By Simon Ludlow, BALPA Accident Analysis and Prevention (AAP) Group On 18 December 2022, a Boeing 777 departing Maui, Hawaii, entered cloud on departure at 2,000ft. Seconds later, the aircraft entered a steep descent from 2,200ft at nearly 8,600ft per minute. The aircraft was recovered at 775ft above the sea in a manoeuvre that recorded 2.7g. Some three weeks later, a Boeing 787 was departing Qatar on a clear night, being manually flown over the sea, when it descended from 1,800ft to 800ft at 3,000 feet per minute, exceeding the flap limitation speed. Both these incidents were close calls. But the crew of a Boeing 767 freighter that crashed on the approach to Houston on 23 February 2019 were not so lucky. The handling pilot inadvertently activated the Go-Around mode on the approach in IMC at 6,200ft. With the engines developing full thrust, the aircraft rapidly accelerated, but both pilots misinterpreted this as pitching up, reacting by making a nose down pitch input. The aircraft reached a 49 degrees nose-down attitude, impacting the ground at 430 knots. The NTSB report cited pilot error and disorientation, specifically the somatogravic illusion. This illusion should be categorised as a human limitation rather than an error. It may occur when the human body is subjected to sustained linear acceleration when visual references are either poor or absent. The organs in the inner ear responsible for detecting gravity also detect acceleration. The human body has only evolved to detect acceleration for about two seconds, the time it takes to reach running speed from standing still. Linear acceleration may be interpreted by the brain as the head tilting in the absence of visual cues. This illusion can be very powerful and, in the absence of a visual horizon, it can lead a pilot to disregard attitude indications in favour of an incorrect attitude perception, with the risk of an inappropriate control input and unsafe flight path. The cause of the somatogravic illusion is a function of the human body, which has evolved to detect acceleration produced by breaking into a run from standing, but not the prolonged linear acceleration achieved by mechanised transport. The balance, or vestibular organs in the ear, detect roll, pitch, yaw, tilt and momentary acceleration, but because of their physiology they can only signal the brain that it has sensed a momentary acceleration, after which it may become confused as to the real nature of the movement. The tilt and acceleration detection mechanisms are the otolith organs the saccule and utricle. They consist of a mass of calcium carbonate pieces on a gelatinous mass, through which hairs connected to nerve endings pass. If the head tilts, the inertia bends the hairs, which send a signal that is processed as a pitching up or down movement by the brain. If the motion is a momentary linear acceleration, then this is correctly sensed. But if the movement persists, the deflected nerve endings continue to signal the brain, which may be falsely interpreted as a pitching motion instead of acceleration. The cause of the somatogravic illusion is a function of the human body, and the signals sent by the inner ear to the brain All pilots are at risk, and it may be assumed that 25% who are not aware of the threat will be more at risk Knowledge is king The best mitigation is training. Knowledge about the somatogravic illusion is included in the Human Performance and Limitations syllabus, which a commercial pilot must pass for the award of their licence. It may also be included in the briefings as part of their instrument flight training. But this is where the training usually ceases. It is practically impossible to induce in the low-powered aircraft used during training, and impossible to replicate in a simulator as to simulate acceleration, the cockpit is pitched to trick the brain into assuming acceleration is present. It is simply not possible to replicate the somatogravic illusion when the very mechanism that senses acceleration is used to make the brain believe it is accelerating. It can even be argued this is actually negative training, leading to more cases of misinterpretation of the signals from the inner ear. For most pilots, knowledge of the illusion is mostly theoretical. A 2016 survey of 585 professional pilots indicated only about 75% had an effective working knowledge of the somatogravic illusion military trained pilots tending to have better awareness as it was often considered only a high-performance aircraft issue. However, an acceleration of 30 knots over 10 seconds can produce a pitch illusion of nine degrees; the illusion is an issue that can affect the pilots of most aircraft. All pilots are at risk, and it may be assumed the 25% who are not aware of the threat will be more at risk. Beating the illusion The effective strategy against the illusion begins with knowledge of what it is, and the circumstances where it can be a hazard. Threat and Error Management (TEM) principles may be applied during departure and approach briefings to identify situations where the illusion could be a factor, and the strategies that can be employed to mitigate it. There are several factors that define the threat, including fatigue, aircraft design, aircraft defects, weather conditions, or any dark night visual conditions. Evolution has yet to equip the human body with gyroscopes immune to the effects of acceleration, but the designer of the aircraft has provided them just for these circumstances. The attitude indicator will be correct. If there is sensation that the pitch attitude is different from that indicated, it is essential that the attitude indication is followed. Any circumstances where the aircraft is manually flown on a departure or arrival over indistinct terrain or water at night, or in Instrument Meteorological Conditions (IMC) while accelerating or decelerating is an invitation to the illusion. The best strategy to avoid it is autopilot engagement. Autopilot use during IMC departures and go-arounds is a sound policy. In the case of an IMC approach, autopilot disengagement should only be considered when landing clearance has been received and the touchdown point is visual, so that the autopilot is engaged for any possible go-around. In the case of aircraft types where the autopilot automatically disengages on initiation of the go-around, re-engagement should be considered as soon as practicable, especially before accelerating. And the autopilot should not be disengaged intentionally on a go-around. The go-around is a little practised high-energy manoeuvre, which when encountered in real life, bears little relation to the heavyweight single-engine event often practised in the simulator. Manual flight skills have been eroded over the years because of increasing automation. The FAA has recently recommended hand-flying opportunities are taken whenever it is safe to do so. However, there are times and places for this. Accelerating while IMC or when visual references are limited are not the right times. On average, a large transport aircraft is lost every two years to the somatogravic illusion. It is a poorly appreciated threat, but easy to counter if identified using TEM and sensible mitigation strategies are adopted. TECH LOG The killer illusion