Dear Alessandro, the income inequality elasticity of demand for air travel is less than one or unity because higher the income inequality the size of population having demand for air travel declines on other hand lower or decline in income inequality raise the size of population having demand for air travel.
Not I am aware, although there are many papers on the effect of income proxied by GDP or per-capita GDP. My recent paper, Lo, W., Y. Wan and A. Zhang (2015), “Empirical estimation of price and income elasticities of air cargo demand: The case of Hong Kong,” Transportation Research, Part A: Policy and Practice, 78, 309-324, is such an example (while the above colleagues focus on pax travel demand).