Elevator vibrations-servo spherical bearing
by Sonia Bouchardie, Engineer Flight Control Systems, Customer Services, Airbus Industrie
Folloowing 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 accounts 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
Two solutions were developed to eliminate these two causes: reduce backlash and
increase hinge moment.
CONCLUSION
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 feedback from the Operators. Therefore as a preventive measure, the incorporation of the Service Bulletins are
highly recommended by Airbus Industrie.
(
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 31075-
27-17 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 orthe new tool.
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 passenger and crew comfort by removing the causes of vibration,
reduce maintenance costs."
Last edited by Jimmy Hoffa Rocks; 18th Aug 2009 at 01:10.