Southwest Airlines says ongoing engine fan blade inspections have not turned up any cracks, as an investigation into the fatal 17 April inflight engine failure on a Boeing 737-700 continues.

The airline has completed inspections on about 8,500 fan blades since the accident after accelerating those checks, says Southwest chief operating officer Mike Van de Ven on an earnings call.

Southwest had been inspecting its CFM56-7B engine fan blades prior to the 17 April accident, following an August 2016 inflight failure of the same engine type on another 737-700 in its fleet. Van de Ven says the carrier had inspected about 17,000 engine blades before last week's accident, which killed one passenger.

"We were on a path to complete the inspections of the remaining 18,000 by year-end and that would have met the recommended service bulletin timeline," he adds, referring to a service bulletin issued previously by engine manufacturer CFM International.

The engine involved in the 17 April accident had not yet been inspected as it had accumulated 10,000 cycles since its last overhaul, below the 15,000-cycle threshold for urgent inspections in a proposed US Federal Aviation Administration airworthiness directive. That directive was still in the process of being finalised at the time of the accident last week.

But the accident prompted Southwest to accelerate the inspections, and to complete them within 30 days or by 17 May. Van de Ven says the airline has roughly 35,500 fan blades, which means the airline has about 10,000 fan blades left to inspect at this point.

An emergency airworthiness directive issued by the US Federal Aviation Administration on 20 April calls for a one-time ultrasonic inspection of fan blades on all CFM56-7B engines with 30,000 or more accumulated flight cycles since delivery within 20 days.

The root cause of the 17 April accident continues to elude the airline, whose executives expressed surprise at the extent of the damage caused by the crack on blade 13 of the 24-blade powerplant that failed on Southwest flight 1380.

"The loss of a single blade just should not have caused such dramatic impact," says Van de Ven. The release of the blade caused significant damage to the engine's inlet cowl, and investigators believe that the ruptured cowl is likely responsible for the damage to the aircraft's fuselage, wing and stabiliser, says Van de Ven.

The impact shattered a window and killed a passenger who was sitting in a window seat.

An inspection in May 2017 found a single cracked fan blade which was subsequently discarded, says Southwest chief executive Gary Kelly. Van de Ven stresses the rarity of engine fan blade cracks on the CFM56.

Referring to the August 2016 accident, he says: "The worldwide fleet of those engines probably had north of 300 million flight hours on them, and it was the first time in history that engine had that kind of event. They [CFM] are very aggressive on trying to understand what happened there."

Southwest is also working with the engine manufacturer to understand how the powerplant's inlet cowl could have been damaged so extensively by the cracked fan blade.

"That's what we are talking about, whether there are opportunities for the inlet cowl to improve its durability so it can minimise the kind of energy that comes out from the fan blade release," says Van de Ven.

Source: Cirium Dashboard