Aircraft Instrument System I

Instrumentation - Introduction 

• Even though earliest aircraft had very few instruments such as a Compass and Air Speed Indicator, it is difficult to imagine modern aircraft without these. With enhanced aircraft capabilities, extended flying hours, difficult all weather flying conditions and passenger safety as a core issues, Pilots must be empowered with as much detail about flight as possible.
• At the same time, these Instruments must be friendly. Invariably Sight and Sound is used concurrently to draw Pilot’s attention. Analog and Digital readouts is also a critical issue. Digital readouts are more accurate but Analog ones are the ones which easily stand out with color bands, shaded zones etc. 
• Ultimately the Pilot should be capable of flying only with the assistance from the Instruments and Communication with Air Traffic Controllers (ATC).

Principle Classification of Instruments 

Measurements of basic quantities is possible in terms of length, weight, time, pressure and temperature. Other quantities which are essential for understanding and negotiating with any real phenomena are derived ones. For example speed = distance/time and density = mass / volume etc. Even though former formula is excellent for calibration and accurate, it is not practicable to use. Inference from Pitot tube is much more convenient for such measurement.  Instruments will perform some internal manipulation to get the desired and understandable quantity. From application point of view following types of Instruments are crucial for aircraft flight.   

• Pressure type instruments 
• Mechanical type instruments 
• Gyro instruments 
• Electrical & Electronic instruments 

Three Principle Categories of Instruments 

• Powerplant - Information concerning operations of engine. Some of them could be Engine rpm, Temperature, Fuel and Oil Pressure Gauge etc. 
• Flight & Navigational Instruments - Information concerning Flight Speed, Altitude, Attitude, Rate of Ascent or Descent, Directional Heading etc. 
• System Instruments - Information concerning various systems such as Hydraulic System & its Pressure, Air conditioning, Electrical system Status etc.  

Safety of Aircraft & Passengers depends on accurate and reliable instruments as well as correct reading of these. At times these Instruments can forewarn the Pilot about impending disaster or calamity while in flight. Instruments and recordings could also be used for trouble shooting. 

Pressure Measurements 

•  One bar Atmospheric pressure corresponds to 760 mm column of mercury & is most accurate measurement one can have. Similarly a simple U tube Manometer permits measurement of differential pressure between two stations.  

•  These basic instruments work well in laboratories. However these are not suitable in actual flight condition. 

High Pressure - Bourdon Tube 

• Bourdon tube which is flat or oval is constructed of spring tempered brass, bronze or beryllium metal.
• Open end is made stationary whereas Close end is free to move and is attached to spring & linkages such as levers and gears.
• Pressure on the close end causes the tube to uncoil & is seen with needle on calibrated dial.
• Pressure is measured in terms of psig (gauge or above atmosphere) or psia (absolute)  

 Bellows and Diaphragm

• Diaphragm consists of two disks of thin metal corrugated concentrically & sealed together at the edges to form cavity or capsule. 
• Through opening on one side, pressure is admitted and the opposite side is connected with bridge-rocking shaftlever arrangement to amplify and display signal.
• Bellows provide greater range of movement and thus pressure. 

Pito Tube

Pitot tube on the aircraft is around 25 centimeters long with a 1 centimeter diameter. Several small holes are drilled around the outside of the tube and a center hole is drilled down the axis of the tube. The outside holes are connected to one side of a device called a pressure transducer. The center hole in    the tube is kept separate from the outside holes and is connected to the other side of the transducer. The transducer measures the difference in pressure in the two groups of tubes by measuring the strain in a thin element using an electronic strain gauge. The pitot tube is mounted on the aircraft so that the center tube is always pointed in the direction of travel and the outside holes are perpendicular to the center tube.  

Since the outside holes are perpendicular to the direction of travel, these tubes are pressurized by the local random component of the air velocity. The pressure in these tubes is the static pressure (ps) discussed in Bernoulli's equation. The center tube, however, is pointed in the direction of travel and is pressurized by both the random and the ordered air velocity. The pressure in this tube is the total pressure (pt) discussed in Bernoulli's equation. The pressure transducer measures the difference in total and static pressure. (pt - ps).  Some practical limitations:    

1. If the velocity is low, the difference in pressures is very small and hard to accurately measure with the transducer. Errors in the instrument could be greater than measurement! So pitot tubes don't work very well for very low velocities.     
2. If the velocity is very high (supersonic), we've violated the assumptions of Bernoulli's equation and the measurement is wrong again. At the front of the tube, a shock wave appears that will change the total pressure. There are corrections for the shock wave that can be applied to allow us to use pitot tubes for high speed aircraft. 

Locating Pitot 

The static line vents the pitotstatic instruments to the outside, or ambient, air pressure through the static    port. The static port (may be located in various places on different types of aircraft and more than one port may be used. Regardless of location, the port is always positioned so the plane of the opening is parallel to the relative air flow. By comparison, the plane of the pitot tube opening is nearly perpendicular to the relative wind. The pressure sensed at the static ports is transferred to the cabin instruments by a tube. 

Pitot Tube Use 

Aircraft constantly encounter atmosphere pressure changes as they climb, descend, accelerate or decelerate. The pitot-static system - sensitive to airspeed, altitude, and rates of altitude change - provides the pressure information displayed on cabin instrumentation. 

An outside air temperature sensor must be installed for air data systems. The airspeed indicator is vented to both pitot and static lines. The airspeed indicator reacts to changes between pitot air and static air. The altimeter and vertical speed indicator, however, require venting to only the static line. Heated pitot tube prevents ice formation.. 

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