For flight, our most vital faculty is our vision, but when the lights go out, sometimes the things we do see aren’t even there, and these illusions take many forms. Yes, the air is smoother, the winds are softer, the lights below can be a less-subtle rendering of the stars above… and both traffic and ATC are less demanding. But we are not creatures of the night; at night, no one has perfect acuity, color vision, and depth perception. Some night visions can seem startlingly real — regardless of a pilot’s experience or skill level — and they can happen during takeoff, cruise flight, or landing. They can also involve some of our other “senses”, such as balance, or touch.
Illusions can arise from the eyes alone (such as an after-image from a bright light), or the brain alone (such as when one misinterprets a false horizon). They can also occur from the innocent cooperation between the eyes and the brain…
- Nystagmus: The involuntary and rapid oscillatory movements made by the eyes when the body experiences vertigo.
- Vertigo can be precipitated by a change in the body’s rotation rate without an adequate supporting reference frame, and can be severe enough to prevent reading one’s own instruments.
- Autokinesis: Tiny, natural, involuntary motions of our eyes can cause an object to seemingly move in irregular arcs. Such illusions often occur when one focuses on a single light in an area with little or any other references. Of course, this can also precipitate vertigo.
Important: Some of the same illusions that can hamper us somewhat during the day also become even more compelling. At night, we no longer have the daytime visual cues to which we are accustomed, and we become particularly gullible (more on this later).
ILLUSIONS BY PHASE OF FLIGHT
Most pilots know about the ear’s semicircular canals and our limited ability to detect low levels of acceleration, which in a way is a kind of illusion related to night flight. But there are other equally clever (and equally fallible) constructs within the confines of our noggins; in this case, the otoliths, our balance organs.
How otoliths work
- : Each ear has a pair of them — one being vertical and the other horizontal. Each comprises small crystals attached to the free ends of sensory hairs that are in turn connected to nerve cells. When our heads are tilted back, the hairs bend and certain nerves send a “
- ” message to the brain.
Problem: Forward acceleration, such as taking off on a dark night over featureless terrain, can cause the otoliths to convey the same tilt message — particularly when there isn’t any visual information to override it after the takeoff roll ends and you climb away from the airport environment. Our neck muscles tell our brain that our head has not tilted back and that we must be climbing quite steeply, so we judiciously push forward on the stick or yoke (which only adds to the acceleration of course, making things worse). This is known as the somatogravic or “false climb” illusion, and has caused many pilots to make their permanently most-recent landing.
Defense: Flying Vx or Vy is probably the safest during the initial stages of a night takeoff, immediately followed by the attitude indicator and VSI.
The lengthiest phase of flight is not immune to a fusillade of potential visual contrivances.
- Scattered lights on the ground can appear to be stars, causing a pilot to lower the nose and descend.
- Unlit terrain with an overcast or dark sky beyond can make the horizon appear to be lower than it really is.
- Sloping clouds can also cause a pilot to enter an unusual attitude.
Several non-visual illusions sharing the province of daylight forays of VFR pilots into IMC can also occur at night:
- Abrupt corrections of a bank entered too slowly for the inner ear to sense (but which is suddenly realized through another reference) can feel like a bank in the opposite direction, causing a return to the original attitude, otherwise known as the leans.
- The so-called coriolis illusion, when a prolonged turn is initiated and the pilot makes a head motion at a 90-degree angle to it.
- The proprioceptive illusions: when rolling out of a turn is interpreted as a descent.
- Graveyard spirals also work the graveyard shift.
Flight Planning: You’ll likely need radio navigation, because pilotage over unfamiliar territory at night becomes difficult, especially if it is unpopulated. And fly higher. You’ll see more, have better obstacle clearance, and have more time if your engine quits. Once over 5000 feet though, bring supplemental oxygen; your retina is the first thing that will go hypoxic. Get a thorough briefing, carefully monitor en route weather, and be prepared to fly on instruments, at least for a short time, should you inadvertently enter clouds.
Fast Fact: Although only 10% of VFR flights are at night, 30% of VFR-into-IMC accidents occur at night.
APPROACH AND LANDING
Once we’ve climbed safely away into the night sky and successfully overcome any illusions particular to departure or cruise flight we’re usually a little less alert, and a good deal more busy during the landing phase — the last thing we need is another trip through the Fun House…
- Low Approach: During the day we’re usually able to use other clues to differentiate between an approach to a narrower runway (or an up-sloping runway) and an approach that is simply too high. At night, there is more danger of making too low an approach or a hard landing.
- High Approach: Similarly, coming into a wider or down-sloping runway at night can make it appear that one is lower than one should be, which would cause too high an approach or too early a flare.
- Wrong Approach: Don’t mistake road lights for those of a runway.
Defense: Use glide slope or VASI systems when they are available. When they are not, and if runway length allows, don’t plant it on the numbers; give yourself some wiggle room by crossing the threshold at about 100 feet, instead… and look for street signs.
Invisible Visual Aids: At night, depth perception is restricted, as is our ability to sense wind drift, and we can be deprived of the additional cues such as the relative motion of nearby objects. One of the worst nighttime landing illusions is the black hole approach.
- Bright Lights, Big City: An airport or destination city includes very bright lights, or when the destination is seen through very clear air (such as a moonless night in the desert southwest) it seems much closer than it really is.
- Unpopulated Terrain that is relatively unlit can cause an unfamiliar pilot to fly too low, because it can appear to our “mental model” as more familiar populated terrain, previously overflown at a lower altitude. Also, tracking relatively lonely lights on the ground can cause unintentional descents.
Defense: The best prevention is understanding and preparation. Give your eyes time to adapt to the absence of light. Dark-adapted cones (on your retina) used for central vision can become 100 times more light-sensitive, but that can take 10 minutes. The rods used for peripheral vision become 1000 times more sensitive still, but that can take a half hour. If you have to look at something bright, before or during the flight, close one eye (the same one, every time).
BOTTOM LINE: There are various strategies for successful night flying, and they vary with the territory, but good flight planning (and flight plan filing), instrument skills, off-center viewing, using oxygen, and radio navigation are always part of the agenda. Don’t fly when you’re tired or anxious, and if you have an instrument rating, use it.