United Airlines is redoubling safety training following a number of incidents in recent weeks, the company's chief executive told customers Monday.

United has experienced "a number of incidents" that are "unrelated" but serve as "reminders of the importance of safety," Chief Executive Scott Kirby said in an email to customers.

Kirby said the company is reviewing the episodes and would use "those insights to inform our safety training and procedures across all employee groups."

The missive comes after the carrier discovered Friday that a jet was missing an external panel following its landing in Portland, Oregon from San Francisco.

In another recent episode, a United flight planned for Florida returned to its Houston departing airport after experiencing an engine fire that was due to plastic bubble wrap ingested from the airfield.

Another United plane bound for Japan lost a tire after taking off from San Francisco, while a second Houston incident involved a plane rolling onto the grass while exiting onto the taxiway.

Kirby said United had already planned an extra day of in-person training for pilots as well as a "centralized training curriculum" for newly hired maintenance technicians and could further supplement these initiatives.

The effort comes as aircraft manufacturer Boeing faces stepped-up scrutiny following problems, especially a January incident on an Alaska Airlines flight that required an emergency landing.