Aerodynamic wing stall accidents have been a concern since the canard was removed around 1910. Approximately 40 percent of General Aviation fatal accidents are due to loss of control with the majority of those being from stalls. One element of stall awareness training that’s not often taught is the details of sensors used to detect an aerodynamic wing stall and then how those sensors are used in a stall-warning system. Let’s fix that shortcoming.
Stalls can happen at any airspeed, but only one angle of attack, the critical angle of attack. The airspeed changes with numerous variables including weight, load factor (bank/ G loading), center of gravity, air density, wing contamination (ice), etc. The wing’s angle of attack is the critical factor in determining how close the aircraft is to an aerodynamic stall. Therefore, an angle-of-attack (AoA) sensor is the ultimate stall warning and protection system.
Angle of Attack Sensors
Angle-of-attack sensors can be as simple as the lift detector switch tab or the Cessna reed. Both of these devices work on the principle that as the angle of attack changes, the stagnation point on the wing changes. The stagnation point is where the wind separates into the flow above the wing and the flow below the wing. As the angle of attack is increased the stagnation point moves aft.
It’s easiest to visualize the lift detector switch tab. In cruise flight, the tab is pushed back/down by the relative wind going under the wing. As the angle of attack increases the stagnation point moves aft (or down) and as the angle of attack is increased further, the stagnation point moves behind the tab. With the stagnation point behind the tab, the airflow will push the tab forward/up. When the tab is forward, a microswitch closes and the stall warning is activated.
Leonard Greene invented the lift detector in the 1940s and modernized versions are still used today. Instead of simply detecting if the tab is forward or back a lift transducer incorporates a sensor to determine the force on the tab and its position. This provides a measurement of the angle of attack around the critical angle of attack (CL-Max).
The movement of the stagnation point is greatest near the stall. This makes a lift detector or lift transducer accurate and repeatable. Of course, the positioning of the lift detector or transducer on the wing is critical. If the lift transducer is part of the certified stallwarning system, the positioning is determined during flight test.
Larger aircraft typically use an angle-ofattackvane on the fuselage. An angleof-attack vane is basically a weathervane. The AoA vane protrudes into theairstream to align with the local airflow.While basically a weather vane, the internaldesign is much more complicated.The external vane is counterbalancedto improve low-speed sensitivity and isdamped to eliminate any jitter or flutter.The external vane itself is heated for usein icing conditions.
Airflow studies using computer aided design tools are used to determine the best position for mounting the AoA vane on the aircraft. After mounting, flight testing will be done to determine the local angle of attack that produces CLA-Max. The actual free airstream differs from the local airstream due to the effect of the fuselage/radome and wing faring. Depending on the aircraft, the free airstream airflow will start to be affected by the airframe several feet in front of the aircraft. However, the AoAmeasured by the vane is the local AoA. The local AoA on the nose of the aircraft is typically two times more sensitive than the free airstream AoA. The vane angle at the stall is recorded for each flap position while the aircraft is in flight-test status.
AoA from Differential Pressure
The SmartProbe from UTC Aerospace uses differential pressure to determine the angle of attack. AoA ports are added to a pitot/static probe to become an integrated air data probe. In cruise flight the pressure on the top of the tube will be close to the pressure on the bottom of the tube. Small ports (holes) are made in the tube to route the air pressure back to a computer to measure the pressure difference and calculate the angle of attack. As the angle of attack is increased, the underside of the tube will have a higher pressure than the top. Again, through flight testing CL-Max will be determined and the differential pressure at the stall will be measured.
Lower-cost differential-pressure angle-of-attack systems are available from Alpha Systems, Garmin, and others.
It’s possible to calculate AoA based on inertial data. Aspen Avionics uses such a derived AoA. Adding AoA to Aspen’s Evolution display does not require any additional sensors. In November 2017 the FAA published a paper titled “Flight Test Results of Direct Measure and Derived Angle-of-Attack Systems for General Aviation Airplanes.” The paper features Garmin’s “sensed AoA” and Aspen’s “derived AoA.”
Because of the unique characteristics of angle of attack, all “artificial” stallwarning systems will utilize AoA as the primary input. The stall-warning system for a light aircraft can be as simple as we’ve discussed. Some Part 23 business jets have a single AoA sensor vane, where more sophisticated stall-warning systems will utilize two independent stall warning computers and two AoA sensor vanes.
Stall-warning systems beyond the most basic will include flap position (CL-Max, the critical AoA, changes with flap position) and an input to adjust the warning for ice accretion. This can be from an ice detector or the anti-ice system being turned on. Landing gear position is incorporated in some systems and Mach compensation is used in most jet stall-warning systems. Weight on wheels logic is usually used to disable the stall-warning system on the ground.
How are dual stall-warning systems better than a single system? First, there is redundancy. One side can fail and you still have stall warning on the opposite side. If either system calculates that a stall is imminent then that side will issue the warning. In a yaw, one system will indicate a higher angle of attack than the other. The higher AoA wins and a stall warning is issued.
Traditionally in air transport aircraft, the stall warning is done with a stick shaker. This device vibrates or shakes the yoke to simulate the feeling of a stall buffet. It is very noisy and the combination of the shaking and noise usually wakes up any pilot. Light aircraft use a stall warning horn while some of the later systems have a voice call out of “Stall”. Early Piper Cherokees had a red annunciator light to indicate a stall, but that is no longer deemed adequate for new aircraft certifications.
What Do You Fly?
We’ve covered the basics of a stall-warning system, but you should dig into the details of the system on your aircraft.
• Is a preflight check required?
• Is it usable in icing conditions? If so, is there an “advancement” of the stall warning to account for aerodynamic changes with ice? (Many older stall-warning systems were certified before this requirement.)
• If it is a later system, what activates the advancement of the stall warning for icing conditions?
This is Important
You might routinely dismiss the stall warning system as insignificant. However, being a simple and reliable system doesn’t make it insignificant. There was a fatal Phenom 100 stall accident on December 8, 2014, in Gaithersburg Maryland that illustrates this point.
The Embraer Phenom 100 stall warning and protection system has an “ice schedule” that adjusts parameters when deicing equipment is turned on. From the NTSB’s report: CVDR data show that, before beginning the descent, the pilot set the landing reference speed (VREF) at 92 knots, indicating that he used performance data for operation with the wing and horizontal stabilizer deice system turned off and an airplane landing weight less than the airplane’s actual weight. Using the appropriate Normal Icing Conditions checklist and accurate airplane weight, the pilot should have flown the approach at 126 knots (a VREF of 121 knots +5 knots) to account for the icing conditions.
The NTSB found that the pilot’s failure to use the deice system during the approach led to ice accumulation, an aerodynamic stall at a higher airspeed than would occur without ice accumulation, and the occurrence of the stall before the aural stall warning sounded or the stick pusher activated. Because the deice system was not activated by the pilot, the band indications (low-speed awareness) on the airspeed display did not appropriately indicate the stall-warning speed. The NTSB’s aircraft performance study found that there would have been sufficient warning of a stall had the device system been used during the approach. Once the airplane stalled, its altitude was too low to recover.