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2018-23-51 THE BOEING COMPANY: ( e m e r g e n c y ) Product Identifier 2018-NM-151-AD.
(a) EFFECTIVE DATE

    This Emergency AD is effective upon receipt.

(b) AFFECTED ADS

    None.

(c) APPLICABILITY

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

(d) SUBJECT

    Air Transport Association (ATA) of America Code 27, Flight controls.

(e) UNSAFE CONDITION

    This AD was prompted by analysis performed by the manufacturer 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. We  are
    issuing 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.

(f) COMPLIANCE

    Comply  with this  AD within  the compliance  times specified,  unless
    already done.

(g) REVISION OF AIRPLANE FLIGHT MANUAL (AFM): CERTIFICATE LIMITATIONS

    Within 3 days after receipt of this AD, revise the Certificate Limita-
    tions chapter of the applicable AFM  to  include  the  information  in
    figure 1 to paragraph (g) of this AD.

        FIGURE 1 TO PARAGRAPH (G) OF THIS AD – CERTIFICATE LIMITATIONS    
    ______________________________________________________________________
                          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.
    ______________________________________________________________________

(h) AFM REVISION: OPERATING PROCEDURES

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

         FIGURE 2 TO PARAGRAPH (H) OF THIS AD – OPERATING PROCEDURES      
    ______________________________________________________________________
                          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 indica-
    tions 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 stabi-
    lizer 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.
    ______________________________________________________________________

(i) ALTERNATIVE METHODS OF COMPLIANCE (AMOCS)

(1) The  Manager,  Seattle ACO Branch,  FAA,  has the authority to approve
    AMOCs for this AD, if requested  using the procedures found in 14  CFR
    39.19. In  accordance with  14 CFR  39.19, send  your request  to your
    principal  inspector  or  local  Flight Standards District Office,  as
    appropriate. If  sending information  directly to  the manager  of the
    certification  office,  send  it  to  the  attention  of  the   person
    identified in paragraph (j) of this AD. Information may be emailed to:
    9-ANM-Seattle-ACOAMOC-Requests@faa.gov.

(2) Before using any approved AMOC,  notify your appropriate principal in-
    spector, or lacking  a principal inspector,  the manager of  the local
    flight standards district office/certificate holding district office.

(j) RELATED INFORMATION

    For further information about this AD,  contact Douglas Tsuji,  Senior
    Aerospace Engineer,  Systems and Equipment Section,  FAA,  Seattle ACO
    Branch, 2200 South 216th St, Des Moines, WA 98198; phone and fax: 206-
    231-3548; email: Douglas.Tsuji@faa.gov.

Issued in Des Moines, Washington, on November 7, 2018.  Original signed by
Chris Spangenberg,  Acting Director,  System Oversight  Division, Aircraft
Certification Service.
PREAMBLE 

DATE: November 7, 2018
AD #: 2018-23-51

Emergency Airworthiness Directive (AD) 2018-23-51 is sent to owners and operators of The Boeing Company Model 737-8 and -9 airplanes.

Background

This emergency AD was prompted by analysis performed by the manufacturer 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. This condition, if not addressed, 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.

FAA’s Determination

We are issuing this AD because we evaluated all the relevant information and determined the unsafe condition described previously is likely to exist or develop in other products of the same type design. Due to the need to correct an urgent safety of flight situation, good cause exists to make this AD effective in less than 30 days.

AD Requirements

This AD requires revising certificate limitations and operating procedures of the airplane flight manual (AFM) to provide the flight crew with runaway horizontal stabilizer trim procedures to follow under certain conditions.

Interim Action

We consider this AD interim action. If final action is later identified, we might consider further rulemaking then.

Authority for this Rulemaking

Title 49 of the United States Code specifies the FAA’s authority to issue rules on aviation safety. Subtitle I, Section 106, describes the authority of the FAA Administrator. Subtitle VII, Aviation Programs, describes in more detail the scope of the Agency’s authority.

We are issuing this rulemaking under the authority described in Subtitle VII, Part A, Subpart III, Section 44701, "General requirements." Under that section, Congress charges the FAA with promoting safe flight of civil aircraft in air commerce by prescribing regulations for practices, methods, and procedures the Administrator finds necessary for safety in air commerce. This regulation is within the scope of that authority because it addresses an unsafe condition that is likely to exist or develop on products identified in this rulemaking action.

This AD is issued in accordance with authority delegated by the Executive Director, Aircraft Certification Service, as authorized by FAA Order 8000.51C. In accordance with that order, issuance of ADs is normally a function of the Compliance and Airworthiness Division, but during this transition period, the Executive Director has delegated the authority to issue ADs applicable to transport category airplanes and associated appliances to the Director of the System Oversight Division.

Presentation of the Actual AD

We are issuing this AD under 49 U.S.C. Section 44701 according to the authority delegated to me by the Administrator.