Unusual inflight vibrations can be a warning of impending failure.
OPERATIONS
Our aircraft have a multitude of components in which some movement is completely normal, and under specific circumstances, they create perceptible noises and vibrations. An example would be the lowering of flaps, spoilers and landing gear. On the other hand, should you be worried if you sense subtle airframe vibrations while in a descent that seem out of the ordinary? Yes. It is important for a pilot to be able to distinguish what is a normal and acceptable aircraft vibration versus vibrations that aren’t normal. It is equally important that pilots know what proper actions should be taken to lessen unwanted vibrations, as well as providing accurate details so maintenance technicians can begin an efficient process of troubleshooting possible sources of the vibration.
Can vibrations compromise the safety of an aircraft, and if so, how? The answer to the first question is a resounding yes, as the following examples exhibit. If engineers did not purposely design a component of the aircraft to withstand an acceptable amount of vibration, the excessive movement will cause accelerated wear. If this happens to a critical aircraft component and the failure occurs in flight, this could quickly create a safety problem. At the very least, the accelerated wear will necessitate inspection and likely replacement of the worn component.
A Canadair CRJ similar to the one involved in the 2007 vibration incident. Credit: Wikimedia Commons/Eddie Maloney
Flight Control Free-Play In general, “cable and pulley” flight control systems are more susceptible to vibrations than hydraulically driven flight controls due to the inherent resistance that hydraulic servos provide against a flight control’s pressures. The interplay between separated airflow behind a Mach wave at transonic speeds and an aileron can cause unwanted aileron buzz (small vibrations). On conventional “cable and pulley” flight control systems, this tendency of aileron buzz, especially at transonic speeds, is heightened if there is any looseness in the cables, pulleys or bushings. The FAA received reports of wing/aileron oscillations from operators of Hawker 800XP and 850XP aircraft at altitudes above 33,000 ft. and speeds over Mach 0.73. When the speed was reduced and the airplane was at an altitude below 30,000 ft., the oscillations ceased.
Investigation of the incidents revealed missing aileron bushings, low cable tensions and improperly installed brackets. If the aileron system, including cable tension, is not properly maintained, wing oscillations could develop into divergent flutter, causing severe damage to the structure. When corrective maintenance brought the aircraft into compliance with the type design configuration, the oscillations did not recur. These oscillations can reduce the fatigue life of the aircraft, and in severe cases, can adversely affect the wing structure by causing cracks in the wing spars and stiffeners. Thus, the FAA issued Special Airworthiness Information Bulletin NM-14-05 dated Nov. 27, 2013, recommending a one-time maintenance check to verify all the bushing in the aileron and aileron tab assemblies are correctly installed and that the free play is within limits, and to ensure that the hinge brackets are properly installed and the cable tensions are correct. While preflightight cable-driven flight controls, pilots should check the free play by gently moving the flight control to determine the slack. The exact amount of free play (i.e., the amount you can jiggle the flight control without restriction) should be stipulated in the aircraft maintenance manual. If excessive free play is found during a preflight, the proper action is to note the discrepancy in the aircraft logbook and not fly the aircraft until the condition is corrected by maintenance technicians. Challenges With Control The flight crew of a Canadair CL-600-2B19, operated by Atlantic Southeast Airlines as Flight 4690, took off on May 8, 2003, from Atlanta en route to Montreal. After doing so, the flight crew sensed a high-frequency vibration with their feet as well as from the flight control column. They glanced at the engine indicating and crew alerting system (EICAS) to see if there were any warning indications from a system on the aircraft to help them ascertain the source of the vibrations, but the EICAS screen remained blank. When the airspeed was increased to 250 kt., the vibrations started again. Initially, the vibrations were slow, but they increased in intensity and lasted about 30 sec. As the flight was cleared to 17,000 ft., the aircraft started an uncommanded 45-deg. roll to the right. The first officer (who was the pilot flying for this leg) immediately disconnected the autopilot and was able to regain control, albeit with difficulty due to the continuing tendency to roll to the right. The captain declared an emergency and ATC issued a clearance to Chattanooga, Tennessee. The pilots continued to sense the vibrations throughout the remainder of the flight at all airspeeds. The flight was successfully landed at Chattanooga without any injuries to the flight crew, flight attendant and 18 passengers. When the NTSB’s systems group began its post-incident inspection, the team members discovered hand movement of the right-aileron control surface allowed free play with the hydraulic systems turned off. The extent of free play was measured at 0.4375 in. In contrast, the left aileron exhibited 0.25 in. of movement. Examination of the components by the NTSB’s materials laboratory revealed wear and polishing on many of the fraying surfaces. Wear locations included the outer diameters of the bolts, both diameters of the bushings, and balls and sockets of the links. The NTSB determined the probable cause to be excessive wear on the bushings and the right aileron’s inboard and outboard power control unit, which resulted in upward movement of the right aileron. This directly led to the aircraft’s uncommanded right roll. A factor was the inadequate maintenance inspection procedure by the aircraft manufacturer for the aileron system’s check performed by the operator. Safely Back To The Ground When an aircraft begins to exhibit unusual vibrations, prudent flight crews must take the proper actions to get the aircraft safely back to the ground in a timely manner. Unfortunately, aircraft flight manuals don’t provide guidance on the proper airspeeds, control inputs and associated abnormal procedures to do so. The NTSB aviation database contains an interesting assortment of events in which aircraft exhibited unusual vibrations. One of the most eye-opening of those reports occurred on March 7, 2005. A de Havilland Beaver DHC-2 airplane was on a sightseeing flight approaching Alaska’s Mount Denali at 11,000 ft. MSL when it started to shake violently. The pilot reported that he could not control the airplane and elected to shut down the engine in the event it was the cause of the problem. He said when the shaking did not stop, he slowed the airplane to about 80 mph, and then it subsided. He said he restarted the engine and flew to Talkeetna at a slow airspeed, with flaps extended. A post-landing inspection revealed that both wings were structurally damaged (NTSB Report ANC05LA046). Normally, an accident investigation of older general aviation aircraft doesn’t have a collection of engineering quality data due to the lack of a flight data recorder. However, in this event the investigators were able to obtain engineering data from a tourist’s camera recording. The audio portion of the recording revealed a vibration for about 3-7 sec. in the 8.2- to 8.4-Hz range. There was nothing on the recording to indicate the airplane was being flown outside the normal operating envelope prescribed by the airplane's manufacturer. The airplane was examined by aerospace engineers from the Anchorage, Alaska, FAA Aircraft Certification Office. Damage to the airplane indicated that the rear spars of both wings oscillated up and down with significant amplitude at span stations outboard along the wings. The bushing holes in the rear spar attachment fittings were elongated, which, according to the engineers, if preexisting, would have been a major contributing precipitator of the flutter. Additionally, the right aileron and rudder were severely under-balanced. They were not able to ascertain if the aileron cable tension was adequate prior to the event. On Feb. 1, 1980, de Havilland Aircraft of Canada Ltd. issued service bulletin 2/29 for the DHC-2 airplane. The bulletin reported instances of aileron/wing flutter, and that at least two or more conditions out of four must be present to facilitate a flutter condition. The four conditions were: ailerons not balanced; aileron cables in the wing slack; deterioration in the stiffness of the aileron mounting structure in the fuselage; and/or the airplane being flown outside the limits of the flight manual. On Feb. 20, 1980, in response to de Havilland's service bulletin, the FAA issued airworthiness directive 80-24-02, which required mandatory inspections of the airplane's wings, spars, and aileron cable tension and balance, within a prescribed time frame, based on service hours and part numbers. Vibrations Can Be a Warning On April 7, 2007, a Canadair CL-600-2B19 (CRJ), operated by Mesa Airlines as Flight 7264, was under flow restrictions for its destination, Chicago O’Hare International Airport (KORD). While holding for takeoff at Capital City Airport (KLAN), Lansing, Michigan, the flight crew received a left thrust reverser unlock master caution and associated EICAS indications. The captain contacted maintenance and cycled the reverser a few times in an attempt to clear the indications. After he had decided to return to the gate, the messages cleared. He subsequently cycled the thrust reversers two or three more times and both appeared to be operating and stowing properly. Therefore, he elected to depart for KORD. The captain reported experiencing a small vibration on climb out. The vibration persisted and the captain became concerned about the thrust reverser. He stated that about 35 mi. west-northwest of Grand Rapids, Michigan, he heard a "loud bang" and the "aircraft pitched and yawed/rolled to [the] left." The autopilot disengaged and the left thrust lever moved to idle during the event. The first officer ran the checklist to stow the reverser. The captain hand-flew the airplane for a time. He ultimately elected to continue to O’Hare because the thrust reverser unlock messages had cleared and the vibrations had stopped. The flight subsequently landed uneventfully at KORD. A post-accident inspection revealed that the left-engine translating cowl had separated from the aircraft. The inboard leading edge of the left horizontal stabilizer was dented and crushed aft consistent with impact damage. The left-side skin of the vertical stabilizer was punctured immediately forward of the center spar. Review of the aircraft's maintenance records revealed a history of anomalies related to the left-engine thrust reverser. On March 11, 2007, the aircraft maintenance log contained the discrepancy, "L Rev Unlock Caution." The entry was deferred in accordance with the Mesa Airlines CRJ minimum equipment list (MEL). On March 18, 2007, the left pneumatic drive unit was replaced; however, operational testing determined that the discrepancy was not resolved. The maintenance record noted binding in the drive assembly to the ball screw actuator. On March 20, 2007, the left-engine thrust reverser flex shafts were replaced. Again, the discrepancy was not resolved. On March 22, 2007, a ball screw actuator and a cascade assembly were replaced. The maintenance record indicated that rigging and operational checks were satisfactory. The MEL item was closed at that time. Routine maintenance was conducted on March 30, 2007, at which time the thrust reverser and ball screw actuators were lubricated. No defects were noted in the records. The NTSB determined the probable cause of this accident was the inflight separation of the left-engine thrust reverser translating cowling due to intermittent binding and jamming of the reverser on the accident flight and on previous flights. Contributing factors were the inadequate maintenance action by the operator due to their failure to properly resolve the prior reverser malfunctions, the failure of the pilots of previous flights in not referring earlier reverser deployment failures for maintenance action, and incomplete company/manufacturer's procedures because they did not address anomalous reverser indications during ground operations.
During a walk-around of an ATR 42-300, components of the right-hand main landing gear’s side brace were found to be migrating from the assembly bore. Transport Canada applauded this example of an observant maintenance staff member who found this defect on the ground. This likely prevented a failure that could have adversely affected the aircraft’s ability to land safely. Credit: Transport Canada “Fixed Wing—Make A to C: Feedback—Canadian Aviation Service Difficulty Reports”
Troubleshooting Process When a flight crew lands and reports “We heard/felt strange vibrations during flight,” a maintenance technician doesn’t have enough information to narrow the search of likely causes. Troubleshooting is further complicated because the vibrations are occurring in flight, not on the ground. Fortunately, all of the incidents cited in this article were concluded when the aircraft landed safely. During the stress of these events the additional workload of trying to remember the aircraft’s parameters at the onset of the vibration is a relatively low priority in comparison to getting the aircraft safely on the ground. The specific checklists utilized by a manufacturer’s maintenance flight crews who conduct test flights for vibration detection and diagnosis are lengthy and specially designed to capture numerous parameters that will help in the post-flight diagnosis. These special checklists are part of the unique procedures for vibration detection and are not normally available to flight crews who are assigned to regular duties. It is helpful for subsequent troubleshooting if you can accurately recall a number of important parameters: aircraft weight, altitude and airspeed. Did you “see” the vibrations in the control column? Did you sense vibrations through the flight controls? Did you observe visible shaking of other parts of the aircraft structure? What phase of flight did it start? Did the aircraft configuration include extension of the flaps and gear? Did the vibrations change, and, if so, did it seem to occur with any change in the speed, altitude, thrust, configuration or flight control movement? Was there atmospheric turbulence? How intense were the vibrations? This troubleshooting process often involves taking the aircraft back into flight. As a former maintenance pilot at a large fractional carrier, this was a duty that I performed on numerous occasions. Seldom were the causes of the vibrations obvious. Many times, we had to do multiple flights as part of the troubleshooting process. Some of the memorable cases involved landing gear doors that were slightly out of alignment. One case involved a flight crew who reported an inflight vibration only to discover a panel on the belly between the main gear wells that was missing 70% of its screws. One problematic case consumed weeks, along with a half-dozen test flights, before determining that a flap fairing was the cause of abnormal vibrations. Vibrations in flight controls were of course taken very seriously. “We,” meaning the team of maintenance technicians and myself, were troubleshooting mysterious rudder vibrations. We flight tested the aircraft on several flights after their maintenance attempts. Finally, after many days, the technicians greeted me with a grin and said, “You won’t believe what we found inside of the rudder!” It was a bird’s nest. We could not fathom how a bird would gain access to the rudder. The next flight test proceeded smoothly. These examples illustrate the importance of pilot training to know specific handling issues such as ground resonance. Operating an aircraft in compliance with its limitations and always within its flight envelope is vital. Excessive wear on critical aircraft components or improper maintenance allows vibrations to occur within the normal flight envelope. Immediate detection by a pilot to prevent further damage to the aircraft’s important structures and flight controls is necessary, which includes getting the aircraft on the ground in a timely manner.
Upon his retirement as a non-routine flight operations captain from a fractional operator in 2015, Dr. Veillette had accumulated more than 20,000 hours of flight experience in 240 types of aircraft, from balloons, rotorcraft, sea planes, gliders, war birds, supersonic jets and large commercial transports. He is an adjunct professor at Utah Valley University.
