We humans are VFR-only creatures. When our bodies are in motion without visual reference, the senses we use to maintain our balance and know “which end is up” are completely unreliable.
Pilots deprived of visual references while flying can quickly lose control of the aircraft and succumb to one of general aviation’s killers: spatial disorientation.
Spatial disorientation is the mistaken perception of one’s position and motion relative to the Earth. Any condition that deprives a pilot of natural, visual references to maintain orientation can rapidly cause spatial disorientation. Pilots can avoid relying on their “feelings” to guide them in flight by learning to fly by reference to their instruments. But a malfunction of flight instruments, such as those caused by a vacuum failure, in conditions of reduced visibility can also end in spatial disorientation, with the same lethal results.
While the physiology and dangers of spatial disorientation are taught during primary and instrument flight training, pilots can still misunderstand spatial disorientation and how to deal with it. And every year, spatial disorientation accidents continue to claim the lives of too many pilots and passengers.
In this video, pilots recount their real-life VFR into IMC experience.
Three sensory systems give us the information we use to maintain our equilibrium and determine where we are and how we’re oriented:
Knowing how each of these systems operates helps explain how spatial disorientation develops, and how to minimize your chances of experiencing it.
Ninety percent of the information we use for point of reference comes from our eyes. The most reliable of our senses, vision overrides conflicting sensations from our other systems. When we fly in visual meteorological conditions (VMC), our vision enables us to keep the airplane properly oriented to Earth by reference to the ground, sky, and horizon. Vision is so powerful that we’re rarely aware when our brain receives conflicting signals from other systems. Even so, vision is prone to illusions. Those mistakes in processing or interpreting what we see can result in spatial disorientation.
The vestibular system is our secondary positioning system and consists of motion- and gravity-sensing organs. These are our internal gyros. The system is redundant; there’s one in each inner ear and each can provide the brain with all the information needed to maintain balance. Despite redundancy, they can be compromised by several factors: sickness, inebriation, a hangover, dizziness, or nausea can all lead to an unreliable inner ear. Also, this system can only supplement—not replace—vision for maintaining orientation while airborne. Each vestibular apparatus has two structures: semicircular canals and otolith organs.
Tip: The vestibular system is also called the kinesthetic senses.
Semicircular Canals
The semicircular canals each have three perpendicular tubes containing fluid and sensory hairs. As the body moves, the motion of the fluid in the canals provides the brain with roll, pitch, and yaw information. This system can substitute for eyesight when on the ground; if you close your eyes, you can still walk, or sense whether you’re upright or lying down.
However, the semicircular canals do have some limitations. When a turn commences in the air, the inertia of the fluid moves in the opposite direction relative to the sensory hairs, and we correctly interpret the turn and its direction. But if the turn continues, the fluid catches up, creating the sensation that the turn has ceased. Therefore, a prolonged constant rate turn results in the false sensation of not turning at all. When the turn finally does stop, the fluid continues moving due to inertia, creating the sensation of a turn in the opposite direction.
Additionally, any bank rate of less than two degrees per second is insufficient to stimulate the fluid in the canals and will not be felt. Considering that a standard rate turn is three degrees per second, you can understand how, without visual reference, it’s possible to enter a bank degree by degree that becomes progressively steeper while feeling that the aircraft is flying straight and level.
Otolith Organs
The otolith organs are small sacs at the base of the semicircular canals. They are embedded with sensory hairs and contain a gelatinous membrane with chalklike crystals called otoliths. As the head or body moves, the movement of the membrane against sensory hairs registers gravity.
The forces of acceleration and deceleration also stimulate the otoliths and, without visual reference, the body can’t tell the difference between the inertial forces resulting from acceleration and the force of gravity. Acceleration may give the sensation of tilting backwards and deceleration may give the perception of pitching forward.
Also called the proprioceptive system, this system is comprised of hearing and nerves in the skin, muscles, joints, and internal organs. The nerves sense pressure differentials. This system remains relatively unnoticed on the ground, but while flying, pilots can feel changes in G-forces and pressure as the inertia of their bodies reacts to the motion of the airplane. These sensations are most acutely felt where the body and the airplane meet, namely on the seat. The ability to correctly interpret these sensations is the source of the term “seat-of-the-pants” flying.
Our binaural hearing can determine our position relative to a sound source. In the air our hearing can also identify conditions such as an overspeeding propeller, air rushing against the airframe, or the unnerving silence of a suddenly quiet engine. all
All three sensory systems are prone to errors. In some cases, we may have the illusion of being straight and level when we’re almost inverted. Or we may be convinced we’re tumbling end over end when we’re straight and level. The following are some of the most common illusions affecting pilots of fixed-wing aircraft that can result in spatial disorientation.
False horizon: When the only or most distinct visual reference is a cloud formation, it can be confused with the horizon or the ground. A sloping cloud deck that extends into a pilot’s peripheral vision will appear to be horizontal. Likewise, a cloud bank below the aircraft that is not horizontal to the ground may appear to be horizontal. These illusions cause a pilot to fly the aircraft in a banked attitude.
Confusing ground and star light: At night, ground lights can be mistaken for stars. This can lead pilots to maneuver the aircraft into an unusual attitude in an effort to put the ground lights “above” them. In areas with sparse ground lighting, isolated lights can also be mistaken for stars, which can make the aircraft appear to be in a nose-high attitude or have one wing low. When overcast conditions block any view of stars, unlighted areas of the terrain can appear to be part of the sky.
Autokinesis: At night, a stationary dim light against a dark background will appear to move if a pilot visually fixates on the light for about six to 12 seconds. Pilots can then mistake the light for another aircraft and attempt to maneuver the aircraft to compensate for the perceived movement of the light.
Tip: You can experience autokinesis from the ground. The next time you’re stargazing, give it a try!
All three sensory systems are prone to errors.
In the absence of visual reference, we rely on our vestibular system to keep us oriented. But this system is unreliable when in motion. Therefore, vestibular illusions create the greatest danger of spatial disorientation.
The leans: This is the most common form of spatial disorientation. It results from a pilot’s failure to detect angular, or banking, motion. If a bank is entered slowly or maintained long enough for fluid in the semicircular canals to stabilize, and the aircraft is quickly returned to straight and level, the motion of the fluid in the canal will give the sensation that the aircraft is banking in the opposite direction. The pilot may try to bank the aircraft into an attitude erroneously perceived to be straight and level.
The graveyard spiral: This is a high speed, aptly named, tight descending turn that results from a failure to detect rolling motion. Since any bank rate of less than two degrees per second is not felt, the wing may drop and the airplane may begin a turn without the pilot realizing it. As the airplane spirals downward and its descent accelerates, the pilot senses the descent but not the turn. The natural tendency is for the pilot to pull back on the yoke to arrest the altitude loss. But with the bank angle having gradually increased, this control input only tightens the turn and increases the descent rate.
Tip: The graveyard spiral is not a spin because the wings are never stalled.
Vertigo/Coriolis illusion: Abrupt movements of the head can set the fluid in the semicircular canals moving in such a way as to create an overwhelming sensation of tumbling head over heels. The sensation can be strong enough to lead a pilot to lose control of the aircraft. Looking down, as you might when searching for a chart in the cockpit or picking up a dropped pen or pair of glasses, and then looking up can cause vertigo.
Inversion illusion: An abrupt change from a climb to straight-and-level flight can excessively stimulate the sensory organs for gravity and linear acceleration, creating the illusion of tumbling backwards.r
While spatial disorientation makes only a modest contribution to the overall accident rate in GA, it is responsible for a high percentage of its fatalities. Spatial disorientation accidents fall into three categories:
VFR flight into IMC is the number one cause of spatial disorientation.
VFR flight into IMC is the number one cause of spatial disorientation. VFR-rated pilots were responsible for most of these accidents, but instrument-rated pilots are not immune.
Following a fuel stop in Indiana, the VFR-rated pilot of a Cessna 210 on a cross-country from Amarillo, Texas, to Washington, District of Columbia, contacted Washington ARTCC. The pilot said “…I seem to be lost…I was heading east into Manassas showing ah sixty miles out and all of a sudden these clouds just fogged in front of me on the mountains so I just turned around and just kind of circling right here above two pretty good sized towns.” The aircraft was given a transponder code and identified at 1511, and then proceeded toward Manassas. At 1517 the pilot radioed ATC and said, “…I’m in clouds right now, you need to get me out…” Asked if he was capable of IFR flight, the pilot answered, “No, I’m not.” The controller attempted to vector the aircraft north to VMC, but communication with the pilot was lost at approximately 1525. Witnesses saw the aircraft descend from an overcast sky at a steep angle and burst into flames upon contact with the ground. The pilot and his passenger were killed. Radar data from the last ten minutes of the flight indicate that it climbed from 5,200 feet to 7,300 feet before entering a descending right turn.
Watch this Accident Case Study about a non-instrument rated pilot who chose to accept an IFR clearance and fly into IMC.
At 1044 a non-instrument-rated pilot called the flight service station in McAlester, Oklahoma, and requested a weather briefing for an afternoon VFR flight to Muscle Shoals, Alabama. During the briefing the pilot was told VFR was not recommended. At 1349 the pilot called for another briefing and was again told VFR was not recommended. At 1645 the pilot, now airborne in his Bellanca 17-30A, contacted flight service and reported he was in deteriorating conditions. He asked for an update on the weather at his destination and was told VFR was not recommended. The pilot landed in Corinth, Mississippi, purchased fuel, and drove into town for food. At 2108 the pilot telephoned for a weather briefing for a flight to Muscle Shoals and was told VFR was not recommended, and that more fog was forming along the intended route. During the briefing, an FBO employee recommended a good local hotel and offered the pilot use of the courtesy car, but the pilot declined the offer. Witnesses reported ceilings of 1,000 feet and visibility of three miles and decreasing at the time.
A search was initiated after the pilot failed to arrive at the destination. The fatally injured pilot, the sole occupant, and the wreckage of the aircraft were found in Corinth. Upon investigation, the engine exhibited no pre-impact failures or malfunctions. Control cables and control surface attachments exhibited failures consistent with overload forces as a result of loss of control.
Tip: Hearing “VFR not recommended” during a weather briefing should be a big red flag that conditions are not conducive to safe VFR flight.
In Accident Case Study: VFR into IMC, a pilot chooses to fly despite a VFR not recommended weather forecast and encounters conditions enroute that outmatch his ability.
Why do pilots flying VFR plunge ahead into IMC despite the dangers they’ve been warned about? An expectation of success appears to play a significant role in these accidents. Such pilots take off without a back-up plan, automatically assuming they’ll successfully complete the flight. Without a Plan B, they have no other course, literally, other than to continue on, developing a kind of tunnel vision that seems to lock up the brain as conditions deteriorate. These habits are reinforced when pilots do successfully complete flights in adverse conditions, leading them to push the envelope more and more relative to the risks they take. But if you keep sticking your hand in the cookie jar, eventually you’ll get caught.
In Accident Case Study: In Too Deep, a non-instrument rated pilot presses on in deteriorating conditions, leading to a fatal encounter with IMC.
Risks vary with the environment we fly in. In many parts of the country, if you stayed on the ground every time marginal conditions or the possibility of thunderstorms were forecast, you wouldn’t fly very often. That’s where experience comes in. As we gain more experience and knowledge, we gain more confidence and expand our horizons. But this confidence also leads us into flight environments where the odds against us can rise, such as marginal VFR, night, and IMC. Add these factors to the flight environment, and the potential for spatial disorientation accidents increases. As the PIC, its your duty to manage that risk responsibly.
During a night flight in VMC from Orlando Executive Airport to Craig Municipal Airport in Jacksonville, Florida, the pilot of a Cessna 172 contacted Jacksonville Approach and requested VFR flight following. At 2142, the aircraft was identified and observed on radar at 2,600 feet, three miles south of St. Augustine, on a heading of 019 degrees, a course that would take it over the ocean. At 2145 the target was observed at 2,000 feet on a heading of 013 degrees. Thirty seconds later the aircraft was observed at 1,200 feet on a heading of 051 degrees, and its speed had increased from 104 to 126 knots. One second later the pilot radioed, “I haven’t any direction finder, I don’t see anything, one-five-six-ro--.” After trying to reach the pilot, at 2146 the controller radioed, “November one-five-six-romeo-alpha, radar contact is lost two and a half miles northeast of St. Augustine, ah, if you can hear me, ident.” The controller was unable to establish further contact with the pilot. The aircraft crashed into the Atlantic Ocean about 4.1 miles east of St. Augustine Airport. Weather at the time was reported as 3,500 scattered, visibility 10 miles. The night was moonless. The VFR-rated pilot, with about 100 hours of flight time, had received his license less than a week before the accident. The pilot’s body was recovered the following day, but the aircraft’s engine, wings, fuselage, and tail section were not located. A 100-hour inspection on the airplane, which had accumulated 151.9 hours total time before the flight, had been conducted about two weeks before the accident.
On descent toward Martha’s Vineyard Airport (MVY) in night VMC, a Piper Saratoga crashed into the ocean, killing the noninstrument-rated pilot and two passengers. The area forecast called for visibility of three to five miles and haze. No adverse conditions were reported. Visibility at MVY was reported as eight miles. Visibility on the mainland at the point where the aircraft turned toward the island was reported as three miles in haze. The accident sequence was reconstructed from radar tapes. The flight had originated at Essex County Airport (CDW) in New Jersey at about 2049 local time. After departure, the aircraft reached a cruising altitude of 5,500 feet and flew along the Connecticut shoreline until turning directly toward the island. At 2133, while over water about 34 miles west of MVY, the aircraft began a descent. Radar data showed the descent rate initially varied from 400 to 600 feet per minute. At about 2138, the aircraft began a bank in a right wing down direction. Thirty seconds later, the descent stopped at 2,200 feet and the aircraft began a climb that lasted another 30 seconds, stopping at 2,500 feet with wings level by 2138:50. At 2139 the airplane entered a left climbing turn to 2,600 feet, then began a descent that reached a rate of about 900 fpm. At 2140 the wings were leveled. Eight seconds later, the airplane banked in a right wing down direction and the bank angle, descent rate, and airspeed began to increase, with the descent reaching as high as 4,700 fpm. The last radar return, at 2140:34, showed the airplane at 1,100 feet. The wreckage of the aircraft was recovered about one-quarter mile north of the last radar return, in 120 feet of water. The pilot had a total of about 310 hours of flight time, including 29 hours in simulated IMC and 9 hours in actual IMC, and about 36 hours in the accident airplane. Pilots flying in the vicinity at the time of the accident reported that no visible horizon existed over the water. Accident investigators also noted that the pilot was recovering from a fractured ankle, which may have affected his ability to use the rudders and thus control the aircraft. Spatial disorientation was ruled the most likely cause of the accident.
Those accidents in VMC prove pilots don’t have to lose all outside visual reference to become disoriented. Spatial disorientation can and does occur in VFR conditions. Haze, darkness, or flying over water can also contribute to a loss of visual reference. In fact, flying over water on a moonless night is tantamount to flying in IMC. Even in daylight, pilots need to remember that marginal VFR, while legal to fly in, isn’t necessarily safe. Three miles of visibility doesn’t guarantee a visible horizon. Remember, “marginal” could refer to the forecast for your chances of survival, as well as the visibility.
An instrument rating is no guarantee of survival when instrument conditions prevail. It’s important to remember spatial disorientation can outmatch the most experienced pilots even in the absence of malfunctioning equipment. However, the high percentage of accidents caused by mechanical failures indicates a widespread inability to fly the aircraft by partial panel. Instrument-rated pilots are required to be proficient in partial panel flying, and these accidents indicate why.
It’s important to remember spatial disorientation can outmatch the most experienced pilots even in the absence of malfunctioning equipment.
During an IFR in VMC cross-country from Pontiac, Michigan, to Providence, Rhode Island, the pilot of a Mooney M20J was contacted by controllers and told he was “going the wrong way.” The pilot reported he had lost his vacuum system. ATC notified the pilot he would encounter IMC enroute, but the pilot elected to continue to his destination, about 180 miles away. During a no-gyro approach to the localizer in IMC, the pilot became spatially disoriented and reported to controllers, “We just lost it.” That was the last transmission from the aircraft. The resulting crash killed the pilot and his passenger. The dry air vacuum pump had been replaced about two years before and accumulated approximately 570 hours of use, under the manufacturer’s recommended replacement time of 700 hours or three years of service, whichever came first.
After a late day business meeting, a pilot called flight service and requested an abbreviated briefing for a trip back to Oklahoma City from Duncan, Oklahoma. The weather briefer asked the pilot if he could go IFR. “I don’t want to but I guess I can if I have to,” the pilot responded. The briefer informed the pilot that IMC was moving toward the destination from the west. After the call, the pilot and his colleagues skipped dinner and went directly to the airport. At 2017, the pilot contacted ATC and reported he’d just left Duncan and was trying to maintain visual conditions. At 2019 he requested and IFR clearance. The instrument clearance was issued at 2020. At 2024, the pilot radioed ATC and said, “I have uh, a vacuum problem and uh panel situation here so I, I’m going to be a little limited on being able to talk to you.” Soon after, radio contact was lost. The C-182 crashed in an uncontrolled descent, killing the pilot and two passengers. Examination of the flight instruments found the gyro bearings for the turn and bank gyro were “heavily corroded and bore no evidence of recent rotation.”
A vacuum pump failure on a sunny day is no big deal, but in IMC—and especially without backup instrumentation—it’s a serious emergency. To see how a Bonanza pilot coped with that situation, and learn how you can do better, watch Accident Case Study: Single Point Failure.
Instruments themselves can fail or the vacuum pump that powers them can fail. Indeed, vacuum pump failures are one of the most common squawks in general aviation aircraft. But pilots don’t train enough for this possibility, and the training they get is often inadequate. An instructor slaps a suction blinder on the attitude indicator (AI) and the directional gyro (DG) and says, “You just had a vacuum failure.” But in the real world, though the pump fails quickly, vacuum instruments themselves usually die slow deaths. The AI and DG – the two vacuum driven instruments – become more and more erroneous as the gyros slow their spinning and coast to a stop. The “No Gyro Approach” accident illustrates that fact. The reason the airplane was “going the wrong way” as reported by ATC, was because the pilot or autopilot was following the directional gyro. The pilot had ample time to divert but chose to continue. Even if you are skilled in partial panel flying, you won’t survive if your secondary flight instruments don’t work. In the second accident, evidence strongly suggests the turn and bank indicator was inoperative, as upon examination its gyro was heavily corroded and showed no signs of having worked recently. In other words, the instrument that would have been primary for maintaining directional control after a vacuum failure didn’t work. This put the pilot and his passengers in a virtually unsurvivable situation.
Vacuum failures are hardest to notice in high workload environments, such as in IMC or immediately after takeoff. When a pilot finally realizes that various instruments aren’t in agreement, they must determine which ones are reading correctly and which aren’t. And once the problem is diagnosed, the pilot must be able to fly the aircraft without the instruments that are normally relied on most. That makes a vacuum failure in IMC, without a backup system, an emergency.
Tip: Practice instrument failures and partial panel approaches regularly.
This Real Pilot Story shows how important it is to be proficient and prepared in the event of an instrument failure during flight in instrument meteorological conditions. As Dr. Donna Wilt can attest, practice flying partial panel in realistic situations, be resourceful, and know when to declare an emergency and use ATC’s help.
Tip: A “rate-based” autopilot gets its information from the electric turn coordinator, not the vacuum driven attitude indicator.videwatch
Should you stumble into instrument weather conditions, follow these steps:
In an emergency, could you rely on synthetic vision to get you safely to the ground? Watch this video where synthetic vision is tested from takeoff to touchdown.
If you experience confusing sensations after unexpectedly encountering IMC, scan all relevant instruments before making control inputs. Start with the attitude indicator (AI). The AI provides the main picture of what your airplane is doing. See where the nose is and where the wings are in relation to the horizon. Note the airspeed, vertical speed, and altitude. Should they indicate improper control of the aircraft, follow these steps:
Tip: If your unusual attitude skills are rusty, go fly with a CFI and sharpen up!
The best way to appreciate the power of spatial disorientation and the speed at which it can develop is by experiencing it yourself.
The best way to appreciate the power of spatial disorientation and the speed at which it can develop is by experiencing it yourself. Fortunately, this can be done without ever leaving the ground. Several training devices can induce spatial disorientation in a supervised setting, providing a vivid demonstration of its effects. We recommend pilots seek out training in a simulator such as the ones listed below:
Tip: Don’t try this on a full stomach. Disorientation can have profound physiological effects!
Tip: Redbird and other partial-motion simulators have also been known to give spatial disorientation.
Maintaining control of your aircraft can be reduced to three simple rules. Observe them and you can immunize yourself against spatial disorientation:
If you’re not instrument-rated, do not enter IMC. If you enter these conditions inadvertently make a 180-degree turn and exit these conditions as soon as possible.
Make a commitment to fly within your capabilities. Maintaining VFR isn’t always enough to avoid spatial disorientation. This is where judgment and discipline come in.
There is nothing you can do, no piece of equipment you can put in the panel, that will do more to protect you from the confusion that kills than the ability to correctly interpret flight instruments and control the aircraft accordingly. Once you earn the rating, or if you already have the rating, here are three additional steps to assure you can use it safely:
