Message from Boeing to all B737 operators
Reference /A/ provides Boeing's previous fleet communication on the subject
event. The US NTSB, FAA, Boeing, the Turkish DGCA, the operator, the UK
AAIB, and the French BEA continue to actively support the Dutch Safety
Board's (DSB) investigation of this accident.
The DSB has released a statement on the progress of the investigation and
has approved the release of the following information.
While the complex investigation is just beginning, certain facts have
emerged from work completed thus far:
- To date, no evidence has been found of bird strike, engine or
airframe icing, wake turbulence or windshear.
- There was adequate fuel on board the airplane during the entire
flight.
- Both engines responded normally to throttle inputs during the entire
flight.
- The airplane responded normally to flight control inputs throughout
the flight.
The Digital Flight Data Recorder (DFDR) data indicates that the crew was
using autopilot B and the autothrottle for an ILS (Instrument Landing
System) approach to runway 18R at Amsterdam Schiphol airport. During the
approach, the right Low Range Radio Altimeter (LRRA) was providing accurate
data and the left LRRA was providing an erroneous reading of -7 to -8 feet.
When descending through approximately 2000 feet the autothrottle, which uses
the left radio altimeter data, transitioned to landing flare mode and
retarded the throttles to the idle stop. The throttles remained at the idle
stop for approximately 100 seconds during which time the airspeed decreased to approximately 40 knots below the selected approach speed.
The two LRRA systems provide height above ground readings to several
aircraft systems including the instrument displays, autothrottle, autopilots
and configuration/ground proximity warning. If one LRRA provides erroneous
altitude readings, typical flight deck effects, which require flight crew
intervention whether or not accompanied by an LRRA fault flag, include:
- Large differences between displayed radio altitudes, including radio
altitude readings of -8 feet in flight.
- Inability to engage both autopilots in dual channel APP (Approach)
mode
- Unexpected removal of the Flight Director Command Bars during
approach
- Unexpected Configuration Warnings during approach, go-around and
initial climb after takeoff
- Premature FMA (Flight Mode Annunciation) indicating autothrottle
RETARD mode during approach phase with the airplane above 27 feet AGL. There
will also be corresponding throttle movement towards the idle stop.
Additionally, the FMA will continue to indicate RETARD after the throttles
have reached the idle stop
Boeing Recommended Action
- Boeing recommends operators inform flight crews of the above
investigation details and the DSB interim report when it is released. In
addition, crews should be reminded to carefully monitor primary flight
instruments (airspeed, attitude etc.) and the FMA for autoflight modes.
More information can be found in the Boeing 737 Flight Crew Training Manual
and Flight Crew Operations Manual.
Operators who experience any of the flight deck effects described above
should consult the troubleshooting instructions contained in the 737
Airplane Maintenance Manual. Further, 737-NG operators may wish to review
737NG-FTD-34-09001 which provides information specific for the 737-NG
installation. Initial investigations suggest that a similar sequence of
events and flight deck indications are theoretically possible on the
737-100/-200/-300/-400/-500. Consequently the above recommendations also
apply to earlier 737 models.
Operators will be notified if further action is recommended .