Introduction 83,000 pounds (38,000 kg) of cargo.PITOT STATIC SYSTEMThe

Introduction                                                                        Fig – A Military Boeing KC-135R refueling a F-16 JetThe Boeing KC-135 Stratotanker is a military aerial refueling aircraft.It was designed specifically for aerial refueling and for 15 years was the only tanker used by the Strategic Air Command (SAC) of the US Army . More than 600 of the 732 tankers built were still in service in the mid-1990s.The KC-135R has four turbofan engines, mounted under 35-degree swept wings, which power it to takeoffs at gross weights up to 322,500 pounds (146,300 kg). Nearly all internal fuel can be pumped through the tanker’s flying boom, the KC-135’s primary fuel transfer method. A special shuttlecock-shaped drogue, attached to and trailing behind the flying boom, may be used to refuel aircraft fitted with probes (See Fig-1). A cargo deck above the refueling system can hold a mixed load of passengers and cargo. Depending on fuel storage configuration, the KC-135 can carry up to 83,000 pounds (38,000 kg) of cargo.PITOT STATIC SYSTEMThe aircraft pitot-static system (Figure 2) shows instruments that operate on the principle of the barometer. The system consists of a pitot tube, static air vents, and three indicators, which connect with pipelines that carry air. The three indicators are Airspeed, Altimeter, and the Vertical Speed. Fig-1 Actual KC-135 Pitot- Static Tube                                                                                                       Fig-2 Generic Pitot-Static System                        The airspeed indicator shows the speed of the aircraft through the air, and the altimeter shows the altitude. The Vertical Speed Indicator (VSI) indicates how fast the aircraft is climbing or descending. All of these indicators operate on air that comes in from outside the aircraft during flight. The pitot tube mounts on the outside of the aircraft (Figure 3) at a point where the air is least likely to be turbulent. The tube points in a forward direction parallel to the aircraft’s line of flight. One general type of airspeed tube mounts on a streamlined mast extending below the nose of the fuselage. Another type mounts on the bottom side extending forward from the leading edge of the wing. Although there is a slight difference in their construction, the tubes operate identically. The Pitot System measures total pressure, which is the pressure of the outside air against the aircraft flying through it. The tube that goes from the pitot tube to the airspeed indicator applies the outside air pressure to the airspeed indicator. The airspeed indicator calibration allows various air pressures to cause different readings on the dial. The purpose of the airspeed indicator is to interpret pitot air pressure in terms of airspeed in knots. Generally, static air vents (Figure 3) are small, calibrated holes in an assembly mounted flush with the aircraft fuselage. Their position is in a place with the least amount of local airflow moving across the vents when the aircraft is flying. Static means stationary or not changing. The static part of the pitot-static system also introduces outside air. However, the outside air is at its normal outside atmospheric pressure as though the aircraft were standing still in the air. The static line applies this outside air to the airspeed indicators, the altimeter, and the vertical speed indicator.AltimeterAn altimeter is an instrument that measures static pressure ( see fig-7) Before we can understand how the altimeter works, we need to understand altitude. Even though the altimeter reads in feet, it is actually measuring pressure. The word altitude is rather generic, so it needs further explanation. The term altitude mainly refers to altitude above Mean Sea Level (MSL) and different type of pressure used on the altimeter like Atmospheric, Absolute and Gauge Pressure. MEAN SEA LEVEL PRESSUREAbout 80 percent of the earth’s surface is water, it is common to use sea level as an altitude reference point. The pull of gravity is not the same at sea level all over the world because the earth is not perfectly round and because of tides. To adjust for this, an average (or mean) value is set; this is the mean sea level. Mean sea level is the point where gravity acting on the atmosphere produces a pressure of 14.70 pounds per square inch. This pressure supports a column of mercury in a barometer to a height of 29.92 inches. This is the reference point from which you measure all other altitudes.Most of the altitude pilot reads from an altimeter in (MSL).                   Fig 3 – Atmospheric Mercury Barometer                Fig 4 – Scale of 14.7 Pounds Square Inch     Atmospheric Pressure Atmospheric pressure also known as barometric pressure is mainly referring to the pressure within the atmosphere . The atmospheric pressure is due to the weight of air in the atmosphere enveloping the earth and is taken as the pressure due to a column of air extending from sea level to the upper limits of earth`s atmosphere. However , atmospheric pressure will not be the same through out all places on earth (see Fig-5) and is always changing . So in this case,once the pilot knows that particular region`s atmospheric pressure , the pilot can adjust accordingly using the baro knob so the altimeter can read correctly.              Fig 5 – Atmospheric pressure                              Fig 6 – Absolute Pressure on a scale                           Of different region on earthAbsolute PressureAbsolute altitude is the distance between the aircraft and the terrain over which it is flying. It is referred to as the altitude Above Ground Level (AGL). Due to variations in terrain, AGL is typically unreliable information. However, it is useful when flying near the ground, such as in a takeoff or landing pattern. You find AGL by subtracting the elevation of the terrain beneath the aircraft from the altitude read on the altimeter (MSL). This is a method of pressure measurement which has a vacuum value for zero, i.e. the lowest attainable pressure as zero pressure therefore all pressure will be positive. (Refer to Fig-6)Gauge PressureThe most common type of pressure measurement is gauge pressure. This is the difference between the pressure to be measured and the atmospheric pressure. The gauge pressureuses a reference point also known as a datum at 0 pounds per square inch (psi). Above which, the Gauge pressure is considered positive , below which it will be considered negative . (Refer to Fig-6). So for example , let`s say the reference for datum used is the mean sea level of 14.7psi and an aircraft has an Absolute Pressure reading of 24.7 psi . Then the gauge pressure will be a positive 10 as Absolute Pressure = Datum+Gauge Pressure. Gauge pressure is easily measured and is obtained by ignoring the fact that the atmosphere is always exerting its pressure on everything. Gauge pressure measurements are simple and widely useful. They eliminate the need to measure varying atmospheric pressure to indicate or monitor a particular pressure situation. Gauge pressure should be assumed, unless otherwise indicated, or unless the pressure measurement is of a type known to require absolute pressure. Operation of AltimeterThe conventional aircraft altimeter works by measuring the atmospheric pressure at the airplane’s flight altitude and comparing it to a preset pressure value. Air pressure decreases by about one-inch mercury for each 1,000-foot altitude increase.Inside the instrument, the casing is a set of three aneroid wafers that are sealed but still able to expand and collapse. These aneroid wafers are calibrated to sea level pressure of 29.92″ mercury inside. An outside static pressure lower than 29.92″ Hg (as experienced with again in altitude) causes the wafers to expand since the pressure inside of the sealed wafers is greater than on the outside. A higher static pressure causes the wafers to collapse . When the static pressure increases or decreases, mechanical connections trigger the altimeter needle to show a corresponding altitude in feet.The appearance of altimeters varies, but a common one is known as the three-point altimeter. This type of altimeter consist of a short, wide needle that shows height in 10,000-foot increments; a slightly longer and wider needle depicts height in 1,000-foot increments, and the longest needle shows height in 100-foot increments (See Fig – 8). Most altimeters in use today include a Kollsman window, which is an adjustable dial that allows the pilot to enter the local pressure values for his flight. Entering a pressure value in the Kollsman window adjusts the altitude for nonstandard pressure and gives a more accurate indicated altitude.   Fig 7 – Cross Section of Altimeter                                Fig 8 – Display of Altimeter        Vertical Speed Indicator                                                  An analog vertical speed indicator (VSI) may also be referred as a rate-of-climb indicator. It is a direct reading, differential pressure gauge that compares static pressure from the aircraft’s static probe or hole directed into a diaphragm with the casing pressure (Pc) surrounding the diaphragm in the instrument case. Air is free to flow unrestricted in and out of the diaphragm but is made to flow in and out of the case through a calibrated leak or metering unit. A calibrated leak or metering unit is a device which limits the rate of airflow in or out of the the VSI Casing`s . The scale reads either up (ascent) or down (descent) of 2000,4000 and 6000 ft per min depending on the type of aircraft . Then these are further sub-divided in 1000 on some instrument into 100 ft. intervals. ( See Fig – 9 )       Fig-9 VSI Scale Pointer                                               Fig-10 VSI MechanismOperation of Vertical Speed Indicator         The capsule or diaphragm is connected by a mechanical link to the sector gear which then connects to a pinion that moves the pointer on the scale VSI scale . Static air is free to enter the capsule/diaphragm  through the the bypass restriction (See Fig-10).However, the static air can only enter the case via a metering unit which limits the rate of air flow in and out of the case . So if the aircraft is climbing , static pressure decreases and this will result in the capsule having a higher pressure than the surrounding . Thus, air in the diaphragm will rush out through the static pressure connection immediately causing the diaphragm  to collapse. Then the air in the casing will also leave through the static pressure connection but at a slower rate compared to the capsule because of the metering unit (See Fig-11).  This resulting differential pressure actuates the mechanism which causes an appropriate pointer movement upwards after magnifying it .. Once the aircraft levels off , the case pressure Pc will be the same of the diaphragm pressure which is directly proportionate to the static pressure Ps , the Vertical Speed Indicator will go back to 0 to show there is no climb or descent action of the aircraft (See Fig – 12). In summary , the whole operation of this VSI instrument is dependent on the changes in altitude , the relationship between air pressure and altitude which is assumed to be standard . However, temperature, density and viscosity of the air will also change with altitude and not guarantee to be with accordance to the standar I.C.A.O atmosphere . Therefore , certain mechanical refinements are necessary to provide a true and correct indications . Fig-11 Pressure difference during ascent              Fig-12 Pressure difference once aircraft                                                                                                    levels off                         Airspeed IndicatorAn airspeed indicator has a cylindrical, airtight case that connects to the static air line from the pitot-static tube. Inside the case is a small aneroid diaphragm. The diaphragm is very sensitive to changes in pressure, and it connects to the pitot  line. This construction allows air from the pitot tube to enter the diaphragm. The side of the diaphragm fastens to the case and is rigid. The needle or pointer connects through a series of levers and gears to the free side of the diaphragm. The airspeed indicator is a differential pressure instrument. It measures the difference between the pressures in the pitot connection and in the static airline. The two pressures are equal when the aircraft is stationary on the ground. Movement through the air causes pressure in the pitot connection line to become greater than that in the static line. This pressure increase causes the diaphragm to expand (See Fig-13). The expansion or collapse  of the diaphragm goes through a long levers , pinion and sector gears to the face of the instrument to regulate needle position. The needle shows the pressure differential in knots or miles per hour (mph) , (See fig-15) .                      Fig-13 Cross-Section of Airspeed Indicator                     Fig-14 Stagnant displaced airOperation Of Airspeed Indicator The airspeed indicator measures the speed of the aircraft by measuring the excess air pressure due to the forward motion of the aircraft. It computes the indicated airspeed in terms of the difference between the pitot and static pressure from the static air line and pitot connection.Pitot pressure is also known as additional pressure produced on a surface of the Pitot-Static Tube when the flowing stream of air is brought to rest or stagnation (See Fig-14) . After the air is brought to rest(stagnant), kinetic energy of the air is converted to pressure energy. This stagnant air pressure energy is the work done (unit in Joules,J) of the air displaced due to excess pressure from the static pressure which is also the normal pressure of the surrounding air. The difference between the pressure energy of displaced air and static is computed using a differential meter and through mechanical linkages shall display in on the instrument`s .Display Of Airspeed IndicatorWhite arc: This arc is commonly referred to as the flap operating range since its lower limit represents the full flap stall speed and its upper limit provides the maximum flap speed Approaches and landings are usually flown at speeds within the white arc. Green arc: This is the normal operating range of the airplane. Most flying occurs within this range.Yellow arc: Caution range. Fly within this range only in smooth air, and then, only with caution.Red line (VNE): Never exceed speed. Operating above this speed is prohibited since it may result in damage or structural failure.                                          Fig-15 Display of Airspeed IndicatorGyroscopic InstrumentsIn aircraft instruments, gyros are applied in attitude,compass(Magnetic Heading)and turn coordinators(Turn and Bank Indicator).These indicators contain motor or rotors which are rotating at high rpm supplied by a power source. These gives it 2 important properties of the gyroscopic instruments, Rigidity and Precession. The rotor or motor can be powered electrically or spinned using high pressure bleed air from a vacuum pump. The basic construction of the gyro is fixed in the instruments with outer and inner gimbals ring which allow certain freedom of motion of the gyro (see fig-16).Fig-16 Construction of gyroscope instruments                                                                                                                                                                                                                                                      Rigidity  Despite the rotor being very small, it must rotate at a very high rpm. Giving it inertia to the instrument , also called rigidity , the rotor maintains alignment in certain space despite applying force to it. A few of the factors that affect rigidity are mass of rotor, it speed of motor(rpm) and the distance between the mass of rotor to the axis of rotation. The heavier the mass of the gyro wheel and the faster the speed of rotor will greatly increase the rigidity.PrecessionPrecession is defined as the angular change in the direction of the plane of rotation when a force is applied. The angular displacement is the result of the gyroscope attempt to maintain about its plane of spin , resistance towards force applied to it and the force within itself. The rate of the angular displacement will depend on mass of rotor , speed of rotor and value of applied force . The co-relationship between Rigidity and Precession is that they are indirectly proportional. The more rigid the gyroscopic instrument is , the less precession it is.Attitude Direction IndicatorThe attitude indicator (also known as an artificial horizon) shows the aircraft’s relation to the horizon. From this the pilot can tell whether the wings are level (roll) and if the aircraft nose is pointing above or below the horizon (pitch). This is a primary instrument for instrument flight and is also useful in conditions of poor visibility. Pilots are trained to use other instruments in combination should this instrument or its power fail.(See fig-17)Operation of Attitude Direction IndicatorFor Attitude Direction Indicator , the rotor is spinning about the vertical axis. So for example, say there is a change in aircraft attitude, lt goes into a climb, the instrument case and outer ring will then turn about the Y-Axis of the stabilized inner gimbal ring see. During the climb action of aircraft, a pivot on the bar will carry the back of the bar upwards causing it to pivot about the stabilized actuating pin. So eventually the , the front of the bar and pointer will move downwards (See fig-18) to show the brown colored horizon ground moving downwards also indicating that the aircraft is climbing.Now in the case of change in lateral attitude of aircraft, rolling/banking , this action will displace the instrument casing about the Z-Axis. This is indicated by movement of the symbolic aircraft with respect to the horizon bar , and also the relative moving of bank angle scale and pointer.(See to fig-18).Fig-17 Attitude Direction Indicator                            Fig-18 Construction of gyroscopic ADIHeading IndicatorThe heading indicator (also known as the directional gyro, or DG) displays the aircraft’s heading with respect to magnetic north when set with a compass. Bearing friction causes drift errors from precession, which must be periodically corrected by calibrating the instrument to the magnetic compass. In many advanced aircraft (including almost all jet aircraft), the heading indicator is replaced by a horizontal situation indicator (HSI) which provides the same heading information, but also assists with navigation.Operation of Heading IndicatorIn the case of Heading Indicator(Directional Gyro), this instrument uses the principle of rigidity which allows stable magnetic heading reference. This gyro is manufactured and design such that it spins on the horizontal axis only , X-Axis (See fig-20). Inside the heading indicator instrument, it consists of mainly an outer gimbal ring pivoted about the Z-Axis and a circulated card with degrees indications printed on it . The circulated card can be seen the pilot because it is connected via mechanical linkages called a lubber line inside. So if for example the aircraft is turning, the gimbal and circulated card will be stabilized. So in actual fact , it is the frame which is actually turning around the circulated card to show the number of degrees turned.     Fig 19- Heading Indicator Display                                    Fig-20 Rotating rotor in Heading                                                                                                                IndicatorTurn and Bank IndicatorTurn and Bank indicators are used to measure how fast an aircraft is turning. On the display of the instrument , it mainly shows a slip ball which is black in color in liquid kerosene and a vertical needle for reading (see Fig-21)Operation of Turn and Bank IndicatorIn the turn and bank indicator it consists of mainly one gimbal ring and a spring connected directly between the gimbal ring and casing to limit or restrain movement about Z and Y axis (See Fig-22) . In the operation of this turn and bank indicator , it highly depends on the principle of precession. Also , the main advantage of this gyroscopic instrument is that no erecting device or error correction is needed it will always be stable and centralized due to the spring control . The best way to explain the operation of this gyroscopic is take to example of a bicycle spinning in suspense in downward . When a force pushes it from the left , the bicycle will swing slightly towards right because as a resultant magnitude of the two vectors – spinning direction and force applied . In this case , the one moving will be the black colored slip ball. The rate of turn will depend on precession . Therefore , it is a must for the rotor speed to remain constant and not at a very high speed because this will it`s rigidity causing less precession which will give wrong information to the pilot.Fig-21 Turn and Bank Indicator                       Fig-22 Turn and Bank Indicator cross section                       Fig- Interior cockpit control panel of the Boeing KC-135R                                                          (Basic T-Layout)Specifications of Boeing KC-135RGeneral CharacteristicsCrew: three: pilot, co-pilot and boom operator. (Some KC-135 missions require the addition of a navigator.)Capacity: 80 passengersPayload: 83,000 lb (37,600 kg)Length: 136 ft 3 in (41.53 m)Wingspan: 130 ft 10 in (39.88 m)Height: 41 ft 8 in (12.70 m)Wing area: 2,433 ft² (226 m²)Empty weight: 98,466 lb (44,663 kg)Useful load: 200,000 lb (90,700 kg)Loaded weight: 297,000 lb (135,000 kg)Max. takeoff weight: 322,500 lb (146,000 kg)Maximum Fuel Load: 200,000 lb (90,719 kg)Powerplant: 4 × CFM International CFM56 (F108-CF-100) turbofan, 21,634 lbf (96.2 kN) eachPerformanceMaximum speed: 580 mph (933 km/h)Cruise speed: 530 mph (853 km/h) at 30,000 feet (9,144 m)Range: 1,500 mi (2,419 km) with 150,000 lb (68,039 kg) of transfer fuelFerry range: 11,015 mi (17,766 km)Service ceiling: 50,000 ft (15,200 m)Rate of climb: 4,900 ft/min (1,490 m/min)Reference