FROM: THE BOEING COMPANY
TO: Boeing Correspondence (MOM)
[MESSAGE NUMBER:MOM-MOM-12-0240-01B] Multi Operator Message
MESSAGE DATE: 12 Apr 2012 1659 US PACIFIC TIME / 12 Apr 2012 2359 GMT
Enhanced Controls Required? No
Summary Notes:
This message is sent to all 747, 757, 767, 777 and MD-11 customers and
respective Boeing Field Service Bases, Regional Directors and Airline Resident
Representatives.
CATEGORY: Flight Operations
SERVICE REQUEST ID: 1-2175453884
ACCOUNT: Boeing Correspondence (MOM)
DUE DATE: No Action Required
PRODUCT TYPE: Flight Operations
PRODUCT LINE: SEVERAL
PRODUCT: 21 - Navigation/FMC/RNP
ATA:
SUBJECT: Information - Magnetic Variation Disagreement at Anchorage
REFERENCES:
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SUMMARY:
The FAA has issued a NOTAM prohibiting ILS approaches to Special Authorization
(SA) Category I or Category II/III weather minima at Ted Stevens International
Airport in Anchorage on all 747, 757, 767, 777 and MD-11 series airplanes
because of potential autopilot flight director system (AFDS) behavior resulting
from a difference between the published magnetic variation for PANC reflected in
the latest FMC Navigation Database (NDB) and the magnetic variation from the
airplanes' onboard inertial reference systems.
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DESCRIPTION:
The FAA issued the following NOTAM for Anchorage (PANC) Airport on 12 April
2012:
QUOTE:
FDC 2/3535 ANC ..SPECIAL.. TED STEVENS INTERNATIONAL AIRPORT, ANCHORAGE, AK. DUE
TO A DIFFERENCE BETWEEN THE ONBOARD MAGNETIC VARIATION (MAG VAR) DATA TABLES AND
THE MAG VAR CURRENTLY PUBLISHED FOR THE AIRPORT, THE FOLLOWING SERIES AIRCRAFT
ARE PROHIBITED FROM CONDUCTING ILS APPROACHES TO SA CAT I OR CAT II/III WEATHER
MINIMA AT ANCHORAGE INTERNATIONAL AIRPORT (PANC); B-747, B-757, B-767, B-777,
AND MD-11. THESE SERIES AIRCRAFT HAVE EXPERIENCED A SHIFT OF RUNWAY CENTERLINE
PRESENTATION AND DEGRADED LOCALIZER TRACKING PERFORMANCE ON ILS APPROACHES. THIS
ACTION IS BEING TAKEN TO MINIMIZE THE HAZARDS ASSOCIATED WITH DEGRADED LOCALIZER
TRACKING PERFORMANCE OR COURSE PRESENTATION/DEVIATION DISCREPANCIES DURING
APPROACH. PILOTS OF ANY AIRCRAFT EXPERIENCING ILS ANOMALIES ARE REQUESTED TO
REPORT THEM TO PANC TOWER. FOR ADDITIONAL INFORMATION CONTACT THE NORTHWEST
MOUNTAIN REGIONAL OPERATIONS CENTER AT 425-227-2000 OR
[email protected]
UNQUOTE
The NOTAM has been issued because of reports of Autopilot Flight Director System
(AFDS) behavior during instrument approaches at Anchorage. These reports
include a shift of runway centerline presentation and degraded localizer
tracking performance.
Boeing and the FAA have determined that the reported AFDS behavior at Anchorage
results from a difference between the published magnetic variation (MagVar) for
PANC reflected in the latest FMC Navigation Database (NDB) and the MagVar value
for PANC from the MagVar tables in the airplanes' onboard inertial reference
systems (ADIRU/IRU).
In November 2011, the published magnetic variation at PANC was changed to E19
degrees from its 2005 value of E21 degrees. At this time, the FMC NDBs were
updated to the new value of E19 degrees. No approach issues were reported after
the November 2011 change. In February 2012, the published magnetic variation at
PANC was changed again to E17 degrees and the FMC NDBs were updated accordingly.
Reports of anomalies during instrument approaches began following the NDB change
to E17 degrees.
The ADIRU/IRU MagVar tables on most of the affected airplanes are based on a
2005 or earlier model. From the 2005 MagVar tables, the magnetic variation for
PANC is approximately E21 degrees, a difference of 4 degrees from the current
FMC NDB value for PANC. The AFDS on most of the affected airplanes controls to
a localizer deviation that is filtered with inertial data translated
geometrically through the use of an adjusted final approach course. The final
approach course can be adjusted for minor localizer beam misalignment, normal
biases in inertial data, and differences in magnetic variation. The 4 degree
difference between the magnetic variation in the ADIRU/IRU and the FMC NDB
reduces the available margin to adjust for minor localizer beam misalignment and
normal biases in inertial data. If these deviations combined with the known 4
degree difference exceed the system tolerance, the AFDS may exhibit degraded
localizer tracking performance during instrument approache!
s.
All 737 series airplanes are not affected because of differences in the
Autopilot Flight Director System design.
All 787 airplanes have updated MagVar tables in the onboard earth reference
system (ERS) and are not subject to this condition.
Boeing is working in concert with the FAA on potential solutions to restore the
capability of the affected airplanes to conduct ILS approaches to SA Category I
and Category II/III weather minima at PANC as soon as possible, including a roll
back of the NDB magnetic variation at PANC to E19 degrees to provide additional
margin for the AFDS and the possibility of an early update of the ADIRU/IRU
MagVar tables to the 2015 model.
If you need further information regarding the subject or if copies of
attachments (when referenced) are required, please access them by logging into
MyBoeingFleet Service Requests. If you have further questions, you may contact
the appropriate Airline Support Manager or call the BCA Operations Center at
(206) 544-7500.