Principal Fly Controls

SAFETY
DAVID CARBAUGH

Modern commercial jet airplanes provide smooth, comfortable travel that typically is free of vibration. Some types of vibration can be expected from time to time and are considered normal. However, isolated cases of abnormal vibration require prompt flight crew response and subsequent timely maintenance action.
MICHAEL CARRIKER CHIEF PROJECT PILOT FLIGHT OPERATIONSBOEING COMMERCIAL AIRPLANES DANIEL HUBER LEAD ENGINEER FLIGHT CONTROLS BOEING COMMERCIAL AIRPLANES ADRIAN RYNEVELD

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CHIEF PILOT FLIGHT OPERATIONS SAFETY BOEING COMMERCIAL AIRPLANES

PRINCIPAL ENGINEER FLUTTER ENGINEERING BOEING COMMERCIAL AIRPLANES

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ccasional airplane vibration during flight is not unusual. There are many causes of airplane vibration, including landinggear 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 willfacilitate 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.

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Buffet is a form ofvibration 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 causesthe 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.

background noise. More noticeable, but also normal, is the reaction of theairplane 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.

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 smoothair; 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.2

CAUSES OF AIRPLANE VIBRATION

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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...