2011-10 · NASA ASRS report 974818
A320 Flight Crew experiences what is described as bump or rhythmic light chop at FL350. While checking the Flight Control page it is discovered that the bump is in sync with aileron movement. Due to the maintenance history of the aircraft; the Captain elects to turn of ELAC1 and the bump immediately disappears.
Ops normal in cruise at FL350. Began to pick up what felt like light chop. We noticed within a few minutes that the characteristics of the chop were unusual. 1.The aircraft only moved in pitch-absolutely no role or yaw. 2.The chop was a very consistent 'bump' that occurred on a cycle of 10-15 seconds. We pulled up the F/CTRL display on the SD. Immediately we noticed that both the left and right aileron indices were moving in sync with the bump. Both indices would move up and down rapidly 1/16 to 1/8 of an inch of total travel on the synoptic as each bump was felt. Applying systems knowledge; we knew that ELAC1 normally controlled the ailerons. During preflight planning; we had noted that this particular aircraft had just had ELAC1 replaced after multiple write-ups for ELAC1 Fault and Ail Servo ECAMS in flight. Accordingly; I pulled out the FM and reviewed the F/CTL ELAC 1/2 Fault Irregular. It reads: One computer failed: CAUTION: Do not reset ELAC if uncommanded maneuvers occurred during flight. ELAC (affected)...OFF THEN ON Based on the uncommanded pitch motion; the maintenance history of the aircraft; my systems knowledge and the caution above; I decided that the safest course of action was to turn off ELAC1. The First Officer disconnected autopilot and was hand flying as I turned off ELAC1. Aileron movement and pitch bump stopped IMMEDIATELY and did not recur for the remainder of the flight. We contacted Dispatch via ACARS and added Maintenance to the patch. I described situation and agreed to make maintenance write up as we continued to destination. I also reminded Dispatch that we were now downgraded to Cat 2 status with the ELAC1 turned off. The rest of the flight was routine to landing.
At FL350 began to experience a rhythmic bump which at first was thought to be light chop; however after a few minutes the chop was occurring on a 10-15 second interval and with a slight vertical bounce to the aircraft. FCTL display showed ailerons moving 1/16 to 1/8 inch in sync with bump. ELAC1 had just been replaced which according to systems knowledge controls the ailerons. After reading the FM for ELAC failure we turned off ELAC1 and the bump immediately ceased and did not return. We contacted Dispatch and Maintenance Control and briefed them on what had happened. We coordinated a write-up to describe the event and our actions; remainder of flight was uneventful.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.
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