Here is an audio/video file with transcript about how the numbers must be pronounced according to the ICAO (International Civilian Aviation Organization) standard. This is how aircrew members, and air traffic controllers should transmit the numbers.
CAUTION – There is not any exception for FL 100, and FL 200 according to the ICAO DOC 9432 Radiotelephony Manual, page 19, chapter 2.4.2, as it is pronounced « FLIGHT LEVEL ONE-ZERO-ZERO », and « FLIGHT LEVEL TWO-ZERO-ZERO ».
However, « Flight level one hundred » follows the French DGAC and the British CAA patterns.
Click on this video:
Here is an example of perception confronted with reality at night, in conditions of fog and poor visibility. What happened at Theodore Francis Green Airport, in Warwick, Rhode Island, could have turned into a major disaster. Listen and watch the video: