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  • #76
    Originally posted by elaw View Post
    Are you implying the EPR indication is erroneous, possibly due to icing?
    I don't dare to "imply" it. I see this as one possibility.

    I haven't read the whole report, but my understanding is airspeed decayed in spite of "Otto" applying increasing throttle.
    Not exactly "increasing throttle". The operatrive parameter in this engines is EPR (not N1 like in others). So Otto increased EPR until it reached the maximum allowed, or......, the (false indication of) EPR increasded themselves to the maximum allowed, and Otto REDUCED throttle to avoid exceeding the limit (although in this case I would have expected a reduction in N1, N2 and FF).

    It strange that en MD-80 at 31000ft with both engines maxed out would not have enough thrust to sustain flight

    Another explanation could be that there is nothing wrong with the engines or their indication but that extremely hot air (for that altitude) was brought up by the vertical convection of the tropical storms, so the air density at 31000ft was greatly diminished and it "felt" as if it was say 43000ft. I haven't checked yet but I guess that the temperature increase above ISA would be ridiculous to make it happen.

    OTOH I think the fuel controllers in those engines meter the fuel based in part on the pressure reading at the engine inlet.
    I think that these engines are pre-FADEC and that the fuel metering valve position is directly linked to the thrust levers position.

    --- Judge what is said by the merits of what is said, not by the credentials of who said it. ---
    --- Defend what you say with arguments, not by imposing your credentials ---

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    • #77
      Originally posted by Evan View Post
      Yes, it can't be. Air Florida Flt 90 on automatics?

      ITCZ phenomonena ice contaminating the PT2 sensor at altitude / The AT reverting to MACH ATL when the mach target wants to add thrust and the EPR limit has been reached, witholding thrust to respect the EPR limit / So N1 and fuel flow decrease but EPR continues to rise...

      Is that what you are thinking?
      Yes, but I am not convinced myself. If the AT was keeping the max allowed EPR, I would expect to see a constant EPR with N1, N2 and FF diminishing.

      --- Judge what is said by the merits of what is said, not by the credentials of who said it. ---
      --- Defend what you say with arguments, not by imposing your credentials ---

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      • #78
        In any case, the problem seems to be two-fold: first, the crew, probably concentrating on the radar and navigation, fails to monitor the engine and airspeed indications which leads to thrust and airspeed reductions and gradually increased AoA; then, a flawed procedural reaction to compressor stall reduces thrust to IDLE while CRM breaks down and the crew, focused on the engines, fails to fly the plane, resulting in a fully developed stall. That's my guess.

        At around 1:45:00 there appears to be a compressor stall, perhaps due to ice finally becoming dislodged or increased ingestion levels. The AT is either shut off manually or quits. Throttles appear to be manually retarded to IDLE (no TLA plots), perhaps as a flawed procedural understanding of the correct compressor stall onset procedure (reduction, but not to IDLE). EGT goes to overtemp (the JT8D doesn't like sustained EGT much above 600°), indicative of compressor stall (core lock?). Below 10,000, it seems they have cleared the stall and effectively firewalled the thrust but without control surface functionality in the stall it is of no use.

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        • #79
          Originally posted by Gabriel View Post
          Yes, but I am not convinced myself. If the AT was keeping the max allowed EPR, I would expect to see a constant EPR with N1, N2 and FF diminishing.
          If the probe was becoming increasingly blocked, the EPR would be gradually increasing despite the thrust reduction. Perhaps there is something in the logic that prevents the EPR limit function from retarding thrust beyond a given limit as well, and therefore unable to absolutely respect the EPR limit...

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          • #80
            Disrupted airflow from the stalled wing is another possible cause for engine's "funny" behavior in low-wing, rear-mounted-engines airplanes.

            --- Judge what is said by the merits of what is said, not by the credentials of who said it. ---
            --- Defend what you say with arguments, not by imposing your credentials ---

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            • #81
              I would like to see AoA and stickshaker in the plots.

              AoA I know that they have the raw data, just that they don't know how to "translate" it yet.

              But this guys seem to be doing a very good job, with a lot of help from the BEA evidently. But kuddos to them for letting the BEA in. Other "developing" countries with mediocre accident investigations are too "proud" to let more experienced agencies in.

              --- Judge what is said by the merits of what is said, not by the credentials of who said it. ---
              --- Defend what you say with arguments, not by imposing your credentials ---

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              • #82
                Good point in a reader's comment in AvHerald:

                "the crew already decided that they were too heavy for FL 330".

                Why did they level off at 310? It's not like it's a super busy airspace.
                Why didn't they climb more, even if just a bit more (like 330 if it's VRSM, or 350 if it's not)?
                Maybe they at FL 310 were not so far from their assumed ceiling for the weight they were carrying, except that warm air brought up by the ITCZ storm raised the temperature enough that they were already a bit over their absolute ceiling, and thus in an unsustainable condition. The only way out was down, and they would go down in one way or another. They did't do it in a controlled manner, so "another" applied.

                Just an hypothesis.

                --- Judge what is said by the merits of what is said, not by the credentials of who said it. ---
                --- Defend what you say with arguments, not by imposing your credentials ---

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                • #83
                  Originally posted by Gabriel View Post
                  Disrupted airflow from the stalled wing is another possible cause for engine's "funny" behavior in low-wing, rear-mounted-engines airplanes.
                  Would airflow disruption really explain the engine plots after 1:45:00?

                  Would the momentary appearance of both high hydraulic temp and low hydraulic pressure indicators be indicative of an engine restart or electrical power restoration? It looks to me like they managed to restore the engines while pulling up relentlessly in a fully developed stall.

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                  • #84
                    Originally posted by Gabriel View Post
                    Another explanation could be that there is nothing wrong with the engines or their indication but that extremely hot air (for that altitude) was brought up by the vertical convection of the tropical storms, so the air density at 31000ft was greatly diminished and it "felt" as if it was say 43000ft. I haven't checked yet but I guess that the temperature increase above ISA would be ridiculous to make it happen.
                    Well I *tried* to check!

                    The service ceiling for that plane appears to be 37,000 feet. I tried a few online "density altitude calculators" to see what temperature would be required at 31K feet to make the density altitude 37K but the result was beyond their limit. Best I could do was 0 degrees C which yields about 36,500 feet. And 0C at 31K feet is damn warm!

                    Originally posted by Gabriel View Post
                    I think that these engines are pre-FADEC and that the fuel metering valve position is directly linked to the thrust levers position.
                    Well... yes and no. I actually have a JT8D service manual but I loaned it to a coworker so can't get "guaranteed accurate" info. But what on this page looks about right: http://cyberpilot.tripod.com/engine_systems.html. About halfway down the page it says fuel delivery is controlled based on thrust lever position, start lever position, N2 RPM, burner pressure, and compressor inlet temperature. It's hard to picture any of those parameters being affected more than slightly by engine icing.

                    FWIW (and pretty far off-topic), only the very earliest of turbine engines had fuel flow based on throttle and nothing else. The problem is that pilots would increase throttle faster than the engines could keep up, causing too much fuel to be delivered for the prevailing conditions (mostly the engine's RPM), causing excessive combustion chamber pressure, causing a compressor stall and a flameout. So more sophisticated fuel controllers were designed to prevent that scenario.
                    Be alert! America needs more lerts.

                    Eric Law

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                    • #85
                      Originally posted by Gabriel View Post
                      I would like to see AoA and stickshaker in the plots.
                      You saw the stickpusher plot? It's a flatline OFF.

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                      • #86
                        Originally posted by Evan View Post
                        You saw the stickpusher plot? It's a flatline OFF.
                        So either they were never "very" staled, or the stickpusher didn't work for other reason (maybe a failure, unlikely, or maybe there is some other logic in it, like "no stickpusher above certain altitude or Mach"). Just inventing, I don't know.

                        --- Judge what is said by the merits of what is said, not by the credentials of who said it. ---
                        --- Defend what you say with arguments, not by imposing your credentials ---

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                        • #87
                          Originally posted by Gabriel View Post
                          So either they were never "very" staled, or the stickpusher didn't work for other reason (maybe a failure, unlikely, or maybe there is some other logic in it, like "no stickpusher above certain altitude or Mach"). Just inventing, I don't know.
                          Or by the time they were 'very' stalled, the engines were 'very' stalled as well and here was no air power for the stickpusher (if it works similar to stick shaker). Or the FDR failed to record it. Also inventing. Very strange there is no stall warning plot on the FDR. 1:45:00 something abruptly happens...

                          If the thrust was manually retarded to IDLE on a normally functioning engine, would you see that EGT signature? The JT8D has a five minute limit at 650C max. I think we are seeing 700+ there.

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                          • #88
                            Hmmm... I remember another incident where (4) engines all lost power for no apparent reason, high EGT... turned out to be volcanic ash. But in that one (BA flight 9) there were other very obvious (after the fact) effects on the airplane: paint "sandblasted" from forward-facing surfaces, windshield "sandblasted", "smoke" coming from air vents in the cabin, etc.
                            Be alert! America needs more lerts.

                            Eric Law

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                            • #89
                              Hey, I just realized that this happened in total darkness. It looks like they applied TOGA thrust just 20 seconds before the impact but only neutral elevator. Could they have been working the engine problem and not been aware that they were in a dive? I would think the g-forces would be pretty obvious but there have been incidents where the spatial disorientation is quite hard to believe.

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                              • #90
                                The TAT plot puzzles me...

                                --- Judge what is said by the merits of what is said, not by the credentials of who said it. ---
                                --- Defend what you say with arguments, not by imposing your credentials ---

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