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The AC646 accident report contains a discrepancy regarding the angle of attack at which the stickpusher of the accident aircraft (a CRJ100, formally a CL-600-2B19, referred to in the aforelinked AAR as a CL-65) would have activated:

The stick pusher mechanism is designed to prevent the aircraft from entering an aerodynamic stall by applying a control column force to pitch the aircraft nose down as the aircraft reaches its computer-calculated pusher (stall) AOA. The Canadair Regional Jet Airplane Flight Manual (AFM) contains a chart (page 06-01-23) which allows calculation of the aircraft stick pusher (stall) speed for various aircraft weights. For 44 180 pounds, the aircraft’s estimated weight at the time of the accident, with the flaps set at 45 degrees and the landing gear down, the pusher activation speed would have been about 109 knots calibrated (109 knots indicated) at one g. At the moment of the aerodynamic stall, the aircraft was at a g loading of about 1.227, and the pusher activation speed would have been about 120 knots, the natural stall speed would have been about 116 knots, and the pusher activation AOA would have been about 13.5 degrees. [...] At the moment of stall, the flaps had retracted a few degrees from 45 and the g was increasing, both of which would cause a slightly higher pusher activation speed; however, there would be no effect on the pusher activation AOA.

Information from the flight recorders indicates the following events occurred during the attempted go-around: the stick shaker activated when the aircraft was at 129 knots as the pitch was being increased through 4 degrees; and the right roll and stall onset occurred and the warbler tone activated when the aircraft was at 124 knots and with the pitch at 9.7 degrees. At the time the warbler sounded, the left and right AOA vane readings were approximately 8.7 degrees and 9.4 degrees, respectively. The pusher did not activate because, while the right AOA reached its trip point, the left AOA did not. [TSB Aviation Occurrence Report A97H0011, page 8 (page 18 of the PDF), my emphasis.]

In the first of the paragraphs quoted, the stickpusher-activation angle of attack is stated as approximately 13.5 degrees (a value which, according to the report, would not have been altered by the aircraft's rapidly-increasing AoA); however, the very next paragraph implies that the stickpusher would have activated at an AoA of between 8.7 and 9.4 degrees (the right AoA sensor, reading 9.4 degrees, is stated as having reached the stickpusher point, while the left sensor, reading 8.7 degrees, had not reached the activation threshold by the time the aircraft stalled, thus implying that the threshold AoA for activating the stickpusher was above 8.7 degrees but below 9.4 degrees1), rather than at about 13.5 degrees.

How to reconcile this discrepancy?


1: Due, presumably, to the potentially-catastrophic results of a spurious stickpusher activation at low altitude, the CRJ100/200's2 stickpusher does not activate unless both of the aircraft's angle-of-attack vanes measure an AoA at least as high as the stickpusher-activation threshold; in contrast, all the automatic high-AoA protections that do not alter the aircraft's flightpath (continuous ignition, stickshaker activation, autopilot disengagement [same threshold as for the stickshaker], stall-warbler activation [same threshold as the stickpusher], and stall-warning-flashing-light activation [also same threshold as stickpusher]) activate even if just one of the two vanes measures an angle of attack equal to or greater than the requisite threshold.

2: Although the accident aircraft was a CRJ100, the CRJ200 is identical to the CRJ100 except for using a slightly-different variant of the CF34 engine; the two even share the same CL-600-2B19 classification (making it somewhat difficult, when reading AARs discussing first-generation CRJs, to figure out whether the aircraft being referred to is a CRJ100 or a CRJ200).

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Your wrote:

a value which, according to the report, would not have been altered by the aircraft's rapidly-increasing AOA

The report did not say that (but I can see how that confusion can arise). It said the 1) flap retraction and 2) increasing g load -- would not alter the AOA trip point of 13.5 -- no mention of rapidly-increasing AOA:

the flaps had retracted a few degrees from 45 and the g was increasing, both of which would cause a slightly higher pusher activation speed; however, there would be no effect on the pusher activation AOA. [emphasis mine]

"Both of which" references the two conditions 1) and 2).


What would alter the trip point however is explained on the page before:

When the AOA is changing, the computer notes the rate of change and, if necessary, applies a correction to activate the protection system ahead of its normal trip points. [emphasis mine]

Which is what the computer did, as explained on page 49 (PDF page 59):

The left-hand and right-hand AOA vane angles as recorded on the FDR were compared with the shaker and pusher boundaries adjusted for the phase advance resulting from the rate of increase of aircraft AOA. This comparison revealed that both AOA vanes reached the shaker trip point, but only the right-hand vane reached the pusher trip point (...) [emphasis mine]

In other words, the trip point would be the AOA phase advanced to account for the AOA rate in the Stall Protection System computer. Which as you noted, was somewhere between 8.7° and 9.4° for the stick-pusher.

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    $\begingroup$ Makes sense, though the report could've been worded more clearly. $\endgroup$ – Vikki - formerly Sean Mar 6 at 21:01

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