American Airlines AAL.O, due to report first-quarter earnings on April 21, on Tuesday revised up estimates for jet fuel expenses. By Rajesh Kumar Singh CHICAGO, April 12 (Reuters) - Major U.S. airlines are enjoying the strongest travel demand in three years, yet investors will focus on how they are mitigating mounting inflationary pressures when they report quarterly earnings starting on Wednesday. After a blip caused by the Omicron coronavirus variant, travel demand has roared back, with some airlines reporting the highest ticket sales in their history.