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old man emu

The Day the Music Died.

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It's interesting to see how comprehensively the crash investigators reported upon the positions that the aircraft instruments were stuck at.

I'm surprised at this, it seems to indicate a lack of understanding of just how unreliable those damaged instrument readings could possibly be.

Gauge needles could fluctuate wildly in the crash forces, before they were jammed by impact from flying debris, not necessarily jammed from ground impact.

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Sometimes the needle on an analog instrument will continue to move in the direction of the "vehicle" at the pre-impact speed as items, such as the face of the gauge, come to a stop. (Inertia - Newton's First Law of Motion) When the needle hits the face of the gauge it can leave an imprint, especially if the backside of the needle is painted. It is a pretty rare occurrence, but is has been observed. 

 

The problem with using these marks to determine pre-impact readings is that an "accurate" reading depends on the direction of the impact force. If the force is applied directly along the longitudinal axis of the vehicle, then the reading is pretty accurate. However, if the impact force is applied at an angle to the longitudinal axis , then the needle will be deflected in the direction of the impact force. If the impact force comes from the right of the longitudinal axis then the needle will deflect to the right. If at pre-impact, the needle at the 12 o'clock position and was pointing upwards, then an impact from the right would result in a mark between 12  and 1 o'clock.

 

From the report of the Holly crash, one could assume that at impact, the aircraft was in a rotating descent. Therefore, the impact force would be expected to be coming from the side of the longitudinal axis. That would cause a wise investigator to note the position of any needle mark, but only conclude, at best, that the gauge was working at the time of impact.

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