Barometric Instruments PDF

WELCOME TO BAROMETRIC INSTRUMENTS 1 I& Bar I o Objectives: At the end of this lesson, you should be able to: 1. State the purpose and the main val...

WELCOME TO BAROMETRIC INSTRUMENTS 1 I& Bar I o Objectives: At the end of this lesson, you should be able to: 1. State the purpose and the main values of the ICAO standard atmosphere. 2. List the main components and explain the principle of operation of:  Airspeed indicator - Define the term `IAS’,`CAS’,`EAS’,`TAS’ and `Mach Number’.  Barometric Altimeter - Define International `Q’ Code by ICAO  Encoding Altimeter  Vertical Speed Indicator: - Diaphragm- base - Air chamber-base I& Bar I o ICAO STANDARD ATMOSPHERE ICAO – International Civil Aviation Organisation ISA – International Standard Atmosphere The purpose of ISA is to provide standard values of the atmosphere as a basis for the calibration of all Barometric ISA states Instruments. the `Standard conditions at MSL are: 1.A temperature of 15° C, 2.Air pressure of 14.7psi / 1013.25 hectopascal (hPa)/ 29.92 inch mercury (in Hg). I& DEFINITION OF ISA Bar I STANDARD o (Pressure,Temperature,Altitu The de) STRATOSPHERE temperature (Height: 36,089 Feet) remains unchanged (Temperature : - at -56.5⁰ C from 56.5˚C) 36,089 feet, where the As height increases stratosphere temperature begins. decreases at a rate of 2˚C per 1,000 feet from MSL. ISA Standard temperature is 15⁰C at MSL MSL (Mean sea level ) = 0 Feet I& Bar I o AIRSPEED INDICATOR Airspeed indicator displays the speed at which the aircraft is moving through the air relative to the surrounding air. The aircraft speed is determined by measuring the dynamic pressure caused by the forward movement of the aircraft Dynamic Pressure = Pitot pressure – Static pressure I& Bar I Airspeed o Indicators CALIBRATION OF AIRSPEED Knots INDICATOR standard Kilometres per hour Glider (Km/h) Miles per hour (mph) Obsolete Figure 1 I& Bar I DEFINITION OF AIRSPEED o Indicated Airspeed I IAS is the uncorrected airspeed which is directly read from the indicator. Calibrated Airspeed CAS is the IAS compensated for installation, instrument and pitot static system errors. C Equivalent Airspeed EAS is the CAS compensated for air compressibility. EAS is found as the ratio between the actual air density and the air density at sea level in the standard atmosphere T E True Airspeed TAS is the EAS compensated for the actual density altitude, i.e. it is the true speed of the aircraft relative to the surrounding air. Mach The Mach number indicates the aircraft’s IAS relative to the speed of sound. I& Bar I o Main Components of Airspeed Indicator Airspeed indicator consists of the following  components: Air tight case Diaphragm. Pointer drive mechanism Temperature compensation (Bimetallic strip) Pressure connection marked - P for Pitot pressure pipe - S for Static pressure pipe I& Bar I Pressure sensing element of ASI o Metalic capsule The purpose of the diaphragm (Capsule) is to measure the dynamic pressure (the difference between the ram or impact pressure and Static (atmospheric) pressure sensed by the Pitot Static Tube The difference between total pressure and static pressure is called Dynamic pressure The greater the difference between the 2 pressure readings, the greater will be the airspeed indication I& Bar I Functional Principle of an ASI o A diaphragm is used as the measuring element. The indication is based on the measurement of a differential pressure, resulting from the pressure inside the diaphragm (= total pressure pt) and inside I& Bar I o Summary on Principle of ASI AIRCRAFT DIAPRAGM AIRSPEED FORWARD SPEED INDICATOR POINTER Increase Expand Move up-scale Decrease Contract Move down-scale Steady airspeed & Remain unchanged Remain unchanged height Airspeed indicator pointer movement depends upon the EXPANSION and CONTRACTION of the diaphragm and it is proportional to the CHANGE OF DYNAMIC PRESSURE 1. Explain the following situ: a. Forward speed increase with constant altitude. P? Pt ? Diaphragm? Indication? b. Aircraft altitude ascending, forward speed constant. c. The diaphragm puncture. InterMISSIO N I& Bar I MACH AIRSPEED o INDICATOR To indicate the speed of the aircraft relative to the speed of sound in Mach number I& Bar I Mach Airspeed o Indicators The Mach Airspeed Indicator displays (in a single instrument), indications of the 3 primary speed functions Indicated airspeed (IAS) Maximum allowable airspeed (Vmo) Mach number (the ratio between aircraft speed and the speed of sound) I& Bar I Main Components of Mach o Airspeed Indicator  air-tight case  diaphragm (pitot pressure)  aneroid capsule (static pressure)  temperature compensation (bi-metallic strips)  Mach number dial  maximum-airspeed pointer Diaphragm  Mach number Aneroid Capsule (Pitot Px) adjustment (Static Pressure) AIRCRAFT PITOTSTA INSTRUME TIC NTS SYSTEM MACHMETER The machmeter indicates continually the fraction of the local true speed of sound the aircraft is flying and indicates as mach number (M). AIRCRAFT PITOTSTA INSTRUME MACHMETER TIC SYSTEM NTS A basic P machmeter has Aneroid the diaphragm Capsule with pitot pressure inside and static When aircraft altitude pressure changes outside. but airspeed is constant, altitude aneroid capsule will change Mach Indication When airspeed changes but altitude is constant, airspeed Pt diaphragm will change Mach Airspeed Diaphragm Indication. AIRCRAFT PITOTSTA INSTRUME MACHMETER TIC SYSTEM NTS VARIATION OF THE SPEED OF SOUND The speed of sound varies as the square root of the absolute temperature i.e. the speed of sound decreases as the temperature falls with altitude. In standard atmosphere conditions, the speed of sound at MSL temperature of 15 deg is 661 knots. Above 36,000 feet (stratosphere) at temperature -56.5C, the speed of sound is 573 knots. An indicated 0.7M at MSL at standard temperature means that the aircraft is flying at 462.7 knots (i.e. 0.7 x 661 ). The same mach number indication at 36,000 feet gives aircraft’s T.A.S. of 401.1 knots. (i.e. 0.7 x 573). I& Bar I Indication of Maximum Safe Airspeed o At high altitudes, the maximum safe airspeed decreases due to the decrease in density of the air. For high-speed aircraft, maximum safe airspeed is provided in order never to exceed An indicated 0.7M at MSL standard temperature of 15°C the aircraft will be this speed. flying at 462.7 knots. With the same 0.7mach number at 40,000 feet the aircraft will be flying at 401.1 knots. Maximum safe airspeed is a function of both the static pressure and the Mach number. I& Bar I Indication of Maximum Safe Airspeed o The effect of the Aneroid static pressure: Capsule 1. Altitude Increase, Static pressure reduces, Aneroid capsule expand. 2. The movement is transmitted via rocking shaft, sector assembly Airspeed Diaphragm and pinion to turn the pointer counter-clockwise As a precaution, mechanical stop is pre- over a scale. set so that the pointer never indicates a speed higher than that being decided 3. This indicates a by the aircraft’s designers to be the lower maximum maximum speed for structural safety. safe airspeed  COLOUR MARKING OF AIRSPEED INDICATORS The purpose of the Colour Marking is to highlight specific limits of operation I& Bar I o COLOUR MARKING OF AIRSPEED AirspeedINDICATORS indicator’s colour markings indicate certain minimum, maximum and/or safety speeds or speed ranges. Depends on aircraft indicator installed position. Modern systems may have movable pointers. I& Bar I COLOUR MARKING OF AIRSPEED o INDICATORS The marking are in the form of arcs, radial lines and sectors. Maximum and RED radial line minimum limits Take-off and YELLOW precautionary arc ranges GREEN arc Normal operating range RED arc Range in which operation is prohibited Speed limit for certain flight control condition. I& Bar I o Colour Markings of Airspeed Indicator I& Bar I o Colour Markings of Airspeed Indicator Tebuan Fly-pass rehersal incident  Bar I&I ALTIMETER o To indicate the altitude of the  Bar I&I o ALTIMETER Calibrated in feet The purpose is to provide an accurate indication of the aircraft’s height above a certain predetermined level (QFE/QNH/QNE). This height is referred to as the `aircraft’s altitude’.  Bar I&I o Main components of a basic Barometric Altimeter Main components: air-tight case with static connection (S) aneroid capsule gear mechanism mechanism with calibration spring temperature-compensating U-bracket pointer group (or counter, drum) dial Sectional View of a Typical Altimeter  Bar I&I Description of the Altimeter o system The altimeter is a barometer that measures the surrounding air pressure in relation to the pressure set in `hPa’ and/or `in Hg’ and indicates this pressure as altitude information.  Bar I&I o Several types of indication can be found in altimeters. The most common ones are: the triple-pointer type, the single-pointer with digital counter and the single-pointer with drum presentation. Prevailing atmospheric pressure is usually set to either QFE, QNE or QNH and the indicator will indicate altitude accordingly.  Bar I&I o ICAO International Civil Aviation Organisation The ICAO Standard Atmosphere Due to the dynamic behaviour of the air mass, the ICAO standard atmosphere has been introduced to provide standard values of the atmosphere. ISA states the `Standard conditions at MSL are: 1.A temperature of 15° C, 2.Air pressure of 14.7psi / 1013.25 hectopascal (hPa)/ 29.92 inch mercury (in Hg). ALTIMETER, TRIPLE P0INTER 1013 1011 10 10 1 09 0 1 00 8 06 7 10 1 00 05 4 9 1 0 0 1 00 3 1 00 8 2 1 002 1 01 10 1000 ALT CALIBRATED CALIBRATED 1000 1009 TO TO 35,000 FEET 35,000 FEET1001 10 10002 7 1 10004 10 0 3 3 06 5 11 6 4 00 8 10 0 0 10111 10 1012 5 1013 7 0 QFE 09 Local Training KANTIN Flight within the country QNH Overseas / Intercontinental Flight QNE  Bar I&I INTERNATIONAL QFE : Prevailing `Q’ CODE o pressure at an airfield, the altimeter would read `Zero’ when aircraft is on ground at the QNH : Prevailing airfield. pressure at an airfield, the altimeter would read Airfield height Local Training above local MSL when being on ground. QNE : Airfield height relates to standard Flight within the country ISA setting or setting of Altimeter to 1013.25 hPa is Overseas / Intercontinental Flight called `Standard Altimeter Setting’.  Bar I&I o PRINCIPLE OF OPERATION (HEIGHT vs CAPSULE MOVEMENT) Remember! An Aneroid Capsule Increasing / Decreasing Atmospheric Pressure An increasing in altitude, will Aneroid cause the capsule to expand capsule A decreasing in altitude, will cause the capsule to contract I&I Summary on Principle of Bar o Altimeter The pressure sensing element of the Altimeter is an evacuated, seal metal (aneroid capsule). ALTITUDE PRESSUR CAPSULE POINTER E Increase Decrease Expand Clockwise (Climb) s Decrease Increase Contract Counter (Descend) clockwise As the aircraft resumes horizontal flight, the capsule’s resilience and the atmospheric pressure will be in a state of balance and the new altitude is indicated.  Bar I&I o Intermissio n  Bar I&I Servo o Altimeter Servo Altimeter vs Barometric Altimeter Servo altimeter operates Barometric altimeter with operates electrical power only on pressure Fast, Easy and accurate reading Both serve the same purpose, to indicate the  Servo Altimeter  Bar I&I Functional Principle of a Servoed o Altimeter The movement of the aneroid capsule is transmitted through a linkage to the l-bar of an 'E' and ' I' inductive pickoff. The movement of the l-bar is a function of the pressure change. Determines the amplitude of the a.c voltage output.  Bar I&I Functional Principle of a Servoed o Altimeter The phase of the output signal of the E- bar will depend whether the capsule expand or contract. This output is fed to a two- Figure 10 phase drag cup servo motor via an amplifier. The motor is coupled by gear train to the pointer and counter assembly, and also to differential gear which drives a cam. The cam with cam follower moves to reposition the E-bar relative to the l-bar.  Bar I&I ENCODING ALTIMETER o Encoding Altimeter also Barometric known as “Altitude reporting Altimeter Altimeter” Encoding altimeter is a Aneroid capsule altimeter indicating the aircraft’s altitude to the pilot. Additionally, it sends the same information to the ground (Air Traffic Control).  Bar I&I Description of Encoding o Altimeter ATC transponder at Mode C, height info pulses are transmitted to the ground ATC (in increments of 100 feet). Principle of Transponder Operation  Bar I&I Main Components of an Encoding o Altimeter Main components  air-tight case with static (S) and electrical connection  2 aneroid capsules  encoder with amplifier  vibrator  altitude counter mechanism  barometric scale with adjusting knob  Maingear planetary parts: mechanism  Anfailure opticalflag. encoder disc (made of glass) Light sources Photo-transistors Electronic power supplies An amplifier Pointer mechanism Functional Principle of Control spindle an Encoding Altimeter Amplifier  Bar I&I To prevent the o stiction or friction Figure 12 Functional Diagram of an  Bar I&I Functional Principle of an Encoding o Altimeter Encoding Disc is made of glass and etched sectors in 10 concentric rings which alternately pass or block light in accordance with the 10 bit code called as “MOA-GILHAM”. One revolution of the disc covers the total indicating range of the altimeter in increments of 100 feet. Fix blocks of sensor modules consist of 10 LEDs and 10 light-sensitive Darlington transistors. The 10-bit encoding covers the altitude range from - 1,000 feet to +63,000 feet Encoding Altimeter’s Warning Flag comes ‘on’ when;  In case of power failure  The altitude information is no longer encoded  The Vibrator is not working VERTICAL SPEED VERTICAL SPEED INDICATOR INDICATOR (Diaphragm- (Airchamber- based) based) VERTICAL SPEED INDICATOR (Diaphragm-based)  Bar I&I o VERTICAL SPEED INDICATOR (Diaphragm-based) The purpose of the Vertical Speed Indicator is to indicate the rate of altitude change of the aircraft. Uses Static pressure of the pitot static system.  Bar I&I Vertical Speed Indicator System o Description It is a differential- pressure gauge designed to detect the change of the static pressure. The diaphragm senses the changes in the static pressure and It measures the indicates as rate of pressure difference climb or descent in feet between the pressure per minute / metre per inside the diaphragm second. (changing static pressure) and the pressure inside the housing/outside the diaphragm (“equalizing  Bar I&I o Main Components of an VSI The typical diapragm-base Vertical Speed Indicator consists of the following main parts: Air-tight case with static connection (S) Diaphragm unit (bellows) Gear mechanism Calibration spring Temperature compensator (bi- metal) Sectional View of a Vertical Velocity Diffusor assembly (capillary Indicator (capsule base) tube) A `zero adjustment ‘screw is located in the lower Eccentric shaft assembly and left corner of the front casing to reset the pointer to zero.  Bar I&I o PRINCIPLE OPERATION OF VERTICAL SPEED INDICATOR (Diaphragm-based) The basic principle of the VSI/ROC is that the static pressure/barometric pressure is a function of altitude. The VSI is a differential pressure gauge. Measures the rate-of-change of the static pressure inside and outside of the diaphragm.  Bar I&I o Unit of measurement of ROC is in feet per minute. Range of indication of a typical ROC is 0 to 6,000 feet/min. The upper scale section indicates climb and the lower section At the end of the indicates descend. indicating ranges the pointer deflection is limited by means of mechanical stops. I&I Functional Principle of a Bar o Vertical Speed Indicator (Diaphragm-based) STRAIGHT AND LEVEL FLIGHT Level flight: zero If level flight is resumed, differential the pressure difference pressure equalizes within 6 to 9 sec across pointer and the pointer returns to diaphragm. indicates ‘0’ `zero rate of climb’.  Bar I&I AIRCRAFT DESCENDING o Aircraft Descend: VSI / ROC Metering unit maintains case pressure lower than diaphragm pressure, at constant rate of The metering differential unit is used pressure to delay across the pressure the capsule entering the case of the VSI during descent DURING DESCEND > - the pressure inside the diaphragm is higher than inside the case due to the lag of the metering unit. - The gear linkage from the diaphragm will  Bar I&I o AIRCRAFT CLIMBING Aircraft Climb/Ascend: VSI / ROC Metering unit creates a constant differential pressure across the diaphragm by maintaining case The metering pressure higherunit delays the pressure pressure in than diaphragm the case of the VSI during climbing. DURING CLIMB > - The pressure inside the diaphragm is lower than inside the case due to the lag of the metering unit. - The pointer will be deflected up to indicate Remembe r ! LEVEL FLIGHT > -pressure inside the chamber and the instrument case is …………... -pointer indicates ‘0’ DURING DESCEND > -the pressure inside the chamber is ………………. than inside the case due to the lag of the metering unit. - Pointer moves down to indicate ‘descend’. DURING ASCEND > -the pressure inside the chamber is …….. than inside the case due to the lag of the metering unit. -the pointer will move up to indicate ‘ascend’.  Bar I&I o VERTICAL SPEED INDICATOR (Airchamber-based) Air Chamber  Bar I&I VERTICAL SPEED INDICATOR o (Airchamber-based) The airchamber based vertical speed indicator consists of the following main parts:  air-tight case with static connection (S)  metering mechanism including an airchamber  cascade filter Air Chamber  ‘zero adjustment’ mechanism (for servicing only)  dial  pointer.  Bar I&I VERTICAL SPEED o INDICATOR (Airchamber-based) Vertical Speed Indicator airchamber-based type is designed to create a more rapid differential pressure effect. Hermetically sealed case with a connection for static pressure line. Air Chamber Calibrated orifice separates the airchamber into 2 separate sections.  Bar I&I o The upper chamber is open to the case and connected to cascade filter via One chamber is a calibrated aperture. Air Chamber supplied with Static pressure and the other with the pressure exiting inside the housing (Equalizing Pressure) The lower chamber is connected directly to the static pressure line via the cascade filter.  Bar I&I o LEVEL FLIGHT Air Chamber Static pressure remains constant. The pressure in both the chambers are equal. ROC pointer reads ’zero’.  Bar o DURING CLIMBING Static pressure reduces. Immediately Lower Air Chamber Chamber pressure reduces. The higher pressure of the Upper Chamber pushes the Calibrated Orifice into lower chamber (low pressure). A balance is achieved between  Bar o DURING CLIMB When the aircraft ceases to climb; Air Chamber >The static pressure remains constant. >Both the chamber pressures equalize through the calibrated orifice and the aperture. >The Spiral spring returns the indicator mechanism to zero.  Bar I&I o DURING DESCEND  Static pressure inside the lower chamber increases and the pointer indicates ’down’.  The indicated rate of descent again increases until the pressure difference and the spring pressure are in balance, i.e. until the true When thedescent rate of aircraftof ceases to descend; the aircraft is indicated.  the static pressure remains constant and both the chamber pressure will equalize through the calibrated orifice and the aperture.  The spring returns the indicator mechanism to zero. SUMMARY `Standard conditions at mean sea level (MSL)’ , ISA states a temperature of 15° C and an air pressure of 1,013.25 hPa or I C E (29.92 in Hg) and a temperature lapse rate of 2 degC./1000 ft. T COLOUR MARKING OF AIRSPEED INDICATORS INTERNATIONAL `Q’ CODE by ICAO ENCODING ALTIMETER PRINCIPLE OF OPERATION  SUMMARY Principle OF Operation : LEVEL FLIGHT > -pressure inside the chamber and the instrument case is equal. -pointer indicates ‘0’ DURING DESCEND > -the pressure inside the chamber is higher than inside the case due to the lag of the metering unit. -the gear linkage from the chamber will deflect the pointer down to indicate ‘descend’. DURING ASCEND > -the pressure inside the chamber is lower than inside the case due to the lag of the metering unit. -the pointer will be deflected up to indicate ‘ascend’.  The End of Barometric System The next subject is on Air Data Computer  Quiz 1. What is ICAO ? 2. What is the value of Standard Atmospheric Pressure 3. What isatthe MSL? temperature lapse rate with altitude? 4. State the purpose of Air Speed Indicator? 5. Air Speed Indicator are calibrated in what unit? 6. Define types of airspeed? 7. What is the sensing device used in Air Speed Indicator? 8. What types of pressure used in Air Speed Indicator? 9. What happen to the diaphragm and pointer of Air Speed 10.Indicator What happen when to the diaphragm aircraft and pointer speed is increased? of Air Speed Indicator when aircraft maintain steady speed and height?