From the MAK Report, Page 169: -
As the information on the landing system was not discussed between the crew and the controller, the landing radar was not requested by the crew, the crew did not read back in most cases the controller’s information and the altitude information was not reported to the controller during the descent on final, the investigation team assumes that actually the crew did not conduct the landing radar+2NDB approach. The crew made the "trial" approach using their own instruments, autopilot and autothrottle. This type of approach is not described in the FCOM so the weather minima and SOP are not determined.
This is an important conclusion from the investigating group. Perhaps it cannot be proven absolutely, but to me it seems entirely logical and as such deserves further consideration because it could be the key to understanding the basis on which the approach was conducted. There are several rather vague references by MAK to the “onboard equipment”. The report determines beyond doubt that LNAV data from the FMS was being used for lateral navigation but says nothing definite about guidance for the vertical profile. The inference is they believe FMS distance was being used like a DME, which would make complete sense.
The use of an LNAV approach (assuming continuous updating by DMEs or GPS) to navigate a non-precision procedure is sensible, provided that the profile conforms to the published and approved procedure. In such a case the approach is included in the FMS data base and can be checked against the appropriate chart. The difference here was that Severny was not even in the data base so the final four waypoints were loaded manually. This could still be helpful in assisting a VMC approach or even a cloudbreak followed by a visual approach. In no way should it be considered a satisfactory standalone IMC approach procedure to low limits.
Let us now consider the ATC position. They believed that PLF101 had some sort of special equipment on board and so specified 100m as the minimum height for the approach. This was also the DH for the Radar+2NDB approach. They were not to know the crew did not even have the correct chart on board (they only had the non-radar plate - MAK unfortunately has not published this) and they were not to know the basis on which the crew intended to conduct the approach. If they had known all this they surely would never have set limits as low as 100m. They expected the crew to conform to the Radar+ 2NDB, then provided the corresponding service. But the crew had their own separate agenda. The essential contract between ATC and crew was never established. Crew and ATC were at cross purposes.
So looking at the approach analysis we see that they did not commence descent at the ATC radar “Entering Glideslope” call, and they made significant adjustments to their descent path that clearly bore no relationship to ATC’s “on glidepath” calls. The answer to the very pertinent question of whether should ATC have provided more timely descent path information may be “yes, but the crew would have not responded to it” .
Now consider the crew perspective. They would have established at or even before their preflight briefing that data on Severny was poor, they had only the non-radar 2NDB approach chart available. Did they even know about the existence of some sort of radar and if so did they dismiss it as old and of poor accuracy ? It is easy to imagine why they may have considered FMS guidance a better method of approaching Severny. And so it would have been if the weather had been better - they did not know about the fog.
It is noteworthy that once they were aware of the fog, the CVR shows no evidence of any crew discussions on limits for the approach, until, with the flight about to commence base turn, ATC stipulates 100m. What would they have done without this ATC intervention?
Next the approach itself. Their destination user-waypoint XUBS was the aerodrome ARP not the runway touchdown zone. This means that if they were using FMS distance to XUBS to follow the published profile, their descent initation point would be approx 1.2km inside the formal FAP (unless allowed for in their calculations).
Further, it looks probable that FMS distance was annunciated in nautical miles, not kilometers (their waypoint 10XUB was a distance of 10nm, 18.5 km). They could have converted nms to kms, but surely an easier mental calculation, 500m from 5nm means 100m per nm, which happens to correspond almost exactly to a 3-degree glideslope . But 5nm is 9.3km and this could bring their descent point a further 700m closer to the airfield.
So total distance discrepancy 1900m i.e. 8.2km to TDZ, , and Fig46 of the MAK report seems to indicate descent initiation in the range 8.7- 8.5 km, and the leaving 500m height at 8.2km. Co-incidence?
In an NDB approach, height at Outer Marker is an absolutely fundamental component of the procedure. PLF101 crossed 120m too high. That is a major discrepancy yet there was no crew comment! Why not ? Was it because they accepted they were flying a different glideslope ? Were they even looking at their 2NDB chart ? MAK believes the dramatic increase in descent rate to 8m/s was a correction to the OM height excess - but that did not commence until 35 seconds after the OM. Is that likely ?
The approach method may or may not have been the direct cause of the accident. They still could have flown their unauthorised unapproved approach and gone around safely. We will never know the judgements and thought processes of the crew and especially the PIC during the final 40 seconds before impact, in particular concerning the initiation of and failure to recover from the excessive descent rate.
Apologies for the length of post in developing this theme It is intended to show how a non-standard approach using “on board equipment”could have been conducted . It seems to fit, but of course it is ultimately unprovable