FAA Airline Transport Pilot (ATM)Meteorology and Weather ServicesHard
A winds and temperatures aloft forecast (FD) for FL390 contains the coded group '731352'. Given that above 24,000 feet, 50 has been added to the coded direction and 100 subtracted from the coded speed whenever wind speeds are forecast between 100 and 199 knots, what are the decoded wind direction, speed, and temperature?
- A280° at 13 knots, -52°C
- B730° at 13 knots, 52°C
- C130° at 135 knots, -52°C
- D230° at 113 knots, -52°C
Show answer & explanationAnswer & explanation
Correct answer: D. 230° at 113 knots, -52°C
Decoding '73': subtract 50 to get 23, giving direction 230°. Decoding '13': add 100 to get 113 knots. The temperature group '52' is read as -52°C because temperatures above 24,000 feet are always negative and the sign is omitted in the code.
Why the other options are wrong
- A. This direction value does not match subtracting 50 from the coded '73' group.
- B. This misreads the direction code as literal tens without subtracting 50 and misapplies the temperature sign.
- C. This incorrectly reads the direction and speed groups without applying the coding adjustment correctly.
Winds Aloft (FD) High-Altitude Coding
For forecast levels at or above 24,000 feet, when coded wind speed would be 100-199 knots, 50 is added to the direction code (in tens of degrees) and 100 is subtracted from the speed to keep the six-digit group format.
- Direction is given in tens of degrees, speed in knots, temperature in °C
- At/above 24,000 ft, temperatures are always negative and the sign is omitted
- Decode by reversing the adjustment: subtract 50 from direction code, add 100 to speed code
Memory trick: Above FL240: subtract 50 from direction, add 100 to speed, and temps are always negative