Following four incidents that United Airlines dubs “safety events and near-misses,” the carrier has issued a warning to its pilots about “significant safety concerns,” and asking them to cut down on potentially disastrous mistakes.

According to the Wall Street Journal, the warning was sent by United’s Sr. VP of flight operations to pilots in early January and cites recent instances — like a plane that would have crashed if the pilots hadn’t executed an emergency maneuver and a plane that was flying with fuel levels below the mandatory reserve threshold — that could have had tragic consequences.

These events, in which “the common thread… is that they were preventable,” have “dictated that we communicate with all of you immediately,” reads the bluntly worded memo.

In addition to calling out the handful of problem cases, the notice reportedly discusses the safety issues involved with the changing makeup of United’s group of pilots. The airline is attempting to hire a total of 700 new pilots this year, which means there will be lots of new faces and some shifting around, with pilots possibly being put in the cockpits of types of planes they haven’t spent years flying around the world.

These changes introduce “significant risk to the operation,” reads the document.

The airline is reminding its aviators that “every pilot must be willing to speak up if safety is in question” and “must also accept the input of their fellow crew members.”

A rep for the airline says that United regularly alerts its pilots to safety concerns and may mention certain incidents in these memos, but some sources tell the Journal that the usual airline updates aren’t so harshly worded — the writers of the bulletin even acknowledge that it’s “brutally honest” — and that the safety events mentioned in such documents are usually much older and more minor than those detailed in the January bulletin.

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