Understanding The Angle-Of-Attack Indicator
Sep 22, 2016 Fred George | Business & Commercial Aviation
Understanding Angle of Attack
The FAA has been actively promoting angle-of-attack (AoA) indication systems as a means of reducing general aviation loss of control (LOC) accidents for several years. An analysis of nearly 2,500 general aviation accidents that occurred from 2001 to 2010, mostly in light aircraft, pointed to LOC as the leading cause of the mishaps, which killed 1,259 people during that period. LOC accidents continue to kill about 175 people in general aviation aircraft every year. An FAA working group concluded that AoA systems could help reduce the incidence of these accidents and save lives.
Government regulatory and accident investigation agencies also believe AoA indication systems have the potential for improving safety in jets. A series of upsets in commercial aircraft, including the crash of Air France Flight 447 in June 2009 that killed all 216 people on board, also piqued renewed industry interest in AoA indicators for civil jets as well as focusing on the need for upset recovery training.
France’s Bureau d’Enquêtes et d’Analyses pour la sécurité de l’aviation civile (BEA), the agency that investigated the AF447 accident, recommends “that EASA and the FAA evaluate the relevance of requiring the presence of an angle-of-attack indicator directly accessible to pilots on board airplanes.”
The pilots of that ill-fated Airbus A330 lost control of the aircraft at FL 350 in night IMC over the Atlantic Ocean north of Brazil, when the pitot tubes likely became obstructed with ice crystals, causing the aircraft’s primary flight displays, digital fly-by-wire (FBW) flight control system and autopilot to become degraded or inoperative.
Pitot tube icing also caused the A330’s digital FBW system to lose its normal control law functionality, including flight envelope protection. The system downgraded to an alternate law in which there was stall warning, but no stall protection.
tsarkar wrote:Note for those who believe that FCS is foolproof
The startled and confused pilots made erroneous control inputs, causing the aircraft to climb quickly above its lift ceiling, according to the BEA. The stall warning system briefly was triggered twice during the ascent. Then the stall warning came on continuously as the aircraft reached about 6 deg. AoA. But the crew was apparently unaware of their hazardous situation.
The nose-up sidestick inputs also caused the elevator to trim nose up. The aircraft subsequently entered into a fully stalled condition, with pitch attitude at 15-deg. nose up and AoA pegged at 35 to 40 deg. as the aircraft plunged toward the ocean at more than 10,000 ft. per minute.
Passing through FL 350, the aircraft decelerated so precipitously that the stall warning system transitioned from the air to the ground mode, causing the stall warning system to silence. The pilot flying finally pushed the sidestick forward, causing airspeed to increase and again triggering stall warning as the stall warning system went back into the air mode.
But it was too late. Little more than 4 min. after the big screens went blank, the aircraft crashed into the water at a descent rate of nearly 11,000 fpm and with a forward speed of only 107 kt.
The BEA notes that the crew of AF447 “never formally identified the stall situation” and that AoA indications were “not directly accessible to the pilots. Thus, “It is essential in order to ensure flight safety to reduce the angle of attack when a stall is imminent. Only a direct readout of the angle of attack could enable crews to rapidly identify the aerodynamic situation of the airplane and take the actions that may be required.”
But few current production civil jets have direct AoA readouts. On most models, AoA mostly is used to drive stall margin and wind-shear escape indicators. Boeing, though, notes that AoA has been “a key aeronautical-engineering parameter” and is “fundamental to understanding . . . performance, stability and control.”
Angle of attack may be defined as the angle between the wing’s mean aerodynamic chord and the relative wind or flight path vector. Wing leading edge upwash and trailing edge downwash, among other factors, determine the true AoA of the wing. But aboard most jets, it’s measured indirectly, as AoA sensors typically are mounted on the nose or forward fuselage of the aircraft, rather than on the wing.
The FAA says that AoA is “a better parameter to use in avoiding a stall” because the airplane always will stall at the same angle of attack for a given configuration. That statement is actually incorrect. In truth, critical or stalling angle of attack also varies as a function of local Mach number over the wing, notes veteran aerodynamicist David Lednicer, vice president-engineering at Aeromechanical Solutions LLC in Redmond, Washington. The stalling AoA during high-altitude cruise may be as much as 6 deg. to 7 deg. lower than at sea-level AoA due to local Mach flow over the wing. Aboard AF447, the BEA notes that the spread between AoA in normal cruise at Mach 0.82 and stall warning was about 1.5 deg. at FL 350.
Available lift indeed is a function of angle of attack, local Mach number over the wing, air density and wing area. Holding constant other parameters, AoA is a direct measure of available lift and stall margin, assuming it’s normalized for configuration and air data inputs. AoA indicators also can respond more quickly than airspeed indicators to the effects of stick and throttle inputs. An AoA-based instrument also can respond more quickly to other variables that affect airfoil performance, such as high lift and drag device configuration changes.
AoA indicators and “indexers,” the three-color, three-symbol displays atop glareshields, long have been used by U.S. Navy aviators to control aircraft performance precisely when making carrier landings. The mantra of “Meatball [improved Fresnel lens optical landing system], Line Up [on centerline] and Angle of Attack” is drilled into the heads of student Navy aviators early in undergraduate pilot training. It’s also used as an effective gauge of lift performance during basic fighter maneuvers.
Normal and Extraordinary AoA at Low Altitude
The maximum lift that a wing can produce at a given AoA very much is determined by secondary flight control configuration, including the position of trailing edge flaps that increase wing curvature, leading edge devices (if installed) and speed brake/ground spoiler extension. Leading edge contamination, most particularly that caused by ice accretion, can cause large-scale degradations in available lift. Aircraft with relatively high wing loadings and having no leading edge devices especially are prone to reduction in available lift at high angles of attack.
At low altitude, a high-performance business jet wing may not stall until local AoA reaches 16 deg. to 18 deg., or more, depending upon high lift configuration. When extended, trailing edge flaps increase lift per degree of angle of attack, but stall actually occurs at a lower maximum AoA because of airflow separation. Leading edge slats, in contrast, enable the wing to achieve a high stalling AoA because they delay airflow separation.
During takeoff and landing, an aircraft flies relatively close to its stall margins. On takeoff, the certified one-engine-inoperative (OEI) V2 safety speed most typically ranges from 1.13 to 1.2 times the stalling speed in a given configuration. So, V2 is considerably slower than best lift-to-drag ratio. If the aircraft were allowed to accelerate to best lift-to-drag ratio, OEI climb performance would be reduced and the aircraft would not climb at the published AFM gradient.
On landing, Vref ranges from 1.23 to 1.3 Vso. As a rule of thumb, lift coefficient increases by about 0.1 for each degree of AoA increase from some reference point, but drag also rises substantially.
Virtually all business aircraft with digital FBW flight control systems have full envelope protection that prevents the aircraft from reaching aerodynamic stall or exhibiting unacceptable handling traits at high AoA. Even with the sidestick or control yoke held fully aft at low speed, the FBW system will nudge down the nose sufficiently to prevent a stall or untoward handling behaviors.
If an aircraft has a conventional flight control system and if its natural pre-stall warning doesn’t provide clear indications of the impending stall, an artificial stall warning system, such as a stick shaker and audible alert, may be required. If the aircraft doesn’t pitch down positively at the stall as a result of aerodynamic forces, a stall recovery system that forces down the elevator may be required. In older Learjets, for instance, the stick shaker is triggered at about 1.07 stall speed and the stall prevention stick pusher fires 5% before stall.
Notably, wings develop more lift per degree of AoA in ground effect, but stall, or CL max, occurs at 1.5 deg. to 2.0 deg. lower angle of attack than in free stream air. Such differences in stalling AoA must be built into the stall barrier system to provide adequate safety margins during all phases of flight.
The Safe Flight SCc Lift Transducer precisely measures the wing’s leading edge stagnation point and airflow. The Indexer Computer is an AoA indicator that incorporates LEDs in the display, making it easy to read in all lighting conditions and features a pilot adjustable Reference Marker for setting AoA targets. Credit: Safe Flight
With a fundamental understanding of AoA, it’s reasonable to ask why the aviation industry hasn’t made the transition from indicated air speed to AoA as the primary performance indicator for target V speeds. Years ago, for instance, Safe Flight Instrument Corp., a pioneer in wing lift sensors and systems since 1946, offered systems that drove the pitch command bars on electromechanical flight directors installed in certain aircraft to help pilots maintain the optimum angle of attack for V2 during a one-engine-inoperative takeoff. But airframe and avionics manufacturers backed away from such systems when they upgraded to glass cockpits and integrated avionics systems and certification rules became tougher.
Of course, Safe Flight continues to manufacture a full line of AoA and stall warning devices for aircraft ranging from light general aviation models to fully integrated ultra-long-range business jets, jetliners and military planes.
Gulfstream veteran test pilot Tom Horne points to two basic reasons for using reference V speeds rather than AoA as primary reference. First, until the advent of the new generation of solid-state pitot, AoA and yaw sensors, such as UTC Aerospace SmartProbes, AoA sensing systems couldn’t match the data integrity, functional reliability and redundancy of conventional pitot-static systems. If an AoA probe or vane iced up or suffered damage, there wasn’t a secondary, cross-side or alternate tertiary backup sensor that could be used to provide AoA information to the pilot flying. The AoA indicator just became inoperative. In contrast, most transport category aircraft have dual air data systems, plus a third backup air speed indication, providing triple redundancy.
Second, reference V speeds not only are based on optimum wing angle of attack, they also must take into account Vmca and Vmcg minimum control speeds for adequate direction control as well as minimum speeds to prevent tail strike on takeoff and/or landing. They also are predicated on maximum allowable pitch, roll and yaw control force and lift performance degradation due to probable ice accretion as defined by the Flight Into Known Icing regulations.
Takeoff and landing reference speeds thus are a balance of wing performance, stability and control requirements, and tail strike protection, among other factors. Yet, a basic tenet remains: As weight increases, so does stalling speed, resulting in an increase in reference V speeds above a minimum allowable point.
Therefore, the aviation industry still uses indicated airspeed as the primary performance reference during normal takeoffs and landings. Horne explains that aircraft manufacturers potentially could make the transition to an AoA system that could be compensated, or normalized, for all the other factors. But abandoning the time-proven V speed system could be complex and costly to develop and certify with limited tangible benefits for operators.
The reliance on reference V speeds, though, gets tossed out in the event that the flight crew has to extract maximum lift performance out of the aircraft or risk a crash, such as during a wind-shear recovery maneuver or low-altitude traffic avoidance maneuver. That’s when AoA indication systems become a primary performance reference.
But AoA indications for stall or upset recovery must be instantly intuitive. This is no time to nuance your interpretation of an angle-of-attack gauge reading as though you were a veteran Navy carrier pilot. The AoA-based PFD airspeed scale low-speed reference tapes and pitch limit indicators on civil jets provide easy-to-read cues for extracting maximum lift performance without stalling the aircraft. Such cues allow flight crews to fall back on so-called “rule-based behaviors,” according to the Netherlands’ Nationaal Lucht- en Ruimtevaartlaboratorium (National Aerospace Laboratory, NLR).
The typical technique for wind-shear escape, for instance, calls for the pilot flying initially to pitch up to 15 deg. and push up the throttles to takeoff/go-around, then follow the guidance of the pitch limit indicator (PLI) on the PFD. The NLR experimented with other techniques during six-axis simulator trials, including using AoA indicators, but nothing proved so simple and consistently effective as this rule-based behavior.
Normal and Extraordinary AoA at High Altitude
Aerodynamically, the minimum drag point occurs at the highest lift-to-drag ratio. Before the advent of modern super-critical airfoils, jet aircraft typically would cruise at or below the critical Mach number, the indicated speed at which local flow over some part of the aircraft, usually the upper surfaces of the wings, reached the speed of sound.
With subsonic local flow over the wings, flying at a constant, optimum angle of attack would yield the best lift-to-drag performance throughout a wide range of cruising altitudes. If, for instance, a first-generation Falcon Jet or Hawker were flown at an optimum angle of attack, it would be possible to eke out more miles per pound of jet fuel than if it were flown at a constant indicated Mach number. (See sidebar.)
In contrast, virtually all current-generation civil jets have supercritical airfoils or semi-supercritical airfoils. This means that the airfoil is designed to operate efficiently with local airflow greater than Mach 1 over a large portion of the wing chord. The wing actually produces more overall lift and lower drag than it would with subsonic flow.
Supercritical airfoils still are most efficient at the best lift-to-drag ratio, as are subsonic airfoils. But the optimum cruise speed is a function of both Mach number and AoA. Flying the aircraft at the optimum Mach number for fuel efficiency thus causes AoA to decrease as aircraft weight decreases. Flying at a constant angle of attack would cost range. Maximum range performance also depends on the thrust output and specific fuel consumption of the engines.
Boeing engineers point out that adjusting cruise speed for winds aloft is more critical to maximum specific range than flying constant AoA. Pilots are advised to fly faster into a headwind and slower with a tailwind to squeeze out optimum range, as shown on the specific range predictions programmed into a full-function FMS.
While cruising at higher Mach numbers increases lift up to some point where drag rises sharply, it also reduces the absolute AoA at which stall occurs. However, there’s usually a 40+ kt. spread between cruise speed and stall warning in modern jets.
That’s all fine for normal cruise. But what about high-altitude upsets? FlightSafety International instructor Mark Scott is fond of saying that integrated glass cockpits have caused pilots to develop instrument stare patterns rather than instrument scan patterns. They tend to fixate on the PFD rather than having to actively monitor the six-pack of conventional instruments. If both PFDs become inoperative, as in the case of AF447, attitude reference, stall margin indication, airspeed, altitude and vertical speed vanish. Pilots must revert to using small emergency instruments or scanning a diminutive integrated standby instrument. AoA awareness information is denied to them, unless the aircraft has a separate angle-of-attack instrument. But if Mach corrections aren’t provided to the AoA system, its usefulness is severely degraded.
Startle factor has the potential to cause pilots to make rash control inputs and thrust changes in the event that the big screens go black. Some potential causes of high-altitude upset can be anticipated, including flight through weather fronts, jet streams and mountain wave, along with pronounced temperature aloft changes, towering cumulus, high-altitude icing and passing through the intertropical convergence zone.
Yet, AoA awareness still is critical during a high-altitude upset, according to the FAA and EASA. The BEA notes that pilots of highly automated aircraft “generally just undertake” flight path and systems monitoring because of the performance and overall reliability of their aircraft, focusing on navigation and fuel management. They’re not necessarily aware of angle-of-attack margins. And they’re not prepared for high-altitude upset recovery.
If the PFD isn’t available, it’s critical to make the transition to the pitch attitude reference on the emergency gyro or integrated standby instrument without delay. Most business aircraft cruise at 2-deg. to 3-deg. nose-up pitch attitude. It’s important to know cruise power settings so that thrust can be maintained or slightly increased to hold altitude. If the aircraft does have an independent AoA indicating system, it can be a useful cross-check to holding target pitch attitude during cruise, assuming pitch and thrust are stable.
If the aircraft does stall inadvertently, it’s essential to reduce pitch attitude smartly, sacrificing altitude for airspeed. During the recovery, it’s also important to keep in mind the considerably lower stalling AoA at typical high cruise altitudes than at low altitudes. The load factor available to recover from a nose-down attitude may be as low as 1.3 Gs.
Bottom line? Angle-of-attack awareness is key to risk management during all phases of flight. It’s critical during low-altitude stall recovery and wind-shear escape maneuvers. We at BCA also believe it’s valuable for flying the appropriate Vref speed, but we recognize that minimum control speed and tail strike prevention, among other factors, must be considered when computing Vref. So, we fly the published AFM Vref speeds instead of AoA, unless the manufacturer otherwise advises.
The optimum aerodynamic lift-to-drag ratio in a modern jet depends both on Mach number and AoA, among other variables. Extracting maximum specific range also depends upon engine thrust output and fuel consumption characteristics.
A stand-alone AoA indicator can be a valuable addition to the cockpit as a backup for Mach and airspeed indications in case of air data system failures. If the gauge is normalized for Mach effect and AoA rate of change, it also has potential to be a valuable stall recovery tool at altitude, assuming pilots are properly trained in its use. But Mach compensation is unlikely to be available in the event of air data system failure.
Thus, controlling angle-of-attack in most jets during high-altitude cruise relies mainly on indirect indications, including pitch attitude, rate of pitch attitude change, airspeed and Mach, plus altitude. AoA awareness is essential in all phases of flight, but it complements reliance on traditional primary flight instruments.
Originally published August 25, 2016