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Prism Effect

  • rmc55005
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rmc55005 created the topic: Prism Effect

Hi Bob/Richard,

Just had a question on the 'prism effect' (heavy rain on windscreen) when coming in to land and the runway appears closer.

On page 5.5 in HP&L it says 'Since runway appears to be closer, the pilot may believe he/she is overshooting (too low on approach) when in fact the correct approach path is being maintained'

Then on page 5.8 under SUMMARY OF VISUAL ILLUSIONS ON APPROACH, the prism effect falls under the category the pilot thinking they are too high on approach and therefore tend to undershoot.

Could you please clear up this confusion for me.

Thanks
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Richard replied the topic: Prism Effect

Hi mc55005,

If the pilot is overshooting, they are too high on approach, not too low. The prism effect indeed makes the pilot feel they are too high since the appearance of the runway is refracted downwards and closer in. The pilot will tend to adjust the approach path and come in too low (i.e. undershoot) but that is because they think they are too high (i.e. overshooting).

To summarise:
If a visual effect makes you feel you are overshooting, you will tend to undershoot to compensate.
If a visual effect makes you feel you are undershooting, you will tend to overshoot to compensate.

Cheers,

Rich
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