Originally Posted by
Aluminium shuffler
Given how many non precision approach accidents have been caused by misset altimeters, it seems a plausible explanation, especially if the local QNH was below standard.
I share your suspicion about a misset altimeter but it appears that the QNH was 1019 hPa according to a weather sequence attached to one of the YouTube videos.
Originally Posted by
RAT 5
I would suspect an A330 flies a CDA. I've never been there, but is the GA flown passing 2nm DME or at 500' QNH. My point being that on a CDA 500' QNH is not at 2DME, but later. At 2nm DME you'll still be above MDA. On a Dive & Drive the GA will be 2nm from level flight.
From the approach plate it lists the MDA (H) (CONDITIONAL) as 500 feet. It's one of those 'find the faces in the picture' exercises they used to love in airline interviews but it looks to me like you can go to 500 feet if you can stay at 205 knots or below on the miss until you are on the 180 degree track. It's note 1 on the bottom, not to be confused with note 1 on the top of the plate about DME required.
If you can't stay at or below 205 knots in the turn, your MDA is 770 feet it appears to me. I'm guessing the B-738 on a straight in approach is Cat C for the right side mins table.
I agree that it seems that the 2.98 degree path puts you higher than 500 feet at the 2 DME fix. I get about 601 feet for the charted 1.9 nm to the threshold, plus 50 feet for the TCH and 14 feet elevation for about 665 feet at D2.0.
So, you would indeed go missed at the D2.0 [MA10] point on a CDA before reaching 500 feet. I haven't trained for or done a dive and drive non-precision approach for many years and I thought they were pretty much extinct in airliner ops by now.
But in the past, there were options to do a CDA on a path in VNAV, speed intervene (to avoid throttle surge on some Boeings), a CDA on a path with V/S or a dive to mins early, level off and then land or go missed at the MAP depending on what you saw. On the CDA's you would go missed at MDA without visual contact and not level off even if you hadn't reached the charted MAP.
Is it possible that the WestJet crew saw that they could go just a little lower with an early descent below path to 500 feet before D2.0 and lost track of the altitude before breaking out so low? Does the B-738 give an audio callout at minimums on path that they were expecting but may not have got due to the setup in the box? Obviously, altitude needs to be closely monitored whether the automatic callouts work or not.