On October 23, 2006, approximately 1400 mountain daylight time, an Airbus A319-111, N924FR, operating as Frontier Airlines flight 539, experienced a nose down pitch during the landing flare at Denver International Airport (DEN), Denver, Colorado. The captain, first officer, 3 flight attendants and 131 passengers were not injured, and the airplane was not damaged. Visual meteorological conditions prevailed at the time of the incident. The scheduled domestic passenger flight was being conducted on an instrument flight rules flight plan under the provisions of Title 14 Code of Federal Regulations Part 121. The flight originated Los Angeles International Airport (LAX), Los Angeles, California, and was en route to DEN.

According to flight crew statements, during the landing flare approximately 10 feet above ground level, the aircraft pitched nose down while the captain applied aft sidestick force. The rate that the nose descended seemed to be commanded and extremely smooth. The first officer wondered if he could have accidentally bumped his sidestick, but he did not think that action occurred. The captain landed the airplane with the main landing gear touching down first, and the captain's sidestick was once again responsive. The entire event took no longer than 2 seconds. No electronic centralized aircraft monitor (ECAM) warnings or other indications were noticed by the flight crew.

At 1353, the DEN automated surface observing system (ASOS) reported the wind from 040 degrees at 4 knots, 10 statute miles visibility, few clouds at 15,000 feet, scattered clouds at 25,000 feet, temperature 21 degrees Celsius, dew point minus 6 degrees Celsius, and an altimeter setting of 30.07 inches of Mercury. A NTSB meteorologist reviewed the weather data available during the time of the incident. According to the meteorologist, no indication of microburst or convective wind shear was noted.

The NTSB was notified of the incident on October 24, 2006. Prior to the notification to the NTSB, Frontier Airlines personnel had downloaded the digital flight data recorder (DFDR). According to the Pilot/Operator Aircraft Accident/Incident Report (NTSB Form 6120.1) submitted by Frontier Airlines, "...a review of the DFDR data indicated the pitch was caused by external atmospheric wind (microburst). It clearly showed the aircraft flying through a microburst where the wind changed from a headwind to a tailwind at the exact point where the aircraft pitch down. The pilot corrected with the sidestick and the aircraft responded correctly. A 90-day review of the aircraft [records] was performed, and there had been no previous similar discrepancies."

A review aircraft flight log revealed the following maintenance corrective actions, "Removed and installed [a new] captain sidestick assembly..." Frontier Airlines submitted a Service Difficulty Report (SDR) referencing the incident. According the SDR, the captain's sidestick control assembly was replaced as precautionary and an operations test was performed with no anomalies noted. An operational check flight was also performed with no faults noted and the aircraft was returned to service.

The DFDR data for the incident flight was requested by the NTSB Vehicle Recorder's Division, and Frontier Airlines provided the data to the NTSB. The cockpit voice recorder was not requested because the incident flight had been overwritten after the aircraft was returned to service. Review of the data by the NTSB recorder's specialist revealed that the captain's sidestick (left) was inoperative for approximately 1 second during the landing flare. During that time, the first officer's sidestick (right) pitch was in the neutral position. Less than one second after the captain's sidestick went to inoperative, both sidestick pitch positions went to approximately 14-16 degrees aft. Pitch data revealed a decrease in pitch of approximately 2 degrees during the event.

The week following the incident, Frontier Airlines contacted Airbus regarding the incident. According to Airbus, the DFRD data was provided to them by Frontier Airlines. Airbus provided the following analysis concerning the event:

"Captain was Pilot Flying (PF). First officer was Pilot Non Flying (PNF). Manual approach and flare input from the PF were nominal. But, it appeared that [the] PNF take-over and priority pushbutton was pushed inadvertently for approximately 1 to 2 seconds during flare. As a warning, white arrow pointing to the right was displayed in front of captain (PF) and F/O (first officer) green light was lit in front of F/O (PNF). With the F/O priority button pressed, EFCS (Electrical Flight Control System) disregarded the captain inputs to the benefit of the F/O one. During this time, as no order (neutral) was applied on F/O sidestick, aircraft elevators returned to neutral position, leading pitch attitude to be reduced by 1.5 degrees. EFCS switched back to captain sidestick when priority switch was released, and captain resumed flare and landing uneventfully."

According to Frontier Airline's Flight Crew Operating Manual for the Airbus 318 and 319, each pilot on his lateral console a sidestick he can use to control pitch and roll manually. Each sidestick is spring loaded to neutral. The hand grip of the sidestick has two switches: autopilot disconnect and sidestick takeover pushbutton; push-to-talk button. According to the sidestick priority logic, when only one pilot operates the sidestick, it sends his control signals to the computers. A pilot can deactivate the other stick and take full control by pressing and keeping pressed his priority takeover pushbutton. A pilot can at any time reactivate a deactivated stick by momentarily pressing the takeover pushbutton on either stick. If both pilots press their takeover pushbuttons, the pilot that presses last gets priority. In a priority situation, a red light (located on the glareshield) comes on in front of the pilot whose stick is deactivated. A green light comes on in front of the pilot who has taken control, if the other stick is not in the neutral position (to indicate a potential and unwanted control demand).

According to Frontier Airlines, the non-flying pilot's hand should not be on the sidestick during critical phases of flight, such as the landing flare; however, the non-flying pilot should be in a position to takeover control, if required.

Airbus reported to the NTSB that they have no record of any inadvertent action on the priority switch reported by an airline operating fly-by-wire aircraft.

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