Date: 201511
Local time of day: 0001-0600
Location: ZTL.ARTCC, GA
Flight conditions: IMC
Aircraft: Heavy Transport
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Cargo / Freight / Delivery
Flight phase: Descent
Airspace: Class A ZTL
ATC / advisory: Center ZTL
Reporter organization: Air Carrier
Reporter function: Captain; Check Pilot; Pilot Not Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Confusion; Situational Awareness; Communication Breakdown
Second reporter function: First Officer; Pilot Flying
Anomaly: Aircraft Equipment Problem Less Severe; Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Clearance; Flight Deck / Cabin / Aircraft Event Other / Unknown
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Returned To Clearance
Contributing factors: Human Factors
Primary problem: Human Factors

Narrative: FL 350; profile; autopilot 1 engaged; .78 Mach; ground speed approximately 480. Optimum alt 350; max 370. Autopilot 2 deferred. FILPZ one arrival to CLT; Line Check Airmen (LCA) Captain giving IOE to FO. Captain was Pilot Monitoring (PM); First Officer (FO) was Pilot Flying. Clearance to cross SKYWA intersection at 330. FO put 330 in FCP; I added a constraint to FMS to cross SKYWA at FL 330. I had unspoken concerns that the FMS computed descent point was too close to assigned constraint. ATC gave frequency change; prior to checking in I directed FO to select immediate descent at approximately 11 miles from SKYWA. I  Checked in with center leaving 350 with clearance to cross SKYWA at 330. They acknowledged. Descent was proceeding normally at 1;000 fpm; shortly after going thru FL 340; FMS picked up its computed profile and increased its descent rate to honor constraint. I recognized at 33;200 feet that the autopilot was not going to capture FL 330. Descent rate was approximately 2;000 fpm and not decreasing. I directed FO to disengage the autopilot and manually level the aircraft. While the autopilot disconnect warning was going off I decided to take control of the airplane. In normal conversational tone I announced I had the aircraft; and I punched off the autopilot warning and began level off.  As I put in back pressure to level off I was surprised that the aircraft entered a buffet. I released back pressure; the buffet stopped; aircraft altitude was now about 32;850. I added a small amount of back pressure back in to climb back to our assigned altitude and the buffet reoccurred. I released back pressure; allowed the aircraft to stabilize 150 below our assigned altitude for about 10 seconds and then manually climbed back up to FL330. During that time I confirmed the speed brake was retracted. I directed the FO to reengage autopilot 1 and to select alt hold.  He completed the descent from altitude with autopilot 1 engaged using level change without further issue. I wrote the incident up in the logbook in detail and also gave CLT maintenance a thorough verbal debriefing. Neither the FO or I felt we induced enough G on the aircraft to induce the buffet; control inputs seemed smooth to both of us with little noticeable G force. During post flight conversation I discovered the FO did not hear me take control of the airplane due to the blaring autopilot disconnect warning and he also did not realize I was putting inputs to the controls. I too did not notice his inputs or ever felt they were contrary to mine. He stated he was also putting inputs in; and from what both of us could tell; his inputs mirrored mine and vice versa. Neither one of us felt at any time that the aircraft response was anything other than what we expected considering the inputs we were applying. The initial manual level off that resulted in the first airframe buffet did not seem aggressive or abrupt to either of us nor did the first attempt to climb back to our assigned altitude which resulted in the second airframe buffet. Stick shaker did not occur at any time. We both completed a visual walk around inspection of the aircraft after landing and noticed nothing unusual. I informed the FO I was submitting [a safety] report and advised him to do the same so causes and errors could be identified and others could gain awareness and knowledge. Maintenance action post flight revealed no system anomalies; ACARS showed lateral g of .15; inspection showed no aircraft damage.


Second narrative: Enroute to CLT we were cruising at FL350 and cleared to cross SKYWA at FL330. I selected 330 in the FCP; the captain edited the FMS to put the crossing restriction in the FMS and I pulled the altitude selector to arm the descent via profile mode. Approximately 11 miles prior to the fix the captain asked me to begin an immediate descent. I selected IMM DESC on the FMS and began a 1;000 foot per minute descent. Passing approximately FL 340 and 4 miles prior to SKYWA the profile mode recaptured the descent and increased the rate of descent to approximately 2;000 feet per minute. At 33;200 with a 2;000 feet per minute descent and the throttles back and no attempt by the autopilot to begin a level off; I realized the autopilot was not going to capture the altitude. At the same time I heard the captain say 'it's not going to capture.' I disconnected the autopilot and began to pull the nose back to the horizon at .78M and 275kts. Almost immediately; I felt a buffet on the aircraft and released back pressure. The altitude decreased to approximately 32;800 feet. After stopping the descent; I attempted to increase altitude by bringing the nose above the horizon. Once again I felt an immediate buffet and released back pressure. The captain put the autopilot back on and I selected alt hold at 32;900 feet. I subsequently selected VS up to bring the aircraft back to FL330 and reselected alt hold. I did not use profile the remainder of the flight. ATC did not mention any altitude deviations. During debrief the captain told me he also disconnected the autopilot at the same time I did and initiated a pull back to the horizon. Due to the loud autopilot disconnect alert; I did not hear him say 'I've got the airplane.' We were both pulling back on the control column same time. After landing the captain wrote up the aircraft for maintenance to investigate both the profile failure to capture and the airframe buffet. Additionally; the number 2 autopilot was MEL and I was using autopilot number 1. The FMS showed the optimum cruise altitude at FL350 with a max of FL370.


Synopsis: While enroute to CLT and beginning descent; the autopilot failed to capture an intermediate altitude. Both crewmembers disconnected the autopilot and corrected back to altitude at the same time which may have resulted in a slight overcorrection and associated airframe buffet.