After landing at 12.37pm HST (10.37pm GMT), Mr Haddad took to Twitter to post images of the damaged engine and shocking footage of it shaking during the trip.
He said: “That looks bad, plain and simple.
“I don’t see anything about this in the manual.”
He continued: “Occurred 36minutes to landing. Seats 24K and 24L had a great view of the debris coming off.”
Maria Falaschi, who was sitting next to Mr Haddad on the plane, also showcased images of the damaged engine.
She detailed that the Boeing 777-200 “shook violently” after the engine casing flew off.
She explained: “It was about 45 minutes until we landed. Heard a big bang and the plane started to shake violently. The pilots and crew did a great job at keeping us updated.
“It was terrifying.”
It is currently unclear what caused the part of the plane to come loose and fly off.
In a statement, United told Express.co.uk that pilots called for an “emergency landing” after the engine cowling fell off.
It read: “United flight 1175 traveling to Honolulu from San Francisco landed safely after the pilots called for an emergency landing because of a loss of the engine cowling (the covering of the engine).
“Our pilots followed all necessary protocols to safely land the aircraft. The aircraft taxied to the gate and passengers deplaned normally.”