Occasional airplane vibration during flight is not unusual. There are many causes of airplane vibration, including landing gear extension and retraction, extension of speed brakes, free play in movable surfaces, and systems malfunctions. Flight crews need to understand the causes and effects of airplane vibration so that they can take appropriate action to maintain flight safety and prevent excessive wear or airframe damage. In addition, flight crews can contribute to accurate reports of in-service events that will facilitate troubleshooting and maintenance activities.

Understanding the possible sources of vibration and the information needed to isolate and correct vibration problems requires knowledge of the following:

1. Types of vibration and noise.
2. Causes of airplane vibration.
3. Detection of airplane vibration.
4. Flight crew response.
5. Maintenance troubleshooting.

1. TYPES OF VIBRATION AND NOISE

The following engineering definitions differentiate various forms and types of vibration and noise:

Vibration is oscillating, reciprocating, or any other periodic motion of a rigid or elastic body forced from a position or state of equilibrium. If the frequency and magnitude of vibration are constant, the vibration is said to be harmonic. When the frequency and magnitude vary with time, the vibration is random.

Buffet is a form of vibration usually caused by aerodynamic excitation. It usually is random and associated with separated airflow. For example, buffet may be felt during the extension of speed brakes or during air turbulence.

Flutter is an unstable condition in which unsteady aerodynamics excite the natural frequencies of the structure over which the air flows. The resulting vibrations can grow to a magnitude that causes the structure to fail.

Noise is a vibration that excites the air and can be heard. When the vibration is random, the noise is unmusical or confused. When the vibration is harmonic, the result is a tone like that produced by a musical instrument. It may sound like the whistling of a drain or a slight leak in a door.

2. CAUSES OF AIRPLANE VIBRATION

Normal and abnormal vibrations occur for several reasons. Aerodynamics, mechanical malfunctions, and external factors such as atmospheric turbulence can cause airplane vibration. All vibrations have associated frequencies and magnitudes that may be readily detected or barely perceptible to the flight crew and passengers. For some vibrations, such as those associated with engine operation, the flight crew has dedicated instrumentation to measure magnitude. Other vibrations are detected by sight, sound, or feel and may depend on flight crew experience for analysis.

Normal vibration.
Each airplane has a unique signature of normal vibration. This is a consequence of mass distribution and structural stiffness that result in vibration modes at certain frequencies. When external forces act on the airplane, such as normal airflow over the surfaces, very-low-level vibrations result. Typically, this is perceived as background noise. More noticeable, but also normal, is the reaction of the airplane to turbulent air, in which the magnitude of the vibration may be larger and thus clearly visible and felt. Engine operation at some spool speeds may result in increased vibration because spool imbalance excites the engine and transmits this vibration throughout the airframe. Finally, the operation of some mechanical components, such as pumps, may be associated with normal noise and vibration.

Most flight crews recognize these normal events, which become the experience base from which flight crews detect abnormal vibration events.

Abnormal vibration.
The most easily identified abnormal vibration is that which has a sudden onset and may be accompanied by noise. The vibration may be intermittent or steady with a distinct frequency, or it may be a more random buffet type. When the onset of abnormal vibration can be associated with a previous action or event, the source may be obvious. However, some vibrations initially are rather subtle and require diagnostic procedures to determine their probable causes.

Abnormal vibration usually is related to one or more of the following causes: engine rotor imbalance, malfunction of mechanical equipment, and airflow disturbances acting over doors or control surfaces that are misrigged or misfaired or that have excessive wear or free play. Abnormal vibration rarely is caused by a structural failure or an unstable power control system.

Flutter.
Aeroelastic instability, or flutter, very rarely causes abnormal vibration. Through design, extensive analysis, and certification tests, all configurations of commercial jet airplanes are free from flutter for all design conditions within the aeroelastic stability envelope. This envelope extends well beyond normal permissible operating speeds and applies to normal operation as well as failures, malfunctions, and adverse conditions. However, when an airplane is operated in a configuration or condition that is beyond these criteria, flutter may result within the operational envelope. Flutter can be differentiated from buffet in that flutter can occur in smooth air; the vibration originates from the airplane rather than from the atmosphere. Closely associated with flutter is limit cycle oscillation (LCO). During LCO, the vibration is self-excited, but nonlinear effects such as friction, clearances, and free play (or backlash) limit the amplitude. LCO most often is caused by excessive free play within the flight control surfaces and associated components.

3. DETECTION OF AIRPLANE VIBRATION

Detection of airplane vibration depends almost entirely on crew sensitivity. The only exception is vibration in the engines, which are equipped with dedicated accelerometers to measure spool vibration. All other airplane vibration is detected by the crew through sight, sound, and feel. It should be noted that flight crews may not sense vibrations in some areas of the airplane, such as the main cabin or tail section, although passengers or other members of the crew usually feel and report such vibrations.

Flight crews use various terms to describe their perception of an abnormal vibration in the flight deck environment. It is very difficult for a flight crew to distinguish among the engineering definitions for vibration, buffet, flutter, and noise. For example, crews often report vibrations as noise because they are carried by the fuselage structure to the flight deck where the crews can hear them. Vibration and buffet both can shake the whole airplane, so it can be difficult for crews to distinguish between them.

4. FLIGHT CREW RESPONSE

The response of flight crews to vibration is fundamentally an exercise in airmanship. Every vibration event is different, and flight crews retain the responsibility to deal with such problems, as they deem necessary. Some general guidelines follow.

The presence of an abnormal vibration or noise usually is cause for flight crew attention. Experience provides flight crews with the ability to judge the severity of the vibration, distinct signatures, and most important, the immediate history of the flight conditions (i.e., the conditions of flight before the event and the parameter changes that occurred as the event occurred).

When unexplained vibrations occur, usually something has changed that is obvious to the crew (e.g., an abnormal engine indication, a change in flap or spoiler position, or a change in airspeed). This differs from the normal vibration felt as flaps come out or speed brakes move up — it is a change in the expected level of vibration.

However, it is possible that something in the airplane can change without an obvious change in vibration; these situations are the most difficult to discern, describe, and avoid.

• What parameter just changed — airspeed, flight control, thrust setting?
  
• Is the vibration getting worse or is it constant?
  
• Will the current flight condition allow for a gradual change of parameters?
  
• What parameter can the flight crew feel or see that provides an idea of the cause?
  
• If the flight crew makes a small, planned change in a parameter, does the vibration or noise, or both, get worse or better?

After these data points are gathered and analyzed, the flight crew can formulate a plan of action to ensure the continued safety of the flight. After completion of the flight, the crew should fill out a vibration report (table 1).

If performance considerations do not override the severity of the vibration, the flight crew should reduce airspeed and engine speed. Essentially, the crew should return to level flight at reduced airspeeds and avoid unnecessary stress on the airplane.

In-flight observations to assist in maintenance troubleshooting.
Identifying and correcting the cause of in-flight airplane vibration often is accomplished through trial and error, which can consume many maintenance hours. The causes of airplane vibration are numerous; however, flight crew observations and detailed reporting can provide very important clues to the potential source of the vibration.

The other type of vibration is of a lower frequency (typically less than 20 Hz) that can be felt by the entire body. This type of vibration usually relates to a large-mass component acting on the airframe, such as the rudder, horizontal stabilizer, or elevator.

Typical crew reports often identify the direction of in-flight vibration as lateral, vertical, or oscillatory. Crews sometimes may indicate where the vibration is most evident within the airframe, such as in the forward cabin, overwing area, or aft cabin. Additionally, crews sometimes estimate the frequency of vibration and supply information regarding the airplane configuration at the time of vibration onset.

Event information has proved to be so valuable for maintenance troubleshooting that a flight deck vibration event log has been attached to Boeing service letters for the 737 and 757 airplanes (table 1). The symptoms recorded on this form can be correlated with specific inspections and tests for applicable system and structure components. The tests and inspections are described in Boeing service letters and aircraft maintenance manuals. Table 1 shows a form that can be adapted to any Boeing airplane model for use in documenting vibration events and assisting engineering and maintenance organizations in resolving vibration problems.

For example, a low-frequency vibration in the vertical direction that is felt in both the forward and aft cabin may be the result of excessive free play in the elevator or stabilizer surfaces. Pitch-control flight surfaces with excessive free play can cause the body to vibrate vertically, with the motion felt most strongly in the forward and aft locations of the airplane. Slight mistrimming of the airplane using the stabilizer and elevator may dampen out this type of vibration because the free play of the surface is removed with aerodynamic loading.

This same low-frequency vibration in the lateral direction may be the result of excessive free play of the rudder. A slight input made to the rudder system may be enough to remove the free play with aerodynamic loading. A low-frequency lateral vibration felt over the wing and in the flight deck at high engine power settings might be engine related. Reducing or increasing the power setting on each engine individually may isolate the vibration to an
individual engine.

A high-frequency vibration with associated sound that can be detected only in the flight deck when the flaps are up could be caused by unrestrained, or unlatched, landing gear doors or access panels located below the flight deck. High-frequency vibrations associated with sound over the wing or in the passenger cabin with the flaps up could be caused by the ailerons, main landing gear doors, trailing-edge flap fairings, or other wing components identified for each airplane model in its respective maintenance manual sections or service letters.

Table 2 lists other typical airplane vibration incidents. These are only a few examples, and operators are encouraged to review applicable airplane vibration service letters and maintenance manual sections to determine how other in-flight observations relate to potential sources of vibration.

5. MAINTENANCE TROUBLESHOOTING

Following a flight crew's report of a vibration incident, a comparison with previously reported incidents on the specific airplane or fleet may reveal the cause of the problem immediately. An important first step is to review service letters containing a compendium of previously reported vibrations. When this does not suggest an obvious problem, the next action should be a careful inspection of the entire airplane.

Control surfaces, spoilers, and flaps should be checked for free play and correct rigging. Doors and access panels must not be loose or out of contour. Landing gear doors must be checked to ensure that they are properly secured and faired with the gear retracted.

To continue the check of the control systems, the control surfaces should first be examined in neutral and displaced positions. Then, the control systems should be energized and the trailing edge of the associated control surfaces checked for rattles and possible bearing damage. If sustained oscillation can be detected, the system should be checked for linkage wear and replacement of the power control unit.

When ground checks fail to isolate the cause of vibration, the investigation should be extended to a nonrevenue flight check. If the cause cannot be determined after all recommended maintenance actions have been taken, Boeing should be contacted for further disposition.

DAVID CARBAUGH
CHIEF PILOT
FLIGHT OPERATIONS SAFETY
BOEING COMMERCIAL AIRPLANES

MICHAEL CARRIKER
CHIEF PROJECT PILOT
FLIGHT OPERATIONS
BOEING COMMERCIAL AIRPLANES

DANIEL HUBER
LEAD ENGINEER
FLIGHT CONTROLS
BOEING COMMERCIAL AIRPLANES

ADRIAN RYNEVELD
PRINCIPAL ENGINEER
FLUTTER ENGINEERING
BOEING COMMERCIAL AIRPLANES