What am I missing here? 18,000 feet at 161 mph is not really very impressive. Many small every day single engine planes can do this.
Practical experience in flight test.
The first test flight is normally just to prove that it will get off the ground and fly in a straight line. Further tests will go higher and faster in increments to see what she can really do. Additionally, most test flights take place with the landing gear down so achieving its highest speed wouldn’t possible.