European airlines’ shares were on track for their biggest weekly declines in years, as investors feared they could be hurt more severely by falling oil prices than their U.S. counterparts.

A slide in oil prices since the beginning of the year could mean European airlines pay more for jet fuel than American carriers. Most airlines in Europe are locked into contracts that set out how much they pay for oil over the next one to two years, said Adrian Yanoshik, a transportation equity research analyst at Berenberg Bank. U.S. airlines…