Flight Crew Operations Manual Bulletin
for
The Boeing Company Seattle, Washington 98124-2207
737
Number: SHZ-11 R1 Date: June 30, 2004 Document Effectivity:
Subject: AFDS Performance Degradation with Radio Altimeter Failure
Reason: To inform flight crews of potential AFDS performance degradation associated with certain Radio Altimeter failure modes. The purpose of this reissue is to provide Service Bulletin information.
Information in this Flight Crew Operations Manual (FCOM) bulletin is recommended by The Boeing Company, but may not be FAA approved at the time of writing. In the event of conflict with the FAA approved Airplane Flight Manual (AFM), the AFM shall supersede. The Boeing Company regards the information or procedures described herein as having a direct or indirect bearing on the safe operation of this model airplane.
THE FOLLOWING PROCEDURE AND/OR INFORMATION IS EFFECTIVE UPON RECEIPT
Background Information
The LNAV function of the flight management computer (FMC) is limited to a bank angle limit of 30 degrees by the flight control computers (FCC) when the radio altitude is 400 feet or greater. Below 200 feet, LNAV is limited to 8 degrees of bank angle. Between 200 and 400 feet bank angle is limited to 15 degrees. The actual bank angle commanded by the FMC will be within these bounds.
Flight testing has confirmed that certain radio altimeter failure modes can cause degraded autopilot flight director system (AFDS) performance. These failure modes may not cause an amber RA failure flag to be displayed.
If a radio altimeter fails while transmitting a valid altitude of less than 200 feet or if the radio altimeter output never becomes valid after power up on the ground, the associated FCC, which uses that radio altimeter as its primary source of data, will use the last valid altitude received or use zero feet if no valid altitude is received after power-up. This will result in the LNAV command on that side always being limited to 8 degrees of bank angle, during either autopilot or flight director operation. Depending on the aggressiveness of the programmed turns of the active LNAV path, this failure could result in the airplane departing the LNAV path if the FMC desired commands exceed the AFDS bank limits.
Flight Crew Operations Manual Bulletin No. SHZ-11 R1, dated June 30, 2004 (continued)
This failure will be indicated by the airplane symbol not following the defined (magenta) path as shown on the Navigation Display.
Operating Instructions
Boeing procedures emphasize the need for pilot monitoring of automated systems to ensure acceptable performance in flight. In this case, vigilant flight path monitoring will reveal the radio altimeter failure as a deviation from the FMC computed path when using LNAV guidance. This failure will also cause a flight director disagreement in LNAV mode. The flight director associated with the failed radio altimeter will be limited to 8 degrees of bank. The flight director associated with the operating radio altimeter will not be bank angle limited unless the airplane is below 400 feet AGL. If these effects occur, the flight crew must utilize other appropriate methods of flight path control at their disposal. These methods include but are not limited to: use of manual flight modes to keep the airplane on the FMC path or use of VOR guidance to track appropriate radials if the LNAV track overlays VOR airway structure. Additionally, the autopilot and flight director LNAV steering commands will be normal from the FCC associated with a functioning radio altimeter.
Administrative Information
This bulletin replaces bulletin SHZ-11, dated December 01, 1998. Discard bulletin SHZ-11. Revise the Bulletin Record to show SHZ-11 as "CANCELLED" (CANC).
Insert this bulletin behind the FCOM Bulletin Record Page in Volume 1 of your FCOM. Amend the FCOM Bulletin Record Page to show Bulletin SHZ-11 R1 "IN EFFECT" (IE)
This anomaly is corrected by Boeing Service Bulletin 737-SL-22-044. This FCOM Bulletin will be canceled after Boeing is notified that all affected airplanes in the operator’s fleet have been modified.
Please send all correspondence regarding FCOM bulletin status to one of the
following addresses:
Mailing Address: Manager 737, Flight Technical Integration & Data Boeing Commercial Airplane Group P. O. Box 3707 MS 20-89 Seattle, WA 98124-2207 USA
Fax Number: Telex: E-MAIL: (206) 662-7812 329430 Station 627 FTID@Boeing.com
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