Elevator 6s1f1m

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Added to potential aerodynamic excitation, two concomitant conditions causing the LCO were discovered: servo control bearing backlash and low actuator load.

SOLUTIONS

AVOIDING ELEVATOR VIBRATION

Two solutions were developed to eliminate these two causes: reduce backlash and increase hinge moment.

A319, A320, A321

TO REDUCE BACKLASH Several cases of excessive play within the spherical bearings of the elevator servo control, due to premature wear of the Teflon liners, were discovered during inspections following reports of in flight airframe vibrations. This condition has now been eliminated thanks to higher performance NMB bearings, introduced on the elevator servo-controls through the LUCAS Service Bulletin 3107527-17 and Airbus Service Bulletin A320-27-1111. This modification incorporates an additive in the existing liner, and chromium and super finishing of the inner ball to reduce the wear rate and friction coefficient. Also the maximum acceptable value for backlash, measured at the elevator trailing edge has been reduced from 10mm to 7mm, as described in the AMM. TO INCREASE HINGE MOMENT The Airbus Service Bulletin A320-27-1114 describes the resetting of the elevator neutral position to 0.5 degree (aircraft) nose up. Accomplishment of this modification ensures that the elevators are aerodynamically loaded in an appropriate manner in order to eliminate vibration, even during flight in turbulent conditions. Those changes have no effect on aircraft performance and there is no change in the handling characteristics of the aircraft, nor is there any penalty in fuel consumption. This modification has been developed to fit easily into the maintenance program. To perform the revised elevator rigging, a new elevator rigging tool, developed by Airbus Industrie, enables the new neutral position to be determined. It is highly recommended that this new tool be used, as it allows more accurate rigging through a simplified procedure. Nevertheless, the elevators can also be set using the previous tool which was developed originally to set the elevators to a 0 degree position. Therefore the Aircraft Maintenance Manual (AMM) procedure now describes how to set the elevators to the 0.5 degree using the original tool or the new tool.

Elevator rigging tool (developed by Airbus Industrie)

by Sonia Bouchardie, Engineer Flight Control Systems, Customer Services, Airbus Industrie

CONCLUSION

F

ollowing reports of in-flight vibrations on the A320 Family, an intensive flight test campaign was launched by Airbus Industrie to determine the different sources of elevator vibrations. They are described in the Trouble Shooting Manual (TSM) Chapter 05-50-00, and each possible cause is associated with corresponding trouble shooting procedures. The TSM also provides a recording sheet to help operators establish the cause of vibration. The main source is the elevator system, which s for more than 70% of all vibrations. Further to the flight test campaign, it was revealed that the phenomenon was in fact a Limit Cycle Oscillation (LCO) which is a sustained vibration at a fixed frequency with limited amplitude and having no impact on flight safety. This article describes how to avoid elevator vibration through the incorporation of a modification on the spherical bearing of the elevator servo control and a new elevator setting.

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ADVANTAGES As a preventive measure, these modifications will: ● improve the fleet reliability due to the new elevator servo spherical bearings and revised elevator rigging, ● improve enger and crew comfort by removing the causes of vibration, ● reduce maintenance costs.

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The extensive work performed by the Airframe Vibration Task Force led to conclusions for eliminating airframe vibration which have since been proven in service. The effectiveness of these modifications has been clearly demonstrated through the positive from the Operators. Therefore as a preventive measure, the incorporation of the Service Bulletins are highly recommended by Airbus Industrie. ✈ REFERENCES • TSM Task 05-50-00, “In-flight airframe vibration” • AMM Task 27-34-00-200-001 “Check of the elevator servo controls and hinge bearings for too much play, and condition” • Video Tape “A320 Family elevator rigging” The Part Numbers are: New Elevator Rigging Tool, 98D27309006000 / Previous Elevator Rigging Tool, 98D27309002000 To order the new Elevator Rigging Tool, please AIRBUS INDUSTRIE, Materiel Center Tel: +49 (40) 50 76 0 - Fax: +49 (40) 50 31 68 For further information or to receive a copy of the video tape please : Airbus Industrie Customer Services AI/SE-E52 - Flight Control Systems - Sonia Bouchardie 1, rond-point Maurice Bellonte - 31707 BLAGNAC Cedex Tel: +33 (0) 5 61 93 22 33 Fax: +33 (0) 5 61 93 44 25

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