Must admit I was somewhat surprised during the Vietnam era the number of US Army pilots who didn't feel comfortable flying above 1,500. Possible MGB failure was always the answer - never asked if it was something drummed into them during flight training. Took one lad to 16,000 to show him the sights. Part of our initial helo training was a height climb to 14,000 (UH-1B) so never really thought twice about it, and when the rare chance of an IMC flight came about 10,000 was regularly used by all company pilots (212/412/76)