When jet engines fall apart, aircraft must stay intact

Two jet engine explosions in one day point to potential dangers underneath the wings of aircraft. But, Andreas Spaeth says, planes must be capable of withstanding such damage.

The long fan blades are made of titanium, the hardest metal available. The funnel-like blades let as much air as possible — as efficiently as possible — into the turbine, where it gets heated up, creating propulsion.

In some engine types, the fan blades are hollow inside with a view to saving weight. The tips of the fan blades, usually between 22 and 38 per engine depending on the class of aircraft, rotate with supersonic speeds inside the inlets. Especially on takeoff, such huge high-tech propulsion devices for wide-body aircraft run at full throttle to lift off aircraft weights of 250 tons or more.

It was during this most critical phase of the flight that two incidents happened on Saturday, one in Europe and the other in the United States.

Broken blades

Initially, a 30-year-old Boeing 747-400 freighter took off from Maastricht in the Netherlands for a flight to New York. It had just become airborne when an explosion occurred on the outer left engine. Broken blades from one of the compressors flew out of the back, slightly injuring an elderly woman and a child on the ground. Worse things could easily have happened as the sharp-edged metal pieces punctured parked cars in the suburb of Meerssen like knives coming down from the sky.

The crew declared an emergency. The plane went into a holding pattern, then dumped fuel before bringing down the crippled Jumbo to a safe emergency landing an hour later in nearby Belgian Liege. The Dutch Safety Board (DSB) initiated an investigation into the cause and circumstances of the incident.

The PW4056 engine in question was manufactured by Pratt & Whitney in the United States, one of the leading global aircraft jet engine suppliers. Developed in the mid-1980s, the PW4000 family is one of the principal types for airliner propulsion. Over 2,500 have been produced. These engines hang underneath the wings of an array of aircraft types, including the Boeing models 777, 767 and 747-400 as well as the Airbus A330 (though not on the ones flown by Lufthansa).

Just a Boeing issue?

Development, testing and certification for commercial operations are done by the engine manufacturers, not the aircraft suppliers. Viewing the incidents as a specific Boeing problem would therefore be misleading, though such events always taint the aircraft manufacturer’s image, too.

That’s especially the case when two such rare incidents occur on the same day, both involving Boeing aircraft, with dramatic photos and video footage of the incidents shared instantly all over the globe via social media.

On Saturday afternoon local time, United Airlines flight UA328 took off from Denver to Honolulu, Hawaii. Shortly after takeoff, the right PW4077 engine by Pratt & Whitney failed and exploded. The huge, ring-like engine inlet with a diameter of over three meters (10 feet) then plummeted to the ground, landing in a front yard in a Denver suburb. Luckily, no one was injured.

Video footage shows metal parts of the engine cowling floating to the ground menacingly before landing next to a soccer field, again without causing any damage.

Meanwhile, passengers on flight UA328 were treated to a dramatic sight: the damaged fan of the engine, clearly missing two blades, turning in the wind as flames flickered around the exhaust and the whole aircraft vibrated. This crew also declared Mayday. It seemed as if the redundancy built into the aircraft and the thorough shielding of the engine core had worked flawlessly.

The crew handled the emergency exactly as practiced in simulator sessions.

Debris can break free from the engine in such uncontrolled failures, with separated blades and stray parts puncturing fuselage and wings like shrapnel, in the worst case disabling vital aircraft functions. This is what happened on Qantas flight QF32 in November 2010, when an engine explosion damaged the Airbus A380 so badly that a the likelihood of a safe emergency landing was called into doubt, though it was ultimately carried out successfully.

On board of UA328 on Saturday, it seemed that the containment ring made of ultraresistant Kevlar had withstood any impact and hindered the emergence of any debris. The aircraft stayed fully maneuverable and came in for an emergency landing in Denver 23 minutes after takeoff. No one on board or on the ground was harmed.

A closer examination of the airframe showed that the people on board were luckier than initially reported. A piece of debris from the engine had torn a sizable hole underneath the root of the wing, only centimeters away from the full main tanks.

First findings

The US National Transportation Safety Board (NTSB) has shared its first findings: One fan blade was fractured near the root, possibly because of prior metal fatigue in the hollow titan blades. An adjacent fan blade was fractured about midspan, and a piece of a blade was found embedded in the outer Kevlar containment ring. The PW4077 engine powers early Boeing 777 air planes from the mid-1990s. The aircraft now affected has been in operation since 1995.

In December, there was a similar incident on a 777-200 operated by Japan Airlines. Almost exactly three years ago, a sister aircraft of the United 777 damaged on Saturday was involved in a similar incident.

The US Federal Aviation Administration, as well as the Japanese and Korean authorities, have now effectively grounded aircraft with this engine, ordering more intense inspections on Sunday. „The inspection interval should be stepped up for the hollow fan blades that are unique to this model of engine, used solely on Boeing 777 airplanes,“ FAA Administrator Steve Dickson said. In addition to the US’s United, only Japan Airlines and ANA in Japan and Korean Air in South Korea operate the affected version of the 777. At last count, 69 were still in active service, while 59 had been parked during the pandemic.

Currently none of those is allowed to fly anymore, and it can be expected that the decline in traffic demand because of the coronavirus pandemic will result in many of them being permanently withdrawn from service.

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