WASHINGTON, April 8 (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 ( LUV ) 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 ( BA ) 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.