United Flight Turns Back to Sydney After Takeoff Due to Maintenance Issue; Latest in String of Problems for Airline

United Airlines flight from Sydney to San Francisco turned back 2 hours after takeoff due to unspecified "maintenance issue"
167 passengers + 16 crew were on the Boeing 777-300 aircraft
At least 6 United flights over the past week have faced problems forcing returns or emergency landings
Issues included blown tire at SFO, hydraulics problem, plane veering off runway after landing, engine fire
Bubble wrap sucked into engine caused plane from Houston to Fort Myers to turn back shortly after takeoff