CNN  — 

United Airlines has issued a stern warning to its pilots after mistakes in the cockpit put passenger planes in potential danger.

A review of United flight operations data, which details safety incidents involving United pilots and aircraft, prompted the warning, according to an airline official.

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The safety memo was sent to United Airlines pilots on Jan. 9th, and was first report by The Wall Street Journal.

“The language was strong, stronger than usual,” said United Airlines spokesman Charlie Hobart.

The tone of the memo was used to “get pilots’ attention,” Hobart added.

A current United Airlines pilot who received the memo says the airline outlined specific incidents that raised alarm among management, including one incident where the pilot was forced to pull off evasive moves to avoid crashing into the ground, performing what’s called an emergency pull up maneuver.

The pilot who discussed the memo with CNN requested anonymity because the he was not authorized to speak publicly.

“The responsibility is in their hands. If there have been so many safety incidents it’s an indication training needs to be improved,” the pilot said.

United Airlines would not comment on what other actions it may take or whether pilots will have to undergo additional training.

The airline says it regularly reaches out to its pilots.

“Our goal is getting customers to their destination safely,” Hobart said.