After fatal accident, senators ask FAA to speed up aviation safety directives The concerns were prompted by the deadly accident aboard a Southwest plane.

In the wake of a fatal accident onboard a Southwest Airlines flight, four Democratic senators are demanding to know why it took the FAA two nearly two years to mandate additional inspections of a suspect engine part.

A fatigued engine blade on an April flight from Laguardia to Dallas led to an engine failure while the Boeing 737 cruised at 32,000 feet.

Pilots performed an emergency landing in Philadelphia, but a woman died when debris broke the window and she was partially sucked out. Other passengers managed to pull her back in, but the medical examiner said she died from blunt force trauma of the head, neck and torso.

The incident raised alarms about the safety of the engines onboard Boeing 737s, an aircraft commonly-used on domestic flights in the United States.

A similar incident occurred in August 2016 on a Southwest flight. While the event was not fatal, it prompted the engine manufacturer to recommend more inspections and the FAA began working on a directive that would mandate a greater frequency of inspections.

Democratic Sens. Wyden, Udall, Leahy and Blumenthal sent a letter to FAA Acting Administrator Daniel Elwell asking why that directive wasn't implemented until April's fatal accident. "While we understand there is no set timeline duration for rulemaking processes, why did the FAA take almost two years to move on an airworthiness directive to ensure CFM56-7B engines were safe to fly?"

An FAA spokesperson told ABC News on Thursday afternoon they had not yet received the letter.

Although two years is longer than many airworthiness directives take to implement, the original proposal by the FAA would not have covered April's fan blade failure. Timelines of these directives vary depending on complexity, the risk of reoccurrence, the severity of occurrences, public comments and number of changes to the proposal in response to the comments.

During this period of comments from airlines, the manufacturer and the general public, U.S. carriers, including Southwest, were already performing more frequent inspections based off the engine maker's warning the blades in the engines could crack, FAA and airline officials have told ABC News.

An FAA official said the agency's action and timeline were consistent with their risk assessment.

Within two weeks of April's incident, the FAA mandated CFM56-7B engines with more 30,000 total takeoffs and landings must be inspected within 20 days. Soon after, regulators ordered airlines to check all these engines with more than 20,000 cycles and to keep checking these blades every 3,000 flights, or about two years.

Last week, Southwest CEO Gary Kelly said the airline had completed engine inspections across its entire fleet. A small number of fan blades were sent for further tests.