Meadowbank
Look at the picture posted by Tarnished 1st September on Mil Aircrew thread entitled
“Typhoons and Reds” .
Apart from being a great shot, look at the mist on the other side of the water – readers not so familiar with localized coastal mist like on the Mull that day can get an idea of the problem using this picture and a bit of imagination, thus:
imagine the middle bit extending across the whole field of view (put your hands over each side to mask the shoreline);
use some tape to mask the aircraft and that one wave;
imagine the top disappears into the cloud base of the same colour;
then you would have the kind of view that would commonly confront crews approaching from NI on the leg to the Mull lighthouse – see the difference?
Common sense dictates that you would have to stay well clear if relying on visual judgment alone – you wrote:
<<There is nothing awkward about this turn so no special arrangement would be necessary. It is the kind of turn made every day by helicopters (and fixed-wing aircraft) on low-altitude flights. Indeed, this appears to have been a "regularly used leg" (your words) - hardly a likely situation if there was any extra element of risk involved.>>
I believe that it was a regular thing to turn in close as described to me by a witness who had reason to know – and while talking to him near the crash site in identical weather conditions one did just that. I believe that it was risky to have approached closer than, say, the waypoint change in these conditions unless they had something else to go on, never mind aiming to turn right on the coastline.
You wrote:
<<Mode 3/A is the standard ATC mode used by all military and civilian transponders.>>
Yes, I apologize for getting confused with some technical information on a commercial (private) system that does it all – in an HC2 (i.e. MK2 equivalent to CH47-D), a separate interrogator is plugged into the (AN-ARC 164) radio (as is fitted to the MK2 Chinook) – mode 3a is significant here only in that it was the correct mode to squawk a code describing the activity of the aircraft. The (US, don’t know NATO at moment) NSN # of such an interrogator is 5895-01-494-5228.
And:
<< I don't know about the sets fitted to Chinooks, but on some military IFF sets, having the first 2 digits set to 77, irrespective of the settings of the 3rd and 4th digits, identifies to ATC as an emergency squawk.>>
Not so:
There are many international references to squawk codes (including one I have found specific on 7760 being associated with ground transponders) but what the UK military uses would be classified and so I cannot myself absolutely confirm the meaning of 7760 to this flight – it would be nice if someone who could would come forward.
Just to counter your broad dismissal that anything starting 77** means an emergency squawk, I can quickly grab this reference:
<<ICAO CAL Edition 2
Supplement to EUR ANP FASID May 2003
Code 7776 and Code 7777 are reserved for SSR ground transponder
monitoring.>> - please note the first 2 digits are “77”.
Another that suggests that 7760 has an interesting meaning is:
<< RRR 7750-7799 ANDOVER RAF SPECIAL TASKS>>
If Boscombe Down had wanted to evaluate a new NavAid on the Mull and had sent its old workhorse up there, it would have been entirely appropriate for that aircraft to have been squawking 7760 (I think it specifically means “Ground Transponders Tests/Trials”, but the exact meaning is moot – it is within a meaningful set of codes); what if they tasked ZD576 (being up there already, newly suitably equipped – in fact the only Chinook in NI so equipped – after the recent upgrade to MK2, and after all it was for the benefit of such helicopters doing that leg) to evaluate a PRC-112 set placed at the Mull helipad as a cheap solution for a DME beacon? Would it not have been entirely appropriate to have been squawking 7760 – just to remind ATC if it did an unusual maneuver, like a closer approach than normal for the conditions?
Perhaps that ZD576 was the first to use it goes a long way to explaining the unfamiliarity of the suggested practice to other pilots; no doubt, after the disaster, it would not have been used for a while – and many would not like it to have been made public. ATC Aldergrove could clear this up by stating what squawk code they had been picking up from ZD576 before the crash – and any specific meaning that it had.
<<This could have been the intention of one of the crew, indicating that some form of emergency had occurred.>>
This was suggested the inquiries; two points:
1 I remember reading somewhere on SSR codes that it may not be desirable or prudent to change (or attempt to change) the code in an emergency in certain situations (I’ll leave it to you airmen to look this up yourselves) – I can’t see what would have been the point in the limited time that they would have had and it may have caused his confusion (possibly one of the conditions where it is not recommended to attempt change?);
2 ATC had them for at least part of that leg – what code was ATC detecting prior to the problem (no more generalizations, please)? – oh, and see above!
You wrote:
<<SuperTANS incorporates GPS positioning so why bother with a separate system that, you tell us, gives a range-only readout?>>
SuperTans is a hybrid system that takes input from both GPS and Doppler radar. The Doppler gives speed vectors to the computer for interpolating between GPS point positions and by way of mathematical filters the computer combines the information.
Each component has its own typical errors and, as the combination of the components is mathematically complex, if one component is poor or spurious the computed position can be much worse than could expected from the other component alone.
The Doppler radar requires a decent return from the ground and is notoriously unreliable over water – it was fortuitous that the TANS was accurate at the point of impact. This crew knew of the inaccuracy of the TANS (one of the pilots having warned the rest of the flight shortly before the final one – and the father of one of the pilots commented on an error of more than a mile in a fly pass he witnessed) – they would certainly not have relied on the SuperTans for a close in turn after a sea crossing.
DME, on the other hand, is normally very accurate and reliable – the ground component would have to be intentionally messed with to give a false reading. If you had doubts over another system (as this crew did in regard to GPS on that day) DME is a good reference to compare with – ZD576s TACAN CU was set to pick up the Machrihanish TACAN (which as you know is part DME) for when it came into LOS.
And:
<< Tandemrotor (a Chinook operator) has not heard of the system you describe so it seems unlikely that it's fitted.>>
Fair enough, ZD576 was the first HC2 in NI (and therefore the first to do this crossing with the avionics upgrade). The upgrade to Mk2 makes the HC2 equivalent to the (US) CH47-D which has the SAR equipment to interrogate the PRC-112 portable ground sets. The SAR equipment (NSN 5895-01-494-5228) just plugs into the AN-ARC 164 radio (as fitted to CH-47Ds).
Just for interest, check out the DME knob on a PRC-112 (stands out like a dog’s balls) – you can get a picture from a manufacturers website – makes it very clear what it can be used for and how easily.
Now to address your comments on my posting entitled “THAT TURN”
I understand the setting of an altimeter – I was making an analogy to try to get the idea over (to readers of this thread) of how having an instrument reading can bias one’s visual judgment. I was not suggesting that they were descending at all – just using a vertical judgment analogy with the problem of judging their distance off the mist covered landmass with the possible influence of a distance measuring readout (that may have been incorrect).
<<The altimeter is only as accurate as the Regional Pressure Setting (RPS) that is set on its subscale. The RPS is forecast by the National Met Office (nr London) and is notoriously unreliable. This flight was carried out entirely within the Belfast Altimeter Setting Region and the same RPS is used all over it, even though the actual pressure could vary by several millibars within this area - the altimeter is therefore not to be trusted.>>
As I tried to explain above, the detail of the altimeter is irrelevant, but what you say is of interest – I have learned something. How things have changed from when I was young – I thought the QFE (“atmospheric pressure at aerodrome elevation”) on the ground at the departure airfield was set by zeroing the altimeter (by way of adjusting the pressure setting dial) so giving good accuracy for local flight.
<< The ground is not featureless - it includes one of the best features available, a coastline.>>
Yeh, right – want to check out the picture referred to at the beginning of this posting? They would have been OK once over and parallel to the actual shoreline (the mist forms over the ground as the air rises and so the actual shoreline is clearly visible below). They could have stayed well clear – it was clear at sea – but how well clear? The problem was closing the gap from an obviously safe distance to being safe parallel above the shoreline without slowing down appreciably. As I have tried to describe so often, now, this one turn would have been awkward for anyone to judge in those all too common conditions – spoiling a nice low level flight with an arbitrary, often arguably unnecessary, early, untidy turn if playing it safe.
<<The countdown you have described is not part of Standard Operating Procedures.>>
The book “Chinook” by David McMullen suggests very active involvement in navigation by the non-pilot crew – I just suggested that one of them may have been watching the SAR readout (I don’t know what you call it).
<<IF the portable IFF that you have described exists, the fact that it might not be in the position at which you believe it to be is a very good reason not to trust it in the way that you suggest.>>
Fixed DME ground equipment (often co-located with VOR, especially as a TACAN) can be tampered with very easily to give a false reading (by altering temporarily the fixed delay) – this does not stop its use as such tampering is not generally anticipated; the portable unit could easily be moved, or possibly switched off and another out of position switched on – well, the debate on this thread should tell you that such action would not have been anticipated!
<<I believe that your suggestion can therefore be discounted.>> and it has been for 10 years! I think more strongly than ever that it deserves consideration, though.
Well, where does all this leave us?
At least most of you can now appreciate the local weather conditions.
And the debate on this thread has made it clear to all that the intended flight path was low level with a slight turn up the coast.
Perhaps now you can also comprehend that this HC2 had the capability to interrogate a portable set in DME mode.
I believe that not only were they using a DME beacon but that they were tasked to do so (the squawk code is the vital clue). Whether such a beacon was correct or not, its exclusion from consideration at the inquiries renders the verdicts on the pilots untenable. As this is one way to clear their names, why not make the effort to get the squawk code as received by ATC (or as known to be allocated)?