Takeoff and Landing Using AOA References
Using the Tone for Everyday Flying
AOA Challenges and Benefits
In 2014, NASA conducted a Review of Research on Angle-of-Attack Indicator Effectiveness. It concluded that “definitive works that determine the requirements for an AoA display were not found” and “definitive works to determine the requirements for training and for and AoA information were not identified in this review.” In addition to a lack of training resources, there are a couple of other challenges to note as AOA systems become more prevalent in a GA environment. First, not all systems are created equal—some do a better job of accurately measuring and displaying AOA and others are simply very good progressive stall warning systems. This is especially true of coefficient of pressure systems which are only as good as the sensor, algorithm used to process the information and how the information is conveyed to the pilot. Second, any system, no matter how good, is only as good as its calibration. The old axiom “garbage in = garbage out” applies.
Perhaps the biggest challenge is educating the GA community on how to apply AOA in daily flying. With the exception of takeoff and landing, most pilots spend most of their time right side up in the heart of the envelope at 1 G, ably pulled along by a reliable power plant or two. AOA is only an academic consideration discussed when practicing stalls. Even then, most pilots think in terms of “stall speed,” not critical alpha. Unusual attitudes and envelope limits are something to be avoided. For these basic operations, airspeed serves as an effective surrogate for AOA, until the pilot inadvertently loses aircraft control. Most GA pilots don’t spend much (if any) time upside down or concern themselves with energy management. If, however, we apply some of the concepts that glider and fighter pilots employ, we can mitigate loss of control risk during takeoff and landing and increase the precision with which we fly our airplanes. While it’s not a cure for cancer, the NASA study noted “...AOA can be a beneficial display and may be used in the following phases of flight: take-off, climb, turning, maximizing cruise, descent, final approach, low speed maneuvers, maneuvers to flare, landing as well as high G turns, approach to stall and identifying and recovering from stalls at low and high altitudes.”
The fighter community in the military was an early adaptor of AOA, and has considerable experience using AOA for aircraft control and energy management, including landing aboard ship. The term ONSPEED and its use come from the fighter community and can be adapted to any airplane with the right equipment on board. This article will explore an aural AOA cuing logic adapted from a successful military system that can provide useful performance and energy cues to the pilot and how to apply those cues during takeoff and landing.
With proper technology, AOA is particularly useful for precise aircraft control and energy management when operating at L/Dmax and slower. The accuracy of a properly calibrated coefficient of pressure AOA sensor increases as AOA approaches stall, whereas the accuracy of IAS decreasesunder the same conditions. We operate in this “maximum performance” region every time we take off and land the airplane. Every pilot can benefit from a better understanding of what’s occurring in terms of AOA during takeoff, approach and landing.
Reviewing the AOA Aural Logic
Reviewing the AOA Aural Logic. Figure 1 depicts the aural AOA logic. The right side of the diagram is “fast”, and the left side is “slow.” Notice I just used the terms fast and slow in conjunction with AOA—alpha is measured in degrees, or even referred to in non-dimensional units; so, it should properly be referenced as “high” or “low.” If, however, our objective is to build a pilot-friendly mental model, it’s much easier to think in terms of ONSPEED, fast or slow when we are operating using AOA cues. In this simple sense, AOA and speed are interchangeable when we think about our energy state.
When the pilot is using the steady ONSPEED tone, the variable pulse rate and frequency difference allows the pilot to easily distinguish between ONSPEED, “slightly fast” and a “slightly slow” condition. Figure 2 is another way to visualize the information the tone conveys and relates the different tones to some familiar 1G approximate airspeed references during takeoff and landing.
Depending on configuration, the aural logic is controlled by a simple mechanical rotary switch that serves as both ON/OFF and volume control or software controls. The volume is fully selectable, and the system may be turned off at any time. Speeds at which the tone becomes active during takeoff roll or shuts off after landing are selectable by the pilot. To achieve usable cues for takeoff, the speed selected should be slow enough to provide a transition to ONSPEED to allow the pilot to listen to the aircraft approaching optimum angle of attack. It’s recommended that this speed should provide 10-15 kts of margin during acceleration and deceleration in takeoff and landing, i.e., be working at Vs -10 to 15 kts. Because I prefer to listen to the entire acceleration and deceleration sequence, my personal setting is on and off at 10 kts IAS.
Takeoff Using Aural AOA Cues
More pilots lose control during takeoff and initial climb segment than come to grief during approach and landing. Having usable, calibrated AOA cues available during takeoff provides performance feedback during this critical phase of flight.
First, let’s consider how the aural AOA logic works as we accelerate through takeoff. The tone becomes active as the airspeed increases during the takeoff roll. In my airplane, that occurs at 10 KTS IAS. At that speed my airplane is deep in the stall region, so I hear a stall tone. But as the airplane accelerates, the stall tone changes to a “slow” tone and the pulse rate of the beeps gradually decreases until the airplane is at ONSPEED AOA. When I rotate to 10-12 degrees, takeoff and continue to accelerate, the tone changes to a “fast” tone until I’m faster than L/Dmax (which is very close to Vfe in my airplane). If performance isn’t critical and obstacles aren’t a factor, this takeoff technique is sufficient in my low power-loaded (HP/pound) airplane that has good climb performance margin over a wide speed range.
What if we need to extract maximum takeoff performance from the airplane? As private pilots, we demonstrate proficiency in short and soft field takeoff operations and learn about best angle of climb (VX) and best rate of climb (VY) speeds. We learn how to read the manufacturer’s charts to determine those speeds based on gross weight and density altitude and estimate performance. The manufacturer provides technique or procedural discussion on how to takeoff. If we fly an EAB type, we may or may not know what those indicated speeds are with precision, because the quality of flight test data for individual airplanes (every one of which is different), varies widely. Takeoff performance charts may or may not be available. There may or may not be a pilot’s handbook with technique or procedural takeoff information. This is why a useable (ergonomic, properly damped) AOA cue can be beneficial in any airplane but in an EAB type in particular: recall that ONSPEED and L/DMAX AOA are designed into the airplaneand known precisely by the designer or anyone that conducts an analysis of the airplane. They are the same AOA for every airplane of that type. Thus, if the pilot knows when the airplane is ONSPEED or at L/DMAX and combines that with some target pitch cues, it becomes relatively simple to precisely manage energy during takeoff and initial climb. Precisely managing energy means converting power into an optimum combination of altitude and airspeed to get the airplane through the initial climb segment as rapidly and safely as possible for a given weight and density altitude—exactly what an airliner does using performance information derived from engineering analysis and flight test. Hopefully, it’s intuitive that if we get to altitude in the most efficient manner, we’ve got more options, sooner if the propeller stops turning. We are also converting BTUs into altitude as efficiently as possible.
Maximum Performance Takeoff Using the AOA Tone
Best angle of climb occurs ONSPEED, and best rate of climb occurs at L/Dmax. If obstacles are a factor, then initial climb should be performed at best angle of climb speed until obstacles are cleared, at which point pitch is reduced and the airplane is accelerated to continue the climb. Figure 3 depicts an AOA-based technique that optimizes energy during takeoff, allowing obstacles to be efficiently cleared and then altitude gained as rapidly as possible to complete the initial climb segment. It is impossible to provide a specific rotation rate (in degrees/sec) or exact pitch angle, because that will vary from airplane to airplane. For example, in my 160 HP RV-4 equipped with a fixed-pitch propeller, I use a 3 deg/second rotation rate to 15 degrees of pitch. In airplanes with less performance, a slower rotation to a lower pitch angle may be appropriate and vice versa for a more powerful airplane. Some experimenting is required to determine the best way to rotate and capture ONSPEED for initial climb in your airplane, but ONSPEED AOA remains constant under all conditions.
Figure 4 is a similar energy efficient technique if obstacles are not a factor. In this case, a lower rotation pitch angle is established as the airplane transitions through ONSPEED, and the airplane continues to accelerate to L/Dmax after liftoff and a positive rate of climb is established.
Loss of Control During Takeoff
The aural AOA logic assists with maintaining aircraft control during takeoff. Some high-performance airplanes (including most of the Van’s types), become less stable in climb at high pitch attitudes with high power. Any tone slower than desired climb speed would tell the pilot to reduce pitch. For example, if the airplane is supposed to be at maximum climb angle (ONSPEED), any “slow” tone would indicate excessive pitch. Similarly, if the airplane is supposed to be at a maximum rate of climb (L/Dmax), any increase in pulse rate tells the pilot that pitch is too high. Another energy problem occurs if there is a loss of power during climb—especially a steep climb attitude. In this case, considerable pitch change may be required to maintain aircraft control and establish a best glide condition. A good energy management rule of thumb is “angles = angles” so if the airplane is pitched up 15 degrees when power is lost, then it’s probably going to take a push to -15 degrees to establish a best glide condition (that’s a 30 degree pitch change!). The aural AOA logic assists the pilot in this case: if power is lost, adjust pitch to establish L/Dmax or ONSPEED (if maximum endurance glide is desired)—the slow tone will keep you honest if you don’t get the nose down far enough.
Approach and Landing Using Aural AOA Cues
Approach and Landing Using Aural AOA Cues. Fly ONSPEED for approach and landing. ONSPEED AOA is always the same regardless of gross weight, G load (bank angle) and density altitude. To land using the aural AOA cues, the airplane is slowed to ONSPEED, configured for landing and ONSPEED is maintained until the flare. Figure 5 shows an energy efficient pattern that allows the pilot to use a simple technique: pitch to control tone (AOA), power to control glide path and bank angle to control ground track. Because the airplane is configured for landing, and a constant AOA is desired, the pilot need only concentrate on flying the airplane from the point the base turn is begun until landing roll-out is complete. The continuous base turn minimizes the chances of over-shooting final and makes wind adjustments intuitive. 10-12 Seconds on final approach allows the pilot to analyze and correct for cross-wind and stabilize parameters prior to touchdown. Exact glidepath flown depends on the airplane and whether or not any power is used during the base turn and final approach. This pattern is very similar to the “180 power off approach” you practiced for your private check ride.
Loss of Control During Approach and Landing
Because most pilots spend most of their time at 1 G and typically utilize limited bank angles for “normal” flight, many aren't well tuned to effects of G load on IAS for stall. The aural AOA logic provides immediate feedback if the pilot increases bank and fails to relax back pressure to maintain a constant AOA. Pilots used to flying IAS without AOA cues or advanced instrumentation don’t always understand when they are eating into the aerodynamic (energy) margin when turning. Some EFIS depict this as a variable airspeed “foot” that moves up and down relative to stall IAS. If properly calibrated and programmed, this type of visual speed indication provides a utility similar to the “slow” tone of the aural AOA logic, although it requires the pilot to look inside the cockpit. Stay faster than the “foot” and you have sufficient margin to avoid a stall. Another consideration is carrying too much energy into the landing transition: too fast can be as dangerous as too slow. ONSPEED is the right energy state for landing transition: not too fast or too slow, and not affected by ambient conditions. When you can hear an ONSPEED condition (and fast or slow cues), it’s caveman simple to manage pitch without looking in the cockpit— any slow tone tells you that you are operating in the margin.
Adjusting Vref/Vapp for conditions is a technique that applies to using IAS as a reference for approach and landing. ONSPEED AOA is not affected by ambient conditions. However, there is a limit to how precisely a pilot can control AOA and airspeed under turbulent or gusty conditions. A “slightly fast” approach (AOA = Vapp [Vref + 5]) may be utilized until transition to ONSPEED landing to assist with mitigating the need for a high gain power or control input that can lead to over-control.
Summary. Let’s see how the aural AOA logic works in areas that NASA thinks AOA information can be helpful. Click on the link to a view a video demonstration of each maneuver: