on a frequency of 124.05, and flight 400 acknowledged. However, possibly due unusual wording used by the controller, Captain Arenas Rodriguez mistakenly believed that the Marina controller wanted him to contact Menhir after reaching Nantes.

Time is running out and the pilots of flight 400 must consider several options — all of them bad. (Own work)

Now well into the Menhir Sector, flight 400 was still trying to talk to Marina. At 1:49, it became clear to Arenas Rodriguez that he could not slow down enough to avoid reaching Nantes before 2:00, and he asked Marina for permission to enter a holding pattern instead. But the Marina controller couldn’t understand his message. In response, the controller simply repeated the order to contact Menhir on 124.05, but flight 400 never received this transmission. At 1:50, Arenas Rodriguez again asked permission to make a 360-degree right orbit, but again, there was no reply.

The pilots of Spantax flight 400 now found themselves in an extremely dire situation. They had been ordered not to overfly the Nantes intersection any earlier than 2:00, but at 1:51, they were only one or two minutes away from it. If the controller wanted them to delay their arrival at Nantes, it was presumably to ensure separation from another plane, so it would be extremely dangerous to cross it early. They needed to make a 360-degree orbit to delay their arrival, but they couldn’t get through to the controller to ask for permission. They were in an impossible situation — no matter what they did, they would be violating an ATC order, and they had less than two minutes to make a decision!

A collision is now inevitable. (Own work)

At 1:51, the Spantax pilots came to the conclusion that it would be safer to make the orbit, rather than continuing straight into a situation where a collision was likely. Captain Arenas Rodriguez announced to air traffic control that they were making a 360-degree right orbit, (although the message never went through) and flight 400 began to turn to the right. What the pilots didn’t know was that they were turning straight into the path of Iberia flight 504, which was inbound to Nantes only a few kilometers behind and to the right of flight 400. Because the two flights had been communicating with different controllers on different frequencies, neither was aware of the other’s presence. The stage was now set for disaster.

Flying through thick clouds at 29,000 feet, neither crew saw the other coming. At 1:52, as the eastbound Coronado proceeded through a banking right turn, it crashed headlong into the northbound DC-9. The tip of Spantax flight 400’s left wing sliced through fuselage of Iberia flight 504 from underneath, instantly tearing the DC-9 in half. Its pilots never knew what hit them, as their crippled plane plunged straight down toward the French countryside, disintegrating further as it fell.

Sketch of the moment of the collision. (Own work)

Meanwhile, the Coronado had lost several meters off the end of its left wing, almost up to the pylon of the number one engine. Flight 400 rapidly lost altitude as alarms blared and the pilots struggled to control the plane. At 1:56, four minutes after the collision, flight 400 issued an urgent mayday call, but neither Marina nor Menhir picked it up. Another Iberia flight did hear the distress call, however, and was able to relay it to Marina. The crippled plane, which was still not in contact with any air traffic control center, initially stated its intention to land in Bordeaux. After this message trickled its way through the pipeline, a French Air Force fighter jet was ordered to intercept the plane and guide it down to the runway.

As the plane headed south toward Bordeaux, shadowed by the fighter jet, it flew within sight of Cognac-Châteaubernard Air Base in Cognac, which was considerably closer than Bordeaux. Unable to communicate directly with the stricken plane, the air base launched green flares into the sky to signal to the Coronado that it could land there instead. As the base had already rolled out emergency vehicles to assist them, the pilots of flight 400 quickly changed course and put the plane down on the runway in Cognac. Miraculously, despite enormous damage to the left wing, Spantax flight 400 had landed safely without a single injury to any of the 107 passengers and crew.

The tail of the DC-9 lies next to a road after the crash. Image source: Presse-Océan

The DC-9, however, was a total loss. Pieces of the plane fell across fields and roads outside the town of La Planche, along with the remains of all 68 people on board, none of whom survived the crash. Iberia flight 504 had become the unfortunate victim of a sequence of events completely outside its control. It was simply in the wrong place at the wrong time.

In the wake of the collision, investigators sought to determine whether the pilots of the Coronado or the controllers were to blame. What they found was a series of minor mistakes and miscommunications that left the pilots of Spantax flight 400 in an impossible situation. Controllers working out of the same building put the two flights on a collision course without any discussion. The Marina controller waited too long to hand over flight 400 to Menhir. And neither the Marina nor Menhir controllers had considered the fact that a passenger jet could not lose enough speed to extend a nine-minute travel time to seventeen minutes.

The crash understandably made for major headlines in a local newspaper. Image source: Presse-Océan

But some parts of the story given by the pilots also didn’t add up. Although they insisted that the Marina controller told them not to contact Menhir until after passing over Nantes, the controller denied that he had done so. Investigators were inclined to side with the controller, because such an order didn’t make any sense. In fact, it would be in violation of regulations to continue speaking to the previous ATC sector so far into the next one. But investigators couldn’t prove either story, because air traffic control communications transmitted through the radio repeater in Cognac were not recorded, and the Coronado did not have a cockpit voice recorder. As a result, it was impossible to know what either party actually said or heard. However, investigators did place some fault on the Spantax pilots for not even trying to call Menhir after they lost contact with Marina.

Another view of the DC-9’s severed tail. Image source: Presse-Océan

This series of miscommunications left the pilots of flight 400 in a highly unusual situation. Unable to avoid reaching Nantes too early, and unable to get permission to make an orbit, they would be violating an ATC order no matter what they did — and they couldn’t tell anyone about it. No procedure existed to tell them what they should do. In the heat of the moment, they believed the safer option was to turn; tragically, they were wrong.

Thankfully, such situations are extremely rare. Today, due to a much improved air traffic control system as well as on-board collision avoidance technology, things are a little clearer. Any pilots who find themselves in this position would be better off holding their current course and setting their transponder to “squawk” the radio failure code, 7600, which would give them a clear trajectory and the unambiguous right of way. If one’s path is predictable, controllers and other planes will have an easier time staying out of the way until radio contact is restored and one’s intentions can be made clear. Much the same principle governs traffic etiquette — don’t be nice, instead be predictable; don’t yield if everyone knows you have the right of way.

Gendarmes inspect a piece of the DC-9’s fuselage, which came to rest in a farmer’s field. Image source: The Bureau of Aircraft Accidents Archives

By the end of the investigation, it had become clear that no single mistake by a single controller had caused the collision. The series of errors that led to the crash originated with flaws inherent to the Clément Marot Plan. The military air traffic controllers who took over the system on short notice were unfamiliar with civilian terminology (“stand by”) and procedures (the 1,000 feet of separation rule), and their technology was less sophisticated. The Marina Sector’s radar equipment in Mont-de-Marsan barely reached to the edge of its own sector, when normally there was some overlap with Menhir; and the Cognac radio repeater used by the Air Force had shorter range than civilian repeaters and was notoriously unreliable. Air traffic control is an unforgiving profession, and when controllers are forced to work with poor technology, their ability to keep airplanes apart decreases significantly.

In its final report, French investigators wrote that because the Clément Marot Plan was an emergency system intended to be used only in exceptional circumstances, it should have had more built-in protections to ensure proper separation of aircraft, and the controllers should have received advance training on civilian air traffic rules. But it remains unclear whether the French accident investigation agency attempted to expand any of the insights of the accident to aviation more broadly.

The nose section of the DC-9 still displayed the plane’s nickname, “Ciudad de Sevilla.” Image source: Christian Vallantin

As was typical of crashes in the early 1970s, little changed as a result of the Nantes midair collision. Three years later, 176 people were killed when two airliners collided over Zagreb, Yugoslavia; it was soon discovered that a failed handover had left the controller unaware that one of the planes was in his sector. Two years after that, 144 died when PSA flight 182 collided with a private light aircraft over San Diego; in that case, a miscommunication led the controller to falsely assume that the PSA pilots had the other plane in sight. All of these collisions occurred because of failures to communicate information in a timely and accurate manner — whether it was due to a poor radio connection, a bad handover in the control center, or a misheard word from one of the pilots. The system was clearly flawed — but it actually wasn’t as bad as it had been just the previous decade, when midair collisions were even more common. Between 1960 and 1971, there were six major midair collisions involving passenger airliners in the US alone. The difference lay in the widespread introduction of secondary radar that could display aircraft altitudes directly on the controllers’ radar screens.

Another view of the DC-9’s shattered fuselage. Image source: Presse-Océan

But this still wasn’t enough to totally eliminate the problem — in fact, both the Nantes and Zagreb collisions occurred when one plane was not being properly tracked by the ATC sector in which it was located. It wasn’t until the 1980s that traffic collision avoidance technology was developed to provide redundancy in case controllers failed to keep planes apart. There appears to be no evidence that French authorities were forward-thinking enough to propose development of such a system after the completely preventable midair collision over Nantes. Today, the presence of the Traffic Collision Avoidance System, or TCAS, on all commercial aircraft greatly simplifies the choice faced by any crew that finds itself in a similar situation to Spantax flight 400 — all they have to do is keep flying straight, and take corrective action if TCAS tells them they’re still on a collision course. Although this isn’t completely foolproof, there has only ever been one midair collision involving two planes that both had functional collision avoidance systems. Chances are, no pilot will ever again find themselves faced with the difficult choice that confronted Captain Arenas Rodriguez. Although hindsight teaches us that he chose wrong, it is difficult to blame him for picking what he thought was the better of two terrible options.

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Analyzer of plane crashes and author of upcoming book (soon™). Contact me via @Admiral_Cloudberg on Reddit or by email at kylanddempsey@gmail.com.