Not-So-Good Vibrations, Part 2
In Part 1, we discussed how vibrations can cause challenges with aircraft control.
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.
In Part 3, we’ll discuss troubleshooting airplane vibrations.