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  1. Comparison of how 2 Air data instruments compensate for the principle changes in the atmosphere.

  2. Selection of a Pitot-static system to be fitted into a modern aircraft (Provide the design considerations, construction and operation that would influence the choice of the system)

  3. Comparison of an analogue to digital air data computer and how they are integrated into an aircraft. Discuss the advantages and disadvantages

  4. Define Construction and Operation director system

  5. Comparison of the 3 types of electronic displays explaining the operation. Advantages and disadvantages of each

  6. Construction and Operation of a typical flight instrument system. How instruments are fully integrated .

Primary Air Data Instruments and Pitot-Static Tube

Due to changes in barometric pressure, input errors would arise as aircraft maneuvers in the air stream.

The air data has the true and indicated airspeed, pressure, altitude, ambient air temperature, angles of attack and rate of climb among others

Now, the earth’s atmosphere is stratified into various zones; notably, the one in which most aircrafts are flown is the troposphere. Other divisions include: chemosphere, ionosphere, exosphere and ozonosphere. Temperature changes are fairly uniform in the troposphere (Wiolland, 2005).).

The air data system mainly comprises the primary air data instruments, and pitot-static tube; The primary air data instruments include : altimeter, vertical speed indicator and air speed

Notably, the ground-based radar are normally used to establish the exact position and velocity of the plane. However, the optical trackers can also be used in this case.

GPS receiver then determines the time, position and velocity minus drift errors. Additionally, the euler angles are mostly measured using mutltiple GPS receiver.

The rate gyroscope is used in measuring angular acceleration rates and linear accelerations. However, in steady flight, linear accelerators are used to measure pitch and roll attitude.

Notaly, therefore, airdata instruments are useful in maintenance of the plane stability hence contributing to safety, navigation and control by the pilot

Figure 1: Air speed indicator (courtesy of USA

Design considerations

Error-free: The system must be able to minimize the effects of error on the indicated readings; too much exaggerated final readings may be risky as pilot receives wrong information that could pose danger to aircraft maneuvers as the pilot is often informed through the readings. Besides, factors such as resolution, accuracy, cost and maintenance and size of aircraft requirements are critical in the integration of the system. The system must be able to repeatedly give the same value in the same condition of performance (FAA, no year).). However, practically, errors would often arise hence accuracy needs to be within a small range that is allowable. Besides, it should be responsive in functionality such that parameters like attitude and barometric pressure changes must be captured in real-time and displayed for the pilot to make informed decision.

The construction and operation

The Pitot-static system is mainly composed of the pitot-static tube, airspeed indicator altimeter and vertical speed are all connected to a static port such that air introduced into the system via the port as the airplane climbs, the altitude changes so is air pressure. The pressure changes are recorded by this system.

The air speed indicator measures speed of air by getting the static difference between the static pressure and the ram air pressure. The display is usually done in a mach number in knob units.

Notably, the air speed indicator is constituted with an expandable diaphragm such that air rushes in, it is filled and expanded while ram air pressure is increased.

The altimeter measures air pressure; the calibration is normally in height. It indicates the static pressure as altitude. The air speed indicator will indicate difference between pilot static pressure and pitot pressure. The vertical speed gives an indication as to the rate at which static pressure changes with either climbing or descending.

In the altimeter, the air pressure increases as airplane descends and decreases as the airplane climbs.

The vertical speed indicator indicates the rate of climb or descends of the aircraft; calibration is done in feet per minute.

There is a needle connected to the wafer such that it rotates as wafer expands and contracts to indicate the rate of climb or descend of aircraft.  

 Figure 2: The Pitot-static tube (courtesy of

Analogue air data computer  

Digital air data computer

This is the traditional air data computer that was used. It mainly comprised of airspeed indicator and altimeter in which primitive pneumatically driven instruments was performed and a nonlinear computation done via a spring mechanism

The digital remote sensing is normally separated from the display feature

Servo-driven cams would compute the parameters like speed, altitude and mach numbers.

Flat panel computer screens are used for display of output information

The sensing and display functions are driven in a single unit.

Special digital data buses are used to carry information signals

Conveying of sensor information is done via wires and pneumatic lines and cams profiles. Numerous display gauges are often used to show the output

It normally maintains optimum performance of aircraft

Attitude capsule was the main airdata sensing used ; it had a simple aneroid to operate it

It provides additional information to the aircrafts system

Varying accuracy at different altitudes

It provides accurate and realtime sensing and computation capabilities

It failed to maintain pressure rates hence system failure was common

Has self-correcting mechanism which can adjust as conditions and altitude change via the static source error correction

This is normally an embedded system within the flight control system in which it provides real-time guidance to the pilot or autopilot along a flight path by selection and computation of various commands. It is linked with an ADC from which it receives the input signal and flight data computer performs computation and supplies data such as attitude, air speed, and flux and air temperature. These signals are then sent and displayed in the altitude indicator (All Star Network, 2000). Notably, in the absence of the autopilot, the FD should still be functional with manual maneuvers. In the manual mode, the pilot directly instructs the FD to carry out the said functions and the system responds by controlling the listed parameters.

Design Considerations

The FD therefore provides a set of commands which are fed from the flight control computer while in steering mode using a command bar located on the altitude director indicator (Bombarider, no year). There are about three status modes, namely: vertical, transfer and lateral modes which are selected accordingly.

The steering commands are in charge of providing visual guidance to pilot as she manually operates the aircraft maneuvers. Therefore, in a nutshell, according to FAA (no year) the FDS provides command options to perform selection of a desired altitude, maintain altitude pressure and ensure aircraft stays at a vertical speed, holds a mach number and ceases a preselected barometric altitude; a part from maintaining the trim options and wing-level (All Star Network, 2000).


Now, the FD is composed of flight director indicator, horizontal situation indicator, mode selector and a flight director computer. The flight director computer comprises features such as altitude, indicator, glide slope, slip indicator, wearing flag for gyro, pitch and bank command and fixed aircraft symbol. The HSI normally facilitates selection and operation navigation aids while both outbound and inbound tracking is made possible. The FDC receives input signals from the radar altimeter, sensors (barometric), and compass and altitude gyro. Certainly, the FDC computes and provides steering commands to enable pilot perform actions such as heading selection, ILS glide sloping and flight track maintenance (Kayton & Fried, 1997).

Figure 3: The PFD system (Courtesy of UND

Basically, the following are the major electronic displays that are predominantly in use:

  • LEDs

  • LCDs

Figure 4: LCDs features (Courtesy of

  • ELDs

The LED stands for Light emitting diodes where they do display arbitrary number of digits and is based on injection luminescence mechanism. The LED has a wide range of operating temperature; it is relatively inexpensive and can fully be integrated with the digital logic units. Besides, its display quality guarantees a wide range of applications where display quality is critical to system performance (The Airline Pilots, no year).

The ELDS stand for electroluminescent display which is similar in construction and operation to ac plas ma display. However, in the place of gas-filled area, a thin film of electroluminescent material is used. An AC voltage is normally applied between the rear and front electrodes and light is emitted to facilitate display property. Normally, zinc sulphide is often used for luminescence. It is mostly relevant in situations where a range of display colors is required. Besides, it can be used in situations that require a wide angle of vision (The Airline Pilots, no year).

Analogue and Digital Air Data Computers

The LCD stands for Liquid crystal displays. It occurs when solid surface molecules interacts with a nematic liquid crystal (Kayton & Fried, 1997)

The system is comprised of display, controls and data processors. The display electronics unit is the symbol generator where data is received from the pilot and signals from sensors such that the data buses are used to ascertain the validity of these signals. There is a graphics generator which undertakes the required computations.

The monitoring unit is comprised of the comparator where warnings for airspeeds, pitch and altitude indications are facilitated. There is also an instrument display source which would automatically reconfigure to catch up on the faults that arise in the system (Bombarider, no year).

Basically, the system comprises the following instruments: airspeed indicator, altimeter, gyro horizon, direction indicator and vertical speed indicator. The gyro horizon is normally centralized such that it provides positive and direct indications of the altitude.  

  • The air speed indicator

This instrument that is often located in the cockpit deck displays real-time speed of the airplane as it is airborne. Traditionally, the displays were analogue but with the advancement in digital technology, the modern air speed indicator is a digital readout

  • Aircraft heading and attitude

They are incorporated with sensors to measure both the attitude and heading with a ground referencing as datum for airborne measurements. It has a self correcting mechanism owing to the errors that it encounters while navigating the airspace. However, this feature may be a challenge to effect in turbulent situations as stability of flight becomes complex to maintain in real-time (Kayton & Fried, 1997). Therefore, pilots are called upon to remain conversant with the relevant aircraft operating procedures. The sensors would relay critical information such as attitude, static and dynamic barometric pressure. This is then integrated (as mentioned earlier) in an air data computer system with processing capabilities to calculate pressure, air speed (both true and vertical speeds). Therefore, air data attitude heading reference system (ADAHRS) combines all of the instruments into a single unit.  

Figure 4: Altimeter instrument (courtesy of

  • The altimeter

As mentioned earlier, this instrument measures the real-time altitude of the plane as it ascends or descends. Mostly it is designated in feet.

Figure 5: The altimeter (courtesy of

  • The vertical speed indicator

The vertical speed is the rate of change in climb or descends of the airplane. This is normally measured and indicated using the vertical speed indicator. However, the error in the instrument is normally enlarged when the airplane flies in turbulent regions (Houston, 2017).

  • Turn coordinator

This instrument is normally used to measure the rate of turn or roll such that when pilot maneuvers the plane by turning the plane, this is registered in the cockpit and assists the pilot to undertake subsequent maneuvering actions accordingly (Haering, 1995).

Figure 6: Turn coordinator (courtesy of

  • The attitude indicator

This is a gyroscopic instrument which helps pilot to know whether the plane is climbing or descending or even turning.  


All Star Network. (2000). Flight Director Systems. Available from:

Bombarider. (no year).Canadair Regional Jet 100/200 - Automatic Flight Control System. Available from:

FAA. (no year).Automated Flight Control. Available from:

Kayton, M. & Fried, W.R. (1997). Avionics Navigation Systems. John Wiley and Sons Publication. Available from:

The Airline Pilots.(no year).Electronic Flight Instrument System (EFIS).

Wiolland,K. (2005). Air Data Computers. Porter-strait instrument co. inc. Available at:

Haering, E.A. (1995). Airdata Measurement and Calibration. National Aeronautics and Space Administration.  Available at:

Houston, S. (2017).Traditional Flight Instruments Pilots Need to Know. Available at:

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