EARLIER this week it emerged that First Minister Nicola Sturgeon had written to the UK Transport Secretary to query an "apparent slippage" in the timetable for publishing the final air accident report on the Clutha tragedy.

In the correspondence, dated August 29, Ms Sturgeon stresses that the families have "already waited two years" and seeks an assurance that it will be published before the second anniversary of the disaster on November 29 this year.

The Air Accidents Investigation Branch (AAIB) had previously indicated that it to publish the final "formal" report in the middle of this year, having completed the draft report in April. Since then it has been circulated among the "interested parties", which is to say the various legal teams of anyone who may be found at fault - Police Scotland, the operator Bond, manufacturer Airbus Helicopters, and the pilot.

At this stage, the lawyers are entitled to challenge the investigators' conclusions if they believe their own client is being unduly blamed.

The apparent delay hints at an element of legal wrangling on the part of lawyers, rather than any heel-dragging on the part of the AAIB whose task is essentially complete. In this respect, there are parallels with the Chilcot report, and it is an understandable source of frustration that the bereaved and the injured - who bear zero responsibility - are, precisely because of that, the last to know the facts.

But it is also fair to note that the duration of the Clutha investigation is not hugely at odds with other UK air crashes.

The AAIB's final report on the Lockerbie bombing was published in August 1990, 21 and a half months after the 1988 tragedy.

The families of those who lost loved ones in the worst North Sea helicopter crash in recent years - the fatal ditching of a Super Puma in April 2009, which left 16 dead - had to wait more than two and a half years for the AAIB's definitive report.

A final report into the Sumburgh helicopter crash, which killed four offshore workers in August 2013, is still outstanding.

In this context, the long wait for answers to Clutha - 22 months and counting - seems less unusual.

What does set it apart, however, is that the AAIB's interim report, published in February 2014, threw up more questions than answers.

If the engines "flamed out" because they were starved of fuel, why was there 76kg of fuel still in the main tank?

Why were crucial fuel switches, which propel fuel from the main tanks to the engines' supply tanks, found in the off position when they should have been on throughout the flight? A bizarre pilot error, perhaps?

And what is the significance of a series of low fuel warnings recorded in the aircraft's memory bank? These are supposed to be triggered by thermal sensors in the left and right supply tanks whenever the volume of fuel hits 32kg and 28kg respectively - yet only 400g remained in the left supply tank at the time of the impact, with nothing at all in the right supply tank.

So did the sensors malfunction, warning the pilot too late?

For now, the blame game continues behind closed doors.