Originally Posted by
Chesty Morgan
Dear Mods, any chance you could give the altimetry geeks their own thread? It's been done to death and we don't need to see reams of calculations cluttering up the technical stuff.
As one such 'geek' (if what happens in modern cockpits doesn't actually need much human mathematical verification?), I don't entirely disagree. I do well appreciate most PP readers here do want to try to understand the foibles of AOA sensors and stab trim runaway with MCAS, and also to be able to scope the data inputs to MCAS so they have some idea of "what's it doing now". PPs will want particularly to be able to answer the related big questions about how MCAS really behaves armed with dodgy AOA and other data, and how to disarm it.
But the reason perhaps why some of us are questioning altimetry may well be because we see very low altitude reports from the outset, and wonder if this accident began with something else entirely e.g. an error during the take-off roll.
Fact is, unless we toss in the benefit of a couple of hundred fictitious feet for doubt, there otherwise seems precious little evidence from the tele-altimetry - which looks very consistent and accurate in so many respects all the way up to rotation - that the aircraft then left the ground at all normally.
If there was a static pressure increase at rotation (if that is a known feature of the aerodynamics at high AOA leading to the "below ground" 05:38:47.714 altitude of 7075') it would affect cockpit IAS, and VS indications too, would it not? And if that is evidence of an aerodynamic anomaly, and it is
not a spurious measurement, then might other static pressure anomalies exist at the static port positions at high AOA with this airframe?
That said, the first ADS-B altitude report that has to be a truly airborne report - because it may be the first that can be truly validated as being above aerodrome level - is timed at 05:39:04.028. Yet that one is well over the dirt which has sloped some 150ft down from the runway by that point. Putting full credence in ADS-B 1013 altitude data, and taking Luc Lion's reminder of 420' differential from QNH, a quick look at that particular datapoint using the "Viewshed" tool in Google Earth, raises a question as to whether a spectator standing behind the other end of the runway could even still see the aircraft. Was it indeed so low that it disappeared off the end of the runway to such a spectator at the 07R threshold end? The previous datapoint is 5 secs earlier at 05:38:59.102, also over dirt not tarmac, and located 1542 feet further back. There is nothing in any of the datapoints along the centreline of the runway to suggest that they are anything other than spot-on location-wise. The two data points indicate an average ground speed of 1542 feet (0.2538nm) in the 4.74 secs between them, or 0.2538 * 3600/4.74 kts = 193kts. The actual reported speeds transmitted as those two datapoints are 200 kts and 207 kts (not sure how they are computed onboard the aircraft before transmission - sourced from onboard GPS or from INS?).
And at that 05:38:59.102 datapoint - the first just beyond the runway - ET302 may have been 100ft (+ or - 25ft) above the by then already downward sloping dirt. The VS between the two points assuming static pressure was then lapsing in book expected linear fashion should have indicated (7300'-7225' + or - 25') * 60/4.74 ft/min = (100' or 50') * 60/4.74 ft/min = between 633ft/min and 1266 ft/min. The lower would seem to reflect what was given in ADS-B reports and that in turn may suggest the aircraft was indeed still fighting for altitude down in the weeds.
IF it was, why was it in the weeds in the first place? It was still only 12-14 secs after initial signs of a rotation. MCAS doesn't explain weeds so soon on its own, does it? Did something happen first on the runway, which has not been much discussed (if something did happen), and did it line up a couple of holes in the cheese? And then did MCAS offer the third or fourth bad thing and the eventual gotcha? Or was the take-off normal and only afterward went pear-shaped?