Map Shift?
The ADS-B data coupled with the nature of the actions taken in the final descent and the location of the crash site have brought out a puzzling issue. The ADS-B position while LMI2933 was holding at RNG VOR/DME are displaced about 10 nm south of the VOR itself. Either the ADS-B IRS position has drifted exceptionally, or is it possible the moving map shifted because of a bad navaid update?
The use of inertial data introduces drift. Normally, aircraft navigation systems account for drift by applying radio-nav updates to null out any accrued errors. I have no expertise with the RJ85 flight management or navigation system, so I can only guess what might have happened based on general experience (and happy to receive any suggestions or corrections).
When a moving map is provided, a navigation data base presents the fixed features, and the IRS provides the position and heading source.
IRS drift can be measured by comparing radio navigation aids to reveal any offset. Inherent in this calculation is the navigation data base that stores the location of the navaid.
The favored update is to take intersecting DME measurements from two VOR/DME. In the event only one navaid is available, a VOR/DME measurement may suffice.
A map shift is when the map moves suddenly to reflect a radio navaid update. Normally a map shift takes out the error, but in this case the question was it putting in an error.
If the DME or nav data base readings were in error on LMI2933, where LMI thought the navaid was in a different place displaced south by about 10 nm, then the flight crew might be confused and follow the map guidance without picking the error (by using DME 1 or DME 2 raw readouts, or looking at the VOR DBI when crossing over and turning outbound).
The flight crew had power when they started their emergency descent and were probably thinking they will make it.
The flight crew were diving for 10,000 feet by the final approach point (FAP), just 0.63 nm north of the VOR. They were setting up for capturing the 3 degree glide slope at the FAP.
If there was no map shift, and the moving map was correct, we have no answer for the decision to operate to the south.
LMI2933 powered rate of descent and configuration would be without regard to the issue of flameout.
If the flight crew knew where they were (assuming ADS-B reports are correct), then they still had 5,000 feet to the FAP in about 10 nm (or about 1,000 fpm).
If they thought they were operating north of where the ADS-B reports portray, because of a bad navaid map shift, then they might have thought they need to lose 5,000 feet to the FAP in just a couple of nm (or about 5,000 fpm).
The last report showed their rate of descent at around 2,500 fpm (prior to flameout). That suggests a map shift may have disrupted situational awareness and that they were thinking that they were much too high for the approach.
The extended glide slope would have been at about 13,000 feet 20 nm out, and 16,000 30 nm out. LMI2933 would have been above the glide slope before the engine flameout, yielding no useful information beforehand.
Note that DME is lost on EMERG AC/DC power, and if the crew looked at the VOR once power was lost (from the moving map), it would have shown the same radial in either case.
The ADS-B data itself is more consistent with a 2 nm error in reported position (noting that the ADS-B data source probably does not reflect radio navaid updates), which suggests a gross error in technical performance to the holding pattern. A map shift is one explanation. This is a quandary that will be cleared up with the official report.