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More on Interesting Times
Dave Jesse on June 3, 2016
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More on Interesting Times

On 20th May I wrote about “Interesting Times” and cited a helicopter accident that happened on 29th April in Norway. This blog is a comment on the investigation process, an update on that accident and a brief explanation of epicyclic transmissions for the non-rotor aviators.

Accident Investigations

There are commonly complaints in the media that, following an accident, there is no information coming out from “The Authorities”. The papers clamour for immediate answers, the television is full of experts expressing an opinion with little or no information to go on. The investigation process is referred to reluctantly with terms like “we will have to wait” and “it may be some time before we know”.
In the case of this accident, the Accident Investigation Board of Norway (AIBN) issued bulletins to report progress of the investigation on 1st, 2nd and 6th May then a preliminary report on 13th May, just two weeks after the accident. Since then further bulletins and two updates to the preliminary report have been issued.  For example, the 6th May bulletin showed the recovered components being spread out at the start of the investigation.

Crash Investigation

It appears to me that anyone interested in the investigation can follow not only the progress of the work being carried out, but also the safety actions being taken.

Probable Cause of this Accident

The cause of this accident appears to be related to metal fatigue in the components of the second stage planet gear. As someone who flew for many hours in a Super Puma (which is an earlier version of the accident aircraft, and has a similar transmission) this is a very worrying finding.

Epicyclic Gearboxes

For those who are saying epi-what-gear? I have to refer you to the best YouTube video of epicyclic transmissions that I have found here. There is some annoying music, so I recommend you turn the sound off, and skip to 2:10 where it deals with Stationary Element: Ring Gear. This is the configuration used on helicopters. The inner face of the gearbox casing supports the outer ring gear, and the sun gear in the middle is driven by the earlier stages of the transmission and engines. The planets can do nothing but roll between the sun and the annulus and the triangular planet carrier forms the output shaft of the gearbox.
On the H225 there are two stages of reduction like this with six and eight planets in the two stages.

Gears Image

Helicopters use epicyclic gears because they provide a high reduction ratio and the forces are shared across the planets, as eight planets will have one eighth of the tooth load of a reduction gear with only one tooth making contact at a time.

This Accident in Context

The world relies upon offshore oil for a significant part of our energy consumption, and the offshore oil industry relies upon helicopters, many of which have epicyclic transmissions. I wish all those investigating this accident, and those looking for a solution to prevent recurrence, good fortune with your endeavours.