A United Airlines Boeing 777 and a Delta Air Lines Airbus A330 both made emergency landings due to engine problems on Tuesday.
Both planes landed safely, but the five passengers on the Delta flight were injured during the evacuation.
Engine malfunctions are increasingly rare these days but do still occur.
Modern twin-engine airliners are designed to fly hours on end using a single engine.


On Tuesday, a United Airlines Boeing 777 landed in Honolulu missing the cowling or cover on the front one of its engines.
United Airlines won't confirm whether the incident rendered the engine operable, but images of the engine after landing in Hawaii along with passenger statements of severe vibrations would indicate it was likely shut down by the flight crew.
Later on Tuesday, a Delta Air Lines Airbus A330 made an emergency landing shortly after taking off from Lagos, Nigeria due to a problem with one of its engines. Five passengers suffered "non-critical" injuries while evacuating the plane from emergency slides.
According to Flightglobal, the United Boeing 777 and the Delta Airbus A330 were powered by different versions of Pratt & Whitney's PW4000 turbofan engine. 
Pratt & Whitney was not immediately available for comment on the matter.
Also, it is too early in the investigation to tell what caused the two incidents. 
Fortunately for the passengers and crew, both twin-engined airliners involved are designed to fly for substantial amounts of time on a single engine.
Should I be worried?
So, should you be worried when an airliner loses an engine?
Sometimes.
Instances of an airliner losing an engine are obviously not unheard-of. It can and does happen. Most of the time, the pilot diverts and no one is injured. Even in the most catastrophic failures, the casing around the engine is designed to contain the damage caused by the failed engine. In other words, a contained engine failure.
In rare and extreme instances, a failure can cause damage beyond the engine casing.
Last October, Air France Flight 66 suffered one of these uncontained engine failures on a flight from Paris to Los Angeles. One of the Airbus A380's four engines malfunctioned causing the fan blade and the entire front of the unit to sheer off mid-flight. Fortunately, the plane was able to land safely in Canada. 
In such cases, the big worry centers around the extent of the damage to the plane caused by flying debris from the engine. It is unclear how much damage the failure did to the aircraft beyond on the engine, but the plane was able to be flown back to Paris a few weeks later.
But there have been documented incidents where an engine failure has caused severe damage to the aircraft.
In November 2010, another Airbus A380, Qantas Flight 32, suffered a catastrophic uncontained engine failure caused by a leaking oil feed pipe. Flight 32 was able to make a safe landing even though the aircraft suffered significant structural damage and damage to several important flight management systems.

It should be noted that the Qantas A380 and the Air France A380 are powered by engines from two separate manufacturers with fundamentally different design philosophies.
The Air France aircraft reportedly involved in the incident, F-HPJE is a seven-year-old Airbus A380 powered by four Engine Alliance GP7200 turbofan engines. Engine Alliance is a Connecticut-based joint venture between Pratt & Whitney and General Electric. While the Qantas Flight 32 was powered by four Rolls-Royce Trent 900 engines.
The take away here is that even in the most severe instances of engine failure from recent memory, the airplane and its crew have been able to bring its precious cargo of passengers to safety. 
While the loud noises and jarring vibrations may be distressing, it likely feels far worse than it actually is. 
One engine? No problem.
Long-distance and transoceanic flights have traditionally been flown by three- or four-engine wide-body airliners. This is because when it comes to the engine count on an airliner, aviation thinking dictates that there is safety in numbers.But as modern turbofan engines have become more reliable, engine failures have become far less common. As a result, most airlines have turned to twin-engined mini-jumbos that are more fuel-efficient.These days, the three-engine airliner has gone the way of the dinosaur, and the four-engine jumbo jets that once dominated the skies are well on their way toward extinction. But engine failures do still happen. As terrifying as they may be for many of the passengers, though, losing an engine on an A380 means there are three more perfectly capable of safe flight. 
Even on twin-engined airliners, an engine failure is not a huge problem. When an aircraft is flying without one of its engines, it tends to fly at a lower altitude and work the remaining engine(s) harder. This makes the plane less fuel-efficient and reduces range. However, the vast majority of twin-engine long-haul airliners can perform this maneuver with no significant reduction in capabilities.Before a twin-engine airliner is allowed to fly long-distance routes over large bodies of water or through uninhabited regions like the Arctic, it must be certified by the Federal Aviation Administration (FAA) for ETOPS or Extended range Twin Operations.When an aircraft is certified, part of the assessment is based on the plane's performance when flying on a single engine.For example, the Boeing Dreamliner is certified for ETOPS-330. This means that the aircraft can fly routes that take it as far as 330 minutes (five and a half hours) of single-engine flying time from the nearest viable airport.Other twin-engine airliners, like the Boeing 777, are also certified for ETOPS 330. The Boeing 767 is certified for as much as 180 minutes of ETOPS. Airbus' popular A330 and new A350 have been certified for ETOPS flying beyond 180 minutes, while the new A350 has been certified for ETOPS of up to 370 minutes. Join the conversation about this story » NOW WATCH: Ken Rogoff on the next financial crisis and the future of bitcoin

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