An Alaska Airlines flight bound for San Diego was diverted back to Seattle shortly after takeoff Monday morning, the airline said in a statement.

Flight 558 “reported an unusual vibration on the left side of the aircraft shortly after takeoff,” an Alaska Airlines spokesman said in a statement to CNN. “The plane returned to the airport and landed safely.”

The flight departed Seattle-Tacoma International Airport at 7:36 a.m. local time and returned to the airport at 8:04 a.m., Alaska Airlines told CNN Travel.

According to the airline, part of the bonnet (removable metal casing of the engine) came off the aircraft during landing.

“I heard pops”

Passenger Greg Anderson was on board Flight 558 on Monday and photographed damage to the plane’s left engine.

He told CNN that the plane took off normally, and then he “heard a little pop” and felt the plane begin to vibrate.

Anderson said he saw pieces of the hood come off.

“We pulled over when the captain showed up and said he felt an ‘unusual vibration’ and we were told he was looking for a place to land,” Anderson said.

He said part of the fairing fell off as the plane landed.

Alaska Airlines said in a statement that there were no injuries among the 176 passengers and six crew members on board.

The guests were transferred to a new flight to San Diego that departed about two and a half hours after they returned to the boarding gate, the airline said in an email.

Anderson said he was booked on another flight and arrived in San Diego around 1:30 p.m.

Experienced Pilots

Alaska Airlines praised the crew on board.

“The two pilots who flew this flight have over 32 years of combined flying experience,” the statement said. “They, along with our flight attendants, handled the incident with great professionalism and care.”

The Boeing 737-900ER is out of service while the security team investigates the cause of the problem, the airline said.