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Getting To Grips With Airbus - General
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  Getting to Grips with Aircraft Performance  INTRODUCTION  9 1. INTRODUCTION The safety of air transportation is a joint effort, regulated by the State on onehand, and practiced by the manufacturers, airlines and Air Traffic Controllers (ATC),on the other hand. The State is responsible for the supervision of civil aviation, toensure that a high safety standard is maintained throughout the industry, and itsprimary means of enforcement is via the establishment and administration of writtenregulations. The control process encompasses a fixed set of rules to secure that allaircraft respect a minimum level of performance, which thereby leads to the definitionof limitations.The State administration generally implies the civil aviation authority, whichcorresponds to the aircraft's country of registration. In the United States, for example,this role is devoted to the Federal Aviation Administration ( FAA) , whereas in France,it is the “Direction Générale de l’Aviation Civile” ( DGAC ).Every country has its own regulations, but the international aspect of air transportation takes into account the worldwide application of common rules. TheInternational Civil Aviation Organization ( ICAO ) was therefore created in 1948, toprovide a supranational council,  to assist in defining the international minimumrecommended standards. The Chicago Convention was signed on December 7,1944, and has become the legal foundation for civil aviation worldwide. Although it is customary for each country to adopt the main airworthinessstandards defined in conjunction with aircraft manufacturers (USA, Europe, Canada,etc.), every country has its own set of operational regulations. For instance, somecountries (mainly European) have adopted JAR-OPS 1, while some others follow theUS FAR 121.The field of limitations is therefore dependent upon an amalgamation of thefollowing two realms: ã Airworthiness: Involving the aircraft's design (limitations, performance dataetc….), in relation to JAR 25  or FAR 25 . ã Operations: Involving the technical operating rules (takeoff and landinglimitations, fuel planning, etc…), in relation to JAR-OPS 1  or FAR 121 .Both airworthiness and operational regulations exist for all aircraft types. Thisbrochure addresses large aircraft”, which means aircraft with a maximum takeoff weight exceeding 5,700 kg. Airbus performance documentation is clearly divided intothe two above-mentioned categories: Airworthiness and Operations. ã   Airworthiness : The Airplane Flight Manual (AFM)  is associated to theairworthiness certificate and contains certified performance data in compliancewith JAR/FAR25.  INTRODUCTION Getting to Grips with Aircraft Performance 10 ã   Operations : The Flight Crew Operating Manual (FCOM)  can be viewed as the AOM (aircraft-related portion of the Operations Manual), which contains all thenecessary limitations, procedures and performance data for aircraft operation.The following table (Table 1) illustrates the large aircraft regulatory basis: ICAOEUROPE (JAA)USA (FAA)Airworthiness  Annex 8to the ChicagoConventionJAR 1  25FAR 2  part 25 Operating Rules  Annex 6to the ChicagoConventionJAR-OPS1FAR part 121 Table 1: Large Aircraft Requirements  All aircraft of the Airbus family are JAR 25 and/or FAR 25 certified. On theother hand, compliance with the operating rules remains under the airline’sresponsibility.This brochure  is designed to address three different aspects of aircraftperformance : ã   The physical aspect  : This brochure provides reminders on flightmechanics, aerodynamics, altimetry, influence of external parameters onaircraft performance, flight optimization concepts… ã  The regulatory aspect  : Description of the main JAR and FAR  certificationand operating rules, leading to the establishment of limitations. For a clear understanding, regulatory articles are quoted to assist in clarifying a givensubject. In such cases, the text is written in italics and the article referencesare clearly indicated to the reader. ã   The operational aspect : Description of operational methods, aircraftcomputer logics, operational procedures, pilot’s actions…  1 JAR:The Joint Airworthiness Requirements are under the European authority called the Joint Aviation Authority (JAA). 2  FAR: The Federal Aviation Regulations are under the US authority called the Federal Aviation Administration (FAA).  Getting to Grips with Aircraft Performance  GENERAL 11 A. GENERAL 1. T HE INTERNATIONAL STANDARD ATMOSPHERE (ISA) 1.1. Standard Atmosphere Modeling The atmosphere is a gaseous envelope surrounding the earth. Itscharacteristics are different throughout the world. For this reason, it is necessary toadopt an average set of conditions called the International Standard Atmosphere(ISA) . 1.1.1. Temperature Modeling The following diagram (Figure A1) illustrates the temperature variations in thestandard atmosphere: 604020-200-40-60 24681012 Altitude subsonic jettransportcruise levelSea level TROPOSPHERETROPOPAUSE = 36089 ft STRATOSPHERE -56.5°C15°C Temperature(°C)(km) 5000   10000150002000025000300003500040000 (ft) Figure A1: ISA temperature The international reference is based on a sea-level temperature of 15°C at apressure of 1013.25 hPa 1 . The standard density of the air at sea level is 1.225 kg/m 3 .  1  1013.25 hPa is equal to 29.92 in Hg, ‘hPa’ meaning hectoPascal and ‘in Hg’ inches of mercury.  GENERAL Getting to Grips with Aircraft Performance 12 Temperature decreases with altitude at a constant rate of -6.5°C/1000m or -1.98°C/1000ft up to the tropopause. The standard tropopausealtitude is 11,000 m or 36,089 feet.From the tropopause upward, the temperature remains at a constant value of -56.5°C.Therefore, the air which is considered as a perfect gas in the ISA modelpresents the following characteristics: ãããã  At Mean Sea Level (MSL):   ISA temperature = T 0  = +15°C = 288.15 K ãããã  Above MSL and below the tropopause (36,089 feet):ISA temperature (ºC) = T 0  - 1.98 x [Alt(feet)/1000] For a quick determination of the standard temperature at a given altitude, thefollowing approximate formula can be used: ISA temperature (ºC) = 15 - 2 x[Alt(feet)/1000] ãããã  Above the tropopause (36,089 feet):ISA temperature = -56.5ºC = 216.65 K This ISA model is used as a reference to compare real atmospheric conditionsand the corresponding engine/aircraft performance. The atmospheric conditions willtherefore be expressed as ISA +/- ∆∆∆∆ ISA  at a given flight level.Example:Let’s consider a flight in the following conditions: Altitude = 33,000 feet Actual Temperature = -41ºCThe standard temperature at 33,000 feet is : ISA = 15 - 2 x 33 = -51ºC,whereas the actual temperature is -41ºC, i.e. 10ºC above the standard.Conclusion: The flight is operated in ISA+10  conditions
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