In the five fatal airline accidents in the first six months of this year 65 people died, while another 150 were killed in the Germanwings aircraft, which was not an accident.

This bears comparison with the first six months of 2014, where there were five fatal accidents causing 28 deaths, plus the enigmatic disappearance of MH370 in which 239 people were lost. Although it cannot be proven, most industry experts believe MH370’s disappearance was a the result of a deliberate act by someone on board.

In terms of fatal accident numbers for the same period each year in the last decade, 2015’s accident figures equal the best. But the “deliberates” are beginning to pose new questions about airline safety, because there was one in 2013 as well – that’s three “deliberates” in three years.

The 2013 “deliberate” involved Mozambique airline LAM which lost an Embraer 190 twinjet and all 33 people on board under the same circumstances as the loss of Germanwings flight 4U9525. That is, one pilot left the cockpit, the other locked him out and deliberately flew the aircraft to impact.

The question is: do three such “deliberates” in three years constitute a trend or a coincidence?

Statistically there’s not a strong case for calling it a trend, but neither can it be ignored.

Look at similar cases before the LAM loss: Egyptair 767 in 1999, Silk Air 737 in 1997, and a Royal Air Maroc ATR42 in 1994. So, in that period 1994-1999 there was one loss every two years. Then there was a long gap – 13 years – with no deliberates. Then between 2013 and 2015 there were three: LAM, MH370 and Germanwings.

Hijacks are also “deliberates”, but since the adoption of the post-9/11 fortress cockpit, plus anti-hijack cabin crew drills, hijacks have been eliminated.

Sabotage is a deliberate act, but security is now so extensive that even those who have smuggled small quantities of explosives on board have failed to detonate them effectively.

So the only “deliberates” against which the industry has no effective defence are those that can be carried out by people in the cockpit or with authorised access to it.

The nearest the industry has come to a defence against this risk is never to leave a pilot alone in the cockpit, so if one of them leaves it, a member of the cabin crew has to replace him or her. This is a useful psychological technique for making it less likely that a pilot in a suicidal frame of mind would initiate a plan when there is a witness to it. Less likely yes, but not impossible.

What is needed is some careful study, probably across other industries also, of people – and their life circumstances – who use their workplace either to end their own life, or for a revenge motive resulting from resentment so embedded that their own survival becomes irrelevant.

If this three-in-a-row set of deliberates is a trend, is it generated by societal changes, including working cultures, or is it just a matter of chance associated with the power and opportunity that control of an aeroplane confers?

Such a study would be complex and may not be conclusive, but that is no excuse for failing to carry it out.