JP1
You wrote:
<<In 94 the GPS on board the Chinook was almost certainly used as a stand alone system, it would not have had its data blended with other kit via kalman filtering etc.>>
ZD576 was an HC2 Chinook with the SuperTANS (as opposed to previous TANS as in Mk1 Chinook) which did indeed blend the data and the presentation did not allow the separate components to be viewed.
The components “blended” were Doppler radar and GPS.
The Doppler radar sends a continuous chain of pulses ahead and to the side of the a/c track so as to get from the Doppler shift of the returns from the ground longitudinal and lateral velocity vectors (hence “Doppler”).
The Kalman filters gave greater weighting to the Doppler components than the GPS – the idea being that the almost continuous and accurate (?!!) ground speed vectors:
interpolated between GPS data points;
modified (to an extent depending upon the filter coefficients) the latest GPS spot position;
and (a tactical consideration in the presence of jamming of the GPS) in the absence of GPS input allowed the nav computer to dead reckon using the ground velocity vector data.
Of course, the great weakness of Doppler navigation radar is that over water either a flat surface gave rise to specular reflection (giving no decent reflected signal back to the a/c) or when the surface was rough the many velocity components of the waves played havoc with it – after a water crossing such as ZD576 had just done, irrespective of the other potential shortcomings mentioned by others, the SuperTANS could be out by ˝ a mile or more and this was to be expected.
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Flt Lt Jon Tapper was well aware of this limitation and had warned other members of his flight.
He would not have relied upon it for an accurate range to go to the Mull – indeed, if relying upon the SuperTANS alone, he would have factored in a safety margin.
The relevance of the SuperTANS in this incident is that had it been in contradiction with another reference, the SuperTANS would have been thought to be in error and therefore could be ignored – and is this not what happened?
Did they not switch to the next route waypoint (thereby dismissing the SuperTANS), make a small corrective steer to the right (into danger), and continue on?
Flt Lt Tapper was very familiar with this area and had landed a helo on the Mull lighthouse landing pad on previous occasions – he would have known the danger of getting too close too fast to the landmass in that location.
As an aside, do you not find it amazing that this officer was accused of being slack in entering the lighthouse coordinates wrongly when the coordinates he had entered are exactly those of the landing pad?
AND EVEN MORE AMAZING THAT IN 12 YEARS NOT ONE PERSON CHALLENGED THIS RIDICULOUS ASSUMPTION OF THE BOI?
I believe that the landing pad was a well used reference/ turning/ way point for exercises and transits in this area – AND I BELIEVE THAT THE LANDING PAD WAS THE INTENDED TURNING POINT/ RV ON THIS FLIGHT AND THAT THE COORDS WERE ENTERED CORRECTLY.
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And another aside: the setting on one of the altimeters was found to be wrong for transit and it was suggested that the setting was the QFE for Aldergrove –
Here is an extract from a post by Cazatou back in 8 Jan 2001 which summarises this well:
<< One of the first things that is taught in Flying Training is that if you are flying below Transition Altitude then you set the regional QNH on the subscale of your altimeter unless you are on approach to an airfield when you set the Airfield QNH or QFE as directed. The AAIB investigation into the crash of ZD576 showed that the Right Hand altimeter was so set but the left hand altimeter was some 10mb lower. There was no evidence that this setting had been altered by the crash. The BOI speculated that it could have been set on Aldergrove QFE.>>
did anyone check if it would have been a suitable value for QFE at the Mull lighthouse landing pad at the time of their journey?
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Looks to me like they were intending to get close, if not actually land, at the landing pad.
Now before you start honking again, get yourselves a map and mark the position of waypoint A;
Imagine walking up the hill ˝ a mile or so, say along the road for a bit and then off a bit just beyond the edge of a little cliff, and plonking yourself down – mark it on the map;
From the approximate estimated position of waypoint change (use the Boeing doc “Anal of Avail Data” – the drawings are useful) to your spot up the hill, draw a line;
Sort of ties up with that steer demand right after waypoint change doesn’t it?
You see, if you were a stooge holding a PRC112 and you were supposed to be at the landing pad BUT were actually up the hill, any unfortunate pilot undertaking an exercise with you would have thought that he had ˝ a mile more to go to get to the landing pad.
In their normal use, this equipment is very accurate (effectively, a portable DME); it also gives approximate azimuth bearing and added confidence is had in its use as the pilot can be talking to the person on the ground who can vector the helo in (the handling pilot’s intercom was found set to the appropriate UHF channel).
DME systems are used widely by helo pilots and are trusted – it is the one system that could override the pilots’ confidence in other systems or the pilots’ visual judgment in marginal forward visibility.
It is a pity that SH pilots who have used this system operationally have not been canvassed for comment or described their experiences with it themselves on the forum.
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You may say that this is wild speculation, but absolutely everything that can be deduced from the available data fits this scenario.
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The equipment in operational use in HC2 Chinooks by 1995 was designed for easy transfer between such Chinooks – easy to fit and easy to remove – as I cannot see there being any significant changes between the first operational HC2 Chinook (ZD576) and those in 1995, I believe that it was possible to fit such equipment at the last minute and retrieve it from the wreckage without leaving much evidence as to its ever having been there – how handy if there was a cock up one wanted to hide.
It remains for someone to come forward who knew of its fitting before the flight.