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I cannot understand why mid point RVR is issued and what's the significance of it

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It is (or was - I'm not sure) relevant because it is limiting:

The touch-down zone RVR is always controlling. If reported and relevant, the mid point and stop end RVR are also controlling. The minimum RVR value for the mid-point is 125 m or the RVR required for the touch-down zone if less, and 75 m for the stop-end. For aeroplanes equipped with a roll-out guidance or control system, the minimum RVR value for the mid-point is 75 m.
Note: “Relevant”, in this context, means that part of the runway used during the high speed phase of the landing down to a speed of approximately 60 knots.

source: JAR-OPS 1.405 - 1.0.5 (f)

The significance, I suspect, is because it's useful to be able to see where you are going when moving at high speed along the mid section of the runway and in manual control of steering.

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    $\begingroup$ Yep, as you thought, one might still be going quite fast after the touchdown zone. It is totally possible to have huge differences in visibility over a very short distance, and the area of poor visibility just might be right after the touch-down zone. You don't want to plow into a wall of thick fog doing 130kts. It is just as easy to lose sense of direction and or place on ground as it is in the air, even if you have runway lights to guide you. Then there' the elevated risk of collision, of course, should a runway incursion occur. $\endgroup$
    – Jpe61
    Nov 9 '20 at 14:31
  • $\begingroup$ Thank you for the answer $\endgroup$ Nov 22 '20 at 14:47

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