Transcript for Engine cover comes loose during United flight, terrifying passengers
the midair merging involving the casing on an engine. Passengers seeing a piece of that engine, the panel there, peeling back. All of it very alarming, obviously. The pilot was then forced to turn back to Denver. ABC's David Kerley covers aviation. Reporter: A frightening scene out the passenger window of this united 737-800. When I heard the pop and the bang of the engine, I knew something was not right. Reporter: Flapping in the wind, tens of thousands of feet in the air, it was the engine cover that had come loose. An emergency for the pilots. We need to get back to Denver relatively quickly. Nature of the emergency? A panel has become detached from the airplane. We have about six hours of fuel and we have 196 people. Reporter: Just 30 minutes into the flight, the orlando-bound jet turns around. Passengers hearing the noise and feeling the vibration from that flapping cover. This pilot really took some pretty heroic, decisive action. Made a big decision with turning us around really quickly. Reporter: On the ground, a clear view for all the passengers. The internal engine exposed. That cover, mangled. Experts say that cover doesn't affect the operation of the engine. Still, not something a passenger wants to see out the window. Definitely not. David Kerley with us now. We know everyone safe on that flight, but another safety issue making news tonight. Boeing warning of a cracking problem, as they say, on some of its next generation 737 aircraft, David? Reporter: Yeah, those next gen, that fleet is still flying right now, David, and in three of the aircraft that are about ten years old, so, kind of old, they found cracking in a critical area. It's where the wings attach to the fuselage. They're concerned about those cracks. They shouldn't be there. So the FAA is asking that 300 other aircraft of about the same age and usage also be checked for these cracks. They want to find out if this is a widespread safety issue. David? All right, we know you'll stay on it for us. David Kerley, thank you.
This transcript has been automatically generated and may not be 100% accurate.