ince the early days of flight, The U.S. National Transportation 1 BASIC PRINCIPLES OF AOA angle of attack (AOA) has been Safety Board (NTSB) has recom- WHAT IS ANGLE OF ATTACK? AOA is one of the most important a key aeronautical-engineering mended visual indication of AOA parameters for understanding parameter and is fundamental in commercial airplanes. This airplane performance and handling to understanding many aspects indication may take the form of (see “What Is Angle of Attack?” on of airplane performance, stability, a dedicated AOA indicator or other p. 13) because a typical wing has a and control. Virtually any book on implementation, such as the PLI. limited range of angles of attack over these subjects, as well as basic texts A dedicated AOA indicator shown which it can function efficiently. In and instructional material written on the primary flight display (PFD) its simplest form, lift is a function of for flight crews, defines AOA and speed, air density, wing area, and recently has been developed in discusses its many attributes. AOA. At a given airspeed, as the AOA cooperation with airline customers. of a wing is increased, lift also will AOA can be used for many indica- The new indicator is offered as an increase (fig. 1). Therefore, at the tions on the flight deck to improve option on the 737-600/-700/-800/ same airspeed, a heavy airplane of flight crew awareness of airplane -900, 767-400, and 777 at this time. the same configuration must fly at a state relative to performance limits. During the development of the higher AOA than a light one. Conversely, Dedicated AOA indicators have been new indicator, discussions with as an airplane decelerates, the AOA used on military aircraft for many must be increased to maintain the airlines, the NTSB, and U.S. Federal years, but this form of display has same lift. So, in the normal oper- Aviation Administration (FAA) pilots not been used often on commercial ational range, there is a relationship and engineers provided a unique airplanes. On Boeing models currently among lift, speed, and AOA. opportunity to examine potential uses in production, AOA is used to drive This relationship will change if the AOA, FLIGHT PATH ANGLE, AND PITCH ANGLE of AOA and the many existing stall warning (stick shaker), stall AOA gets too high (fig. 1). The air uses that have evolved in recent margin information on airspeed indi- flowing over the wing will separate decades along with advances in cators, and the pitch limit indicator from the upper surface, resulting in display and indication technology. (PLI) on the primary attitude dis- a loss of lift, or a stall. It should be plays. AOA information is combined This article discusses the following: noted that this stall condition could occur at a wide range of speeds with other data and displayed as an 1. Basic principles of AOA. Angle of attack integral part of flight deck displays. (depending on the airplane weight or 2. Airplane performance and AOA. load factor, or g loading) and at any Velocity Pitch angle Recent accidents and incidents 3. AOA measurement. attitude (depending on the flight (flight path vector) have resulted in new flight crew path angle). What is important is the 4. AOA indications and flight crew Flight path angle training programs for upset recovery AOA. Therefore, it is imperative to Angle of attack (AOA) is the (still air) procedures in current Boeing and terrain avoidance, and these in know when the wing is approaching angle between the oncoming air or production models. the stall AOA and to take steps to turn have heightened industry relative wind and a reference line Horizon interest in AOA as a useful flight 5. Design and uses of a separate avoid it. on the airplane or wing. Sometimes, parameter for commercial aviation. AOA indicator. However, many other parameters the reference line is a line connect- influence the lift that a wing produces. ing the leading edge and trailing relative to the ground will be less The most basic is the configuration Flight path angle is defined in 1 LIFT VERSUS AOA edge at some average point on the two different ways. To the aerody- than that referenced to the air. On of the wing, specifically the position of wing. Most commercial jet airplanes FIGURE namicist, it is the angle between the newest commercial jet airplanes, the trailing-edge flaps, leading-edge use the fuselage centerline or flaps or slats, and spoilers. As the the flight path vector (where the this angle can be displayed on longitudinal axis as the reference Maximum lift trailing-edge flaps are extended, the airplane is going) and the local the primary flight display and is line. It makes no difference what curvature (or camber) and area of atmosphere. To the flight crew, calculated referenced to the ground Lifting range Stalled the wing are increased, and the wing the reference line is, as long as it it is normally known as the angle (the inertial flight path angle). is used consistently. will produce more lift at the same between the flight path vector and AOA is the difference between Lift AOA (fig. 2). Note that although the AOA is sometimes confused with the horizon, also known as the pitch angle and flight path angle maximum lift is increased, the AOA at pitch angle or flight path angle. climb (or descent) angle. Airmass- when the flight path angle is refer- which stall occurs is actually less Pitch angle (attitude) is the angle referenced and inertial-referenced enced to the atmosphere. Because of because the wing cannot sustain the between the longitudinal axis flight path angles are the same only the relationship of pitch angle, AOA, higher lift levels up to the same AOA. (where the airplane is pointed) in still air (i.e., when there is no and flight path angle, an airplane The airflow separates earlier. and the horizon. This angle is dis- wind or vertical air movement). For can reach a very high AOA even with Wing-mounted speed brakes or played on the attitude indicator example, in a headwind or sinking the nose below the horizon, if the spoilers have the opposite effect. Angle of attack or artificial horizon. air mass, the flight path angle flight path angle is a steep descent. They reduce the lift at a given AOA;

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