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Fri, Nov 09, 2018

FAA Issues Emergency AD For 737 MAX AOA Indicators

Follows Lion Air Accident Involving One Of The Airplanes

The FAA has issued an emergency AD for Boeing 787 MAX-8 and MAX-9 airplanes following analysis performed by Boeing showing that if an erroneously high single angle of attack (AOA) sensor input is received by the flight control system, there is a potential for repeated nose-down trim commands of the horizontal stabilizer.

The FAA issued this AD to address this potential resulting nose-down trim, which could cause the flight crew to have difficulty controlling the airplane, and lead to excessive nose-down attitude, significant altitude loss, and possible impact with terrain.

The AD applies to all The Boeing Company Model 737-8 and -9 airplanes, certificated in any category.

Investigators recovered the airplane's Flight Data Recorder from the ocean floor last week. CNN reports that analysis of the data from that "black box" shows the airplane was intact with both engines running when it impacted the water. Indonesian officials say that the flight crew that had flown the accident aircraft the previous day had reported similar problems with the airplane.

The FAA is requiring the following revisions to the airplane's operating manual:

Required by AD 2018-23-51 - Runaway Stabilizer
In the event of an uncommanded horizontal stabilizer trim movement, combined with any of the following potential effects or indications resulting from an erroneous Angle of Attack (AOA) input, the flight crew must comply with the Runaway Stabilizer procedure in the Operating Procedures chapter of this manual:

  • Continuous or intermittent stick shaker on the affected side only.
  • Minimum speed bar (red and black) on the affected side only.
  • Increasing nose down control forces.
  • IAS DISAGREE alert.
  • ALT DISAGREE alert.
  • AOA DISAGREE alert (if the option is installed).
  • FEEL DIFF PRESS light.
  • Autopilot may disengage.
  • Inability to engage autopilot.

Within 3 days after receipt of this AD, operators are required to revise the Operating Procedures chapter of the applicable AFM to include the information in figure 2 to paragraph (h) of this AD.

Required by AD 2018-23-51 - Runaway Stabilizer

  • Disengage autopilot and control airplane pitch attitude with control column and main electric trim as required. If relaxing the column causes the trim to move, set stabilizer trim switches to CUTOUT.

If runaway continues, hold the stabilizer trim wheel against rotation and trim the airplane manually. Note: The 737-8/-9 uses a Flight Control Computer command of pitch trim to improve longitudinal handling characteristics. In the event of erroneous Angle of Attack (AOA) input, the pitch trim system can trim the stabilizer nose down in increments lasting up to 10 seconds.

In the event an uncommanded nose down stabilizer trim is experienced on the 737-8/-9, in conjunction with one or more of the indications or effects listed below, do the existing AFM Runaway Stabilizer procedure above, ensuring that the STAB TRIM CUTOUT switches are set to CUTOUT and stay in the CUTOUT position for the remainder of the flight.

An erroneous AOA input can cause some or all of the following indications and effects:

  • Continuous or intermittent stick shaker on the affected side only.
  • Minimum speed bar (red and black) on the affected side only.
  • Increasing nose down control forces.
  • IAS DISAGREE alert.
  • ALT DISAGREE alert.
  • AOA DISAGREE alert (if the option is installed).
  • FEEL DIFF PRESS light.
  • Autopilot may disengage.
  • Inability to engage autopilot.

Initially, higher control forces may be needed to overcome any stabilizer nose down trim already applied. Electric stabilizer trim can be used to neutralize control column pitch forces before moving the STAB TRIM CUTOUT switches to CUTOUT. Manual stabilizer trim can be used before and after the STAB TRIM CUTOUT switches are moved to CUTOUT.

(Source: FAA. Image from file)

FMI: AD

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