Photos and videos posted on social media by several passengers on board showed the cover ripping off the Boeing 777-222 airplane's engine mid-air.
"Our pilots followed all necessary protocols to safely land the aircraft", United said in an emailed statement on Tuesday evening.
The airport's Aircraft Rescue and Fire Fighting personnel stood by as a safety precaution, according to a Honolulu County information officer.
The plane was taken out of service and inspected, but the cause of the issue was still unclear as of Tuesday night. The 363 passengers say the 10 crew members did their best to keep people calming, warning them it could be a rough landing.
"I didn't see anything about this in the mannual", he joked. It is also working with United Airlines to get the flight data and cockpit voice recorders sent to its lab.
Other passengers said the plane shook violently.
Passenger Haley Ebert told the "New York Times" that she heard a "huge bam" as part of the casing came away from the engine. By the time the plane landed safely in Honolulu around 40 minutes later, the engine was bare as its insides were on full display.
The flight was forced to make an emergency landing in Honolulu after passengers notified staff about the missing cover, according to officials. "They looked them straight in the eye and that was real".
Sudiacal had been flying with her four-month-old-son and feared the worst.
No one was injured and the plane landed at Daniel K. Inouye International Airport and taxied to the gate.
United is fully cooperating with the NTSB and FAA investigation of the event.