Four Ryanair jets that landed at Stansted on the night of April 24th last should have diverted to another airport, the Irish Aviation Authority (IAA) has found. The landings were investigated after the UK Civil Aviation Authority (CAA) received a complaint about the arrival of 15 Ryanair jets in heavy fog when most other aircraft diverted.
The investigation found that visibility, which is described as Runway Visual Range (RVR), had improved sufficiently at times to enable 11 of the aircraft land within the regulations.
They had an RVR of 550 metres, said Kevin Humphries, director of safety regulation at the IAA. This was acceptable, he said.
But four of the Ryanair jets had an RVR of less than 500 metres and should have diverted, Mr Humphries said.
However, he said the IAA was regarding the four landings as a technical breach of the regulations rather than what he termed a "critical safety issue".
Stansted had been operating to what is known as category three that night, which in normal circumstances allows fully automated landings in almost zero visibility thanks to a sophisticated instrument landing system, Mr Humphries explained.
But the runway lights in the touchdown zone and in the centre of the runway were switched off for maintenance and that meant that greater visibility was required. Only lights to the sides of the runway and on the approaches remained to guide pilots.
"It's easy to see how they could have been misled. A pilot could easily think that because it's category three, an RVR of 550 metres doesn't apply. But lighting is an integral part of the instrument landing system and with less lights available, visibility needs to be greater," he said.
"In those fog conditions and without those lights, normal category three didn't really apply."
The news comes just weeks after two consecutive reports found Ryanair pilots breached safety regulations in serious incidents at Knock and Cork airports.
Penny Guy of the UK's National Air Traffic Services, which provides air traffic control at Stansted, said 34 aircraft diverted to other airports due to the fog that night.
The CAA obtained official reports, including voice tapes and radar records, on the incidents from the Stansted control tower and passed them on to the IAA, which regulates Ryanair.
"No other airline was involved," said James Hotson of the CAA.
"Once we established that the Stansted airport authorities had issued an appropriate NOTAM we passed the matter on to the Irish authorities," he said.
NOTAM stands for Notice to Airmen, and is issued to all airlines using an airport whenever significant work that may affect air operations is taking place.
However, Ryanair's chief pilot, Ray Conway, has claimed that the Stansted NOTAM was misleading because it did not emphasise the fact that the RVR was affected.
In a statement Ryanair said that it would not comment further until the investigation was complete.
The investigation was still continuing because there were other matters to be resolved, Mr Humphries said.
However, the IAA was satisfied that Ryanair had taken appropriate action, including additional training for the pilots, to ensure the incidents would not recur.