Hang glider pilots are among a unique set of individuals who are willing to accept personal risk in order to continue to pursue the thing that we love.... FLYING! There is no question that hang gliding involves a certain amount of risk, yet flying can be done quite safely if a flyer is good at managing risk. PIO, or Pilot Induced Oscillation is one of many things that pilots should familiarize themselves with in order to effectively manage risk. This page is intended to provide information useful to pilots for understanding PIO and ultimately hopes to make flying safer.
PIO, or Pilot Induced Oscillation, describes the act of repetitively overcontrolling the glider while flying at 'faster' speeds inducing a sequence of back and forth movements that can escalate in severity and can become quite dangerous. PIO is particularly dangerous in that it often occurs during a landing final approach which can result in banking turns while close to the ground while flying fast which then leads to the danger of a wingtip striking the ground causing the glider to cartwheel into the earth planting the pilot head first into the dirt. PIO is also common while aero-towing due to the high speeds and need to stay centered behind the tug. There are many circumstances besides landing that require flying at speed for safety. In any situation where speed is desireable the occurence of PIO can be potentially dangerous.
Here are a couple vids:
http://www.youtube.com/watch?v=IQHUXO5N2Mc (right after launch)
http://www.youtube.com/watch?v=brxZPs_rdYs (at 3:30)
http://www.youtube.com/watch?v=AvHimZNKgCg (at 1:30)
http://youtube.com/watch?v=jr3iWM34KRM (at 1:15)
Dealing With PIO
The time tested method for dealing with PIO once it has become established is to relax and center oneself on the control frame. Then gently reduce speed until the oscillations go away. Relaxing includes relaxing one's grip on the control frame for only when the hands are relaxed can a pilot feel the subtle messages the glider is sending.
Avoiding PIO requires that the pilot get tuned into their glider while flying at a variety of speeds. The trick is to learn exactly what control inputs your bird requires at each different speed in a safe circumstance so that when you're burning a landing into a turbulent LZ you can maintain a straight line of flight without risking PIO and without having to slow down to get out of a PIO situation thus putting yourself in potential danger from gradient, wind shadow, or turbulence near the ground.
In short, practice flying fast while at altitude. Fly fast in a straight line. Fly fast and execute a gentle turn. Fly fast in smooth air first. Fly fast in turbulence when you are more experienced because the rough air will change the dynamics. Try to induce PIO and then recover. All of these tasks will help the pilot learn the intimate details of handling their specific wing. Be aware that a different wing may be more or less subject to PIO and that it will respond differently to control inputs.
PIO Information Collected From the Forum
Understand the enemy:
"Second, it helps to really understand what is going on in a PIO (which this guy obviously didn't). The important thing to know is that it happens when flying FAST (for example, while he was on tow). Even when this guy slows down the PIO goes away. So now you know how to STOP it if you ever have it happen to you, just relax and slow down.
Learning to fly FAST and avoid PIO at the same time is a bit more difficult. If you're aerotowing I would recommend putting a vertical tail on the glider, this helps reduce the tendency to PIO quite a bit. But if you really want to understand it, think of it this way: it's an oscillation, not a TURN. In a hang glider you have pitch control, and roll control. Can't do much about yaw. So if it's a mellow oscillation, simply ignore it... that's far better than reacting and adding the PI to the O.
What this guy did is make a correction and hold it until he starting seeing results. By this time he held it for WAY too long, and the glider would roll the other way. Then he'd do the same thing back. The best rule of thumb is to ignore it rather than react to it, but that said people always want to know what they can do to counter it... so... you have to be so dialed in to the glider that you know exactly how much roll input to give and how long to hold it. If you hold it too long you will PIO, which is why it's better to ignore it."
This thread contains Zig's accident report and has a whole discussion about PIO. This one is very useful:
"I'm getting the impression by some of the comments that not everyone knows or understands what 'PIO' is (or what it even stands for).
PILOT INDUCED oscillation.
Some gliders are (much) more susceptible to it than others, I believe it usually stems from a disconnection between the roll and yaw coupling...
It's similar to when the tires on your car are out of balance, there's a 'magic' speed where the glider will do it, and faster or slower will decrease it. But like your car, even with the steering wheel shimmy-ing, the car drives straight...
What happens in PIO that causes the PI part is one wing yaws behind and the glider might even roll to that side, so the pilot instinctually corrects by leaning to the other side, but since it's just an oscillation and not a turn, the pilot only worsens the following oscillation as the other wing drops.
For those of you with lesser experience with PIO, it's important to recognize it and fight the natural instinct to try to correct it, because you will only add to it. Instead, relax, center yourself, and slow down. I've never seen ANY glider PIO at trim speed...
Again, the common response is to try to dampen the oscillation, add more PI to the PIO... and if you don't stay relaxed, the other tendency is to squeeze the bar tighter and fly faster, which usually makes it worse.
If you know that you and your glider are susceptible to PIO, it is wise to come in slower so that you do not put yourself at that 'magic' speed near the ground. If possible, do a high flight and practice flying fast up high and getting a handle on it. If nothing else, you'll learn that even if your glider is banking as much as 90' left and right, you pretty much stay on a straight heading without doing anything... although the glider is banked, it's not really TURNING very much at all.
Zig, thanks very much for your thorough accident report... it was very precise as to the lead up to the accident, and I think a lot of people will be able to learn from it and avoid being in the same situation...
get better quick, now that you have been in the air it is there that you'll long to return... us pilots can't be happy being ground-bound for too long..."
"The way I get PIO going on my MkIV is to pull in suddenly. When I do that, it's easy to not be exactly even, and the glider starts to turn one way or the other. Then the defective component (the one between my ears) automatically corrects, but overdoes it a little too much. Next thing I know, left-right-left-right... I've never had it go for more than a few cycles because I just let the bar out. If I pull in more gradually, it doesn't happen. It's basically a combination of the quicker response when the wing is flying fast, and getting too far off of the straight line so that I feel like I need to make a major correction. A properly designed control system (or a more skilled pilot) could apply an appropriate correction, and the oscillations would not occur. On my Falcon, presumably because there's more damping, I've never really had it happen, though when I tried flying as fast as possible once, I could feel little suggestions that it might be lurking.
Get some altitude, stuff the bar, and make it happen. Get experience with it so that you'll be able to recognize it and not be afraid of it. Same with a stall (which can be kind of fun, if you like roller coasters). I suggest at least 1000 feet of altitude, well away from the hill for this kind of hijinks, and look around thoroughly for traffic before you start."
"So are you saying there is NO way to dampen an oscillation, or that if you dont know how to, you will make it worse(PIO)?
Why does one wing yaw in the first place? Is it from weight shift increasing the wing loading and speeding up that wing? Is PIO at its worst when this Yaw speed/frequency syncs up with the roll speed/frequency?"
"I won't say there is NO way to dampen PIO oscillations, but I will say I have never been able to through weight-shift roll inputs. The most reliable fix is to SLOW DOWN... also, if you're up high, you can put the glider into a coordinated 360 and that'll dampen it out.
As for what causes it in the first place... it can be a pilot input (combining sudden pitch and roll movements will usually do the trick), or it can be instigated by the air... but what keeps it going is the same... the pilot. Some pilots simply cannot get a glider to do it, while others can't keep a glider from doing it. Don't feel bad if you're the latter- I used to get it a lot on my old Super Sport... I'm not smart enough to say what factors of glider design lead to it and what doesn't... I'm inclined to say a little too much anhedral (or not enough dihedral), but that's probably only one of many factors..."
"I agree, slow down and relaaaax... let the glider work it out... is the best solution for most. But sometimes you dont have the time to do this.
But you CAN dampen PIO with weight shift, if you understand the dynamics, can think in slow motion, stay calm, and time the weight shift correctly.
I actually believe someone code program a simple PIO simulator video game where people can practice getting out of PIO with various ranges of lag, ranging from beginner glider, all the way up to a topless with full VG on.
But basically, if you are in PIO, the lag makes you over react by making you wait to see that your input is having an effect. Very understandable in a panic situation.
So for example... you are PIO-ing and your wing is currently turning to the right. You give it a hard left input and hold it until you see the wing react. You already messed up. If you wait for the wing to start coming around to the left, you have already put way too much input into the wing which will cause it to over shoot... and the cycle starts again. You are stuck in PIO.
The key is to understand how much input it takes to get the wing back without overshooting, giving the input, AND TRUSTING that the wing will level out *before* you see it react at all. This requires that you really are in tune with your wing and know exactly how much input to give it for any situation. This simply takes experience, which is why PIO is so dangerous for pilots on news wings.
A few weeks ago I launched off crestline in windy conditions. Hit the strongest elevator ride ive ever experienced. I considered myself VERY RUSTY on my wing, and not perfectly in tune with it anymore. I launched as hard as I could, with as much speed as I could, for maximal control. I was stuffing the bar into the head wind.
I went up like a rocket and started to PIO. I recognized the PIO, but refused to slow down because I wanted to keep the speed more than anything at that point. The reason I didnt slow down is because my PIO was not increasing in severity. Otherwise I would have had no choice but to slow down. I was using my counter-bump technique to keep the PIO at bay, but because im so rusty and wasnt perfectly in tune with my wing anymore, I didnt correct it perfectly, but maintained a slow PIO that didnt increase in severity until the elevator ride ended and I slowed to finish killing off the PIO. Fortunately, I was still in tune just enough to keep the PIO at bay until I felt safe slowing down.
The other thing that throws pilots off is YAW. Sometimes pilots get the wing to yaw and try to weight shift to fix what they perceive as a turn that gets them into a PIO situation. You gotta learn the difference between YAW and PIO. If your wing is just yawing real bad, just leave it alone it will straighten up. Reacting to it is a great way to start a PIO cycle.
Granted... there are weight shift techniques to "break" a YAW as well, but they are subtle and require good timing as well, so you dont put yourself into a turn."
The solution... simple and effective... for resolving PIO once it has begun:
"I took the Xtralite off the training to make sure the damn thing would fly. I had a nice launch and landing. I then went up to Crestline for a high altitude flight….with 30 other pilots. I found that launching the Xtralite as easy. I had a nice strong launch and flew away from the hill. I noticed I was very gently Dutch rolling (PIO), so I slowed down and the PIO went away."
Something to do to overcome PIO:
"...Carrie castle was around when i was bitching about pio and she told me to get high and pull the bar in as far as i could and get used to flying real fast in turbulence,we will call this setting the bar as high as u can.The natural progression of thought then made me get used to flying my bird near stall(this can be risky so always pull in when things get to slow,sorry for stating the obvious) we will call this setting the bar as low as we can.Now i have to say this,if you get in trouble,providing your cg is correct,a light touch on the control bar will get u out of trouble....
A really good way to avoid PIO in the first place:
"My biggest problem was pilot induced oscillation. In other words, as we were towed I tended to move back and forth because of over corrections. Malcolm told me to loosen up, but it was Mike Barber (he of the distance record fame) who, one morning at breakfast, really cured me of my rigidity, and thus my PIO.
Mike had me grip a couple of salt shakers very tightly, and then asked me to identify which was heavier. I could not tell which shaker was the full one when I was holding the shakers tightly. He then told me to hold them in a relaxed grip. The difference in their weight was immediatly apparent when I relaxed my grip.
It probably seems ridiculous and self evident that too much tension interferes with your ability to distinguish subtleties, but it was a very helpful lesson for me. Once I loosened up I was able to feel the glider's inputs and respond both more quickly and more appropriately, avoiding the oscillations."
A technique for avoiding PIO on final approach
"One of the problems with going to the uprights with speed pulled on, is the infamous bobble, where an assymetric load is momentarily applied to only one downtube, leading to either or both a pitch up and a direction change. Now you've got to scramble to get the glider straight again and back to speed.
Instead here's a better way IMHO. If I plan on going to the uprights before ground effect (safest plan), here's what I do.
Let's say you're turning final in a left hand bank... During the turn, transition your right hand from basebar to the right upright. You can do this easily, because the left hand is doing all the work of banking left. To roll out of the left turn, use that right hand which is now on the right upright, to weight shift right...
Again this is easy to do, since you now have way more leverage to pull yourself in that direction... plus it also allows you to more easily stay pulled in a bit, for speed. During this weight shift, your left hand is on the basebar and is completely neutral since you're using the right hand for all input... so you just let go with the left and transition that hand to the upright as you roll straight. Now you should be on final, both hands on the uprights, still pulled in, and no need to do the bobble."
- Bob Franklin
Another pilot demonstrating PIO on final with some good discussion
"Re "relaxing" & PIO'ing, various instructors have told me three things that are more specific than "relax" and which helped me:
1. Floppy elbows
2. Wriggle your fingers (on the uprights ie, you can't have a death grip) and
3. re PIOing, don't correct your corrections."