PIOs explained: what makes a test pilot nervous

I have just learned how to force a test pilot into that dreaded syndrome known as PIOs (pilot induced oscillations). In simple parlance, the results of persistent overcontrolling when flying.

Apparently helicopters are just as – if not more – prone to PIOs than fixed wing aircraft, and it happened to be at a rotary wing forum that I gained some intriguing psychological insight into what makes even test pilots over-control. I find myself perversely pleased at the thought that test pilots struggle with flying sometimes, like the rest of us.

I was in Liverpool a week or so ago. No, not cruising down Penny Lane on the Magical Mystery Tour double-decker bus inspired by an intra-aural infusion of Lucy in the Sky with Diamonds. I was attending the Royal Aeronautical Society’s 34th European Rotorcraft Forum, a conference top-heavy with rotary wing science and academia and rather light on helicopter manufacturers and operators.

But I still understood some of it. I think.

Especially when, among equations heavy with Greek alphabetical symbols, one lecturer provided the audience with an analogy I understood: what makes you get into the cycling equivalent of PIOs when riding a bike (I have blogged on the humble bike’s connection with aviation before).

Back to rotary wing and PIOs. Picture a helicopter pilot flying a satellite guided precision approach, with the trajectory presented to him, on his LCD primary flight display, as a perspective of a generously proportioned rectangular “tunnel in the sky” leading to the landing site.



There’s plenty of space in this tunnel, and the pilot flies down the middle of it without a twitch.

All you have to do to induce PIOs, according to experiment, is to make the “tunnel” narrow and shallow – to crowd the boundaries in on him.

The academics have a name for what’s going on. When a pilot is flying a profile that doesn’t put him under stress he follows a process they call “point tracking”. He aims his aircraft where he wants to go, and goes there smoothly. If, on the other hand, he’s given very close lateral and vertical boundaries, he stops “point tracking” and starts “boundary avoidance”; he keeps twitching away from the boundaries and ends up in a PIO. Even test pilots do this, I am delighted to hear.

The bike analogy is simple. You can easily cycle in a pretty straight line along the road and need little lateral space for your track. But if you were riding along the top of a wall, even a low one, suddenly you would stop unconsciously aiming your bike steadily at a constantly receding point ahead and start twitching frantically to avoid the edges.

Easy to understand, isn’t it? Sometimes, something you’ve known instinctively all your life – but didn’t bother to rationalise – gets explained to you.  But you still can’t cycle along a wall without twitching.