Troubleshooting Airframe Vibrations

Today, the Airbus fleet benefits from many years of accumulated in-service experience, and is relatively free from reports of airframe vibration during flight. However, airframe vibrations are still sometimes reported.

When an airframe vibration occurs, it can be identified by people inside the aircraft. Depending on the source of the vibration, it may be experienced either as a physical movement, or as noise, or as both a movement and noise. These experiences can cause passenger concern and discomfort. Additionally, any vibration indicates increased wear of components. For both these reasons, identification of the cause of the vibration should be established quickly.

Due to the size of the fleet, the majority of reports of airframe vibrations on Airbus aircraft are received on A320 Family models.

To identify the causes of vibration, Airbus organised a four year working group with airlines and equipment manufacturers, which focussed on the A320 Family fleet. This work identified that the majority of vibrations arise in the aircraft tail section, including 57% of vibrations from the rudder, and 15% from the elevator. Moveable control surfaces in the wings together account for only 11%, whereas sources in belly fairings, passenger and landing gear doors account for 17%.

(fig.1)
Main sources of vibration on A320 Family aircraft, identified by the Airframe Vibration working group

The main contributor to vibrations, particularly on flight control surfaces, is free-play of servo-control bearings, servo-control attachments, and and/or surface hinge lines (bearings & attachment). Free-play is primarily caused by wear.

When free-play is present, the flight control surface or door will have a tendency to oscillate slightly within the space created by the free-play whenever the surface is at zero hinge-moment. When in this condition, an observable vibration will only start if an energy input is provided, typically from aerodynamic effects of a sufficiently high air speed. This phenomenon is called a ‘Limited Cycle Oscillation’ (LCO).

LCO are characterised by a stable and non-divergent vibration of constant amplitude and frequency, after initiation by the triggering input. LCO do not create any handling or performance concern, since surfaces and systems remain fully efficient during the vibration. It is a stable self-sustained non-diverging phenomenon.

An LCO vibration cannot diverge into flutter because whenever the LCO amplitude increases, the damping term involved in LCO mechanics also increases and leads automatically to a decrease of amplitude. The extra damping comes from the increased stiffness caused by the increased amplitude on the involved free-play area; the force of components pushing against each other.

(fig.2)
Waveform of a Limited Cycle Oscillation (LCO).
An LCO vibration cannot diverge into a flutter phenomenon because any increase in vibration amplitude is damped by the force of components pushing against each other.

Upon experiencing an airframe vibration, quick action is recommended in order to identify and resolve the cause of the vibration. It is therefore important that flight crew report the vibration to their maintenance personnel.

Maintenance personnel are provided with appropriate procedures in the Trouble-Shooting Manuals (TSM) for resolving the issue. However, since airframe vibrations only occur during flight, maintenance personnel will need pilots to make observations of the vibration.

To collect pilot observations, a ‘Vibration Reporting Sheet’ (VRS) is provided within the TSM procedure ‘Identification of the cause of In-Flight Airframe Vibrations and/or Noises’. A well completed VRS will provide sufficient information to maintenance crew to help them complete a Decision Tree and Decision Table, so that they can identify the specific part of the aircraft which is vibrating.

As can be seen in (fig.3), the VRS is split into four sections as follows:

1. Flight conditions when the vibrations and/or noise occur
2. Observations when the vibrations and/or noise occur
3. Parameter changes with AP ON that have an effect on vibration
4. Parameter changes with AP OFF that have an effect on vibration

Section 1 of the VRS collects basic flight information. Sections 2 to 3 include further data collection fields which do not require pilots to make any specific control inputs.

(fig.3)
The Vibration Reporting Sheet (VRS)

The information needed in section 2 can be found when the vibration occurs, by observation of the aircraft, its instruments, and the vibration. Corroboration of flight crew with cabin crew observations of where the vibration is the strongest is recommended for a higher reliability of the reported information.

Vibrations can be caused when a control surface is in the zero hinge-moment position. Therefore, the principle for the information collected in sections 3 and 4 is to monitor the vibration when a control input is made and a control surface is moved out of the zero hinge-moment position. If a control input is made and the vibration changes, this gives a useful indication of the surface involved in the vibration.

However, there is an important difference between the pilot actions necessary for section 3 and the actions necessary for section 4.

The information needed in section 3 of the VRS can be collected by observing the aircraft with the autopilot ON, whereas the information in section 4 of the VRS can only be collected with the autopilot OFF. The goal is to observe any change in the vibration, including whether it becomes weaker or stops, or becomes stronger.

In section 3, observations are made whenever the autopilot itself commands a change in thrust setting, turn, climb or descent. The only manual action listed in this section of the VRS is selection of the speed brakes by a few degrees. A change of the vibration due to speed brake extension can indicate that the vibration originates in the elevator.

Either elevator or rudder would be implicated as the source of vibration if a change in the vibration results from a change in altitude setting or in thrust setting. Ailerons would be the principle structural element impacted if the vibration is changed during a turn.

Section 4 of the VRS is only intended to be used if sections 2 and 3 do not succeed in helping identify the source of the vibration. Observations of the vibration are made when the pilot flying directly makes small and smooth flight control inputs, using the side-stick for pitch and roll inputs, or the rudder trim for yaw inputs.

A change in the vibration due to a pitch input indicates that the elevator is the most likely source of vibration. A change in the vibration due to a yaw input indicates the rudder is the most likely source of vibration. And finally, a change in the vibration due to a roll input primarily indicates that the vibration comes from the ailerons

Today, the vast majority of commercial aviation operations takes place within Reduced Vertical Separation Minima (RVSM) airspace. A regulated requirement of conducting operations in RVSM airspace is to maintain an Auto Pilot (AP) engaged in order to ensure that the aircraft does not deviate from its assigned altitude.

Sections 1 to 3 of the VRS can all be completed with the AP ON. However, section 4 can only be completed with the AP OFF, and therefore cannot be performed in RVSM airspace. This condition means that completing section 4 of the VRS may not always be appropriate in all airline operations.

Although flying with AP OFF is a normal task for pilots, some operators prefer to have only technical pilots complete section 4 of the VRS because it may involve non-routine manoeuvres. Some operators prefer to conduct VRS evaluations on a non-revenue flight.

In whichever way an operator chooses to complete section 4 of the VRS, the associated instructions in the TSM clarify the appropriate technique for implementing the procedure. This includes the following points:

Appropriate technique for applying section 4

• When permitted by flight conditions and airline policy, and when not in RVSM airspace, the flight crew can disconnect the Auto Pilot to try to identify the source of the vibrations
• All inputs must be smooth and follow the Flight Director (FD) bar guidance
• Usually only very small inputs are sufficient to stop the vibration
• Large control inputs are neither required nor recommended for the purpose of VRS evaluation, especially when flying with passengers on-board
• Apply the procedure in the sequence pitch, roll and then yaw
• If vibrations do not stop, apply small rudder trim inputs of +/- 1.5° MAX (yaw)
• Do not use rudder pedals
• When the reporting is completed, AP should be set back on again as required