SuoerTANS
I am not sure what relevance the navigation errors of the SuperTANS might have had on a VFR flight where it would appear that the crew had changed waypoints based on visual identification, but taking JP1’s point the following information may be relevant.
The CA Release words for the RNS 252 say:
The RNS 252 system requires the input of the relevant aircraft compass deviations to minimise Doppler/GM9 errors.
(a) RNS GPS (Ext) Position. In addition to paragraph 2.2 above, (the GPS words I put on post 2639) the GPS is highly susceptible to jamming of which the only crew indication is loss of GPS. There is no perceptible difference in accuracy between 2D and 3D navigation. The "Err" figure displayed, which has conventionally been taken as a measure of GPS performance, is meaningless and so no indication of the accuracy of the GPS is available to the user. The RNS 252 suffers from an average two second processing delay on the displayed GPS position. There is no significant degradation of the GPS with manoeuvres up to +/- 60 degrees roll and up to +/- 20 degrees pitch. In "Ext" position, Doppler reversion should be selected to minimise navigational errors during GPS outages.
(b) RNS252 Doppler/GM9 Position. The RNS 252 Doppler/Compass 95 percentile radial error on trial was 2.35% of distance travelled using an RNS 252 with aircraft compass deviations entered. Improved aerial alignment on all other fleet aircraft (presumably including ZD576) should result in a reduced error.
(c) RNS 252 Doppler/INS Position. The accuracy of the Doppler/INS navigation system on trial was 2.81% of distance travelled, overland, with a recently harmonised INS.
The initial AAIB finding, given below was that the RNS252 could have been off at the time of impact, but the Racal report showed that this was not the case:
The unit was recovered from the cockpit wreckage area slightly fire scorched but with little apparent impact damage (Fig 16.1). The ON/OFF switch was found at OFF, and an absence of substantial damage to the lift-toggle type switch, including its ramp mechanism, together with ground fire sooting patterns suggested that this had been the setting at impact. However, subsequent examination and memory read-out by the manufacturer, Racal Avionics Ltd, reportedly clearly showed that unit had been operating until impact and it appeared that the switch position had in fact been altered by the effects of the crash. The detailed investigation of the unit was covered in a Racal Report.
The Racal report shows that at the time of impact the differences in the system were:
Crash Site: N 55 18 67 W 005 47 65 (It is not clear if this is the initial impact point or the centre of wreckage distribution, but I assume power down and the initial impact would have been simultaneous given the catastrophic break up of the aircraft)
GPS at power down: N 55 18 61 W 005 47 80 (GPS was the selected mode)
Doppler at power down: N 55 18 65 W 005 47 49 (The Racal report calculates the Doppler position to be approximately 330 M east of the GPS position.)
The Racal report says that at power loss the SuperTANS display page would have been showing “TacB” (Tactical steer to waypoint B), with a steer command left 10 degrees HTS (Heading to Steer) = 025 degrees magnetic, Dis: 85.7NM (distance from aircraft position to WPB) and TTG 32.0 (Time to go in minutes to intercept waypoint B)
So, apart from an explanation as to how the gated switch got to "off" without leaving any damage or trace, the system errors would, to my unskilled mind, appear to be OK, and well with the expected RTS parameters - but we do not know what they were at the time of the waypoint change, or, indeed, at any other point in the flight. We also do not know what reliance the crew were placing on SuperTANS at that instant, but the WP change had been made at least 0.95 NM before impact and the display page would immediately have been demanding a left turn as would their visual identification and their route planning. It seems to me that this is the point where, as always, we move to the realm of “we shall never know”! However, instead of assuming aircrew gross negligence at this point why not think of what might have gone wrong with the aircraft from the list of known problems with the Chinook Mk 2 and ZD576 in particular. Sadly the BoI never did this, although the Stn Cdr Odiham did go some way down this road.