The three most common causes of helicopters crashing are pilot error, lack of fuel, and mechanical failure.
The probability of pilot error being a factor in Tuesday morning's crash are extremely remote, given the enormous amount of training and practice routinely performed by Coast Guard crews.
A fuel shortage is also unlikely given the sophistication of the engine management and warning systems on the latest Sikorsky helicopters. And if fuel was about to run out the crew would have been trained to put their machine down on the sea in a controlled landing and escape into the water. They were also drilled in the need to notify the authorities immediately they encountered any difficulties whatever.
Sadly, the lack of a Mayday distress call, or even a less serious Pan-pan call to warn of a potential flight difficulty suggests a sudden and catastrophic mechanical failure occurred aboard R116 in the air off the coast of Mayo.
No back up
Mechanical failure is the peril most feared by helicopter crews because of its severity. While a twin-engined passenger jet can fly on one engine, the failure of a key helicopter component such as the rotor and rotor shaft, main gearbox or tail rotor often proves fatal. In no other type of flying machine is so much stress placed on single components.
Airliners, for example, are required to have what regulators call redundancy in vital components. This means that, as far as possible, there needs to be alternative methods of propulsion and flight control so that if one system fails, another takes over. Helicopters lack redundancy in most vital components. Although there may be two engines on a large helicopter, their power is directed to the main rotor through just one gearbox and one rotor drive shaft. An airliner can lose an engine yet continue to fly safely: there is no such safety valve in a helicopter.
Giving the amount of power passing through them, and enormous stresses placed on them, gearbox failures are almost ubiquitous in the world's fleets of large helicopters and Sikorsky's experience has been no exception. It was introduced around the turn of the century with a price tag of $90 million (€85m) and proved to be a very safe machine until a 2009 tragedy en route to a Newfoundland oil platform killed 17 people. A gearbox problem was blamed. A gearbox problem was also the cause of an emergency forced landing in Australia in 2008 when all the gearbox oil was found to have leaked away due to a flawed filter.
Black boxes
Several S-92 helicopters operating in the North Sea were later found to have cracked gearbox mounts but the most dramatic recent mechanical problem was highlighted in an official UK air safety report published in January which described how a worn bearing almost caused a serious accident on an S-92 landing on a North Sea oil rig. All S-92s, including the aircraft which crashed on Tuesday morning, were grounded for an emergency inspection of the bearing.
But the leaking gearbox oil issue uncovered in Australia in 2008 continues to cause controversy. Safety advocates say it may have also been at least partly responsible for the 2009 Newfoundland crash and they question why the US Federal Aviation Administration (FAA) allows the S-92 to operate without proving that it can fly safely for a limited time in the event of a sudden loss of gearbox oil.
When first certifying the aircraft, the FAA said that such a loss of oil is so unlikely as to be safely discounted. The Australian incident proved them wrong but the FAA has still to insist that Sikorsky modifies both the gearbox and its oil system.
Meanwhile the Irish Air Accident Investigation Unit will continue its search for the missing aircraft's black boxes: the flight data recorder and the cockpit voice recorder. The flight data recorder contains a digital record of the missing aircraft's last flight. It would reveal how the engine was operating and the power it produced, its height above sea level, and its speed and direction. Its digital data, fed into a flight simulator, will enable investigators to "fly" the doomed aircraft's last moments on a computer as they attempt to find out why it crashed.
Answers
But the flight data recorder may not hold all the answers: it will say what the aircraft was doing when it crashed but not necessarily why it fell out of the sky. The final report into the accident may take upwards of a year. Crash investigators will also need to recover as much wreckage as possible from the seabed. The condition of the gearbox and other transmission components will yield valuable information. If the gearbox is in perfect condition it will prompt investigators to search elsewhere for clues. If it is mangled they will want to know why.
The cockpit voice recorder is often a surprisingly useful instrument for crash investigators. Sometimes the pilots’ last words describe a flight problem which they cannot solve. A change in engine tone and unusual noises in the background often provide useful clues that have helped solve air-crash mysteries.
Strange grunting noises made by one pilot before a 1994 US airline crashed were later found to have been produced as he strained to free the jammed rudder which , it was discovered, caused the accident.