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Spatial disorientation is defined as the inability of a pilot to correctly interpret aircraft attitude, altitude or airspeed in relation to the Earth or other points of reference.
Spatial disorientation, if not corrected, can lead to both loss of control and controlled flight into terrain. The possibility of becoming spatially disorientated is hard-wired into all humans. In fact, it is the proper functioning of our spatial orientation system, which provides the illusion; and because this is a system we have learnt to trust, it is particularly difficult for some people, in some circumstances, to accept that their orientation isn’t what it appears to be. Despite the capability, accuracy, reliability and flexibility of modern flight displays and instrumentation, pilots can still find themselves questioning what the aircraft is telling them, because the “seat of their pants” or “gut feeling” is saying something else. No one is immune.
Therefore, learning, and regularly refreshing one’s knowledge, about spatial disorientation, how and why it happens, how to recognise it, and what to do to about it, is essential in improving and maintaining flight safety. This Article should be read in conjunction with Visual Illusions.
Spatial orientation is the ability to perceive motion and three-dimensional position (for pilots we could include the fourth dimension – time) in relation to the surrounding environment. Humans (and most animals) are able to achieve this by automatic, subconscious, integration of multiple sensory inputs, such as:
- the key senses of sight and hearing provide broad peripheral awareness as well as focused attention on details
- pressure and touch, through the somatosensory system (the whole body) provide proprioception, and
- the vestibular system in the inner ear provides three-dimensional movement and acceleration sensation.
There are three aspects to spatial ‘’position’’ orientation:
- knowing where the extremity of our body and limbs is
- knowing what is up, down, left and right, and
- knowing our position in relation to our immediate environment.
This is then complicated by factoring in, for each aspect, awareness of direction of movement, change in direction, speed of movement and change of speed.
This automatic system and process has evolved to help us run, walk, sit, stand, hunt, climb, balance etc. and, it even provides for stabilised eyesight (our most convincing sense) whilst doing all these things. This system even works when one or more sensory inputs are degraded. Such that many blind, deaf, and disabled people are also able to achieve incredible things naturally and effortlessly. However, the key point is that this adaptation has occurred on the ground, and under the constant force of gravity, and not in-flight!
Spatial Orientation in Flight
Fully functional flight instruments must be the primary source for pilots to ascertain their spatial orientation. This, of course, relies both on good eyesight and good use of that eyesight; provided we use our sight to look at and read, regularly, those flight instruments that will tell us our attitude, altitude, position, heading and speed. Even pilots flying VFR will need to consult their flight instruments regularly.
Because in everyday life our vision is mostly correct, we naturally and habitually trust our vision implicitly above all other senses. It can therefore be compelling, when flying visually, to believe what we see, despite what our instruments are telling us. This makes us prone to several visual illusions, especially during landing.
There are many occasions in-flight when we cannot use, or rely on, our vision at all, such as when flying in Instrument Meteorological Conditions (IMC), when there is no visible horizon and at night. Furthermore, there are many situations when flying in VMC when a pilot should not rely on his vision, such as when flying an Instrument Approach, Instrument Departure, or in response to an ACAS Advisory alert etc.
When our sense of sight is degraded, then our “natural” sense of spatial orientation becomes dependent on proprioception (pressure on muscles, joints, ligaments and nerves) and the vestibular system. Without any (or any reliable) external visual references pilots will subconsciously become more sensitive to their proprioception and vestibular systems, and this is where spatial disorientation can manifest itself.
It must be noted that flight instruments will provide the same information regardless of the meteorological conditions!
Spatial Disorientation in Flight
Being in flight means that we may be subject to motion, speed, forces and variations in gravity (both positive and negative) which our orientation system will be unfamiliar with. This can lead to a false perception of our orientation and relative movement.
Spatial disorientation is more likely to occur when there is no visible horizon - on a dark night or in Instrument Meteorological Conditions (IMC). If malfunctioning flight instruments, high workload or a breakdown in CRM are present, then the risk of spatial disorientation is increased.
There are two main types of spatial disorientation “illusions” that humans are susceptible to in flight:
- somatogravic – experiencing linear acceleration/deceleration as climbing/descending.
- somatogyral – not detecting movement or perceiving movement in a different (mostly opposite) direction to reality.
Both are a result of the normal functioning of the vestibular system, in the relatively unusual environment of flight. The most common somatogravic and somatogyral illusions are explained in more detail below.
The vestibular system (or apparatus) sits within the inner ear and provides evidence to the brain of angular accelerations of the head in three-dimensions (roll, yaw and pitch) and also linear acceleration/deceleration of the head. It consists of three semi-circular canals and two otolithic detectors.
The semi-circular canals consist of:
- Anterior (or Superior) canal – combines with the posterior canal to detect roll.
- Posterior canal – combines with the anterior canal to detect detect pitch.
- Lateral (or Horizontal) canal – detects yaw.
The two otolithic detectors, utricle and saccule, provide the brain with a sense of the head’s position in relation to gravity, and they combine by detecting accelerations in the horizontal and vertical planes.
Whilst there are some physiological and anatomical differences between the canals and the otoliths, their operation can be described using the same model. Contained within each organ is a free-flowing fluid, such that whenever the head is turned, tilted or accelerated, the fluid (under the influence of gravity, and with its own mass and momentum) will not move with the head immediately, but lag behind somewhat. However, hair-like detectors, attached to the walls of each organ, do move with the head; the resulting force that the deflected hairs are subject to by the lagging fluid is proportional to the angular acceleration.
It should be noted, that once the acceleration (or deceleration) ceases, and a constant velocity is reached (including zero velocity), the fluid “catches-up” with the head and becomes still, closely followed by the hair-like detectors. With no force exerted by the fluid on the detectors the “head” experiences no movement until there is a change in speed or direction. Much like the body detecting an accelerating aircraft at take-off, through the pressure on the back of the seat, once a steady speed is reached, there is no longer the extra pressure, only the feel of gravity on the bottom of the seat.
In the same way that our body (proprioception) is unable to detect small accelerations, our vestibular system components also have thresholds of detection, below which we do not “sense” any acceleration. It is therefore possible to be gradually accelerated or decelerated to very high or low speeds respectively without “sensing” any change in speed. Similarly, it is possible to enter a roll, pitch or yaw movement without being able to “sense” any change.
Generally the only force experienced in straight and level flight is the vertical force of gravity. If a linear acceleration or deceleration occurs in straight and level flight, then the “sensed” vertical reference of gravity will move back or forward, giving an illusion that the aircraft is climbing or descending respectively. Furthermore, when in a turn the body will be pushed back into the seat, also giving the illusion of climbing. When exiting a turn the opposite can occur, giving the sensation of descending.
If a pilot reacts to any of these sensations without reference to a true visual horizon and/or flight instruments, then the pilot is likely to start an unnecessary descent or a climb depending on whether the aircraft is accelerating or decelerating. Such a reaction can lead to a fatal conclusion.
Illusion of Climbing – The illusion of climbing is most likely experienced when accelerating at take-off, initiating a go-around with full power, pulling out of a dive, levelling off from a climb and entering (or tightening) a turn.
An automatic somatic reaction to the illusion of climbing is to push the nose forward with the intent of stopping the illusory climb or to initiate a descent. When the pilot considers that the illusory climb is dangerous i.e. possibly leading to a stall, or “busting” a level, then the reaction is liable to be a fast and large “bunt” forward. Another automatic reaction may be to apply more power. Unfortunately, both reactions (bunting forward and applying more power) will increase the sensation of climbing and therefore motivate the pilot to increase the rate that the aircraft nose is lowered; thereby setting up a dangerous positive feedback loop.
A large bunt forward can reduce the experienced vertical force of gravity, which moves the sensed vertical reference backwards, as if climbing. Therefore, in the case where an abrupt change is made from climbing to level flight (note that this is an opposite scenario to those outlined above), the reduced G-force experienced can give the illusion of climbing, causing the pilot to push forward even more, making the situation worse. This particular scenario is often referred to as illusion of tumbling backwards
The application of power and elevator to maintain a level turn can also give the illusion of climbing, or of the nose rising too fast and too much. Any reaction here to lower the nose and/or reduce power can quickly result in a loss of height and an increase in bank angle.
Illusion of Diving – The illusion of diving (or descending) is most likely to occur when decelerating the aircraft i.e. when reducing power quickly, deploying air brakes or lowering undercarriage. It can also occur when recovering to level flight following a banked turn.
The automatic somatic response to a perceived dive is to increase the aircraft’s attitude. If the pilot considers the situation immediately dangerous i.e. when close to the ground, perhaps even over the threshold, then any pull-up response will slow the aircraft further and increase the risk of stalling or a heavy landing and tail-scrape.
There are three common somatogyral illusions, each of which involves the normal functioning of the semi-circular canals in the vestibular system:
- the leans – a false perception of the horizontal
- illusion of turning in the opposite direction, and
- coriolis – a sensation of tumbling, or turning on a different axis.
Either of the first two illusions above, if not corrected, can lead to what’s known as a “graveyard dive” or “graveyard spiral”.
The Leans – When entering a turn the vestibular system will usually pick up the initial rolling and turning movement. However, once stabilised in a steady rate-of-turn and angle of bank (usually around 30 seconds), the vestibular system will “catch-up” with the aircraft (see above) and the pilot will “sense” only that the aircraft is straight and level. The pilot may even adjust his body, and the aircraft, to this new neutral position, hence the term the leans. Only a look at a true horizon and/or the flight instruments will confirm that the pilot is suffering an illusion. The leans can often occur when an aircraft is not trimmed correctly and starts to roll or turn at a rate so slow as to be undetectable (below the detection threshold).
The illusion of turning in the opposite direction will often occur when returning to the straight and level from an established turn that was long enough (>30 seconds) to re-set the pilot’s internal horizontal reference – as described in “the leans” above. Because the vestibular system is no longer detecting a turn, when the pilot initiates a return to straight and level flight, the vestibular system detects a bank and turn in the same direction of movement. So, when recovering from a left-hand turn to straight and level, the body “senses” a turn from straight and level to the right, and the pilot will be tempted to turn again to the left in order to correct his perception.
Graveyard Dive – If, because of the leans or other spatial disorientation, the pilot does not detect a turn, eventually the nose will lower (depending on power management) thereby increasing the speed. The pilot who senses that the wings are level, but the nose is dropping, will pull back on the elevator to stop the descent and reduce the speed. However, as the aircraft is actually banked, the turn will steepen, which in turn increases the likelihood of the nose dropping further. This positive feedback scenario, if not corrected, will result in an uncontrolled spiral dive.
Coriolis – this occurs when the pilot makes an abrupt head movement (such as reaching down and over to collect a chart) whilst the aircraft is in a prolonged turn. Once a turn is established (around 30 secs) the fluid in all three semi-circular canals will be “neutral” waiting to detect any difference in movement. If the pilot makes a sudden head movement one, two, or all three semi-circular canals will suddenly “sense” the turning aircraft, but because the pilot’s head is at a random angle, the brain will compute an illusory movement. Such an illusion can produce a sensation of tumbling, or merely a turn in a different direction, or at a different rate. The pilot’s instinctive reaction might be to correct any perceived movement.
Vertigo and dizziness can occur as a result of illness, such as a cold or possibly other long-term health issues.
Usually associated with high altitude flights, and during periods of low stimulation, some pilots have been known to suffer from various “out-of-body” experiences, where they “sense” that they are on the wing looking back in at themselves flying the aircraft. Under similar conditions, some pilots have also reported feeling that the aircraft is precariously balanced on a knife edge and extremely sensitive to small control inputs, or sometimes being “held” or restrained somehow, such that the controls become ineffective.
These events are often one-off, and pilots will benefit from sharing this information in the right forum. However, to rule out any risk of recurrence, pilots experiencing any unexplained form of spatial disorientation should consult their AME without delay.
Loss of Situational Awareness
The flight crew might become spatially disorientated in relation to an aerodrome or runway when flying an approach. This is called loss of situational awareness. Although of a different nature to somatogravic and somatogyral illusions, believing that the aircraft is in a different location (in the air) than it actually is can also be called spatial disorientation. Furthermore, the potential consequences, if not corrected, are the same.
Avoiding and Recovering from Spatial Disorientation
Whether avoiding or recovering from all types of spatial disorientation and visual illusions the remedy is the same, and that is always scan, read and follow serviceable flight and navigation instruments. In the case of visual illusions there are extra recommendations concerning visual approach aids; read the SKYbrary Article Visual Illusions for more details. In a multi-crew aircraft, recovery usually means a prompt alert from the pilot monitoring and if their is no instant response, their taking control.
Recommendations for Operators
The following activities can be implemented by air operators in order to reduce the risks of pilots reacting inappropriately to spatial disorientation:
- aviation medicine training to include understanding of the vestibular system
- human factors training to include understanding of the causes of all forms of spatial (and visual) disorientation
- safety information discussions to include those accidents and incidents attributed to spatial disorientation
- SOPs for recovery from any suspected case of spatial disorientation
- Standard Operating Procedures (SOPs) for flight instrument scanning, flight display management, cross-checking and monitoring, for all phases of flight
- Standard Operating Procedures (SOPs) to ensure adequate briefing of critical phases of flight (departure, descent, approach and landing) to also include contingency measures in case of unforeseen event, such as balked landing
- Standard Operating Procedures (SOPs) for flying, managing and monitoring, stabilised approaches
- Standard Operating Procedures (SOPs) always favouring instrument approaches in preference to visual approaches, and perhaps even banning night visual approaches
- Standard Operating Procedures (SOPs) for flying, managing and monitoring go-arounds
- where possible, exposure to disorienting conditions in the flight simulator, and practicing recovery SOPs
- safety reporting system that encourages self-reporting of human factors, including spatial disorientation
- regular refresher training that covers all elements discussed above.
Concerning the issue of self-reporting, there may be some resistance from pilots who fear that they will lose their medical category; hence the need for effective education, and possibly an anonymous reporting system.
Accidents & Incidents
- A320, en-route Karimata Strait Indonesia, 2014 (On 28 December 2014, an A320 crew took unapproved action in response to a repeating system caution shortly after levelling at FL320. The unexpected consequences degraded the flight control system and obliged manual control. Gross mishandling followed which led to a stall, descent at a high rate and sea surface impact with a 20º pitch attitude and a 50º angle of attack four minutes later. The Investigation noted the accident origin as a repetitive minor system fault but demonstrated that the subsequent loss of control followed a combination of explicitly inappropriate pilot action and the absence of appropriate pilot action.)
- B733, vicinity Sharm El-Sheikh Egypt, 2004 (On 3 January 3 2004, a Boeing 737-300 being operated by Flash Airlines on a passenger charter flight from Sharm el-Sheikh Egypt to Cairo for a refuelling stop en route to Paris CDG crashed into the sea 2½ minutes after a night take off into VMC and was destroyed and all 148 occupants killed. The Investigation was unable to establish a Probable Cause but found evidence of AP status confusion and the possibility of distraction leading to insufficient attention being paid to flight path control.)
- PRM1/CRJ2, Nice France, 2012 (On 29 March 2010, a Raytheon 390 operating a passenger charter flight failed to follow acknowledged taxi instructions in normal visibility at night and entered the departure runway at an intermediate intersection and turned to backtrack against an opposite direction CRJ200 which had just started its take off roll. There was no ATC intervention but the CRJ crew saw the aircraft ahead and were able to stop before reaching it. The Raytheon flight crew stated that they had “encountered considerable difficulties finding out where they were while taxiing” and ended up on the departure runway “without realising it”.)
- B744, Taipei Taiwan, 2000 (On 31 October 2000, the crew of a Singapore Airlines Boeing 747-400 taxiing for a night departure at Taipei in reduced (but not 'low') visibility with an augmenting crew member present on the flight deck failed to follow their correctly-confirmed taxi instructions and commenced take off on a partially closed runway. The subsequent collision with construction equipment and resultant severe post crash fire destroyed the aircraft killing over half the 170 occupants and injured 71 others. All three flight crew survived.)
- SW4, en-route, Taranaki Province New Zealand, 2005 (On 3 May 2005, Fairchild-Swearingen SA227 (Metro III), operated by Airwork (NZ) Limited, was on a night air transport freight flight when it suffered a loss of control which developed into a spiral dive. The crew did not recover the control and the aircraft became overstressed which resulted in an in-flight break up and terrain impact, killing both crewmembers.)
- Flight Safety Foundation – PowerPoint presentation. Understanding Spatial Disorientation.
- Flight Safety Foundation – PowerPoint presentation. Managing Visual Somatogravic Illusions.
- Flight Safety Foundation – PowerPoint presentation. Understanding Visual illusions and Disorientation.
- Flight Safety Foundation – ALAR Briefing Note 5.3. Visual Illusions.
- Flight Safety Foundation – Human Factors and Aviation Medicine. Vol 44, No6. Nov-Dec 1997. Inadequate Visual References in Flight Pose Threat of Spatial Disorientation.
- FSF Human Factors and Aviation Medicine Vol. 39 No.1. Inflight Spatial Disorientation.
- FAA Safety Brochure - Spatial Disorientation
- Drug Use Trends in Aviation: Assessing the Risk of Pilot Impairment, NTSB study on which examines trends in the prevalence of over-the-counter, prescription, and illicit drugs identified by toxicology testing of fatally injured pilots between 1990 and 2012. Published in Sept. 2014.
- Spatial Disorientation - A Perspective, by Alan J. Benson
- Aircraft Loss of Control: Causal Factors and Mitigation Challenges, by S. R. Jacobson, NASA, 2010
- An overview of spatial disorientation as a factor in aviation accidents and incidents, by Dr. David G. Newman, ATSB, 2007