Originally Posted by
slast
Unfortunately not as simple as it sounds as the chart DME heights also need to be cold-temp corrected.
It appears that Air Canada got the approval of Transport Canada for a chart in the QRH (not shown in the report) that gave low temp corrections for FAF altitude and the MDA, rounded up to the nearest 100ft, and for the FPA itself. The numbers were apparently correctly extracted for these. But there is no mention of DME altitude corrections, which could be different for each approach and not easily amenable to 100 ft rounding since they would actually become less than 100ft as the aircraft descends.
Once the descent was started the aircraft was ALWAYS below the intended flight path. But using the indicated altitudes against the basic chart values would have produced the following indications:
6 DME indicated alt 2080 = 140 ft HIGH when actually 40ft LOW
5 DME indicated alt 1650 = 70 ft HIGH when actually 70ft LOW
4 DME indicated alt 1240 = 10ft LOW - near correct but actually 110ft LOW
3 DME indicated alt 840 = 80 LOW when actually 150 ft LOW.
Which brings to mind the question of what procedures AC used to deal with this situation for older aircraft, without a FPA facility?
The way I see it is to comply with that "Second Gate" they have in their procedures, the AC crew could have chosen one of the DME fixes from the chart that most closely matched a point that was either at 500 feet AGL, or 100 feet above the MDA, computed it's cold temperature corrected indicated altitude, and used it to evaluate where they were relation to the required slope. If too low, as AC does not allow FPV corrections after the FAF, then they would have called for a Go Around, as per the SOP.
The crew might have done all of this mind you. We just don't know, for the TSB authors, or the lawyers that later edited the original report, decided it was not necessary to include this information in the report.