Daytime Vision

Of all the senses, vision is the most important for safe flight. Pilots must be aware of several illusions that can cause problems during daylight and must understand the most effective way to scan for other aircraft.

Daytime Illusions

Different runway widths, changing terrain, lighting, and other factors, can trick the eye. Some of the common daytime illusions include:

1. Runway width illusion: A narrow runway can create the illusion that the aircraft is higher than it actually is. A pilot who does not recognize this illusion flies a lower approach, with the risk of striking objects along the approach path or landing short of the runway. A wide runway can have the opposite effect, with the risk of leveling out high and landing hard or overshooting the runway.
2. Runway and terrain slopes illusion: An up-sloping runway, up-sloping terrain, or both, can create the illusion that the aircraft is at a higher altitude than it actually is. A pilot who does not recognize this illusion flies a lower approach. A down-sloping runway, down-sloping terrain in the approach path, or both, can have the opposite effect.
3. Featureless terrain illusion: An absence of ground features, as when landing over water, dark areas, or snow, can create the illusion that the aircraft is at a higher altitude than it actually is. The pilot who does not recognize this illusion flies a lower approach.

Scanning for Other Aircraft

Scanning the sky for other aircraft is a key factor in collision avoidance. Pilots must develop an effective scanning technique to cover the entire sky and divide attention between cockpit duties, checking the instruments, and watching for other traffic.

Scan in Sectors

Because the eyes can focus only on a narrow viewing area, effective scanning involves a series of short, regularly spaced eye movements that bring successive areas of the sky into the central visual field, which has the most acute vision.

It's best to scan by

1. Moving the head and eyes in increments of 10 degrees or less.
2. Observing each area for at least one second.

When flying under VFR, you should devote no more than 4 to 5 seconds to scanning the instrument panel for every 16 seconds scanning outside the cockpit.

NOTE: To challenge your ability to detect and avoid other aircraft, you can add air traffic to Flight Simulator. For more information, see the topic "Displaying Dynamic Scenery" in the Using Flight Simulator section of Pilot's Help.

Nighttime Vision

The human eye is optimized for daytime vision. At night, human vision is much less acute. Depth perception, of critical importance during landing, is also drastically reduced at night.

Seeing in the Dark

The human eye perceives light through special nerve cells called rods and cones arranged along the retina, the back surface of the eye. Cones, grouped in the center of the retina, are excellent at determining color. The rods, arranged around the outside of the retina, don't distinguish colors well, but they readily detect movement and are more sensitive in low light.

In fact, the rods can still function in light just 1/5,000 the intensity at which the cones cease to function. They are 100,000 times as sensitive in the dark as they are in sunlight. However, rods need more time to adjust to darkness than the cones do to bright light. Your eyes become adapted to sunlight in 10 seconds, but they need 30 minutes to fully adjust to a dark night.

Because the rods are grouped around the edges of the retina, the eye has a blind spot at night in the center of the field of view.

Nighttime Illusions

You must be especially aware of several potentially dangerous nighttime illusions, including

1. Autokinesis: Involuntary eye movements make a stationary light appear to move about if you stare at it for several seconds. Trying to use such a light as a reference to maintain aircraft attitude and direction can lead to a loss of control.
2. False horizon: If the horizon isn't distinct, the light from stars and ground objects can merge into one pattern. Without a stable horizon reference, a pilot may have difficulty maintaining the proper aircraft attitude.
3. Ground lighting illusions: You may mistake lights along a road, railroad, or other straight path for a runway. Bright runway and approach lights may make the runway appear closer than it really is.

Scanning for Traffic at Night

To take advantage of the rods' sensitivity at night, scan for traffic by looking slightly to the side of each area you want to see. Don't stare directly at the area; it will fall within the blind spot at the center of your field of view.

TIP: You can practice night flying skills in Flight Simulator. For more information, see the topic "Changing the Date and Time" in the Using Flight Simulator section of Pilot's Help.

Tips for Night Flying

Night flying requires more preparation than flight during the day. Here are a few things to keep in mind:

1. Spend at least one-half hour in dim, preferably red, light prior to your flight. Do not go from a well-lit briefing room directly to your airplane and take off.
2. When taxiing around the airport pay special attention to staying on the taxiway centerline.
3. Always carry at least one flashlight.
4. Rely more on the flight instruments than you normally would during daytime flying.

Night Landings

Limited depth perception and nighttime illusions become most critical during landings. When landing at night remember that you have little depth perception in the darkness. Landing at an unfamiliar airport without the assistance of a visual approach slope indicator (VASI) or precision approach path indicator (PAPI) may be dangerous.

Here are a few tips to help you make safe, smooth, night landings:

1. Fly the approach and landing using normal speeds. Many pilots tend to fly faster at night.
2. Whenever possible, use VASI systems to help you maintain the proper descent path on final approach.
3. To determine the proper point to begin the flare just before landing, continue the descent until the landing light reflects on the runway and skid marks and centerline stripes are clearly visible. If you're not using a landing light, start the flare when the runway lights at the far end of the runway first appear to be rising higher than the airplane.