A series of recent low-fuel incidents for Ryanair, the Ireland-based carrier known for its no-frills planes and low fares, has prompted a safety investigation.
In three cases, the airline was forced to make emergency landings because aircraft were drawing close to the minimum legal level of fuel. Another two incidents were due to engine fault and a technical issue, according to The Telegraph.
Each incident took place in Spanish airspace. An investigation is being conducted jointly by Ireland's Department of Transport, Tourism and Sport and the Spanish Ministry of Development.
The Telegraph reported that the Irish pilots union claims that flight crews are under pressure from Ryanair to carry the minimum amount of fuel required under European regulations and that the European Cockpit Association said that pilots were being encouraged to make decisions based on "factors other than safety."
Ryanair has rejected both of those claims.
The less fuel an airplane carries, the greater the savings for the carrier. Ryanair in the past has suggested such innovations as pay toilets and stand-up seating.
"The Irish authorities gave an assurance of the Irish Aviation Authority's rigorous oversight of Ryanair's operations and of their satisfaction with Ryanair's safety standards which are on a par with the safest airlines in Europe," said the Irish department of transport in a statement.
However, Spain's minister overseeing aviation safety has called for tighter safety rules at low-cost airlines. A spokesman for Ryanair told the paper it had "invited the Spanish Ministry to send a team of inspectors to Dublin to correct any (misplaced) concerns about Ryanair's compliance with Europe's highest operating and maintenance standards by providing them with unfettered access to Ryanair operating, maintenance and flight training facilities and unlimited access to Ryanair's safety, flight management, engineering and maintenance personnel."
Ryanair did not respond to an ABC News request for comment.