Planes sit on the tarmac at the Newark International Airport in Newark, New Jersey
CNN  — 

The largest US airline pilots union is counseling its members to be more focused to avoid additional close calls. There have been multiple close calls on runways this year which have raised questions about air safety.

The Air Line Pilots Association issued a safety alert encouraging pilots to “increase vigilance” and “prevent complacency” in themselves and others. “Maintaining vigilance and promoting vigilance in others is always vital, but it is especially critical during periods of dynamic change,” the alert said.

This alert has been sent just two days after the Federal Aviation Administration told airlines in an industrywide bulletin to step up vigilance at airports because of the close calls on or near runways.

Friday’s alert noted that the rapid hiring for aviation positions, and with pilots upgrading into new aircraft and shifting between captain and co-pilot roles, meant many people are performing new roles.

“Reducing distractions where possible, disciplined procedures and communications, and effective crew resource management help manage strains on the system,” the alert said.

The bulletin encouraged pilots to submit voluntary safety reports and be observant of any loss of situational awareness in fellow pilots to help ensure flight safety.

On Thursday, the head of the nation’s air traffic controllers said there have been too many near collisions at airports and laid out steps to avoid more, including more supervisor oversight in control towers and extra controller training for “unusual circumstances.”

“Even though we all know that multiple levels of safety are built into our system, there is no question that we are seeing too many close calls,” said Tim Arel, chief operating officer of the Federal Aviation Administration’s Air Traffic Organization.

The five steps laid out by Arel in an agencywide memo follow last week’s FAA safety summit focusing on the recent series of near-collisions. The National Transportation Safety Board is investigating six incidents this year alone.

“Our dedication to continuous improvement demands that we dig deep to identify the underlying factors and address them,” Arel said. “With the summer travel season just around the corner, airlines and the traveling public have high expectations.”

The most serious types of near-collisions on US runways this year are about double the rate seen in the past, Transportation Secretary Pete Buttigieg said on Thursday.

“In past years, they’ve occurred at roughly a rate of once per month,” he told a Senate panel. “Right now, they are this year occurring at a rate that is double that.”

CNN’s Pete Muntean contributed to this report