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CB - FLT CONTROLS SYSTEM 4/4
P I LOT T R A I N I N G GU I D E
FLIGHT CONTROLS
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Sept 04
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BOEING COMMERCIAL AIRPLANE GROUP
FLIGHT OPERATIONS TECHNICAL BULLETIN
NUMBER: 737-09-2
DATE: March 19, 2009
These bulletins provide information which may prove useful in airline operations or airline training. This
information will remain in effect depending on production changes, customer-originated modifications, and Service
Bulletin incorporation. Information in these bulletins is supplied by the Boeing Company and may not be approved
or endorsed by the FAA at the time of writing. Applicable documentation will be revised as necessary to reflect the
information contained in these bulletins. For further information, contact Boeing Commercial Airplane Group,
Chief Pilot, Flight Technical, P.O. Box 3707, Mail Stop 14-HA, Seattle, WA, USA 98124-2207, Phone (206) 544-
9610, Fax (206) 544-9687, SITA: SEABO7X Station 627.
SUBJECT: Flight Crew Monitoring During Automatic Flight
ATA NO:
APPLIES TO: 737 All
Background Information
An erroneous Low Range Radio Altimeter (LRRA) has been identified in connection with
a recent 737-800 accident.
The Digital Flight Data Recorder (DFDR) data indicates that the crew was using both the
autopilot and the autothrottle during an ILS approach. The right LRRA was providing
accurate data to the first officer’s display and the left LRRA was erroneously providing a
reading of -8 feet to the captain’s display. No amber RA flag was displayed because the
left LRRA system did not declare the data invalid.
On some 737s, the autothrottle logic uses left radio altimeter data regardless of the
autopilot selected. In this event, when the airplane descended through approximately
1950 feet on the approach with flaps extended beyond 12 ½ degrees, the autothrottle
erroneously sensed that the airplane was in landing flare. The thrust levers were retarded
to the idle stop where they remained for approximately 100 seconds. For the first 70 of
the 100 seconds, idle thrust was sufficient to maintain the selected airspeed. During the
next 30 seconds, airspeed decreased below the selected MCP speed to approximately 40
knots below the selected approach speed.
The two LRRA systems provide height above ground data to aircraft systems which
include the displays, autothrottle, autopilots and configuration/ground proximity warning.
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If one LRRA provides erroneous altitude readings, the associated flight deck effects may
typically include:
• Large differences between displayed radio altitude.
• Inability to engage both autopilots in dual channel approach (APP) mode.
• Unexpected removal of the Flight Director Command Bars during approach on
the pilot’s side with the erroneous radio altimeter display.
• Unexpected Configuration Warnings after takeoff, during approach, or during goaround.
• Inappropriate Flight Mode Annunciation (FMA) indication of autothrottle
RETARD mode during approach phase with the airplane above 27 feet AGL.
There will also be corresponding thrust lever movement towards the idle stop.
The FMA will continue to indicate RETARD after the thrust levers have reached
the idle stop rather than change to ARM.
Boeing Recommendations
Whether in automated or manual flight, flight crews must carefully monitor primary
flight instruments (airspeed, attitude etc.) for aircraft performance and the FMA for
autoflight modes.
The following information is taken from the Flight Crew Training Manual (FCTM) and
has been adapted to provide Flight Crews and Operators with guidelines which should be
followed if a flight crew encounters any of the above mentioned indications.
General Guidelines
Condition:
• Large differences between displayed data.
Crew Resource Management (CRM) involves the effective use of all available resources
to operate a flight safely. It is important that all flight deck crewmembers identify and
communicate any situation that appears potentially unsafe or out of the ordinary.
Experience has proven that the most effective way to maintain safety of flight and resolve
these situations is to combine the skills and experience of all crewmembers in the
decision making process to determine the safest course of action.
Situational awareness, or the ability to accurately perceive what is going on in the flight
deck, requires ongoing questioning, crosschecking, communication, and refinement of
perception.
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