We can see on the stockcharts bellow, there was ample advance warning as to the viability of airlines as an investment: 1) the Relative Strength of The Airlines Index (XAL) to the S&P 500 Index (SPX), crossed below, on a 200 day average, on December 27th, 2019. 2) The Airlines Index (XAL), was barely maintaining trend prior to the market crash this month. It broke below trend on February 18th., as shown on the stockchart below. Trend as defined as maintaining an annualized rate of growth over a 200 day average, of at least 20%.
Chart of the Airlines Index (XAL) relative to the S&P 500 Index, over 200 days:
Chart of the Airlines Index (XAL) relative to the trend of 20% annualized over 200 days: