In a strongly worded internal memo, Air France has warned its pilots to be more vigilant about safety procedures and upbraided those blaming flight equipment for the crash of Flight 447 into the Atlantic in June.

No one knows what caused the accident, which killed all 228 people aboard and was Air France's deadliest crash. Pilots' unions said Saturday the company is trying to distance itself from blame — and shift attention to the possibility of human error — as the investigation drags on.

"Enough Scandals and False Debates about Flight Security!" reads the memo, sent to pilots Tuesday and obtained by The Associated Press on Saturday. It dismisses calls by pilots for new safety procedures following Flight 447's crash. "It suffices simply to apply our doctrine, our procedures," the memo says.

Erick Derivry of the SNPL union said he was "shocked" by the letter and that pilots were being made into "scapegoats."

Air France said in a statement that the memo was meant to be an internal document and insisted that it "has total confidence in its pilots."

The memo details the company's responses to concerns about Flight 447's airspeed sensors, known as Pitots. Air France replaced older models of the sensors amid concerns they could have iced over and sent false speed information to the pilots as the Airbus 330 ran into a thunderstorm far off Brazil's mainland.

Air France revealed in the memo that it has stopped a training program for pilots on how to manage a Pitot malfunction like that.

Planemaker Airbus told the airline that the simulation "does not loyally reproduce the chain of consequences in real situations," the memo says, adding that the exercise misled pilots into thinking such a chain of events was more likely than it is.

The memo also outlines recent safety lapses by pilots that could have posed dangers, including deviating from the takeoff trajectory and not reporting technical problems immediately. It warns against "overconfidence" and thinking that safety measures are overkill.

Derivry described those incidents as everyday occurrences in any airline and "very largely exaggerated."

The memo appeared to be a response to a strike threat by two unions representing a minority of Air France's more than 4,000 pilots that have demanded new safety procedures.

A pilot with one of those unions, Alter, said Saturday it was maintaining the threat and dismissed the memo as a failed effort to assuage flight staff. The pilot spoke on condition of anonymity out of concern of repercussions in his job.

Investigators may never determine what happened to Flight 447 because the flight recorders have not been found after extensive searches deep in the Atlantic.

The families of two Americans killed in the crash filed a lawsuit in Houston last month claiming the airline and the plane's various manufacturers knew the aircraft had defective parts — including Pitots — that could have caused the accident.

The Air France memo came the day before pilots of a Northwest Airlines jet missed its destination in Minneapolis, saying they forgot to land, an incident that renewed concerns about flight safety.