Cruising at FL390, M 0.84 is found to give a TAS of 499 kt. The ISA deviaon at this level will be:a.-17b.+17c.+19d.-19

To find the ISA deviation at FL390, we need to calculate the difference between the true airspeed (TAS) and the expected airspeed based on the International Standard Atmosphere (ISA) conditions.

Step 1: Convert the TAS from knots to meters per second (m/s). 499 kt = 257.06 m/s

Step 2: Calculate the expected airspeed based on the ISA conditions. The ISA conditions at FL390 state that the temperature should decrease by 2 degrees Celsius per 1,000 feet. FL390 is approximately 39,000 feet.

Temperature deviation = -2°C/1,000 ft * 39,000 ft = -78°C

The ISA temperature at sea level is 15°C, so the expected temperature at FL390 would be: 15°C - 78°C = -63°C

Using the formula for calculating the speed of sound in air: Speed of sound (m/s) = 20.05 * √(temperature in Kelvin)

Temperature in Kelvin = -63°C + 273.15 = 210.15 K

Speed of sound = 20.05 * √(210.15) = 295.97 m/s

Step 3: Calculate the Mach number (M) using the TAS and speed of sound. Mach number (M) = TAS / Speed of sound M = 257.06 m/s / 295.97 m/s = 0.868

Step 4: Calculate the ISA deviation. ISA deviation = M - 1 ISA deviation = 0.868 - 1 = -0.132

Step 5: Convert the ISA deviation to a percentage. ISA deviation (percentage) = ISA deviation * 100 ISA deviation (percentage) = -0.132 * 100 = -13.2%

Therefore, the ISA deviation at FL390 is approximately -13.2%.

None of the given options (-17, +17, +19, -19) match the calculated ISA deviation.