While the aircraft descended in instrument conditions through 6,300 feet msl, the go-around mode of the autopilot and autothrottles was accidently triggered, likely by the first officer’s hand on the speed-brake control being jostled in light turbulence. The thrust levers then advanced from idle to go-around thrust while the pitch attitude increased to five degrees nose up. The first officer reacted by pushing forward on the controls while saying “we’re stalling,” actions the NTSB concluded were likely caused by the pilot’s experience of spatial disorientation due to the somatogravic illusion. Despite rapidly increasing airspeed the pilot further increased the nose down attitude, sustaining 45 to 50 degrees nose-down pitch until the aircraft descended below the cloud base—only then were the controls moved to full nose up. Tragically, there was insufficient altitude remaining to effect recovery, and the aircraft was destroyed upon impact with the surface. The captain, first officer, and a pilot riding in the jumpseat died in the crash.
The somatogravic illusion is one of the myriad vestibular illusions causing spatial disorientation, something that pilots dutifully study during initial training. As with other medical terms memorized and regurgitated during the knowledge test and oral exam, pilots often simply memorize a rote definition of the somatogravic illusion, as necessary to pass the exam, then promptly purge the term from working vocabulary post-checkride. This is unfortunate, as the somatogravic illusion has been implicated over and over in fatal accidents, striking even experienced airline pilots, and is an illusion that pilots of general aviation light jets may be at especially high risk of experiencing.
The organs in our inner ears that help maintain orientation and balance are subject to “instrument errors” just as those in our aircraft experience. Germane to the somatogravic illusion is the inability of our vestibular senses to differentiate between acceleration and a tilt in the same axis—a rapid forward acceleration of an aircraft, such as might be experienced during takeoff or go-around, stimulates the inner ear in the same manner as if the head is being tilted backward. If outside visual references are poor, the combination of simultaneous acceleration and pitch-up can cause an overpowering feeling that the pitch attitude is dangerously high, leading the pilot to push the nose down into a level, or even diving, attitude.
A recent study found that events with the pilot exhibiting confusion about aircraft state during takeoff and go-around have become more common with the replacement of three- and four-engine transport jets with twin-engine aircraft.Consider a lightly loaded Embraer Phenom 300 that rotates at 103 KIAS and can easily accelerate to 200 KIAS in under 30 seconds after liftoff. This rate of forward acceleration will combine with the downward force of gravity to result in a perceived pitch attitude of over 10 degrees nose up. But the actual pitch attitude is level. Combine the added perception of pitching up with the actual initial pitch attitude of 15 to 20 degrees necessary to prevent overspeeding while the takeoff flaps are retracting, and the pilot may be perceiving a pitch attitude over 30 degrees nose up—an uncomfortable feeling that may induce a strong need to do something by pushing forward on the controls.
A recent study of nearly 40 years of spatial disorientation mishaps among transport airplanes found that events with the pilot exhibiting confusion about aircraft state during takeoff and go-around have become more common with the replacement of three- and four-engine transport jets with twin-engine aircraft. Because of operational requirements pertaining to climb ability following loss of an engine, two-engine jets have much more performance with all engines operating than older jets with three or four engines. This extra performance leads to higher acceleration rates and corresponding susceptibility to the somatogravic illusion. Given that high-performance light jets such as the Phenom 300 and Cessna Citation CJ4 can have a thrust-weight ratio 50 percent higher than these three- or four-engine transports, it’s apparent the problem can be even more severe for the GA pilot.
The study, published in Aerospace Medicine and Human Performance, also found a striking correlation of flight conditions with the occurrence of severe spatial disorientation events: Only one percent of events leading to fatalities occurred during day visual meteorological conditions, while 71 percent of the fatal events were during night flight in instrument meteorological conditions. Considering the likely denominator difference due to relatively lower total hours flown in night IMC, this disparity is especially notable. Other conclusions drawn by the study are also relevant to light jet pilots. The researchers offer as explanations for the year-over-year increase in spatial disorientation mishaps an overall decrease in pilot experience. Pilots, particularly co-pilots, may be less experienced thanbefore. Recent changes to international pilot certification have allowed airline co-pilots with flight experience on the order of 200 hours to fly as second in command. This is a historically low total time to serve as second in command, with a corresponding decrease in the time pilots spend hand-flying, and a corresponding dependency on the autopilot—plus training deficiencies in recovery from pitch upsets.
When I conduct training, I frequently see task-saturated pilots display signs of experiencing the somatogravic illusion, even in day VMC. A theoretically straightforward maneuver, the all-engine-operating go-around, is often mishandled in a surprising manner—the go-around is initiated, but after a short climb of just 100 to 200 feet, the aircraft is pitched down to a level attitude, resulting in rapid acceleration rather than a climb. While trying to recall the go-around profile from memory (e.g., flaps and gear up when, and in what order, flight director reprogrammed with what modes) the scan deteriorates and the pilot falls back on the most instinctual of references, their vestibular perceptions. When I point out the energy state—flying level at 500 feet agl while rapidly approaching a bust of the 200 KIAS speed limit in Class C and D airspace frequently used for training—the pilot sheepishly pulls the nose up to a proper climb attitude and often says something to the effect of “I don’t know what happened.”
Knowing the significance and prevalence of disorientation caused by the somatogravic illusion, what mitigating actions can a pilot take? First should be a briefing, as appropriate to the takeoff or approach phase of flight, that includes an explicit acknowledgement of those times this illusion will be more likely. For example, a pilot may include during their takeoff briefing a statement such as “I will be departing into night IMC, which can significantly increase risk of spatial disorientation. Furthermore, temperatures are cold and I’m departing 2,000 pounds under maximum takeoff weight, which will result in rapid acceleration, worsening any disorientation. Entering the clouds at 500 feet agl I will focus on my primary instruments and ensure my pitch attitude remains between 5 and 10 degrees nose up until I begin to level off.”
Primed to expect disorientation, the pilot may be less overcome and panicked should it occur. Even forewarned, though, the somatogravic illusion can be so compelling a pilot can be overcome with certainty that the pitch is problematic, when it is evidentially not. Here I advise pilots to take the classic advice of “focus on the instruments” one step further: Faced with a compelling feeling that the pitch attitude is wrong, the pilot should momentarily focus entirely on pitch attitude alone. Quickly cross-check the other attitude displays to rule out bad information, then stay laser focused on the attitude depiction of the primary flight display while controlling the aircraft. In the panic of disorientation, trying to interpret more than one instrument is nowhere near as successful as staying focused on the most easily interpreted presentation—the one on the attitude indicator.