When passengers disembarked the aborted flight KL685 in Amsterdam on Friday they were understandably upset.

Not only had they spent 11 hours on a plane to nowhere, the reason for their return to the Netherlands while half-way through a transatlantic flight to Mexico didn't seem to add up.

Aviation enthusiast Tom Podolec spotted the unusual diversion when the KLM flight unexpectedly reversed course in Canadian airspace over Quebec. It then continued 5400km back to Schiphol Airport.

At the time the official reason for this return was due to the eruption of volcano Popocatépetl.

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Popocatepetl volcano south of Mexico City was initially to blame for the diversion. Photo / Roberto Carlos
Popocatepetl volcano south of Mexico City was initially to blame for the diversion. Photo / Roberto Carlos

On November 28th, at 9am the Volcanic Ash Advisory Center in Washington issued an advisory that ash had reached 7300m. However, this ash had largely dissipated by the following morning.

What seemed odd and particularly infuriating to passengers on the plane to Mexico was, firstly, that there seemed to be no other planes affected by the troublesome volcano and, secondly, that the plane could not divert to any other airport in Mexico. Or on the right side of the Atlantic for that matter.

While the volcano Popocatépetl continued to be active, other arrivals such as AeroMexico's Boeing 787 from Paris arrived apparently unaffected.

Disgruntled passengers had their own theories why the plane had been diverted.

As a Combi Aircraft used to transport both passengers and cargo – particularly difficult cargo such as live animals – they speculated that there was more to this 5000km diversion than initially thought.

Artur Fityka who claimed to be a passenger on the flight said the diversion had more to do with the plane's hidden equine passengers.

"Officially, we turned back because of volcanic ash. We could not land in any other airport 'in Mexico, USA or Canada' due to 27/28 horses the plane had on board. We stuck on the Airport w/o support," he tweeted.

The airline was quick to respond to the claims of live cargo onboard, issuing a statement that it was their policy and IATA's advice to avoid areas of volcanic activity.

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"The reason for the return was the unfavorable flying conditions above Mexico after activity of the volcano Popocatepetl. Also, please know that we don't transport horses on board."

This statement was quickly retracted and a spokesperson for KLM said that there were in fact a large number of horses on the aircraft.

Talking to the BBC the Dutch airline "admitted the equine cargo may have been a factor" in returning to Amsterdam.

The airline issued a tweet apologising for the situation, but insisting that the volcano and "unfavourable flight conditions" were the main reason for the diversion.

"Landing on another airport was not possible, because of the visa requirement for passengers and because there were also horses on board," the Airline said in a statement.

The passengers were rebooked onto the next flight to Mexico City, and the service flew the next day without incident.

KLM's fleet of 747 Combi aircraft carry everything 'from horses to elephants'. Photo / Supplied, KLM
KLM's fleet of 747 Combi aircraft carry everything 'from horses to elephants'. Photo / Supplied, KLM

Boeing 747-400 Combi planes are used by airlines to transport both passengers and large or difficult cargo.

KLM's website says it has used its Combi fleet to transport everything "from small packages to large animals, like elephants or horses." There are five such Combi jumbo jets in their fleet flying routes from Schiphol to Mexico City, Chicago and Hong Kong.

Few passengers flying the 747 Variant are ever aware that they are sharing their flight with a stable of 24 horses.

KLM has been contacted for statement.