Originally Posted by
POBJOY
TUC Thanks for that, I assume that 'normal' or good practice would require a new nut and bolt fitted as directed and then checked for free movement. hardly a difficult operation and seemingly in the general scheme of things not expensive for what it does. I can see that in place seat servicing would be a real loss of good practice, and also a loss of a good opportunity for a loose article check at the same time. Considering the location of the only operating handle being low down in a restrictive place that itself makes a cogent inspection difficult in situ. With that mind set running the show it beggars belief that no one saw a big red light and flagged up what was quite obviously a dangerous practice for such a complex piece of safety equipment. No wonder they b..... up the glider fleet.
There's a few separate issues there.
Yes, a new nut and bolt is required every time the Drogue Shackle assembly is dismantled. MoD's report did not address this, and the Routine Technical Instruction being followed at the time does not mention it. The proper assembly instructions (not the illegal RTI) say it is to be assembled in the parachute bay as part of the duplex drogue assembly, which is later fitted to the seat in the seat bay. At that point any jamming or resistance would be obvious, as two checks are required to ensure the shackles can freely disengage. AND THEN ARE NOT TO BE DISTURBED. If they are, then you start again. Yep, it's hassle, but its an escape system that is only required to work once. If you get it wrong, someone dies. But it appears someone spotted an opportunity to save money by eliminating that hassle, without thinking through the impact. I cannot speak for Martin-Baker, but I understand they expressed grave concern at this not-so-cunning plan. So too did MoD armourers. But there's a certain type of person in MoD who sees this as the company being bolshie and awkward, that MoD knows best. Nope, they're satisfying legal obligations.
A free movement check is carried out by pilots, but it gives false confidence. The shackle lugs can be pinched by over-tightening which (as illustrated by the SI in an image of the XX177 shackle), but when this happens the lug faces are no longer parallel. This can allow free movement at the bolt, but not at the ends of the lugs. The effect is variable, and in this case the SI calculated a speed of 50kts would be required to overcome this resistance. It follows that it is likely the problem had occurred before, but only manifested itself when the seat was required at the extreme of its design envelope. (Zero-Zero). Hence, the instruction that the nut just touches the shackle when attached. That eliminates any problems caused by slight tolerance differences in bolt lengths and lug gap.
The handle issue was addressed very well by the SI, who confirmed that the seat height mechanism was required to be motored to the top after use, to allow easier visibility of the handle/pin. However, it reported that the Red Arrows maintainers had been instructed to leave it as is; the implication being this was to shave seconds off a start-up regime, which the Panel said was shorter than normal for other Hawks. I know nothing about Hawk operations, so cannot comment.
My last comment actually highlights one of the main errors made by the HSE and Judge (and MoD in general). I've never been in or worked on a Hawk. But that doesn't stop me understanding basic fitting practices taught when a sprog. Or prevent me from reading and understanding the manuals and APs (which are both excellent). The Judge rejected expert evidence from a highly qualified mechanical engineer on the basis he was not an aviation engineer. As you rightly said Pobjoy, the basics apply to other domains, such as marine engineering. Similarly, the HSE denied that the scissor shackle assembly was in any other seat, because these other seats pre-dated Hawk. A 2 year old can destroy that logic, but.... Similarly, it said information relating to previous Marks of seat was irrelevant, because they were not Mk10s. The fact that the many parts of the design remained the same was also irrelevant - although in this case the HSE went further and denied the scissor shackle was in any other seat. The Judge confirmed it WAS, since 1952, but the HSE just ignored her and continued with its claims even after the trial. What am I saying? The HSE was incompetent, and actively misled the courts and family. MoD was complicit, as it knew this and said nothing.