WASHINGTON (Reuters) - An engine cowling on a Southwest Airlines Boeing 737-800 fell off on Sunday during takeoff in Denver and struck the wing flap, prompting the Federal Aviation Administration to open an investigation.

Here's what we know about the incident so far.

What is an engine cowling cover? An engine cowling is a protective covering that houses an airplane engine made of metal or composite material. The cowling is part of the nacelle, which is separate from the fuselage and houses the engine and other equipment such as fuel tanks or electrical systems.

An engine cowling reduces drag, increases speed and smoothes airflow, allowing for better engine cooling and protects engine controls. Cowlings are required to be resistant to extreme temperatures and heavy vibrations.

What did Southwest say?

The company said Southwest Flight 3695 returned to Denver International Airport and landed safely after experiencing a mechanical issue. "We apologize for the inconvenience of their delay, but place our highest priority on ultimate Safety for our Customers and Employees. Our Maintenance teams are reviewing the aircraft," Southwest said in a statement.

Why did the engine cowling cover fall off?

We do not know the reason this particular cover fell off.

The Seattle Times reported in 2022 that dozens of similar accidents have happened in the past three decades on 737s and Airbus A320s but none resulted in injuries. The paper said almost every cowling incident had been traced to maintenance errors tied to a fan cowl door latching failure missed during preflight checks.

Airbus has said it developed several additional devices to prevent incidents including improving fan cowl latches and adding sensors.

What else has fallen off a plane recently?

In January, the door plug from a new Alaska Airlines Boeing 737 MAX 9 fell off in mid-air. The National Transportation Safety Board said four key bolts appear not to have been installed when it left the Boeing factory.

Last month, the FAA said it was investigating how a United Airlines Boeing 737-800 lost an external panel before landing safely in Oregon Friday. The FAA said a post-landing airline inspection of the 25-year-old airplane revealed a missing panel.

Those incidents appear unrelated to the cowling cover problem.

Is it safe to fly?

The last fatal U.S. passenger airline crash occurred in February 2009 - more than 15 years ago. Transportation Secretary Pete Buttigieg said last week that air travel is the "safest form of travel in America."

What happens next?

The FAA will conduct interviews and review maintenance records to determine what went wrong. The FAA can take enforcement actions or issue fines for airlines failing to follow safety rules.

(Reporting by David Shepardson; Editing by Lincoln Feast.)