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set the THS trim at -2.2°UP rather than the correct setting of 2.0°DN.
Although the investigation into Northwest Airlines flight 985 accident is ongoing,3 the
Safety Board identified a safety issue regarding the procedures used by some airlines for setting
the THS trim on the A320. The Board also identified a safety issue regarding inconsistent
formats for presenting trim setting data to flight crews.
Airbus Industrie equips its A319, A320, and A321 airplanes with two index scales
located adjacent to the THS trim wheel. (See figure 1.) One scale indicates the CG as a
percentage of mean aerodynamic chord. In the accident airplane, the CG scale showed the values
10.5, 17, 20, 25, 30, 35, and 41.4 The CG scale is not graduated and does not show intermediate
values. The other scale indicates the number of degrees of THS deflection above or below zero
(neutral), followed by “UP” or “DN” to indicate the corresponding pitch direction of the airplane.
These values are 4DN, 3DN, 2DN, 1DN, 0, 1UP, 2UP, 3UP … 13UP, 13.5UP. Like the CG
scale, the degree scale is not graduated and does not show intermediate values. Both the CG
scale and the degree scale are fixed in relation to each other and move together when the trim is
set.
2 Postaccident simulator flight tests have shown that, even with the improper trim setting, the airplane would have
been controllable if the takeoff had continued. According to Airbus Industrie, the airplane is controllable on takeoff
as long as the airplane’s CG is within the limits of the green band, and the trim, regardless of whether it is set
incorrectly, is also within the green band. (The “green band” is the range of CG and trim positions approved for
takeoff.)
3 The description for this accident, CHI01FA104, can be found on the Safety Board’s Web site at
<http://www.ntsb.gov>.
4 Values shown on the CG scale differ depending on the type of engines installed on the airplane.
3
CG Scale
Green Band
Degree Scale
Index
(set at 1.7 UP)
Trim Wheel
Figure 1. Airbus THS Trim Wheel Indicator Scales.
The trim setting (in degrees) is also shown on the flight control page of the electronic
centralized aircraft monitoring (ECAM) display. This display, which correlates to the degree
scale on the trim wheel, shows the trim in tenths of degrees followed by “UP” or “DN” to show
the corresponding pitch direction of the airplane. On the ECAM display, pitch-up trim values are
preceded by a minus sign (“-”), but pitch-down trim values are not preceded by a plus sign (“+”).
The ECAM display for this accident would have shown the THS trim as “-1.7°UP.” If the trim
had been set correctly, it would have shown as “1.7°DN.”
The A320 flight manual issued by Airbus Industrie specifies use of the CG scale for
setting the THS trim. However, Northwest Airlines’ Flight Operations Manual at the time of the
accident called for the first officer to set the THS trim in degrees by turning the trim wheel while
looking at the ECAM display.5 The proper trim setting was provided to the crew on the load data
sheet from the aircraft communication addressing and reporting system (ACARS) but was not
followed by “UP” or “DN.” In this case, the trim setting was given as “1.7.” According to the
manual, the captain was to cross-check the trim setting during the taxi checklist by looking at the
“pitch trim wheel index.” Northwest Airlines’ pilots indicated that this cross-check was
5 Because it displays the trim setting in tenths of degrees, the ECAM display provides a more precise way to
cross-check the trim setting than the degree scale does.
4
supposed to be accomplished by the captain looking at the ECAM display (in degrees) and
calling out the setting shown.6 At the time of the Lufthansa event, Lufthansa’s procedure for
setting the trim was similar to Northwest’s procedure.
After the accident, Northwest Airlines changed its procedures. Although the first officer
continues to set the trim using degrees, the revised procedure requires the captain to cross-check
the trim setting as indicated on the CG scale against the CG information contained in the load
data sheet provided by ACARS.
Safety Board staff contacted three other major U.S. carriers that operate Airbus Industrie
A320s regarding their procedures for setting the THS trim. One carrier reported using the CG
scale to set the trim. The other two carriers reported using the degree scale without requiring the
crew to cross-check the setting on the CG scale against the airplane’s calculated CG.
　
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