Pilots of the Lion Air Boeing 737 Max 8 which crashed after take-off from Jakarta last year would not have received an alert regarding the disagreement between the angle-of-attack sensors, because the carrier had not selected an optional angle-of-attack indicator for its aircraft.
Boeing analysis, carried out after the accident, showed that some 20% of airlines had opted to have the indicator installed on the type.
Aircraft fitted with the indicator also generate a 'disagree' message if the values of the left- and right-hand angle-of-attack sensors, transmitted by the air data computers, differ by at least 10° for 10s.
The ill-fated Lion Air aircraft had departed Jakarta on 29 October last year with sensors misaligned by 21°.
Boeing initially implemented angle-of-attack 'disagree' warning messages on the previous evolution of the 737 – the 737NG family – and this feature was installed on all newly-manufactured aircraft from 2006, with an option to retrofit. It was then carried over to the 737 Max.
But Indonesian accident investigation authority KNKT says that, in 2017, a few months after 737 Max deliveries began, Boeing found that the Max display software "did not correctly implement" the 'disagree' alert.
Although the 737 Max display system requirements initially called for the angle-of-attack 'disagree' alert to be a standard feature, the software delivered to Boeing linked this alert to the angle-of-attack position indicator – which was only an optional feature on the 737 Max.
"Accordingly, the software activated the [angle-of-attack 'disagree'] alert only if an airline opted for the [angle-of-attack] indicator," says the inquiry.
It points out that pilots who had been trained on earlier versions of the 737 would have been aware of the possibility of a 'disagree' alert, but would not necessarily be aware that the warning would not appear on the Max.
"This would contribute to [pilots] being denied valid information about abnormal conditions being faced and lead to a significant reduction in situational awareness," says the inquiry.
Lack of the 'disagree' message "did not match" the Boeing system description that served as the basis for certifying the 737 Max, it adds.
Boeing considered that the absence of the 'disagree' alert "did not adversely impact aircraft safety", the inquiry states, because the stick-shaker and pitch-limit indicator served as primary alerts for the crew at excessive angles of attack – whereas the 'disagree' warning amounted to supplementary information.
Although Boeing planned to revert to the originally-intended functionality – through a display system software upgrade scheduled for the third quarter of 2020 – it felt that the optional status for the angle-of-attack indicator, and therefore the 'disagree' alert, was acceptable in the interim.
Lion Air had not chosen the optional indicator, which meant the crew of the crashed aircraft did not receive a 'disagree' alert despite the conditions for such a warning being met.
Boeing convened a review board to examine, in the aftermath of the crash, whether the absence of the 'disagree' warning amounted to a safety issue – but concluded that it did not, and the airframer also chose not to accelerate the planned software update.
But it has since advised that new software, incorporating the 'disagree' alert, will be available before the 737 Max returns to service following the type's worldwide grounding.
"All customers with previously delivered [737 Max] aircraft will have the ability to activate the ['disagree'] alert [through] a service bulletin to airlines," the inquiry says.