Answer
Altitude Gain = 84.98 km/h
Work Step by Step
Step 1: Information
Airspeed is 275 km/h
Airspeed (AS) is the hypotenuse
The angle of ascent (AA) is 18 degrees
To get altitude gain (AG), $AG=sin(AA)\times AS$
Step 2: Substitute values
$AG = sin(18)\times275$
$AG = 84.98$
