European airlines are flying with clear skies ahead, but solid share price performances for the year to date place a question mark over whether they can still make investors money.

The International Air Travel Association recently raised its forecast for 2017 net income for the global airline industry at the end of June to "over $31 billion" from a previous estimate of $29.8 billion. Higher traffic volume growth of 7.4% and better load factors are likely to support the bottom line result, according to IATA, driven by a brighter outlook for global GDP growth.

June passenger data from many of Europe's airlines confirmed a favorable environment for the industry over on the other side of the Atlantic.

London's easyJet (EJTTF) reported 11.3% growth in the proverbial footfall and a 0.8% improvement in its load factor, which rose to 94.8%. That data came closely on the heels of Ryanair (RYAAY - Get Report) having declared, on July 4, volume growth of 12% and a 2% increase in its load factor to 96%.

Over on the continent, British Airways owner International Consolidated Airlines (ICAGY) said June passenger numbers were up by 3.2% while its load factor rose 0.3%.

Germany's Lufthansa (DLAKY) and KLM Air France (AFLYY) will both report traffic statistics over the course of next week.

All European airline stocks have posted high double digit gains the first-half, after similarly strong returns in 2016, as investors have rewarded what was a record year for revenues, earnings and margins across the industry.

KLM and Lufthansa are up 138% and 63% respectively, aided by diminishing concerns over the impact on traffic to come from recent terror attacks in Europe, while Ryanair and easyJet have posted 24.1% and 33.9% returns respectively.

With such strong share price performances already in the bag and considering that the IATA's improved forecasts still imply a small fall in profitability from 2016's record, investors could do well to consider how much longer Europe's airline stocks will be able to remain at such high altitude.

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