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The Evolution Of Basic Flight Instruments History Essay


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In the pioneering days of aviation, flight was restricted to times of good visibility and good weather. In early flight operations, the pilot maintained visual contact with the ground below him at all times and used it as a reference point for executing all maneuvers. The design of most early aircraft positioned the pilot in the front of the aircraft affording him a high degree of visibility. Currently, there was little thought regarding cockpit design, and the pilot was just seated on the aircraft in a completely open fashion. This open design allowed the pilot to receive full sensory input from the chill of the blowing wind, the exhaust of the turning engine, and even the vibration through 'the seat of his pants' (Siberry, 1974). He was in an ideal position to make use of his only flight instrument--his body. These perceived sensory inputs superimposed on the visual backdrop of the earth combined to form the early pilot's mental model of reality. Two broad cognitive principles that negatively affected the pilot's mental model require definition at this point.

Spatial disorientation (SD) and Loss of Situational Awareness (LSA) are both difficult concepts to define. Both involve a degraded awareness of reality resulting from the mismatch between the pilot's mental model and the visual backdrop used to create his perception of the environment. Unfortunately for the pilot SD and a LSA can result in the generation of misinterpretation and faulty prediction about his current situation often resulting in disastrous flight control inputs (Boers, 1996/97). SD is the faulty perception of position, direction of travel, or speed relative to the ground. LSA is closely related to SD, but deals with the pilot's more specific confusion over his actual geographic location at a specific point in time (Mortimer, 1995). Now that the cognitive principles of SD and LSA have been defined, let me return to the discussion of the pilot's use of his body as an exclusive sensor for environmental inputs, and the inherent limitations of this practice.

Under visual flight rules (VFR) conditions, the early aviator was able to extract enough input from his senses to adequately pilot the aircraft. However, during continued experimentation with flight, poor visibility and poor weather conditions often were encountered. Pilots were robbed of their conventional sensory inputs under these conditions and many aircraft accidents resulted. Reliance on visual, vestibular, and "seat of the pants" acuity to control the aircraft was a major weakness and it prompted flight instrument development. Equipment designed to display aircraft heading, speed, and altitude information quickly evolved to counter man's sensory vulnerabilities.



The compass was one of the first flight instruments used by early aviators (Allstar, 1995/00). Aviators quickly took advantage of the magnetic compass to overcome their past reliance on visual cues for establishing a heading. Compasses in use today are of two basic types, magnet, and gyro-magnetic (USCGA, 00). Both types can trace their ancestry back two thousands year to Chinese magicians. By accident, these magicians had discovered the properties of magnetism while playing a board game similar to chess. Unknowingly they had constructed their game pieces from metal containing lodestone, a natural magnetic ore. When they started their game by dropping the pieces on to the board, they noticed that they would spin and align themselves in the same direction every time. Quickly realizing the potential of their discovery, the magicians employed the magnetic properties of the lodestone to develop a liquid filled compass similar to the ones in use today (A History of the Compass). Advancements in steel casting methods a thousand years later made permanent magnets possible by enabling compass needles to stay magnetized making the compass more practical and reliable. Early pilots simply made use of borrowed land and sea navigational compasses in their cockpits. However, liquid filled compasses were not perfect instruments. Steep turning and diving maneuvers caused the compass to present inaccurate heading information. In addition, metallic objects in the aircraft created magnetic interference that could corrupt the compass heading display. Some pilots made use of small magnets of opposite polarity positioned near the compass to counteract this harmful interference. Another solution for overcoming the interference was provided by the compass correction card, and it is still in use today (Gum and Walters, 1982). The card displayed magnetic headings from zero to 330 degrees, at 30-degree intervals. Underneath the magnetic heading readings were the appropriate corrected steering headings that compensated for the magnetic interference. Although the compensations were only minor, the pilot needed to ensure they were made as they could result in a LSA, as the magnitude of error would increase with the distance traveled. In addition, pilots occasionally failed to compensate for geographic differences in magnetic variation and this caused further cases of LSA. Fortunately, compass accuracy and stability took a leap forward on September 24, 1929, when Lt. James Doolittle performed a successful test flight of a directional gyro manufactured by the Sperry Company (Allensworth, 2000).

Gyrocompasses combined the gyroscopic phenomenon that keeps a rapidly spinning wheel stable in space, and the property of magnetic polar attraction to overcome the limitations of the purely magnetic compass. Pilot alignment and compensation input requirements, that could be subject to human error, were required less often for the gyrocompass and this positively impacted flight safety. Today, the development of laser and other technologies have led to refinements in the basic gyrocompass enhancing its performance. Despite these advancements, a heading instrument of any type is a valuable tool for preventing pilot LSA. However for it to be effective its limitations must be known and observed. More importantly, to be effective it must be used. The collision between a Korean Air Line (KAL) DC-10, and a Piper PA-31 on a foggy runway in Alaska highlights this point. Lacking SA the KAL pilot continued to taxi to what he believed was runway 32, and began his takeoff roll. Sadly, the DC-10 pilot had been fooled by his sense of direction, and struck a Piper aircraft holding for take off on runway 6L/24R. If the KAL pilot had only compensated for his lack of visual perception by confirming his runway heading with a compass (NTSB 1983) the accident could have been avoided. Obviously the compass has proven to be a vital resource for pilots and the development of speed measuring instruments would prove to be just as significant.


Knowing exactly how fast you are traveling is important for many reasons. Most importantly, the passage of air over the wings of the aircraft generates lift, and to remain airborne the pilot needed to maintain a minimum speed, or the aircraft would stall, ceasing to create lift. Additionally, you need the ability to measure speed, to make use of the navigational technique known as dead reckoning (DR). DR could greatly increase pilot SA by providing him a reliable indication of his location based on his rate of travel in conjunction with the passage of time and heading information. Unfortunately, the human body is poorly equipped for this task. Using his vestibular sense, the pilot had a rough feel for changes in speed due to acceleration and deceleration. Visual acuity also afforded him a limited sense of speed based on his perceived rate of closure with distant objects. However, poor weather, darkness, and visual illusion further degraded both these senses. A speed-indicating instrument immune to all these factors was required to improve pilot SA and make DR navigation a reality.

Early airspeed indicators were of two types mechanical and differential. Some mechanical devices were very simple producing only limited information, much like the mechanical stall warning indicators in use today on small private aircraft. Other mechanical airspeed indicators grew to be quite elaborate and their evolution started on a beach in North Carolina.

Orville Wright held in his hand a mechanical anemometer on the first heavier than air flight. As Orville flew, Wilbur recorded his flight time with a stopwatch (Hunt and Stearns, 1923). Using the distance measurement from the anemometer, and the elapsed time from the stopwatch, they were able to calculate their speed. Strapping the anemometer to the wing of the aircraft further refined this speed measurement technique. Rubber tubing was then used to connect the anemometer to an indicator in the cockpit known as an air log. Variable suction was produced based on the anemometer's speed of rotation. The resulting suction deflected the needle inside the air log to produce a reading. Unfortunately, the reading it produced was not airspeed. Separate elapsed time measurements still had to taken and combined with the air log reading to calculate air speed. Airspeed would not be readily calculated until new equipment using a commutator-condenser was devised.

The new equipment employed a commutator-condenser unit that worked similar to an electrical motor, only in reverse. By connecting it to a spinning anemometer, varying levels of current were produced that could be displayed on an ammeter. Improvements to this technology was made by incorporating a timing signal with the electrical output to produce a true measurement of air speed (Beij, 1933). In fact, this measurement was still not a true measure of air speed but a measure of indicated airspeed. To measure true air speed, accurate compensations for altitude and temperature needed to be made, and differential speed measurement instruments would be better suited to this task.

Worldwide experimentation with measurement instruments that collected air pressure to determine airspeed had begun. The most common system designs used funnel like collectors known as pitot tubes positioned facing forward on the aircraft's wing to collect the maximum amount of air possible. Care was taken to ensure the pitot tubes were placed away from any propeller generated air stream disturbances that might corrupt their air collection. An additional air collection device named a venturi was positioned perpendicular to the forward flow of air to capture the ambient or static air pressure (Hunt and Stearns, 1923). Both of the separate pressure inputs were routed via metal tubing to an indicator gauge that subtracted the static pressure from the pitot pressure to produce a dynamic pressure (Siberry, pg. 49). The resultant dynamic pressure inside the gauge actuated levers, and wheels to drive an indicator needle. A measurement scale was drawn on the face of the indicator and the speed of travel could be found by reading the needle's position against the scale. These systems were known as differential for their subtraction of the static and dynamic pressures. Furthermore, they proved to be a great improvement over early anemometer based measurement instruments because of their flexibility. Eventually, scales were devised that allowed the pilot to calculate appropriate altitude and temperature offsets to produce true airspeed readings (TAS) (Hunt and Stearns, 1923). Once the offset was found it could be dialed in to airspeed indicator to display TAS.

Certainly the airspeed indicator has greatly improved the pilot's ability to navigate at night and during periods of poor visibility by making use of DR. Nonetheless; one should not become complacent to the measurements they provide. Today with the advent of integrated systems that arbitrarily use airspeed data provided to them to calculate flight control inputs, it is even more important for the pilot to challenge instrument readings that fall outside expected limits. Undoubtedly the airspeed indicator provides vital information to pilots about the aircraft's ability to maintain lift and has had a dramatic impact on flight safety. Moreover, the airspeed indicator has proven to be a valuable tool in the prevention of pilot SD. Its ability to detect increases in airspeed is far superior to that of any pilot relying on perception alone. Today, precision flight instruments are a reality, and yet pilots still suffer from SD, primarily from their inability to use them. Being without flight instruments and being unable to use them are one in the same. Therefore, the importance of past instrument development can be inferred from a recent study. The study involved accidents from 1987-1996 that implicated spatial disorientation as their cause, and it revealed that 90 percent of them resulted in fatalities. As a rule, these accidents involved non instrument-rated pilots caught in instrument meteorological conditions that became disorientated and lost control of their aircraft. This loss of control resulted from their inability to use their flight instruments and over come their LSA (Air Safety Foundation, 1999). Without doubt, heading and airspeed instruments combined with adequate training to use them are critical to pilot safety and the prevention of LSA. To further safeguard pilot's from LSA, instruments that could accurately indicate altitude were required.


The early pilot's only tools for estimating his height above the ground was his eyes. He made use of the eye's ability to detect changes in the size of objects as he flew to estimate his altitude (Hawkins, 1987). In addition, the eye's capability for detecting subtle changes in the texture of the landscape below provided him with altitude clues, also the eye's inherent binocular nature afforded him a certain degree of depth perception. Unfortunately, for these techniques to be remotely accurate perfect weather conditions and daylight were needed. Flying in the darkness and relying on bonfires, and beacons for navigational cues was a difficult task. Subsequently, focusing on the distant flickering lights against a black background could result in vertigo and cause LSA (Haines, 1992). An altitude indicator, resistant to man's sensory flaws was required to further combat pilot LSA. Like the compass, altitude-measuring equipment were also used in a variety of fields before the first heavier-than-air flight. Subsequently, the first altitude-measuring instruments used by aviators were adaptations of the aneroid barometer used by meteorologists. These early altitude indicators were of two types barograph, and altimeter (Hersey, 1923).

The barograph was a precise instrument that contained a gradually rotating cylinder with paper on it. Changes in atmospheric pressure would force a pen attached to the aneroid to move up and down in relation to the pressure changes. The changes in pressure would then be recorded by the pen throughout the flight (Williams, 1999). It is no surprise that the barograph did not catch on as a permanent flight instrument and was primarily used to test and calibrate more practical altitude indicators like the altimeter.

Early altimeter were nothing more than a modified aneroid barometer, displaying altitude rather than air pressure. Following in the footsteps of pioneering balloonists aviators simply changed the scale on the barometer from a measure of pressure to a measure of height above the ground, subsequently giving birth to the first aviation altimeters in the process. Bimetallic strips were incorporated inside the sealed unit of the altimeter to compensate for changes in temperature that could cause inaccurate readings. Later the simple dial and needle style altimeters grew more complex. Some made use of movable dials to compensate for daily variations in atmospheric pressure, and others allowed the pilot to input a pressure compensation factor (Mears, 1923). However, the overall indicator design changed very little-a simple rotating needle from zero to the maximum operating altitude of the aircraft, or the current theoretical safe maximum altitude. The pilot now had a tool that could be used in any weather and at any time of day accurately to indicate his altitude. For example, the altimeter in conjunction with navigational charts dramatically improved the pilot's SA by making him ware of potential collision hazards.

As aircraft engines became more powerful, and airframe construction materials improved, high altitude flights became more frequent. The single needle display of the altimeter evolved to one that used three pointers to better cope with the range of higher altitudes. The longest of three pointers indicated hundreds of feet, the medium one indicated thousands of feet, and the smallest one indicated tens of thousands of feet (Siberry, pg. 60). The new device was intended to improve pilot SA by making it easier for him to read a broader range of altitudes and readily identify possible hypoxic environments. Despite the good intentions of its designers, the three-pointer altimeter became a liability. The mingled hands of the indicator was difficult to decipher, and in environments that would not allow the pilot's eye to linger over any one instrument for too long, mistakes were made. A P-47 pilot's testimony from a combat mission dramatically emphasizes this point. "I was flying at 25,000 feet on my first combat mission, but had mistakenly read the hands on my altimeter and was under the impression that I was at 35,000 feet. I called in some unidentified aircraft which were level with our formation and, consequentially, actually at 25,000 feet. Since I mistakenly reported them at 35,000 feet, they were believed to be enemy aircraft … a good deal of confusion resulted. I believe some improvements could be made in our present altimeter" (Sinaiko, 1961). His words proved to be very profound. However, the three-pointer and other altimeters relying on multiple pointers were in use for over 50 years before safer ones that incorporated drums or digital displays numerically to represent the altitude replaced them. Altitude indicators have proven to be a useful addition to the pilot's arsenal in his battle with SD and LSA. Nonetheless, the altimeter was not a cure-all for overcoming man's sensory deficiencies. Just like the early compass, and airspeed indicators the resulting benefits of the altimeter must be contemplated.


Without doubt, the evolution of basic heading, speed, and altitude indicating instruments has had a positive impact on pilot SD and LSA. Even today, this group of three instruments is of great importance for conducting safe flight operations under VFR conditions, and subsequently is the legal minimum requirement established by the Federal Aviation Administrations for private aircraft. At times, pilots have become complacent by placing total faith in their instruments, and have flagrantly refused to believe their own sensory inputs telling them that their equipment has failed. In addition, poor design has hindered the pilot's ability to maximize their benefits. Although these occurrences are rare, grounds for concern exists when safety is at stake. However, these concerns should not overshadow the rewards brought by the evolution of cockpit instrumentation, and the rewards have been numerous. For example, aviation reaped the rewards of instrumentation and became a competitive mode of transportation with the advent of around-the-clock all weather flight capability. Additionally, the process of refining the integration between the pilot and these early instruments in the fledgling airborne environment served as catalyst for subsequent Human Factors research and development. Not only has this research positively impacted aviation safety, it has also contributed significantly to technological advancements gained refining SHEL model interfaces in other fields tackling man-machine integration dilemmas (Edwards, 1972). Today, problems still exist in man's ongoing marriage with machine but fortunately, they can, and will be reconciled with further application of human factors engineering intervention.is

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