Stick Shaker & AOA System Anomalies - RECOGNITION AND RECOVERY TECHNIQUES

Stick shaker. An artificial stall warning system is required for airplane certification if the natural prestall buffet characteristics of the airplane are insufficient to warn the flight crew of an impending stall. This warning must be in a form other than visual to be effective, even if the flight crew is not looking at the instrument panel. Beginning with early commercial jetliners, standard practice has been to equip these airplanes with a stick shaker as a means of stall warning. Some airplanes also have employed stick nudgers or stick pushers to improve stall avoidance and stall characteristics. All these indications have been driven by an AOA threshold, which is usually a function of flap configuration, landing gear configuration, or both.

Because of the effect of Mach number on stall AOA, the stall warning AOA typically was set at a conservative level to accommodate gross weight and altitude variations expected in the terminal area.

It should be noted that the stall warning schedule does not follow the buffet boundary at very high Mach numbers. The buffet here is caused by Mach buffet, or too high a speed. Setting the stall warning system to activate at this point may lead the flight crew to believe the airplane is near stall and increase, rather than decrease, speed.

The early stall warning system thresholds were not set to be effective at cruise altitudes and speeds because they did not correct for Mach number. This kept the system simple. The stick shaker was set at an AOA effective for low altitudes but at too high a value for cruise. Natural stall buffet was found to give satisfactory warning at higher Mach numbers.

Later stall warning systems used Mach number from the pitot or static air data system to adjust the stall warning AOA threshold down as Mach number increased. This provided the flight crew with a stall warning related to the actual available performance. However, it also made the stall warning system dependent on good pitot and static data, a factor that will be considered in the next section on the dedicated AOA indicator.

SYSTEM ANOMALY RECOGNITION AND RECOVERY TECHNIQUES

Regardless of the nature of erroneous flight instrument indications, some basic actions are key to survival. The longer erroneous flight instruments are allowed to cause a deviation from the intended flight path, the more difficult recovery will be. Some normal procedures are designed, in part, to detect potential problems with erroneous flight instruments to avoid airplane upsets. Examples are the 80-kn call on takeoff and callouts for bank angle exceedances. In some cases the flight crew may need to recover the airplane from an upset condition: unintentional pitch greater than 25 deg nose high or 10 deg nose low, bank angle in excess of 45 deg, or flying at airspeed inappropriate for conditions. As the condition deteriorates, it becomes more dynamic and stressful. This stress increases the difficulty flight crews experience in determining, believing, and adjusting to using the correct instruments and ignoring the faulty instruments. Regardless of the situation, good communication between crewmembers is essential, and several basic actions are paramount:

Recognizing an unusual or suspect indication.

Keeping control of the airplane with basic pitch and power skills.

Taking inventory of reliable information.

Finding or maintaining favorable flying conditions.

Getting assistance from others.

Using checklists

ACCIDENT AND INCIDENT CASE STUDIES

Erroneous flight information such as the many and varied symptoms of pitot-static anomalies can confuse an unprepared flight crew. Because of the confusion caused by multiple and sometimes conflicting alerts and warnings, the flight crew may not recognize an air data error and may fail to respond appropriately. The following accidents and incidents show what can happen when a crew is confronted with unreliable or erroneous flight information.

In December 1974, a Boeing 727 crashed 12 min after takeoff while on a positioning flight from Buffalo, New York, in the United States. Three crewmembers were killed and the airplane was destroyed. The U.S. National Transportation Safety Board (NTSB) determined that the probable cause of the accident was flight crew failure to recognize and correct the airplane's high angle of attack and low speed stall. The stall was precipitated by the crew's reaction to erroneous airspeed indications caused by atmospheric icing blockage of the pitot probe. The pitot heat switch had not been turned to the ON position.

 In April 1991, the crew on a large corporate jet survived the following incident. On the previous leg, the captain's airspeed/Mach indicator and the standby airspeed/Mach indicator were erratic. The ground crew was unable to duplicate the problem. The next leg was at night in visual conditions. It was uneventful until the crew observed the first officer's airspeed/Mach indicator begin an uncommanded increase as the airplane climbed through FL310. Passing FL330, the captain's airspeed remained steady, but the first officer's airspeed pointer exceeded "barber pole," and the high-speed aural clacker activated. The autothrottles were disconnected, and at that point the captain's airspeed indicator began to show a decrease in airspeed that coincided with the standby airspeed/Mach indicator. Because of problems reported on the previous leg, the crew assumed that the captain's instruments were faulty. As the first officer's airspeed/Mach indicator kept increasing, the crew pulled the power back to silence the clacker, but the first officer's airspeed continued to increase and the captain's airspeed indicator continued to decrease. The airplane began to shake, which the crew assumed was high-speed Mach tuck. At FL340, the pitch was increased and stick shaker activated. The crew suddenly realized that they were entering a stall. While performing stall recovery procedures, they experienced severe vertigo, spatial disorientation, and confusion over determining the actual airspeed. Though the clacker was still sounding, fuel flow, attitude, and N1 were calculated for descent. Appropriate checklists were run and the circuit breakers were pulled to silence the clacker. Using calculated attitude and power settings, a descent, instrument landing system approach, and uneventful landing were accomplished. Maintenance later confirmed that the first officer's central air data computer had failed.

 In February 1996, a Boeing 757 crashed after takeoff from the International Airport of Puerto Plata, Dominican Republic. After climbing through 7,300 ft, the airplane descended until it crashed into the Atlantic Ocean about 5 mi off the coast of the Dominican Republic. All 189 people on board were killed, and the airplane was destroyed. Data from the cockpit voice recorder (CVR) and flight data recorder (FDR) indicate that the airspeeds displayed to the captain during the takeoff roll were incorrect and that the captain was aware of this during the takeoff roll. Nevertheless, the captain decided to continue the takeoff, and the first officer notified the captain when the airplane reached V1 and Vr. Shortly after takeoff, the captain commented that his airspeed indicator had begun to operate, even though it indicated unrealistic airspeeds. A normal climbout ensued, and the captain engaged the center autopilot. During the climb, at an altitude of 4,700 ft, RUDDER RATIO and MACH/SPD TRIM advisory messages appeared on the engine indication and crew alerting system display unit. For the next several minutes, the crewmembers discussed the significance of these advisory messages and expressed confusion about the airspeed. At an altitude of about 7,000 ft, the captain's airspeed indicator showed 350 knots, and an overspeed warning occurred, immediately followed by activation of the stall warning system stick shaker. Flight crew confusion about appropriate airspeed, thrust setting, and proper pitch attitude was evident as the airplane stalled, descended, and then crashed. The erroneous readings from the captain's airspeed indicator are consistent with a blocked pitot tube. Comments by the first officer recorded on the CVR suggest that his pitot probe was not obstructed, and he was seeing correct airspeed indications on his display.

 In October 1996, a Boeing 757 crashed into the Pacific Ocean about 30 mi off the coast of Lima, Peru. The flight crew declared an emergency immediately after takeoff because of erroneous airspeed and altitude indications and was attempting to return to Lima when the accident occurred. Data from the CVR and FDR revealed that the airspeed and altitude readings were normal during the takeoff roll. However, as the airplane began to climb, the flight crew noticed that the airspeed indications were too low and the altitude indications were increasing too slowly. Shortly after takeoff, the windshear warning activated, despite calm wind conditions and no significant weather activity. The flight crew declared an emergency and expressed confusion about the airplane's airspeed and altitude displays. Analysis of FDR data indicates that the airplane subsequently climbed to a maximum altitude of approximately 13,000 ft. When the airplane descended, the captain's altitude and airspeed displays were still erroneous, but at that point they indicated higher-than-actual conditions. During descent, the first officer's displayed airspeed slowed to the point of stall warning stick shaker activation. Meanwhile, the captain's airspeed read over 350 knots, and the overspeed warning was sounding. Flight crew confusion about airspeed and altitude was evident as the airplane continued its final descent. At impact into the Pacific Ocean, the captain's flight instruments were reading approximately 9,500 ft and 450 kn. The erroneous indications recorded by the FDR are consistent with a partial blockage of the captain's static ports.

 Three valuable lessons emerged from the investigations of these events. First, the effects of flight instrument anomalies appear during or immediately after takeoff. Second, flight crews must overcome the startle factor associated with rare anomalous events and immediately begin to implement specific corrective procedures and techniques. Finally, flight crews should acquire enough system knowledge to be able to determine the difference between valid and faulty display information.

AOA – Angle of Attack probe has been used as a primary performance parameter for years on some military aircraft, particularly on fighters. There are many good reasons for this.

In general, fighters operate more often at the extremes of the envelope, often flying at maximum lift for minimum radius turns. For other applications, AOA minimizes the pilot (usually single-place) workload by giving a simple target to fly. AOA is accurate enough for these applications. In addition, the higher sweep and lower aspect ratio of the wing reduce the sensitivity to AOA errors.

AOA has proved particularly useful for approach to aircraft carriers, where it is important to maintain a consistent approach attitude for each landing. In this case, 'backside' approach techniques are used, where glide path is controlled primarily by changes in thrust while the aircraft is held at a fixed AOA. Use of this technique during approach on commercial jet airplanes would be contrary to the pitch commands provided by the flight director bars, and to the speed hold mode of the autothrottle, which is often used during approach.