Once airborne, AF-447, very close to its maximum take-off weight, flew north for some distance, inland of Recife before proceeding out over the Atlantic. They climbed to their assigned flight level of 350 (35,000 ft.) The flight crew conversed with air traffic controllers at various points. There were three flight crew on board because of the duration of the flight so that they might rest, as needed. Their qualifications allowed for someone with proper command authority to always be in the cockpit if the Captain was taking a break.
The aircraft was flying on autopilot and the Captain had left the cockpit for a rest. Shortly after midnight and about two hours into the flight, the crew made a slight course adjustment to the left, probably to avoid weather spotted on the radar. Two minutes later the distinctive “Cavalry Charge” aural alarm sounded and the autopilot disengaged. The flying pilot said, “I have the controls.” It’s likely that the aircraft was flying in some turbulence and once off autopilot the aircraft rolled to the right causing the pilot to make pitch and roll corrections. The stall warning sounded twice and the left flight display showed an airspeed drop from 275 knots to 60, though that was probably not an accurate speed indication. The standby instrument system showed the same marked decrease.
The pilots noted that the various speed indicators were providing different readings. This occurrence caused the auto thrust system to also disengage while remaining at the previous thrust setting. At the same time, the loss of valid speed indications caused the flight systems to revert to an “alternate law” mode where some protections, notably those preventing the pilot from “over-flying” the aircraft, were lost. The aircraft was now under manual control at 35,000 ft. without a valid speed indication, in at least moderate turbulence.
Reports note that aircraft flying at both high speed and high altitude are especially vulnerable to stall conditions. AF-447 was at mach .82 (about 550MPH) and 35,000 feet when the emergency commenced. Even modest increases in the angle of attack would create a condition known as buffeting (turbulence felt in the pilot’s seat) which is a precursor to a stall. Cruising at Mach .8, the differential between the normal angle of attack and the angle required to initiate stall warning is only 1.5 degrees.
After the flying pilot’s initial actions, the aircraft was now nose up, climbing at a rate of 7,000 feet per minute and he continued to make inputs for roll and pitch. The non-flying pilot was attempting to reach the Captain so that he could return to the cockpit.
Forty-six seconds after autopilot disconnect the stall warning sounded for the third time as the angle of attack reached six degrees. “Take Off-Go Around”, a high thrust setting, was selected and the flying pilot continued to make pitch-up inputs. Seconds later the speed reading stabilized at 185 knots but the angle of attack continued to increase to 16 degrees.
One minute and forty seconds into the emergency, the Captain reentered the cockpit. The aircraft was out-of-control, stalled and dropping at 10,000 feet per minute. Theoretically, he was tasked with instantaneously understanding the immensely complex situation he encountered and determining which immediate actions to take. At that moment the aircraft was “pitch-up” with an angle of attack of more than 40 degrees.
Each time the speed indications became inaccurate the stall warnings would cease even though the aircraft was, in fact, stalling. (The flying pilot was holding the airplane nose high without a stall warning because of invalid speed readings.) When he pitched the aircraft nose down and the speed indications once again matched and became valid, the stall warning would again sound. The aircraft was rolling violently, up to 40 degrees, left and right.
Data and voice recording ceased at 2 hours, 14 minutes and 28 seconds into the flight. The aircraft was falling at 10,912 feet per minute, still nose high and with a ground speed of 107 knots.
Tomorrow- FF Safety: AF447: Investigation (Part Four)