Former F-16 pilot Nate “Buster” Jaros shows us how Angle of Attack (AOA) is extremely important in basic fighter manuevers and how optimizing it can help kill the enemy.
Angle of attack (AOA) is an important tool for the combat fighter pilot. Understanding how it works and what it provides is key to predicting and optimizing aircraft performance. This is something extremely vital when in a turning air-to-air combat engagement as well as the pilot faced with landing on the deck of a carrier or any critical landing environment actually.
I remember my first HABFM (High Aspect Basic Fighter Maneuvers) training sortie clearly. It was 1998 at Luke AFB Arizona and my instructor pilot (IP) and I launched out to the Gladden/Bagdad MOAs (Military Operating Areas) west of Luke and Phoenix. The objective of HABFM is for each fighter to approach in a neutrally advantageous position, and then begin the fight. This is done with a head-on pass.
At the pass a “fight’s on” is declared and the objective is to maneuver to an advantageous position behind the other fighter to be able to employ weapons. But remember, the other fighter is turning too. It quickly becomes a contest of who can ‘out turn’ the other guy, and exact better turning performance from their aircraft. The winner is the pilot that can maneuver his or her aircraft faster and more efficiently. We do HABFM with what’s called either a one-circle or two-circle fight, and the pilot who can turn the most resourcefully will win.
I didn’t know this.
At the pass, I was determined to crush my IP and show him how good I was. I pulled hard. The Gs came on heavy and fast, my Viper’s nose rated around the circle and for a few seconds I was gaining the advantage. I kept pulling hard, I was turning faster than him and things were looking really good. I was pretty sure I was about to win my first ever HABFM engagement!
Then it all went downhill for me and my shortsighted bravado. The nose of my Viper suddenly stopped turning. The wings became heavy and mushy, and I couldn’t get the plane to turn or accelerate. What the heck happened!? It was as if I had flown into a bubble of molasses.
My IP’s jet continued to trend further and further behind me while I struggled to figure out what occurred, and before I could recover my sloppy-handling Viper, I was thrashing to keep sight of him as he closed in behind me. The next words on the radio are the ones you hear a lot in initial training.
“Tracking, Kill Viper, left hand turn, 15 thousand feet.”
Damn it…I had lost.
What happened? BFM and turning performance in fighters is all about AOA. I re-learned this again in the debrief and saw just how my turn performance diminished as my F-16 dropped below optimum AOA and airspeeds.
At some point, my airspeed bled off (as is predictable in a combat engagement where you trade speed for angles and geometric advantage) and the wings of my trusty Viper stalled. In the Viper however, “HAL” our friendly flight control computer doesn’t let the aircraft truly stall. He (She? It?) just quickly takes more and more control from the stupid pilot. The resultant effect is an airplane that feels mushy and slow.
What is AOA? And why is it important?
AOA is a direct measurement of what angle ‘the wind’ is hitting the wing. Predictably, we can describe an expected performance for that wing (and thus the airplane) at every conceivable angle the wind hits the airfoil. At some ‘critical’ angle, the wing can no longer produce lift, lift over the wing fails, and the airfoil stalls.
Interestingly, this concept is known as the Critical Angle of Attack, and it never changes. Lift fails at that angle every time for that particular wing. This critical AOA stays the same for that aircraft at any weight, density altitude, temperature, and G.
By knowing what this critical angle of attack is, a pilot who wants to optimize his or her performance in a turn, can maneuver right up to the limit, just short of the critical angle of attack and reap the most benefits of their aircraft’s abilities.
In the cockpit, there are many ways to see this AOA, and thus how close to the performance edge you really are. In the F-16 we had three AOA indicators, four with the gear down.
AOA vs Critical AOA
Think of it this way. AOA and critical AOA is like walking out onto a semi-frozen lake where the center of the lake is not quite solid ice, but the shoreline edges are. As you walk further and further out, the ice below your feet gets thinner and thinner. At some point, if you keep walking out, you will break through the ice and fall into the freezing water below.
Think of that as the critical AOA and resultant stall of the aircraft.
A good AOA indicator shows the pilot exactly how thick the ice is below, and allows them the ability to walk out to the very extreme without falling through. A simple stall indicator only chirps as you are breaking through that ice…it doesn’t allow you to see your performance on that spectrum and thus maximize it.
AOA systems have been on fighters for decades, and on heavy transport planes as well. But AOA has only recently gained more attention from the FAA and legislators for applications in General Aviation (GA) and business aircraft. One such highly publicized event that highlighted the significance of AOA was the crash of the Air France flight 447 Airbus in June 2009 that killed all 216 people on board.
Investigators concluded that the aircraft’s airspeed probes had iced over providing erroneous indications to the pilots, and that the AOA system and it’s “gauge” was not available to pilots in the cockpit. They also concluded that having an AOA gauge could have saved the stricken crew.
Additionally, Loss of Control (LOC) accidents in GA aircraft kill about 175 people every year. It is the leading cause of fatalities in GA. An FAA working group concluded that AOA systems could help reduce the incidence of these accidents and save lives, by helping pilots see and identify that critical AOA before entering a stall. The FAA just recently made it easier to install AOA devices in GA aircraft by removing some of the proverbial red tape. Makes sense to me.
One of my first modifications in my Bonanza was to add an Alpha Systems AOA. It’s just like the fighter AOA systems!
AOA is an amazing tool for the pilot of any aircraft. Whether it is used by the transport pilot for assessing landing speeds, the GA pilot for helping avoid a stall, or the fighter pilot during a combat engagement, AOA provides a fast and effective way to see the performance of the wing and optimize the aircraft’s capabilities…and keep the pilots from falling through that ice.
Top Photo credit: FighterSweep.com