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After having conducted additional certification test flights in July, the design review obviously has been on-going and yesterday, the FAA has released a NPRM (Notice of Proposed Rule Making). The NPRM procedure is a normal procedure before issuing a final rule. It is intended to allow there public and stakeholders to comment and the FAA to review comments and, if deemed appropriate, incorporate them into the final rule.

The expected relevant commenters in this case would be foreign aviation safety authorities like EASA, Transport Canada and Chinese CAAC. Note, the latter one was the first to ground the type.

THE NPRM is attached (with explanatory bookmarks) for interested to download.

The NPRM's airworthiness content has four technical elements in the form of design changes, which will likely all be in the final rule (AD)

1/ Revised FCC (Flight Control Computer) software

  • Utilises input from both available angle of attack (Ao) sensors instead of one in previous configuration. This in order to prevent erroneous MCAS activation based on erroneous signal from a single AoA sensor

  • Compares signal outputs from both AoA sensors and when beyond limits, disables the Speed Trim System (STS) (including MCAS) and displays a [AOA DISAGREE] message on the primary flight display

  • Would only activate MCAS once during a high AoA event

An MCAS activation drives down the horizontal stabilizer a few degrees to the aircraft nose down position. In both fatal accidents that triggered the grounding of the type, MCAS activated repeatedly due to the fact that it utilised the signal of one of the two available AoA sensors and that these AoA sensors generated an erroneous signal. The repeated MCAS activation drove the aircraft so far out of trim that it was not manually controllable anymore

2/ AFM (airplane Flight Manual) revision, that proposes

  • add a step to allow flight crew to determine reliable airspeed without use of reference tables

  • Improve procedure for go-arounds, allowing for increased use of automation

  • Add a step to ensure that erroneous altimeter info is not transmitted to ATC via transponder signal

  • Add erroneous AoA as potential cause for unreliable airspeed conditions

  • Runaway stabilizer checklist is used when there is underside stabiliser movement. FAA proposes to revise criteria and contain explicit recall for thumb switch use to bring the aircraft in trim.

  • Stab trim ions checklist would be revised to better align with other non-normal checklists

  • Add a speed trim fail checklist to be used in case of a speed trim deactivation by the new FCC software.

  • Addition of stabiliser out of trim checklist

  • Addition of AoA disagree checklist

  • Addition of an ALT (Altitude) disagree checklist

  • Addition of a IAS (Indicated Airspeed) disagree checklist

3/ Separation of horizontal stabiliser wiring

As a result of the design review of the entire stabiliser control system, it was concluded that the horizontal stabiliser trim arm wiring and horizontal stabiliser trim control wiring did not comply with Part 25.1701, EWIS, electrical wire interconnect systems standards (stemming from 2007), stipulating separation of wiring such that electromagnetic interference and failure will not affect nearby components, creating hazardous situations.

As a result, the NPRM proposes to separate the affected wiring in order to meet EWIS requirements.

4/ Maintenance related item

In order to ensure that both AoA sensors are operating before a 737 MAX aircraft is returned to service, the NPRM proposed to conduct a AoA sensor system test on each airplane prior to return to service. This involves placing the AoA vanes in a calibrated position by means of a fixture and read and assess the signal outputs to be within specified limits.

5/ Mandatory operational readiness flight of each airplane before (re)entry into service after above changes have been embodied

Please comment!

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