The Germanwings flight from Vienna to Cologne-Bonn airport on 19 December 2010 appeared to be going normally. Take-off had been delayed by two hours because of heavy snow at the destination airport, but otherwise everything seemed fine.

Then, as they started their descent towards Cologne-Bonn, the two pilots suddenly became aware of a "strange, very strong, unpleasant" smell in the cockpit, an investigation report of the incident revealed. They checked with the chief purser, who said there were no unusual smells in the passenger cabin.

Shortly after this the co-pilot felt so sick that he grabbed his oxygen mask, and the pilot felt he was "quite literally, losing my senses." There was a sudden, strong tingling sensation in his hands and feet, his field of vision shrank, and he felt dizzy.

Summoning their last reserves of strength, the two pilots succeeded in safely landing the Airbus, which had 149 passengers on board. Later, however, they told the German Federal Bureau of Aircraft Accident Investigation that they had had extreme difficulty in concentrating, and even in thinking clearly. Everything had seemed "surreal" and "like in a dream."

Obviously they had only barely succeeded in averting a disaster. But it's only now, almost two years later, that the incident has been made public, in the Bureau's interim report on its investigation.

Poisonous gases from the engines

Jörg Handwerg from the pilots' association Cockpit told DW what he believes happened. "As far as we're concerned, all the evidence points to this having been an oil vapor incident. Oil vapors from the engines got into the cabin air via a faulty seal."

Since the 1960s planes have usually been constructed in such a way that they suck in air for the cockpit and the passenger cabin via the two engines. Obtaining the air this way can be risky, said Handwerg, "If a seal breaks, it's possible for a lot of oil vapor to get into the cabins. In fact, for construction reasons, these seals are never 100 percent closed. Small amounts of oil vapor always seep into the cabin."

A well-known problem

Handwerg said that, unfortunately, this is not the first incident of its kind, although it is "extremely rare" for it to have such dramatic effects. Green party delegate Markus Tressel told the German parliament on Friday that over the past three years the authorities in Germany had logged 67 incidents involving contaminated air in the cabin.

In fact, the same Germanwings plane had earlier been involved in another unexplained incident. A flight from Dublin to Cologne in May 2008 was forced to turn back after the pilot complained he had lost sensation in his arm and that he and the cabin crew felt unwell.

After landing in Cologne in December 2010 the plane was immediately thoroughly checked by Germanwings engineers. They too noticed the smell in the cockpit, but attributed it to the de-icing fluid with which the Airbus had been treated several times that day.

Handwerg said he found this explanation unconvincing. It's not uncommon for de-icing fluid to get into the cabin in winter, he said. When this happens, the evaporating alcohol creates a kind of "white fog," and doesn't have such serious effects on the pilot's ability to function.

"The tingling in the fingers, the signs of paralysis, as well as the reduced ability to think clearly correspond precisely with experiences we've had of incidents involving oil vapor, and not with experiences of de-icing fluid," he said.

Business over security?

The pilots' association Cockpit has for years called for planes to switch over to systems that don't draw in cabin air across the engines. "Technically speaking, new planes should not be built in such a way that it's possible for something like this to happen. And we need filter technology or sensors for the existing planes to reduce the problem as much as possible."

Handwerg was critical not only of Germanwings, who he claimed are playing down the severity of the incident, but also of the European Aviation Safety Agency. Early in 2012 the EASA said it could see no causal connection between health-related complaints made by some pilots, crew members and passengers and contamination of cabin air with oil vapor.

Germanwings has defended itself against the accusations. It insists that it reported the incident in the proper manner to the authorities responsible, the German Federal Bureau of Aircraft Accident Investigation and the German Federal Aviation Authority.

The airline's description of the incident was much less dramatic: "The captain told Germanwings that despite his physical impairment he had 'everything under control at all times.'"