Boeing says 737 Max flaw rooted in software not hardware

The plane manufacturer says it can fix the problem with an update

Grounded Boeing 737 MAX aircraft parked in an aerial photo at Boeing Field in Seattle, US. The plane has been grounded globally since March. Reuters

US government test pilots running through various flights scenarios for the Boeing 737 Max over the past few weeks revealed a potential failure in the grounded aircraft, but the plane maker and industry insiders claim it is a software rather than a hardware fault.

The jet's flight system attempted to push down the plane's nose repeatedly during simulator tests, prompted by incorrect flight data, Bloomberg reported on Saturday. The Federal Aviation Administration (FAA), the US regulator, concluded that pilots may not have enough time to react and avert a tragedy in a real scenario.

This flaw, revealed by the FAA last month, created further uncertainty about when Boeing's best-selling aircraft will return to service and prompted engineers to find a fix.

The failure triggered multiple aggressive movements to lower the plane's nose, which alarmed FAA pilots and other officials, but the nose-down motion was not the result of a computer hardware problem, Bloomberg reported.

This backs up Boeing's assertions that it can fix the problems with a software change.

"We are confident that is a software update, not a hardware update," Dennis Muilenburg, Boeing chief executive, said on Wednesday. "It's an understood update and we're in the middle of working our way through that."

The 737 Max has been grounded globally since March after the model was involved in two deadly crashes in Ethiopia and off the coast of Indonesia within a span of five months. Mr Muilenburg said Boeing can complete a software patch by the end of September, but cautioned the timeline remains uncertain. The FAA, which must sign off on any fix in the glaring spotlight of the 737 Max investigations,
has not set a deadline or agreed with Boeing's assessment that software changes alone will suffice.

Bloomberg, citing sources briefed on the flight tests, reported more details of the failure.

A wing at the tail of the Boeing jet known as the horizontal stabiliser was rotating in a way that lowered the nose, it said. This same scenario happened during the two fatal accidents when a safety feature known as Manoeuvring Characteristics Augmentation System (MCAS) activated during a malfunction.

But the newly discovered problem was not triggered by MCAS, according to Bloomberg. It was instead triggered by multiple erroneous data streams in a flight computer that occurred simultaneously. It was simulated in tests even though it has never been documented to have occurred during flight.

Projecting that pilots would mitigate a hazard from a malfunction is common on aircraft, but that was part of the reason the FAA approved MCAS initially, a now-controversial decision that is being reviewed by US Congress and other panels. Even though it was possible for pilots in both fatal crashes to have counteracted MCAS, the pilots were unable to do so.
When the newly discovered computer failure began trimming the nose down in the recent test, it was more difficult than expected for test pilots to counteract. 
Another source could not confirm that faulty data streams triggered the nose-down movements.
One of the ways pilots are taught to respond to a so-called "trim runaway", which is what the computer issue prompted, is to activate switches on the control column that move the horizontal stabiliser. Doing so can counteract the malfunction, even if only temporarily, so that pilots have more time to perform other emergency actions.
Using the trim switches to halt the horizontal stabiliser movement proved difficult, though test pilots were able to respond to the failure and maintain control. As a result, they concluded that a typical pilot might not be able to respond adequately.
Because the fault was triggered by specific streams of erroneous flight data, a new software patch can be devised that monitors the computer for that highly unusual condition and prevents movement of the stabiliser when it occurs, according to the Bloomberg report.