Ryanair is putting undue pressure on its pilots to cut fuel consumption, a practice that forced three Ryanair jets to make emergency landings in Spain recently, a German labour union said Thursday. A spokesman for the pilots\' union Vereinigung Cockpit, Joerg Handwerg, told AFP that the three Ryanair airplanes were compelled to land in Valencia rather than Madrid because their kerosene tanks were almost empty. \"Fuel is the biggest cost factor, particularly with budget airlines,\" Handwerg said. And the more fuel an aircraft is carrying, the heavier that aircraft is, thereby pushing up consumption, Handwerg argued. Nevertheless, it should be left up to a pilot how much fuel they take on board, because it is ultimately the pilot who is responsible for the safety of a flight, he argued. \"This infringes on the pilots\' rights,\" Handwerg said. Ryanair was exerting heavy psychological pressure on pilots whose jets showed the highest fuel consumption, he complained. The Spanish transport ministry said earlier this week it is investigating a complaint by airport operator AENA against Ryanair. Airline spokesman Stephen McNamara said Ryanair supported the investigation and was ready to cooperate with the authorities.