I would like to add to the discussion on "
Why the pitch up?" by quoting from and otherwise referencing from two sources -
HtBJ by Davies, (1rst & 3rd ed, discussion on the "super-stall"), and from one of the papers to which I provided links a while back on this thread, entitled, "
The Effect of High Altitude and Center of Gravity on the Handling Characteristics of Swept-wing Commercial Airplanes" published in the "Flight Operations" [FO] section in the April, 1998 issue of
Boeing's AERO magazine.
The goal here is "continuing education" if you will, because the serious discussion on stalling heavy transport aircraft has become necessary as have some points regarding high altitude, high Mach number flight. There are some worthwhile papers which discuss this in great detail but are really tough sledding and the mathematics will be beyond most! I can't lead the discussion as HN39, gums and others might but I do wish to rejuvenate the discussion on the two items which continue to interest us all: Why the initial pitch up?, and the behaviour of a heavy transport aircraft in approach to, and in, the stall. Specifically, I am wondering if the low-damping forces of high altitude flight had anything to do with the eventual loss of control. -
PJ2
First, from the Boeing document:
"Maneuvering Stability
"Maneuvering stability, like static stability, is influenced by CG location. However, when the CG is aft and near the neutral point, then altitude also has a significant effect. Since air density has a notable impact on the damping moment of the horizontal tail, higher pitch rates will result for the same elevator deflections as altitude increases. From the flight crew's perspective, as altitude increases, a pull force will result in a larger change in pitch angle, which translates into an increasing angle of attack and g. While a well-designed flight control system, either mechanical or electronic, will reduce the variation of stick force with CG and altitude, it is very difficult to completely eliminate the variation due to design limitations.
"For example, for the same control surface movement at constant airspeed, an airplane at 35,000 ft (10,670 m) experiences a higher pitch rate than an airplane at 5,000 ft (1,524 m) because there is less aerodynamic damping. The pitch rate is higher, but the resulting change in flight path is not. Therefore, the change in angle of attack is greater, creating more lift and more g. If the control system is designed to provide a fixed ratio of control column force to elevator deflection, it will take less column force to generate the same g as altitude increases.
"This principle is the essence of high-altitude handling characteristics for RSS airplanes. Unless an RSS airplane has an augmentation system to compensate its maneuvering stability, lighter column forces are required for maneuvering at altitude. Longitudinal maneuvering requires a pitch rate, and the atmosphere provides pitch rate damping. As air density decreases, the pitch rate damping decreases, resulting in decreased maneuvering stability (see figure 2 and "Maneuvering Stability" below).
"An additional effect is that for a given attitude change, the change in rate of climb is proportional to the true airspeed. Thus, for an attitude change for 500 ft per minute (fpm) at 290 knots indicated air speed (kias) at sea level, the same change in attitude at 290 kias (490 knots true air speed) at 35,000 ft would be almost 900 fpm. This characteristic is essentially true for small attitude changes, such as the kind used to hold altitude. It is also why smooth and small control inputs are required at high altitude, particularly when disconnecting the autopilot.
Summary
"The use of wing sweep and stability augmentation on modern commercial airplanes makes them more fuel efficient. However, flight crews must understand the effects of CG and altitude on performance and handling qualities. For example, operating at an aft CG improves cruise performance, but moving the CG aft reduces static longitudinal and maneuvering stability. Many modern commercial airplanes employ some form of stability augmentation to compensate for relaxed stability.
"However, as long as the CG is in the allowable range, the handling qualities will be adequate with or without augmentation. An understanding of static and maneuvering longitudinal stability is an essential element of flight crew training." (my bold/underlining).
. . . .
"
Static Longitudinal Stability and Speed Stability
"STATIC LONGITUDINAL STABILITY
"The term "static longitudinal stability" is the name of the stability coefficient (Cm-alpha) for the pitching moment due to a change in angle of attack. In a stable, conventional airplane, the CG is forward of the neutral point of the airplane (wing plus tail). An increase in angle of attack from trim increases the amount of lift generated by the wing and results in an increasing pitch-down moment. This drives the airplane back toward its original angle of attack. If the CG is aft of the neutral point, increasing the angle of attack causes the airplane to pitch up, away from its original trimmed condition."
Next, from Davies; (NOTE 1: We have seen some of these illustrations from Davies before, posted by others during discussions on AoA, the stall and so on. I am providing pages 121 through 128. By referencing the so-called "deep stall", I am not implying that we have such here in AF 447 - I don't have the background to determine that. What I wish to provide is Davies' discussion on the broader elements of the deep stall, a discussion which I think is relevant to the behaviour of a heavy transport, regardless of kind of AFS installed. NOTE 2: "
Cm" is referenced in the last paragraph of the Boeing document, above.
*
Handling the Big Jets, D.P.Davies. 3rd ed. 1971. Civil Aviation Authority, London (OP)
