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Introduction Mach Numbers • At high altitude and high true airspeed, aircraft may approach Mach 1 • Mach numbers are used because pilots need to know if they get too close to the speed of sound • Problems resulting from speeds close to Mach 1: • Increased drag • Airframe buffet • Control problems • Machmeters are used to measure the Mach number
Temperature & Airspeed International Standard Atmosphere (ISA) • These are like ‘average’ figures for the world • ISA temperature values include: • Sea level temperature = 15°C • Troposphere lapse rate = 1.98° drop per 1000 ft • Tropopause is at 36,090 ft • Tropopause temperature = -56°C • Lower stratosphere (FL360 - FL660) lapse rate is Isothermal at -56°C 66000 -56° Stratosphere -56° Pressure Altitude (ft) Tropopause 36090 1.98°/1000’ Troposphere +15° MSL -70 -60 -50 -40 -30 -20 -10 0 10 20 30 40 Temperature (°C)
TAS and Mach Numbers TAT and Mach number TAS Calculation Method 2: Method: (Given CAS, FL and TAT) Eg: An aircraft is cruising with a CAS of 300 kts, at FL290. If the TAT is -22°C, find the TAS. 1. Align the Calibrated airspeed (300 kts) against the Pressure altitude(FL290) in the CAS window. • The Mach number is indicated above the single Mach number arrow M = 0.775
TAS and Mach Numbers TAT and Mach number TAS Calculation Method 2: Method: (Given CAS, FL and TAT) Eg: An aircraft is cruising with a CAS of 300 kts, at FL290. If the TAT is -22°C, find the TAS. 1. Align the Calibrated airspeed (300 kts) against the Pressure altitude(FL290) in the CAS window. • The Mach number is indicated above the single Mach number arrow M = 0.775 2. Find the intersection of the actual TAT (-22°C) and the the black index line • Position the cursor with the recovery coefficient (1.0) over this intersection 3. Read TAS below the aqua cursor arrow TAS = 450 kts • If desired, the Ram effect temperature rise is under the innermost aqua arrowhead 27° • OAT = TAT – Temp rise OAT = -22 – 27 = -49°C