If only we could pull out our brain and use only our eyes. [Pablo Picasso]
Semicircular Canals, Aeromedical Factors - Private Pilot Ground School
The human body has a remarkable set of motion sensors, which are capable of detecting linear and angular accelerations, in addition to the haptic sensors that detect tactile pressure applied to the skin. These sensors enable us to walk up a flight of stairs, jump off a chair or run after a bus, without endangering ourselves. They also provide orientation in three dimensions, for example, an athlete can jump over a 2-m bar or a diver can perform several somersaults, jumping backwards from a diving board. These same sensors detect accelerations during conventional flight and in aerobatic manoeuvres. For the flight simulator designer, an understanding of the human motion sensors is essential; it can identify the limits of the brain to detect motion and possibly exploit this information to reduce or simplify the motion applied to a simulator platform. If the dynamic responses of the human balance sensors are known, filters can be designed to match the combined response of the motion platform and pilot to the response of the aircraft and pilot. In attempting to provide realistic motion cues, the motion inputs applied to a flight simulator platform are constrained:
The structural limitations of a simulator motion platform determine the maximum forces that can be applied to the pilot in a simulator; it is necessary to establish the limits where lack of motion may affect the simulator fidelity;
Reducing the motion inputs may allow a lighter and less expensive structure to be used in a motion platform; alternatively, the mass of the platform and cabin determines the power needed from the actuators;
If acceptable platform motion can be achieved with reduced power, considerable savings can be made in the running costs of a flight simulator; the use of electrical actuation may reduce both power requirements and the environmental problems associated with hydraulic systems.
The vestibular system of the human body senses the orientation of the head and dynamic movement of the head. As the head moves, the eyes are stabilized, so that the vision is not blurred by the head movement; the vestibular system also provides signals for the eye muscles to accommodate this movement. In effect, the vestibular system provides a stabilized platform as it is capable of detecting both linear and angular accelerations about three axes. The angular accelerations are detected by semicircular canals and the linear accelerations are detected by the otoliths. One set of these sensors is located in each inner ear.
The semicircular canals are arranged as an orthogonal set of canals, in three mutually perpendicular planes. The canals are attached to the skull and filled with a fluid, known as endolymph. In each canal, there is an expanded section called the ampulla, which is sealed by a flap known as the cupula. Angular acceleration of the head about one of the three axes causes the fluid in the canal to move (with a short lag) deflecting the cupula by a small amount. The nerves in the cupula detect this movement, sending signals to both the brain and the oculomotor muscles in the eye (to stabilize eye movement)
Linear accelerations are detected by the otoliths. The sensor consists of hair cells in a gelatinous fluid containing particles of calcium carbonate. As acceleration is applied to the head, the calcium carbonate particles lag slightly behind the head movement, deflecting the hair cells. Movement of the hair cells is detected by nerve cells which transmit signals to the brain and the oculomotor muscles in the eye. Studies have shown that the otoliths detect the tangent component of applied forces.
Clearly, the pilot’s vestibular system detects accelerations before the effect of the accelerations are perceived on the aircraft instruments. In particular, attitude and altitude are second integrals of acceleration, introducing a lag before the initial acceleration takes effect. There is, arguably, an inner control loop in which the pilot detects and responds to accelerations, which occur in a full-motion simulator and also in an aircraft, but is omitted in a fixed-base flight simulator. This lack of acceleration cues in a fixed-base simulator is cited by some pilots as a potential cause of negative training transfer in transitioning from a fixed-base simulator to an aircraft. In other words, pilots apply one technique in the simulator and another technique in the aircraft. Certainly, there have been instances during in-flight refuelling exercises and also with vertical take-off aircraft, where there have been noticeable differences between the pilot’s performance in the simulator and in the aircraft. In refuelling applications, such differences can be attributed equally to the visual system (potentially the reduction of the vertical field-of-view or the projector focal length with near objects), poor turbulence modelling or incorrect aerodynamic interaction with the tanker aircraft.
Middle ear and sinus problems
Above is of prime importance especially for simulation study and significantly affect pilot behaviour under different simulation environments. However, in real flights Climbs and descents can sometimes cause ear or sinus pain and a temporary reduction in the ability to hear. This not just apply to pilots, but there are many of us who have experienced this at least once in their life. The physiological explanation for this discomfort is a difference between the pressure of the air outside the body and that of the air inside the middle ear and nasal sinuses. The middle ear is a small cavity located in the bone of the skull. It is closed off from the external ear canal by the eardrum. Normally, pressure differences between the middle ear and the outside world are equalized by a tube leading from inside each ear to the back of the throat on each side, called the eustachian tube. These tubes are usually closed, but open during chewing, yawning, or swallowing to equalize pressure. Even a slight difference between external pressure and middle ear pressure can cause discomfort.
During a climb, middle ear air pressure may exceed the pressure of the air in the external ear canal, causing the eardrum to bulge outward. Pilots become aware of this pressure change when they experience alternate sensations of “fullness” and “clearing.” During descent, the reverse happens. While the pressure of the air in the external ear canal increases, the middle ear cavity, which equalized with the lower pressure at altitude, is at lower pressure than the external ear canal. This results in the higher outside pressure, causing the eardrum to bulge inward. This condition can be more difficult to relieve due to the fact that the partial vacuum tends to constrict the walls of the eustachian tube. To remedy this often painful condition, which also causes a temporary reduction in hearing sensitivity, pinch the nostrils shut, close the mouth and lips, and blow slowly and gently in the mouth and nose.
This procedure forces air through the eustachian tube into the middle ear. It may not be possible to equalize the pressure in the ears if a pilot has a cold, an ear infection, or sore throat. A flight in this condition can be extremely painful, as well as damaging to the eardrums. If experiencing minor congestion, nose drops or nasal sprays may reduce the chance of a painful ear blockage. Before using any medication, check with an aviation medical examiner to ensure that it will not affect the ability to fly. In a similar way, air pressure in the sinuses equalizes with the pressure in the cockpit through small openings that connect the sinuses to the nasal passages. An upper respiratory infection, such as a cold or sinusitis, or a nasal allergic condition can produce enough congestion around an opening to slow equalization. As the difference in pressure between the sinus and the cockpit increases, congestion may plug the opening. This “sinus block” occurs most frequently during descent. Slow descent rates can reduce the associated pain. A sinus block can occur in the frontal sinuses, located above each eyebrow, or in the maxillary sinuses, located in each upper cheek. It will usually produce excruciating pain over the sinus area. A maxillary sinus block can also make the upper teeth ache. Bloody mucus may discharge from the nasal passages.
Sinus block can be avoided by not flying with an upper respiratory infection or nasal allergic condition. Adequate protection is usually not provided by decongestant sprays or drops to reduce congestion around the sinus openings. Oral decongestants have side effects that can impair pilot performance. If a sinus block does not clear shortly after landing, a physician should be consulted.
This is not Enough
Unfortunatly, engineers aren’t happy with above subjective descripitions of both problems, especially firt (motion factor). Thus, to quantify the influence of motion of pilot behaviour, it is best to develop mathematical model of human vestibular system, technically a general this model forms a transfer function in terms of percieved displacement and angular displacements, and can be solved by second order approximation (2nd order ODE). Under laboratory conditions, it has been shown that angular acceleration cannot be detected below a minimum acceleration, which is between 0.12 and
4.0 deg/sq.s. Threshold values of 0.5 deg/sq.s have been reported for pitch and roll under flight simulator
conditions. For example, although the maximum pitch angle of the platform might be 15deg, as the simulator motion approaches this limit, the platform can ‘leak’ 10deg of pitch in 6.3s, enabling the simulator to achieve a further (perceived) pitch up of 10deg in response to subsequent pilot input.
Further details for mathematical model can be accessed in any relevant aeromedical book, or (Allerton,2009 “Principles of Flight Simulation”) provides a brief overview of motion influence. For technical results and analysis, please keep following the space.