A compact 'black box' holds the key to the mystery of why a state-of-the-art helicopter and its crew of four crashed into the sea off Blacksod Bay, Co Mayo, on a routine medical evacuation mission.
Irish Coast Guard Sikorsky S-92A helicopters are typically equipped with a Penny & Giles Multi Purpose Flight Recorder (MPFR).
Weighing less than seven pounds and just 10 inches long, the MPFR serves the dual purpose of a cockpit voice recorder and a flight data recorder. All this data is recorded on a crash-proof, solid-state memory unit. It holds 25 hours of flight data, height, speed, engine temperatures and other details as well as up to two hours of audio data.
Its recovery is a top priority for investigators from the Air Accident Investigation Unit from the Department of Transport. If it is recovered intact, investigators will be able to download its data and digitally recreate the doomed last flight of Rescue 116.
The probe will exhaustively examine all elements of the flight: from the medical history and training of its four-person crew, to weather conditions at the time, to the aircraft's systems, including the performance of its two General Electric CT7-8A turbo shaft engines.
Five of the sophisticated helicopters were in service with the Coast Guard. They replaced older, less capable S-61N helicopters. The S-92A is specially configured for search and rescue missions around Ireland with a cruising speed of 151 knots, auxiliary fuel tanks extending its range to 800 nautical miles and carrying an extensive array of thermal imaging cameras, searchlights, radio altimeter and an automatic identification system.
Some 280 Sikorsky S-92s are in service around the world. They have a generally excellent safety record. A version of the helicopter, the VH-92, is destined to become US President Donald Trump's new "Marine One" helicopter in two years time.
The Irish Air Corps was slated to become one of the first military forces in the world to operate the S-92A, with five on order: three for search and rescue and two to support the Army Ranger Wing of the special forces. But the order was cancelled as the Government decided to outsource the search and rescue function to a private company, which provided a fleet of five: four second hand S-92A and one "new build", based at four locations around Ireland.
In January, safety inspections were carried out on the five Irish Coast Guard helicopters after the US manufacturer, Sikorsky, issued a global 'All Operators' Letter' followed by an alert service bulletin.
This followed an incident involving an S-92A at the West Franklin Well Head platform in the North Sea on December 28 when it experienced "unexpected control responses" while mid-air on a routine flight.
An initial investigation by the UK Air Accident Investigation Branch pointed the finger at a bearing in the Tail Rotor Pitch Change Shaft. There had been a previous incident in 2007 where a degradation of the TRCPS bearing had occurred, "leading to reduced tail rotor control in flight", the AAIB said.
The Air Accident Investigation Unit inquiry will also consider why there was no warning of the catastrophic event that caused the Sikorsky to crash. It had comprehensive communications systems on board, including, VHF, FM, Tetra radios and satellite communications which featured a flight-tracking facility.
It also carried emergency locator transmitters and life rafts in the event it had to ditch in the sea. None of these was triggered when the aircraft went down.
Divers searching for the aircraft's black box will have the aid of advanced underwater detection equipment carried by ships like the Naval Service's LÉ Róisín, which is on site and can deploy remotely piloted submersibles.