The condition formerly known as Inadvertent Instrument Meteorological Condition (IIMC) has been described using many terms: Unintended Flight into IMC (UIMC), Controlled Flight into Terrain (CFIT), Loss of Situational Awareness (LOSA) and Loss of Control Inflight (LOC-I) to name a few. Regardless of the term used, Degraded Visual Environment (DVE) induced Spatial Disorientation (SD) accidents are a leading cause of fatal accidents in helicopter aviation.
A DVE is any environment that causes reduced visibility. Most discussions focus on a weather phenomenon, but this is only a portion of the issue. Low visibility can be caused by low contrast environments such as a flight over featureless terrain or water, as well as into the sun or moon.
Statistics based on Federal Aviation Administration (FAA) and National Transportation Safety Board (NTSB) reports show that no pilot is exempt from the risk of SD. The average fatal accident pilot had over 2,600 total flight hours and more than 600 in the accident make and model. For years, training has focused on avoidance, but lacked effective methods of managing conditions that lead to SD. This is like a baseball coach telling his pitcher to throw strikes. They know what they should do, but require guidance on how to be successful. As an industry, we must provide pilots with proper techniques on how to avoid conditions that cause SD, and methods to get out of them, if encountered.
Research into the accidents, including the aircraft telemetry and cockpit transcripts, typically indicate the pilots were actively flying, but unable to make the correct flight control inputs. Each flight, the control input was incorrect, or if the correct input was made, it resulted in a greater amount of disorientation. Many crews will mention how visibility has deteriorated, but fail to acknowledge and respond to the condition of IIMC/UIMC.
Statistics based on FAA and NTSB reports show that no pilot is exempt from the risk of SD
A team of volunteers set out to work on H-SE 127A Recognition and Recovery from Spatial Disorientation. Along the way, it requested to change the name to Spatial Disorientation Induced by a Degraded Visual Environment. This was a result of research indicating that the actual recognition of SD was challenging at best – and a focus on the conditions that led to it would be more effective in accident reduction.
From a survey, it became clear that pilots were misunderstanding the process. SD is caused by illusions, but not all illusions will result in SD. For example, a pilot that experiences the leans, but recognizes the false sensation, is able to use instruments to confirm the condition of the aircraft, affirming that they had experienced an illusion. With SD, however, they would be unable to determine their position (or aircraft condition) in relation to other objects.
Pilots are taught a wide variety of terms related to illusions and resultant SD. Many of the explained illusions are not the typical causes of fully developed SD. Those like vection, the crater illusion and autokinesis are important for pilots to understand, but are not the leading causes of SD accidents. Effective training doesn’t need an in-depth aeromedical block of instruction, but rather a pointed focus on illusions such as:
- False horizon – when a pilot confuses a wide sloping plane of reference, such as cloud tops, mountain ridges or so-called ‘cultural’ lighting at night (such as a coastline or highway), with the true horizon (US Army Aeromedical Training for Flight Personnel TC 3.04.93 August 2018)
- Confusion with ground lights – a pilot mistakes ground lights for stars. They can place the helicopter in an extremely dangerous flight attitude if aligned with the wrong lights (FAA Helicopter Flying HandbookFAA-H-8083-21B)
- Leans – the most common vestibular illusion, caused by a sudden return to level flight following a gradual and prolonged turn that went unnoticed by the pilot (Pilots Handbook of Aviation Knowledge)
- Elevator – occurs if an excessive amount of power is applied, causing a rapid climb rate, the pilot to experience a nose-up sensation and incorrectly applies forward cyclic
- Oculogravic illusion – inertia from linear accelerations and decelerations cause the otolith organ to sense a nose-high or nose-low attitude. During acceleration, the pilot will sense a nose-high attitude and may push the nose of the aircraft down.
Along with a more refined list of illusions, instruction needs to go beyond rote memorization to an understanding of how the illusions interact with one another and impact on cognitive brain function. During low visibility conditions, a pilot’s brain becomes stressed, changing how information is processed, and works rapidly, depending on previous training, which has historically lacked effectiveness. Each encounter with conditions conducive to SD can produce a different result for each person.
Data from one of the researched accidents shows when the pilot announced they were committing to IMC. They applied an excessive collective input, which resulted in an excessive climb rate. This was immediately followed by a pilot-induced pitch forward of the aircraft, likely a result of the elevator illusion the climb would have caused. Following the initial acceleration, an oculogravic illusion caused another pitch forward. This accident example was primarily a vestibular illusion. However, without the low visibility condition, the result would likely have been different.
Academic training is the bedrock of any program, but should be coupled with practical experience. This can be conducted in a simulator, or an aircraft with or without a visibility simulation system.
Simulators provide a safe environment to train procedures. From virtual reality to Level D, they are effective at training the visual illusions, but lack the range of motion to provide the 20 seconds of sustained motion the FAA discuss in its spatial disorientation pamphlet. Simulation has seen advancements in creating environments for vestibular illusions with focus-built systems that provide the sustained motion required.
In-aircraft training can be broken into two categories: with and without visibility simulation systems. The absence of visibility systems sees the use of hoods or foggles. This can be effective for basic instrument training and vestibular illusions, but lacks the ability to train visual illusions. Visibility simulation systems use controllable films to vary pilot visibility. In-aircraft training has an increased risk over simulators. In-aircraft SD training will also add increased risk. The average SD encounter lasts less than 30 seconds from the point of disorientation to the accident. An NTSB report into an accident in California highlighted the concern for ‘a rapid development of a rapid climb, steep descent and other inflight upsets’. This risk can be mitigated with systems that provide automation and safeties. The automation prevents the pilot from being distracted by operating the visibility simulation system, and the safeties provide the clearing of the visor film if an unsafe flight condition is determined to be present by the training system.
In-aircraft training can be broken into two categories: with and without visibility simulation system
Regardless of the method, selected training should have a demonstration of varied visibilities and visual and vestibular illusions, as well as a scenario-based event. The culminating training should be treated as a monitoring of learning, rather than a punitive flight check.
Training is critical pilot preparation for degraded visibility – and planning will reduce the likelihood of encountering these conditions. This should include weather brief, familiarity with local weather phenomena and, most importantly, the Enroute Decision Trigger (NEMPSA’s Enroute Decision Point). EDTs are a variety of conditions which will trigger a predetermined decision. Most common are decreasing airspeed by a predetermined amount (i.e. 30 knots or 30 per cent), decrease in altitude, reduction of the collective due to weather or deviation from route for weather. While setting EDTs, the decision to be executed should be selected. Land the aircraft, commit to instrument conditions, or turn to known good conditions. Triggers will vary for each pilot.
During application of an EDT, the pilot should view these triggers as CAUTION and WARNING lights. A pilot would not second guess a warning in their Rotorcraft Flight Manual if it directed them to land as soon as possible; deteriorating weather conditions have the same level of importance.
Recovery from developed SD differs from upset recovery training, which is based around a pilot who is in an unusual attitude, but not spatially disoriented. A technique for recovery, or commitment to IMC conditions, is Power, Attitude, Balance.
Power: set cruise power, taking care to not apply excessive power that can create an elevator illusion
Attitude: level wings, turn only to avoid known obstacles, this attitude with cruise power will prevent a descent and allow the pilot to gain and maintain control
Balance: aircraft in trim.
Combining Power, Attitude and Balance minimizes the scan requirement during the initial entry and aircraft stabilization.
Through a better understanding of the academics, the pilot is better prepared to understand their body, while training allows the pilot to recognize and recover while in a state of disorientation. The application of EDTs will have the pilot acknowledging the conditions that will trigger an action and identifying what to do next. Preventing the development of disorientation increases the pilot’s chance of surviving low visibility and low contrast conditions.