Understanding Air France 447 (9 page)

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The pitot icing lasted for about a minute and five seconds. But 30 seconds later the airspeed indications again fell to extremely low levels.

Consider that in normal operation, the angle of the airflow along the fuselage is no more than a few degrees. At 02:11:45, as the airplane was descending through 35,000 feet, the angle of attack started to exceed 45° on a regular basis. At the same time the indicated airspeed fell to values that were well below its actual forward speed. If it were only a matter of the air striking the pitot tube at the 45° angle, geometry tells us that the resulting ram pressure would be 70% of its actual value, but the indicated airspeeds were often below 60 knots for the number one air data system and near zero for the standby instrument.

This created a situation where the air was pushing into, in addition to flowing over, the static ports. Dynamic pitot pressure is only calculable by subtracting the static pressure component. If the air is directed at the pitot inlet and the static port inlet at the same angle then the differential will fall to zero, or perhaps beyond. This dynamic accounts for the repeated falling of the airspeed indications to invalid values.

In addition to airspeed, altitude and vertical speed indications were also compromised because of this effect. At this same time, the recorded vertical speed indications become erratic, and changed at rates that had no corresponding change in the vertical g load.

Further evidence of this phenomenon is that sometime during the 02:11 minute (the ACARS-transmitted fault reports were not recorded more precisely than to the minute) the comparison between the static and pitot pressures were “out of bounds.” That is the static pressure was greater than the pitot-tube sensed pressure. This caused a hard speed/Mach function error in the standby instrument (“hard” meaning that it persisted over a period of time). In normal flight regimes this would be a nonsense situation, where static pressure was greater than pitot pressure, even if the pitot tubes were completely blocked. But the correlation of this message with a time period where the angle of attack became consistently excessive lends credence to the explanation that the angle of attack was responsible for the airspeed values at ridiculously low readings, long after the icing issue ceased to exist.

Airspeed vs. Angle of Attack

There have been many calls for the installation of angle of attack (AOA) indications on transport category airplanes. This accident would seem to be the perfect example to make that case. Unfortunately, it is not that simple.

Airspeed/Mach is an excellent indication for a number of reasons. It provides a direct indication of limit speeds for the airframe and flaps/slats. In cruise flight, it provides a higher degree of precision for performance than AOA alone, and an indirect indication of AOA within the normal envelope. Cruise performance is more related to Mach number than AOA. Lift is increased and stall AOA decreased with increasing Mach number, even at the same airspeed and AOA.

Angle of attack indications are no panacea. In cruise, one degree of angle of attack change is equivalent to up to 25 knots of airspeed change. The stall AOA is also not a constant, at least not at Mach numbers above 0.3. Therefore, to act as a replacement in case of loss of airspeed/Mach number, the Mach number actually needs to be known to know the stall AOA, or conservative assumptions made. It is a catch-22. That is not to say it would be useless. In the case of AF447, it would have shown an obviously excessive AOA, and perhaps would have allowed the crew to answer the question both first officers posed: “What’s happening?” It might have led to earlier attempts to recover from the stall with pitch. However, in so much as the pilot flying seemed to be ignoring the more fundamental indications of pitch attitude and altitude, along with numerous stall warnings, one could question what difference a rarely used AOA gauge would have made.

AOA indications are more useful at low altitudes (where the stall angle is constant) for higher AOA flight regimes like approach. Precision in the approach and climb phases is more critical and an AOA reference is appropriate from a aerodynamic perspective. Military jets equipped with AOA indications use them in those flight phases and high performance maneuvering, but not at high-altitude cruise. The military attitude/AOA critical carrier approach also uses a “back side of the power curve” technique not compatible with transport category flight director and autothrust operating.
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Additionally, the stall AOA is also influenced by flap and speed brake position. The addition of flaps actually reduces the stall AOA.
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Other factors such as CG, required body angle clearances, gust factors, and minimum control speeds (not AOA related), combine so that no single AOA can be targeted to ensure proper speed or landing attitude margins.

Measuring AOA is also more complicated than it may first appear. The airflow around the fuselage, where transport airplane angle-of-attack vanes are mounted, is not identical with the airflow experienced at the wing. Boeing cites Mach number, flap and gear position, side-slip angle, pitch rate, ground effect, fuselage contour, radome damage, installation error, sensor inaccuracies, contamination, and damage among the factors that add errors to the measurement of AOA.

Some transport category aircraft do have AOA indications (e.g., late models of B-737, 767, and 777). The indication provides a green approach band which represents the normal range for approach operations. The band is intended not as a target reference for the approach, but a tool to detect configuration errors, reference-speed calculation errors, and very large errors in gross weight, as not all approach speed parameters are related to or sensed by AOA.

Airbus does offer an angle-of-attack based speed replacement display called the Back Up Speed Scale (BUSS). The BUSS provides a green target area based on angle of attack and replaces the barometric altitude display with GPS altitude data. However the display only comes on after all three ADRs (Air Data Reference units) are shut off by the pilot, and its use is not recommended above 25,000 feet.

Boeing notes an additional hazard: “Pulling to stick shaker AOA from a high-speed condition without reference to pitch attitude can lead to excessive pitch attitudes and a higher probability of a stall as a result of a high deceleration rate.”

For a more complete discussion of these AOA integration issues for transport airplanes, see the excellent issue of Boeing AERO magazine at:
http://www.boeing.com/commercial/aeromagazine/aero_12/attack_story.html

As part of the investigation’s certification recommendations, the BEA recommended that “EASA (the European Aviation Safety Agency) and the FAA evaluate the relevance of requiring the presence of an AOA indicator directly accessible to pilots on board airplanes.”

I think it is indicative of the complexity of the issue that the agency that does not concern itself with the cost or technological barriers associated with many of its recommendations has only called for the evaluating the relevance of AOA indicators, and not their outright installation.

Chapter 6: "I Have the Controls"

As soon as the autopilot disconnected First Officer Bonin announced “I have the controls.” At that moment his skills and knowledge were put to the test. When the automatic systems stop functioning (‘the magic goes away’) and flight control laws degrade, a pilot must identify and understand the situation, and consolidate many areas of understanding into his actions. An understanding of aerodynamics, the characteristics of the A330’s fly-by-wire control system, performance, procedures, and raw instrument flying skills must be applied simultaneously.

First Officer Bonin’s inappropriate pitch up, attempts at stall recovery solely with power, misidentification of a over-speed situation, difficulty handling the airplane in Alternate Law at high altitude, and other failures highlight many the areas of understanding that must be fully grasped by every pilot crewmember to operate safely.

Understanding the Machine

Would this accident have happened in a Boeing? Some say no, but history does not necessarily agree.

The accident happened in a Airbus A330-200. A marvel of modern technology, without question. But the Airbus has its own unique qualities that pilots must understand to operate it properly and safely.

There is no question that the Airbus is different from any other civilian aircraft. Its flight control handling is different, its autothrust system works differently, and it has sidestick controllers instead of conventional control wheels, which is definitely different. I do not think it is a dangerous or bad design. In fact, overall I think it is a good design.

When I was learning to fly gliders, already an airline pilot at the time, my glider instructor pointed out that the glider was not just an airplane without a motor. It was in fact a whole different
category
of aircraft. Pilots will recall that the word “category” divides aircraft into airplanes, balloons, rotorcraft, gliders, airships, etc., so it is in fact a legal definition too. But while all aircraft obey the same laws of physics and aerodynamics, they have their own unique handling characteristics.

Due to a glider’s long slender wings and the slow speeds that they often operate at, a glider pilot’s coordination of rudder and aileron inputs can be quite different from a regular airplane. In tight slow turns, such as when climbing in a thermal, a glider pilot may actually have opposite aileron and rudder applied - a virtual sin in the airplane world. These differences are not unsafe nor difficult to learn or even master, but they are different. It takes understanding the principles involved and practice, and that is why there is separate license for each category.

I think that the different handling qualities of an Airbus fly-by-wire airplane have a similar degree of difference from a conventional airplane, as an airplane has to a glider. While not designated as its own category, the Airbus, like any large or jet powered airplane, requires training and a specific type rating for that model in order to operate it as a pilot, as it should.

These differences may have played a part in the failure of the AF447 pilots to recover from their loss-of-airspeed incident. They may not have fully understood what inputs they were making to the flight controls, or what they were really asking for. But they should have.

When everything is working right, as it is more than 99.9% of the time, the Airbus fly-by-wire system provides excellent protection from inadvertent stall, flight envelope exceedance, wind shear recovery, and more. When something is not working properly it is important for the pilot to understand what has changed and that he is now fully responsible for not exceeding normal limits. That responsibility is something most pilots take for granted anyway.

Flight Control and Stalls Review

In conventionally controlled airplanes, the pilot moves the controls to directly command the position of the flight control surfaces. In most larger aircraft, and some smaller aircraft as well, the pilot is not directly moving the flight control surface, but is doing so through some mechanical means to activate a hydraulic or electric servo that moves the control.

As an airplane has a wide range of speeds, the effect of a specific amount of control deflection has differing results depending on that speed. At low speed, there is little airflow and the controls are easy to move, yet require more deflection to achieve a given performance, such as a given roll rate. At high speeds there is greater airflow over the control surface which will have a greater effect, and the same roll rate can be achieved with much less control deflection. At higher speeds the force needed to displace the controls into the faster relative wind is greater as well. These forces cannot be felt through a hydraulic system; therefore, an artificial feel system is incorporated to mimic the natural feel as if there was a direct connection.

At high altitude, a given control deflection provides a faster airplane response than at low altitude due to less aerodynamic damping (i.e., it is easier to move the airplane around in the thinner air). This creates an opportunity for over-controlling the airplane if large control inputs are made.

In pitch, there is a balance between the center of lift from the wings, the center of gravity, and the aerodynamic forces created by the tail of the airplane. The airplane pivots around the center of gravity, which is about 20-30% of the way back from the average front of the wing (the mean aerodynamic chord, to be precise). This balance is dynamic and changes with the loading as well as the speed of the airplane. The two horizontal components of the tail, the horizontal stabilizer and the elevator hinged to its aft half, move to control the pitch attitude of the airplane.

The elevator is the primary pitch-control surface and moves immediately with pitch commands. The stabilizer is adjusted to reduce prolonged elevator deflection for both efficiency and controllability. This adjustment is called trim, and the stabilizer is often referred to as the “trimmable horizontal stabilizer” (THS) or simply the “stab,” and the adjustment of it as
stab trim.