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Police helicopter crash in Glasgow city centre

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  • #16
    Originally posted by Petertenthije View Post
    Is it possible for this safety net to malfunction, or (accidentally) disabled? Also, is it possible for this to work wrong way round, so it stops the rotor blades? Similar to reverse pitch on a prop plane.
    Whenever one ask "is it possible", the answer is almost always "yes", except in the rare event where it's impossible. But impossible is a too strong word that hardly applies.

    However, the speed control of the helicopters works only when the engine is running.

    In the variable-pitch, constant-speed propellers of the airplanes where the governor controls the pitch to keep the RPM constant as the pilot moves the throttle (to increase or reduce the power) or the airspeed increases or diminishes. If the airplane loses power, the pitch will go to a finer pitch to keep the prop turning at the same RPM up to the point where the plane reduces its speed so much that even the finest pitch will not be able to keep the RPM.

    In a helicopter, the governor (or other device or technology, sometimes the pilot himself in helicopters not fitted with these devices) controls the throttle to keep the rotor RPM constant as the pilot increases or diminishes the pitch. In an event of loss of power, the pilot must lower the pitch to prevent the rotor speed to diminish. Typically it involves moving the collective all the way down to the minimum pitch position, which is already set to be the right autorotation pitch. Then will use the cyclic to establish the right autorotation speed and steer the helicopter during the "glide". The collective will be kept all the way down until the flare, where the pilot will combine the collective ("pull up") and the cyclic (pull up too) to flare and slow down before touching down. The rotor will immediately start to slow down so you only have so much energy stored in the rotor to perform this maneuver for a few seconds, and you only have one chance to get it right. Once the rotor slows down to the critical speed you better be already on the ground.

    If you lose power and, instead of pushing the collective down, you pull up on it, the governor (or equivalent) will try to "throttle up" to keep up with he RPM, but it will of course fail and the rotor will slow down and eventually stop.

    Now, why would a pilot do that? It's as if an airplane loses power and the pilot, instead of establishing a glide, pulls up, stalls, falls and crashes.

    --- 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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    • #17
      Investigation report on this accident has now been published. According to the BBC it says that both engines flamed out, despite there being 76kg of fuel on board, no fuel leakage, both fuel pumps working and no fuel line blockages.

      Main rotors and tail rotors were not spinning on impact. No emergency transmissions were sent.

      BBC Article

      AAIB Report

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      • #18
        Originally posted by sjwk View Post
        Investigation report on this accident has now been published. According to the BBC it says that both engines flamed out
        Okay, but that doesn't explain why:
        Main rotors and tail rotors were not spinning on impact.

        --- 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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        • #19
          Originally posted by sjwk View Post
          An initial report on the crash, which was released on 9 December, said there was "no evidence" of major engine or gearbox failure.
          "As there is no indication of problems with the engines, gearbox or flying controls, it appears that a serviceable aircraft crashed due to some form of fuel starvation, despite having 76kg of fuel in its tanks. This is something that should have never happened."
          No, and the outcome shouldn't have been this even after the two engines failed.

          So the AAIB, Airbus Helicopters and Bond Air services won't speculate on the causes, but I will:

          Gross pilot error who failed to establish a proper autorotation and, instead, pulled up on the collective, thus aerodynamically stopping the rotor, after both engines flamed out for reasons yet to be determined.

          --- 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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          • #20
            Originally posted by AAIB
            The operations manual provides pilots with guidance and procedures for use following a double engine failure. It states:

            ‘Immediate Actions following total power loss in cruise or accelerative flight
            ● Lower collective immediately and flare aircraft to conserve and/or recover NR
            ● Select attitude for 75kt
            Selection of speed and RRPM [Rotor RPM] in Autorotative flight
            ● Normal Autorotation 100% RRPM, 75kts
            ● Range Autorotation 85% RRPM, 90kts
            ● Min RoD [Rate of Descent]100% RRPM, 65kts

            The investigation will seek to determine why a situation arose that led to both
            the helicopter’s engines flaming out when 76 kg of fuel remained in the fuel tank group, why no emergency radio transmission was received from the pilot and why, following the double engine failure, an autorotative descent and flare recovery was not achieved
            .

            --- 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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            • #21
              Originally posted by Paraphrased
              But that doesn't explain why the rotors were not spinning on impact.
              Indeed.

              It would seem that something is amiss with the story.

              To parlour speculate- I thought the aircraft was reasonably in-tact- which tells me he did autorotate but botched it and therefore crashed "slightly gently"- (and botched autorotations are pretty common).

              OTOH- is there a sad analogy to running out of airspeed, altitude and ideas...which might be running out of RPM's and altitude and ideas....

              ....he was autorotating but on "pull up" ran the rotors all the way down to near zero RPM (again, while having the crash be a lot more gentle than a 100 ft, zero-lift plunge.)????

              Nah, I think something's amiss with the report.
              Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.

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              • #22
                Report on this incident now published.
                Investigators worked out that both fuel transfer pumps in the main tank had been selected "off" for a sustained period before the accident, leaving the fuel in the main tank, unusable.
                The report said that "despite extensive analysis of the limited evidence available, it was not possible to determine why both fuel transfer pumps in the main tank remained off during the latter part of the flight".
                It also could not work out why the helicopter did not land within 10 minutes of the low fuel warnings and why a MAYDAY call was not received from the pilot.
                The report into the crash, which took place on 29 November 2013, said:
                the fuel pumps were switched off when the helicopter was somewhere between Dalkeith in Midlothian and Bothwell in South Lanarkshire
                the first "low fuel" indicator activated somewhere before Bothwell
                the audible "low fuel" warning was acknowledged by the pilot, David Traill, five times. Guidelines say he should have landed within 10 minutes, but did not
                there were about 32 seconds between the first engine flaming out and the second, but the single engine emergency shutdown checklist was not completed in that time
                there was no evidence of any technical malfunction and there was fuel left in the tanks.
                Source: http://www.bbc.co.uk/news/uk-scotlan...-west-34606835
                Summary of report: http://www.bbc.co.uk/news/uk-scotlan...-west-34615340

                Seems funny to me. Whether the fuel pump switches were switched off deliberately or accidentally, the pilot clearly was aware of the situation but appears to have made no attempt to deal with it.

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