DENVER — The sound rumbled across the sky above Broomfield Saturday afternoon. What followed was something equally unnatural and dangerous: small and large pieces of a massive turbine engine began to rain down on neighborhoods below the flight path of United Airlines Flight 328.
"We looked up in the sky and saw a lot of dark smoke, and things raining down from the sky," Lisa Hill told Denver7, who was walking in an open space near her home. "We thought maybe birds hit the engine and those were the small things we saw falling, but they were in fact big things, they just looked small from a few miles away."
The Boeing 777-200, heading to Honolulu, just experienced a right-engine failure shortly after takeoff. The plane turned around and returned to Denver International Airport, landing safely. Nobody on board or on the ground was injured.
But what caused such a spectacular uncontained engine failure? Investigators will be searching for that answer to such a rare event in the coming months. But Tom Haueter, ABC News consultant and former NTSB Director of the Office of Aviation Safety, has some insight into what they might be looking for.
“What investigators will look at, collect all the parts, look at the photographs, take a look at the maintenance records of the engine, the history of the engine, when was it made, what’s the service life, how many hours are on it, how many cycles, how many flights were on it, start collecting all the information,” Haueter told ABC News.
Soon after the incident, photos and videos began appearing on social media. To aviation experts like Haueter, those posts offered a clue as to what might have happened.
“Looking at the photographs I’ve seen, it appears that a piece of one of the fan blades, those are the large blades you can see when you’re looking from the engine from outside, there’s a piece of a fan blade missing, and I can’t tell from another photograph if there’s another fan blade completely missing,” Haueter said.
Debris from the engine fell onto a wide area of Broomfield, affecting homes and property in the Northmoor and Red Leaf neighborhoods. Parts were seen scattered in parks, lawns and on rooftops. But Haueter said one of the most important pieces authorities need to find is the missing fan blade.
“The really important piece to get to get back would be the fan blade, or pieces of the fan blade. You want to have two sides of the failure so you can say, ‘OK, here’s what happened.’ Was there a nick in the blade? Was there a flaw in the blade? Was there something else going on? There’s a lot of pieces, but only a few pieces are really critical to the investigation,” he said.
Video captured by a passenger on board a Flight 328 Saturday showed the inflight engine failure before pilots made an emergency landing. Passengers likely felt the explosion and vibrations all the way back to DIA.
“Well, basically when something like this happens, the engine is extremely out of balance. Turbine engines are designed with all that massive rotating to be very smooth, obviously. When you lose a piece of that, now you have a lot of vibration going on. Parts start rubbing together that normally don’t rub, you start vibrating parts loose for the fuel system, so suddenly you can have a fuel line crack,” Haueter said. “You’ve got friction from this massive engine grinding still rotating and it’s destroying various pieces of metal as it’s doing it. It looks pretty dramatic, let’s be honest, but that, unless the engine really catches fire and has a major fire going on, it looks worse than it is.”
Investigators won’t know exactly what happened until they tear the engine apart. A team from the National Transportation Safety Board is headed to the area to take over the investigation, police said.
Although Haueter said what happened Saturday is very rare, some are not taking chances. Japan's Transport Ministry instructed Japan Airlines and All Nippon Airways, which operate aircraft equipped with the same series of engines, to ground the Boeing 777's in their fleet.
The transportation ministry also refers to a "serious incident" that occurred on a Japan Airlines flight on December 4, last year where the same type of engine (Pratt & Whitney PW4000 series) was damaged.