What is ONSPEED and How Does the AOA Tone Work?
ONSPEED is an angle-of-attack (AOA) that is used for approach, landing and maneuvering. ONSPEED AOA is always the same. It’s not affected by bank angle (G load), gross weight or density altitude and is designed into the airplane. ONSPEED AOA is not an airspeed. It is optimum AOA for approach and landing: not too fast and not too slow. It helps turning base to final and achieving stable parameters for landing. The AOA tone lets the pilot hear ONSPEED and provides easy to interpret cues to make pitch corrections. When the airplane is banked, the tone automatically compensates—all the pilot has to do is adjust pitch based on tone cues to maintain a safe, optimum AOA for maneuvering, approach, and landing. If the pilot maintains ONSPEED, the airplane cannot stall.
ONSPEED AOA provides an optimum energy condition. Optimum energy occurs when the blend of airspeed, power and turn performance is “just right” in Goldilocks terms…pull any harder on the stick or yoke and you are expending energy (usually airspeed) for no gain in performance, don’t pull hard enough and you are too fast for landing. It’s about the AOA at which maximum power is available in a propeller-driven airplane.
Not too many topics can start as much discussion amongst pilots as the age-old discourse of angle of attack vs airspeed. Like most topics in aviation, the unemotional answer depends on what you are trying to accomplish (or what mental model you’re building). If you are fast and maneuvering, say descending from cruise altitude or starting a loop, then airspeed is your primary energy reference. But if you are slow, banked up and turning base to final with an overshooting wind, then using optimum maneuvering AOA as a reference makes more sense. Optimum AOA isn’t affected by gross weight, G load/bank angle or density altitude—it’s always the same: no cockpit math required. With the right calibrated AOA system, it’s easier to maneuver and land the airplane than computing and referencing airspeed—just do what it takes to maintain ONSPEED. The best way to frame the AOA vs airspeed discussion is to think of the two as complimentary concepts—know when to apply each one, and how to blend them. As a basic rule, if you are faster than L/Dmax it makes sense to think in terms of airspeed and G, and if you are L/Dmax or slower, then it makes sense to think in terms of AOA. If you are slowing down, it’s usually is a matter of transitioning from airspeed to AOA references.
Energy management is another term that can either confuse or engender more “what the hell are you talking about?” discussion. It’s a skill set that is hard-wired into fighter and glider pilots due to the nature of the flying that those folks do, but not really well taught or applied in other aviation communities. In its simplest terms, energy is nothing more than airspeed, altitude or some combination of the two. We can generally use the terms “energy” and “airspeed” interchangeably. Technically, energy is power converted into airspeed and altitude. The pilot “manages” energy with pitch, power and roll inputs. In powered airplanes, sometimes we have “excess power” and we can use the throttle to accelerate or climb, sometimes we don’t, and all of the throttle available still won’t keep the airplane from slowing down, going down or stalling. Like AOA, we can think in terms of the power-required curve when we assess energy: if we are faster than ONSPEED (the bottom of the curve), then we’ve likely got some excess power (or the ability to gain energy) and if we are slower, then likely we don’t.
All pilots are familiar with the maneuvering envelope of the airplane. Some pilots explore the limits of that envelope flying aerobatics, soaring or dogfighting, and all pilots explore the aerodynamic limit every time we take off and land—or try to extract maximum performance from the airplane. If we combine what we know about the maneuvering envelope with what we learned about “drag” curves in training, we can develop some helpful maneuvering rules of thumb that can keep us out of trouble if we are trying fly a perfect final approach, handle an engine-out situation, execute a maximum performance climb or extract optimum turn performance from the airplane.
Maneuvering AOA References
Let’s start by talking about the four AOAs that we care about when we maneuver: Carson’s speed, L/Dmax, ONSPEED and stall. These AOAs are designed into the airplane—we know where to look for them in any airplane with a bit of engineering math. Carson’s speed is the angle of attack at which the airplane achieves the greatest increase in velocity for the least increase in fuel flow—roughly analogous to “best MPG.” Expressed as a speed, it’s 32% greater than L/Dmax velocity. In a typical airplane, it’s generally slower than normal cruising speed. It might be handy for holding, initial segment for instrument approach, or optimum cruise climb; but not a reference most pilots use on a regular basis. It is, however, very helpful when wecalibrate an AOA system in an airplane. All pilots should understand the value of L/Dmax. Lots of good things happen at that AOA: maximum endurance, best rate of climb, and maximum range glide. Knowing when the airplane is at an L/Dmax condition is helpful in daily flying. ONSPEED occurs at a speed equal to approximately 80% of L/Dmax velocity flaps up. It is where maximum thrust power is available point for a propeller-driven airplane and about the point the wing is producing 60% of its total lift capacity. ONSPEED is the most useful AOA reference for the pilot: it is used as a Vref (approach speed), is coincident with best angle of climb, maximum endurance glide and provides optimum turn performance. All pilots are familiar with stall. For an AOA system, we use it to calibrate the system, and from an operational perspective, we want accurate, progressive stall warning so we don’t lose control of the airplane. Three of these AOA’s are on the “slow” side of the power-required curve—AOA can be a great reference when operating in this region.
The "Drag" Curve
When we learned to fly, we studied the drag curve—the sum of parasite and induced drag, and we learned about the region of “reverse command” where it takes more power to fly slower. Most of us have heard the expression “back side of the power (or drag) curve.” If you are a jet pilot, then it is proper to refer to the drag curve for your airplane. Jets have thrust levers (jet engines produce thrust directly). If your airplane has a propeller, it’s correct to refer to the power-required curve. Piston (and turboprop) engines produce power in the form of torque, and torque is converted into thrust by the prop. Propeller-driven airplanes have power levers. Figure 1 shows a drag curve for a jet airplane on the left side. The most important thing to note is that L/Dmax occurs at the nadir of the curve. Figure 1 depicts a power-required curve for a propeller-driven airplane on the right. L/Dmax occurs where the line from the origin is tangent to the curve, and minimum power required (ONSPEED) occurs at the nadir. Recalling two of the four forces of flight we learned about in ground school, thrust equals drag in 1G, unaccelerated flight.
The Flight Envelope
The expression “envelope” was originally applied to what is technically called a VN diagram, since it looks somewhat like an opened envelope. VN refers to velocity plotted “normal” to load factor, commonly referred to as G (modern texts also call this a VG diagram). Figure 2 is the maneuvering envelope for a typical aerobatic EAB airplane. The curved line on the left side of the envelope is the aerodynamic (stall) limit of the airplane. Since critical angle of attack for the airfoil is constant, this line represents critical AOA. Aeronautical engineers and pilots often refer to AOA as “alpha” since AOA is represented by the Greek letter alpha. The terms AOA and alpha are interchangeable. The aerodynamic limit is parabolic, since stall velocity increases proportionately to the square root of the ratio of weight to the coefficient of lift. Herein lies an important concept: stall speed changes with weight, G load/bank angle and density altitude, but stall AOA remains constant. The top horizontal line of the diagram is the design load limit of the airplane, expressed in G. The right side of the diagram is the airspeed limit of the airplane, usually referred to as red line since this speed is represented by a red line on the airspeed indicator. In Figure 2, the yellow area of the diagram is defined by maximum structural cruising speed (Vno) on the left and never exceed speed (Vne) on the right and corresponds to the yellow arc on a properly marked airspeed indicator in a piston-powered airplane. Like a drag or power-required curve, there is an obvious point on the flight envelope that we can use when are building a mental model to help safely maneuver the airplane: the top left corner of the diagram. This point corresponds to maneuvering speed for the airplane. Since it’s at the corner of the envelope, fighter pilots refer to this speed as “corner velocity.” The terms maneuvering speed and corner velocity are interchangeable. At speeds slower than maneuvering speed, the airplane will stall before structural limits are exceeded. If we are slower than Va, then we are likely concerned with AOA (i.e., not stalling if we are maneuvering), but if we are faster, we are likely concerned with airspeed and G (i.e., not exceeding airspeed or G limits if we are maneuvering).
On Figure 3, I’ve plotted something called the ONSPEED band on the flight envelope. This is the AOA associated with ONSPEED, plus or minus about 1 degree, and represents the Goldilocks “just right” AOA for optimum energy maneuvering. The obvious question is why it’s just right? In simplest terms it’s just right because if you increase AOA above ONSPEED, you will lose energy (either airspeed, altitude or both). Since excess power is negative in this region, you can do whatever you like with the throttle and it won’t help the situation. On the “the back side of the power curve,” if you push the power up, you will still keep slowing down. This effectively creates a part of the envelope (highlighted in yellow in Figure 3) that it makes no sense to fly in (unless your intention is to expend energy or stall). There is one important exception: emergency dive recovery when you want to get right to the aerodynamic limit of the airplane to avoid hitting the ground using maximum instantaneous turn performance. Since most of us don’t fly too many emergency dive recoveries, this leads us to an important rule of thumb for most flying: don’t get slower/pull harder than ONSPEED. If you do, you are unnecessarily approaching the aerodynamic/stall limit for no real performance benefit. Remember, when you are ONSPEED, things are balanced, and the airplane achieves optimum turn performance for a given power setting. The other benefit of knowing ONSPEED AOA is that it is an optimum energy state for approach and landing (not too fast and not too slow), and although there is no aerodynamic relationship with stall, ONSPEED happily occurs at a speed approximately equal to 130% of stall speed—which should sound awfully familiar. This is why fighter pilots have used AOA for decades as a primary reference for landing, including carrier landings.
Listen to your AOA: Flying the Aural AOA Logic
If we are flying faster than L/Dmax, we are generally concerned with airspeed (and G) limits, and if we are flying L/Dmax or slower, it makes sense to reference AOA if we want to optimize performance. The two key performance AOAs we are concerned with are L/Dmax and ONSPEED. There isn’t usually a reason to fly any slower (or pull any harder) than ONSPEED, since that will only bleed energy for no increase in performance. For most non-aerobatic flight, we may want to reference ONSPEED or L/Dmax for takeoff/initial climb, and then think in terms of airspeed, power setting and/or fuel flow during cruise and descent, and then transition to ONSPEED AOA for approach and landing. Remember, ONSPEED AOA is always the same, it’s not affected by gross weight, G/bank angle or density altitude. For a best angle of climb takeoff, rotate and climb on the steady tone until obstacles are cleared. For best rate, climb at L/Dmax: the start of the “fast” tone. For a maximum performance takeoff, use ONSPEED initially for best climb angle and then transition to L/Dmax for best climb rate. If we are landing on a short strip, we definitely don’t want to be any faster than ONSPEED for final approach. The aural AOA logic makes AOA management simple and allows the pilot to listen to “the backside of the power curve.” For a normal landing, slow to the steady tone, configure the aircraft and adjust pitch and power to maintain the steady tone to landing. Avoid the slow tone until the final alignment/flare. The aural logic is depicted in Figure 4. Note that fast is to the right and slow is to the left so if you are slowing down, you’d hear the logic from right to left on the diagram. For example, if you were trying to maintain an ONSPEED condition for approach, and you heard a “slightly slow” tone (1600Hz slow beeps), you would reduce AOA slightly (by easing back pressure on the stick or yoke) to re-establish an ONSPEED condition, simultaneously adjusting power (if necessary) to control glidepath angle. The aural logic is simple to use—it takes longer to read about it and decipher the diagram than it does to learn how to use it in the airplane.
Why an Aural Cue for AOA?
Flying is primarily a visual endeavor and hearing is an underutilized cockpit resource. Translating AOA into sound taps into that resource and frees up the pilot’s eyes for other tasks. One advantage of an auditory cue is that the brain processes aural cues faster than visual cues, so the feedback loop between the tone and your wrist is reduced. But the substantial benefit of aural AOA cuing is that the pilot’s eyes need not be in the cockpit. Because the tone is always present at AOAs above L/Dmax, the presence of the tone indicates that the system is working. Operationally, we “trust but verify” when using the logic for landing, i.e., cross check ONSPEED against a known airspeed to confirm proper operation at 1G. Once proper operation is confirmed, the tone can be used as a primary reference. The tone is readily internalized after first exposure, so it peacefully coexists with radio chatter and other cockpit sounds. Volume is fully adjustable, and the tone may be turned off if desired. The tone is sufficiently damped (i.e., not “noisy” in engineering terms) so that small, low gain control inputs are sufficient for AOA control. Under turbulent or gusty conditions, a “slightly fast” aural cue may be used until landing transition. The bottom line is that flying with the aural AOA cues is like flying with a flight instructor or another crew member—it’s there to back you up when you do that “pilot stuff.”