Understanding Air France 447 (10 page)

The trim setting is such that at a given load distribution, the airplane’s trim setting is valid for a particular speed. If the airplane slows down, left on its own the nose will tend to pitch down, which will increase speed, causing the nose to pitch up, causing the airplane to slow down to eventually become stable at the speed the trim is set for. An airplane with this configuration, like commercial jet transports, is considered dynamically stable (it will return to its stable speed).

Therefore, if an airplane is trimmed to fly at a given speed and the actual speed falls much below that, the pilot has to exert additional control force to keep the nose of the airplane from pitching down. Either that or adjust the trim to the new speed.

When a new speed is desired, the pilot or autopilot adjusts the trim, so that the airplane is stable at the new desired speed. This occurs repeatedly throughout any flight.

An additional factor that plays in this balance is the thrust from the engines, especially when they are mounted below the wing and not in line with the center of gravity. An increase in power from below the center of gravity will induce a pitching-up moment and a new balance must be achieved with the trim.

Operation of Trim

On Airbus fly-by-wire aircraft, the trim is normally automatic. The automatic trim moves the stabilizer so that the elevator is neutral on average (aligned with the stabilizer). This is both aerodynamically efficient and it provides for a range of elevator movement, and thus control, on either side of the trimmed setting.

A trim wheel is located next to the thrust levers on the center pedestal. The trim wheel provides a direct link with the THS hydraulic control. Whenever the THS moves the trim wheel moves, and vice versa. Its operation is silent and usually goes unnoticed. The pilot can adjust the stabilizer manually with the trim wheel, and manual movement of the trim wheel control overrides the automatic function. However, except in case of a malfunction, there is no reason to do so.

But Airbus fly-by-wire airplanes are operated by a different set of rules than a conventional airplane. Those rules are called flight control laws: Normal, Alternate (with two versions), and Direct.

In practicality, the trim operation is completely behind the scenes. Trim operation in Normal and Alternate Law does not affect the way the airplane handles, which is quite a departure from conventional airplanes.

Assuming the pitch controls laws are operative (Normal, Alternate 1, or Alternate 2), the elevators will move to maintain the g-force/pitch rate requested by the pilots via the sidestick. The trim does
not
play a part in the feel of the airplane. If the pilot pitches up to 15° and lets go of the sidestick, the pitch will remain there, whether the stabilizer has trimmed or not. The only difference is how much elevator the flight control computer will command in order to maintain that attitude. As the stabilizer trims, less elevator will be held, but the pitch attitude and sidestick position will remain unchanged.

In a case where the automatic trim fails, the pilots (directed by procedure) will trim the stabilizer by referencing the flight control display on the ECAM, and trim the stabilizer until the elevator is in the neutral position. When this is done, the sidestick remains in neutral and no change is felt with the sidestick or actual aircraft pitch. Once the stabilizer and elevator are aligned, the elevator’s ability to carry out a pitch up or pitch down order is assured. If this were not done, and the elevator were near the limit of its range (due to stabilizer being positioned in the opposite direction), the elevator would not be able to move further in order to carry out a pitch change. We describe this scenario as running out of elevator.

If the pitch laws have degraded to Direct Law, the elevators are directly controlled with sidestick position. If the sidestick must be held against the centering spring then trim is called for. When properly trimmed the sidestick will be in the center position and the elevator and stabilizer aligned. If other than neutral elevator is required, the pilots must hold the sidestick deflected and move the stabilizer with the trim wheel until sidestick no longer needs to be held out of center.

Stalls

There are limits to how slow an airplane can fly. The slower an airplane moves through the air, the greater amount the wing must be deflected against the direction of movement in order to achieve the required amount of lift to stay in the air. The angle between the wing and the relative wind is the angle of attack (AOA).

Above a critical angle (in the lower atmosphere it is typically around 15°) the air can no longer flow smoothly around the wing, the airflow becomes turbulent and lift rapidly decreases with any increase in the angle of attack. This is called a stall. The stall angle of attack is considered constant for a given configuration, but is decreased by flap extension and generally for Mach numbers above 0.3 (about 200 knots).

The stall is accompanied by characteristic behaviors of the airplane such as loss of effectiveness in the controls and a buffeting of the airplane from the turbulent airflow. Pilots experience this in training, albeit briefly.

In primary flight training, pilots are taught that an airplane always stalls at the same angle of attack (for a given wing configuration), no matter what the speed, weight, or attitude. That is true enough throughout the normal operating range of airplanes that pilots learn to fly in. However, that is not necessarily the case for high performance aircraft operating at high altitudes.

To recover from a stall, the angle of attack must be reduced so that the air can flow smoothly around the wing again. This is properly accomplished by pitching the nose down. However, if there is sufficient engine power available, an application of a large amount of thrust may be sufficient to change the airplane’s direction of movement an thus indirectly reduce the angle of attack.

On the high speed end, there are two limit speeds to consider. The first is a straight airspeed limit that limits the maximum force on the structure from high airspeeds. The second is a Mach number limit (Mach 1.0 is the speed of sound). The Mach limit references the point that airflow around parts of the wing become supersonic, forms a shock wave, and also disturbs the flow of air around the wing, which results in a large increase in drag. The Mach number that this occurs at is the
critical Mach number.
The supersonic flow and shock wave formation is also accompanied by a buffet, known as Mach buffet. Most pilots will never experience Mach buffet in airline operations or in simulator training. For the A330, this critical Mach number is beyond the maximum operating speed imposed by other speed limiting factors.

In older generation aircraft, the onset of this supersonic airflow could also result in
Mach tuck
, a dangerous loss of control. Mach tuck is a strong pitch down force due to the redistribution of airflow and forces resulting from shockwave formation. In a Mach tuck situation the center of lift is shifted aft toward a swept wing's tips, inducing a pitch down moment. The shock wave may also reduce the effectiveness of the tail reducing its normal pitch-down moment, which may then make a pitch up recovery impossible.  In the early days of commercial and private jet operation, a number of accidents occurred due to this phenomenon.

In order to allow for high cruise airspeeds, and avoid the effects of Mach buffet, airplanes of the A330's generation employ a “supercritical” airfoil. These air-foils typically have a larger leading edge radius, a flatter upper surface, and a rather distinctive cusp at the trailing edge. These air-foils were developed by NASA starting in 1965 and have improved over time.

The supercritical airfoil is not “extra critical”, but one with a high critical Mach number (super, meaning high). It allows efficient cruise speeds at relatively high Mach numbers before incurring a large increase in drag due to shock wave formation.

Modern aircraft with supercritical wing profiles offer numerous advantages, which include:

• Improved aircraft control characteristics at high speed
  ²¹
  
• The position of the aerodynamic center is virtually stable for supercritical profiles, and therefore less susceptible to adverse high Mach effects such as Mach tuck.
  
• They have a higher drag divergence Mach number and greatly reduce shock-induced boundary layer separation.
  
• Their geometry allows for a thicker wing and/or reduced sweep angle, each of which may help reduce the weight of the wing structure.
  
• The increase in drag above a given speed is so great that it is extremely unlikely, or even impossible, to fly faster than the demonstrated dive speeds that ensure the absence of flutter in flight testing (typically set at maximum operating Mach +.07).
  

Therefore, the airfoil is better behaved at near Mach speeds than older generation air-foils, as the critical Mach number is higher, and the buffet effects less. As a result, the threat of loss of control due to an over-speed is much less than in older generation aircraft.

Unfortunately, the characteristics of these new air-foils and the reduced possibility of Mach buffet are not well known to pilots.

High Mach Stall

At higher Mach numbers, the stall angle of attack is considerably decreased. Despite the flight school admonition that an airplane always stalls at the same angle of attack, the stall angle of attack is greatly influenced by Mach number. So much so that while the airplane could pull more than 2.5g’s without stalling at low altitude and normal operating speeds, at the maximum recommended cruise altitude and Mach, the stall buffet may occur at only 1.3g’s.  This is the result of the compressibility of the air, and it starts to become a factor at around 200 knots of calibrated airspeed (the effect is more pronounced at higher altitude). 

To use the stall warning as a point of reference, at 0.3 Mach the A330 stall warning comes on at an angle of attack of about 10°, at M.82, it is only 4°.