Visual illusions are perhaps the greatest source of danger pilots confront when flying at night – especially if they are proficient on instruments. That proficiency can lull them into thinking a night flight is just like a day flight, when in fact it’s anything but.
Some common visual illusions:
• The ability to estimate distance fades with the light. Humans are used to judging distance by comparing the size of one object with another or by noting something’s size, color or clarity. Because it’s common for closer things to be brighter, the tendency is to accept this at night as well – but of course it depends on the power of the light.
• Beware the false horizon. A few lights on the ground can appear indistinguishable from the twinkling of stars. A pattern of lights on the ground can appear to be a horizon. Moonlight on the edge of a cloud can create a false horizon.
• The movement of lights can fool you. If you bank away from lights, it may appear they are farther below you and moving away. If you bank toward them, you may feel like you are descending. If you are nose-down, you may think you’re lower than you are. Pitching up will “fix” your perceived attitude, but you may still be descending.”
• Don’t undershoot dim runways. Dim objects are perceived to be farther away, and that means you may think you’re higher and farther from the runway than you are – prompting you to land prematurely. Conversely, if the runway lights are particularly bright, you may think they’re loo close and land long.
• Moisture means trouble. Fog, haze, mist or light rain can lead to variations in the perceived brightness of the runway environment and make you unsure of whether you’re above or below your desired flight path. The illusion may change every few seconds. Use guides such as the VASI or an ILS lo help.
SOURCE: Aviation Safety 2003
