Understanding Air France 447 (17 page)

The angle of attack sensor is a weather-vane like device on the side of the airplane. There are three of them. They measure the angle of the airflow relative to the airplane fuselage and therefore the wings. Each sensor feeds its angle of attack information to an associated air data computer which then makes the data available to the flight control and warning systems.

Why was the stall warning intermittent even when the angle of attack was critically high? The warning is inhibited whenever the indicated airspeed is less than 60 knots. The design logic is that the airflow must be sufficient to ensure a valid measurement by the angle of attack sensors, especially to prevent spurious warnings such as alarms due to gusty winds and unusual vane angles while on the ground. When the airspeeds values from all three air data computers are less than than 60 knots the angle of attack readings are therefore considered invalid.

The airplane’s actual airspeed was never below 60 knots. However, there were two reasons that the indicated airspeed was below 60 knots, thereby rendering the angle of attack values invalid and inhibiting the stall warning.

The first is the outright blockage of the pitot tubes by ice crystals. This inhibited the stall warning for only about 7 seconds. The pitot tubes were blocked for about 30 seconds for the left side and 40 seconds for the standby (the right side instruments are not recorded), but during most of that time the angle of attack was not high enough to trigger the warning anyway.

The second reason occurred in phase 4 where the angle of attack was so high that the pitot-static system could not effectively measure the forward airspeed.

In the center of the graphic below the brown and purple lines indicate the actual angle of attack, and the green line represents the stall warning angle of attack. The period of 02:10:10 to 02:10:20 corresponds to when the PF aggressively pitched up after the autopilot disconnected, causing the angle of attack to momentarily exceed the stall warning threshold. The blue area indicates the time period where the stall warning should normally have sounded, but was inhibited due to low indicated airspeed as a result of the clogged pitots.

As the airplane climbed in phase 1 and 2, the actual airspeed and Mach number decreased. Along with a lowering of the Mach number came an increase angle of attack threshold for the stall and stall warning (the slower Mach number allowed for a higher angle of attack without stalling). Even though the angle of attack continued to increase throughout phase 2, the stall angle of attack also increased, preventing a stall from occurring. The airplane remained slightly below the stall warning angle of attack until phase 3, when the angle of attack rapidly increased.

At the beginning of phase three (02:10:52) the stall warning activated and sounded continuously for the next 53 seconds. The PF pulled back on the sidestick and pushed the thrust levers full forward to the TOGA position (02:10:56).

The stall buffet was felt, as recorded on the flight data recorder, and heard as a vibration noise in the cockpit. This buffet is a more rapid shaking of the airplane than turbulence. In the absence of the stall warning, the buffet itself should have acted as a stall indication to the crew.

As the aircraft’s descent rate increased, the g load fell below 1g and a sense of falling would have been felt, like the initial feeling after pushing the down button on a fast elevator. As vertical speed continued to accelerate downward, the g load dropped further and larger nose-up inputs were made, probably to counteract the seat-of-the-pants feeling. Due to the combination of the flight control laws degrading to Alternate 2 and the loss of airspeed data, there were no protections or positive pitch stabilities to make the airplane pitch down on its own. Instead, the airplane attempted to follow the pilot’s orders for an increase in g load, as commanded by the sidestick position, and pitched the nose up further.

The airplane’s angle of attack continued to rise as the nose of the airplane porpoised between about 10° and 18° nose up.

During phase three, the sidestick was held on average about half way back, telling the flight control computers to pitch up in order to provide a g load above 1.0. The airplane increased both the elevator and stabilizer deflection in order to comply with this demand.

At 02:11:10 the airplane’s altitude peaked at 37,924 feet. The angle of attack was at 12° and increasing, and the g load was only at .75g’s.

In phase 4 the angle of attack reached extreme levels and the stall warning became intermittent.

At 02:11:32 First Officer Bonin said, “I don’t have control of the airplane any more now.” His sidestick was full left, and remained so for 47 seconds. The airplane’s bank angle increased to the right. This is possibly because the downwardly deflected aileron on the right wing caused its angle of attack to be greater than the left.

At 02:11:38, with the airplane descending through 36,000 feet, First Officer Robert moved his sidestick full left in an attempt to correct the bank angle which was approaching 30° to the right. He said, “controls to the left.” It is not clear if Robert was commanding Bonin to let him fly the airplane or simply move the controls to the left to counteract the steep right bank. Regardless, Robert pushed his takeover push button, momentarily disabling Bonin’s sidestick. But Bonin neither acknowledged the takeover nor released the controls. Instead he then pushed his own takeover push-button, disabling Robert’s sidestick while he continued to hold full left sidestick. Then also moved his sidestick full back.

The elevator moved to full nose up to comply with Bonin’s order. The stabilizer moved automatically to reduce the need for elevator deflection over time, and due to the constant up elevator demand, the stabilizer drove to the full nose-up position.

At 2:11:41 the stall warning was sounding when First Officer Robert again said, “what is that?”

Bonin replied “I have the impression we have the speed.” Most interpret this to mean that he believed the airplane was going too fast, or that he had regained any lost airspeed - and therefore the stall warning was false. In fact, the exact opposite was true.

At 02:11:45 the angle of attack passed 45° and the stall warning silenced. The indicated airspeed dropped to 40 knots and the angle of attack information was therefore declared invalid. The low airspeed indication was not due to another pitot clogging problem, instead it was because the angle of attack was so excessive that it made the pitot-static system unable to measure the airspeed. At this extreme and oblique angle of attack, the air pressure sensed by the pitot tube was almost equal to air pressure impacting the static port at nearly the same angle. This cancelled out the ability of the air data system to sense sufficient difference in pressure between the two ports that register airspeed.

At this same time (02:11:45) the captain entered the cockpit. He was not informed of the nearly 3,000 foot climb and he arrived to find the airplane descending through its original cruise altitude of 35,000 feet at 10,000 ft/min. The pitch attitude was 15° nose up, banked 32° to the right and increasing. The PF’s sidestick had been full left and full back, and the stall warning had silenced.

The thrust levers were retarded to idle and as a result the nose pitched down from about 15° nose up to about 12° nose down. We can be sure that this was the result of the thrust reduction as the sidestick was held full back this entire time, and the elevators were at full nose up as well. The decreased pitch reduced the angle of attack slightly to around 38°, which was enough to push the indicated airspeed up to about 80 knots, revalidating the angle of attack information and activating the stall warning again. Meanwhile, the airplane was descending at 15,000 feet per minute.

Even though the stall warning sounded again, the sidestick remained full back, the elevators full up, and thrust at idle. The nose then pitched up to about level with the horizon and the angle of attack increased, causing the indicated airspeed to fall again and the stall warning to silence.

Almost 20 seconds later at 02:12:04 Bonin said, “I have the impression we have some crazy speed, what do you think?” and he started to deploy the speed brakes. This validates the theory that he thought they were going too fast. Robert commanded, “No, above all don’t extend the…”

“No? Okay,” Bonin replied.

“Don’t extend,” Robert said, and then they were retracted.

At 02:12:10, descending through 29,000 feet, the thrust levers, having been at idle for the last 20 seconds, were then advanced to the climb detent, and the engines spooled up. The aft deflection of the sidestick was relaxed, but the elevator remained full nose up. With the additional thrust from below the wings the nose pitched up from 10° nose down and reached 8° nose up over the next few seconds.

02:12:20 The bank angle, which had been to the right for the past minute, momentarily reached wings level, but continued to oscillate between 20° right and 18° left. Each rolling motion was counteracted with full lateral stick input.

02:12:33 The thrust levers were then pushed forward to the TOGA position again, but little additional thrust was available. The stick was moved halfway forward. The elevator moved from its full-up position, and the pitch reduced from 8° nose up to 2° nose down. The stab trim also started to move from its full nose-up position. The indicated airspeed rose above 60 knots yet remained below 80 knots, and the stall warning triggered again for about 2 seconds. This indicates that some pitch control, or at least pitch influence, was still available.

02:12:40 Another pitch oscillation brought the nose from 2° down to about 8° nose up and then about 8° nose down. The indicated airspeed rose briefly to over 150 knots and the stall warning sounded for about 5 seconds.

02:12:42 A brief exchange illustrated First Officer Bonin’s uncertainty about the altitude. With the stall warning sounding, he said, “What are we here? On alti what do we have here?”

The captain said, “it’s impossible,” and Bonin repeated the question, “In alti what do we have?”

First Officer Robert replied, “What do you mean, on altitude?”

Bonin said, “Yeah, yeah, I’m going down, no?”

Robert replied, “You’re going down, yes.”

At 02:12:45 the airplane passed through 20,000 feet. The bank angle increased to 40° to the right and Bonin held full left stick again to try to correct the excessive bank angle. The sidestick averaged approximately neutral in pitch but the nose pitched down to about 8° below the horizon, as it rolled right. The airspeed rose, angle of attack data was declared valid again, and the stall warning sounded for about 10 seconds. With the nose pitched down and the stall warning on, the sidestick was moved further aft.

The captain and First Officer Robert instructed Bonin to ‘get the wings horizontal’ three times, and Bonin replied, “That’s what I’m trying to do. I’m at the limit with the roll.” He had been holding left stick for most of the previous 90 seconds.

02:13:00 The captain suggested, “The rudder bar. Wings horizontal, go gently, gently.”

02:13:02 Bonin initially and briefly input left rudder to counteract the right bank, and as the wings returned towards horizontal, a right sidestick input was made and rudder input also changed to the right. The yaw damper’s rudder commands intensified and Bonin’s rudder input had little effect.