Ground-Reference Illusions
During my career, I’ve spent a good bit of time analyzing the perceptual illusions to which we humans are prone, as they affect us pilots. It’s easy to find examples of simple or classic illusions, but when piloting an airplane, the things we see and feel are so complex that they don’t allow us to easily recognize an illusory situation.
Another factor is that our perception, especially visual perception, is designed to create a very detailed image of our surroundings. This fools us into thinking that the image in our mind is accurate. Our peripheral vision is blurred, and when in motion our surroundings are continually changing, so at any instant what seems so precise is at best a little behind. (Not all that is precise is accurate, especially in measurement.)
Then there’s the fact that we tend to understand things as constant until change is forced upon us.
The turn about a point is a valuable exercise in building pilot skills, especially in integrating the moving airplane and the wind with respect to true course.
But I’ve never seen a discussion of the perceptual effects of the turn about a point. I think that this is a very important factor in the all-too-common spin during the turn to final.
The vection illusion is an illusion of false motion. For example, we are sitting in a train in a station next to another train, and it seems to move. Are we moving, or is the other train? Many years ago, my younger brother was driving a car up a hill in Duluth, Minnesota, and stopped at an intersection. The driver of the car ahead did not keep his foot firmly on his brake pedal and slowly drifted back into our front bumper. That driver was irate at my brother because the vection illusion persuaded him that we had crept forward and struck him. As no damage had been done, and he could not be persuaded, we let him fume and drove away.
When we are piloting an airplane, the ground moves past us most rapidly during taxi, takeoff and landing because we are close to it. As we all know, at altitude we seem falsely to be still, creeping along at 100 or 150 or 200 knots. This, too, is the vection illusion.
The turn about a point is crucial in this regard because above that altitude, in a steep turn, the wing moves backward across the ground, helping convince us that we are indeed turning steeply.
At the altitude at which we are turning about a point, the ground seems not to move at all at the tip of the wing. This is the key to finding that altitude.
The danger is that below the pivotal altitude, the wing appears to move forward across the ground, even in a steep turn. This convinces us that we are not turning as much as expected and that we are going very fast. The fact that we spend almost all our time in the airplane well above the pivotal altitude means that the forward-sweeping wing is an unexpected change.
The illusion of not turning enough may reflexively be answered with bottom rudder. This is a spin-entry maneuver.
The illusion of rapid movement below the pivotal altitude is reflexively answered with a pull on the yoke, especially if we feel as though we’re above glide slope. This is a stall-entry maneuver.
These two illusions make the turn from base to final hazardous.
Having explored these illusions thoughtfully while landing, my conclusion is that I am least prone to these illusions if I make a descending 180-degree turn from the downwind leg to the final approach instead of a rectangle with two separate turns at the corners. This way the sense of speed and the sense of turning change gradually; I am able to frequently glance at the airspeed indicator to ensure that it’s locked on the approach speed, and can complete the turn sufficiently far from the runway to correct the glide slope after completing the turn.
I don’t know any pilots who practice ground reference maneuvers except when preparing for a checkride. Perhaps more didactic attention to the perceptual change that occurs at the pivotal altitude would enhance safety.
—Daniel L Johnson, M.D., via email
Thanks, Dan. There’s clearly a lot more going on here than most pilots acknowledge.
Passenger Briefings
The sidebar on page 9 of September’s issue, accompanying Ryan Motte’s “Managing Emergencies,” highlighted evacuation of the airplane as something to plan for. It also should serve as a reminder that passengers may not be at all familiar with the egress routes, only the ingress process. The opportunity to remedy this shortcoming is in the preflight briefing all passengers should receive, whether a Piper Cub or an A380. And it’s required under the FARs.
Thanks!
—Mike Stevens, via email
Good point. I give a briefing to all new passengers that includes how to open the door and get away from the airplane, but I’m sure I’m missing something. We’ll tackle this topic in an upcoming issue.
Metal In The Filter?
I’m new to airplane ownership and am learning to do some of the preventive maintenance tasks myself. I appreciate August’s “DIY Oil Changes” article. So far, so good with my efforts, but I’m curious about how much metal in the filter pleats is too much. Also, are the dark, crushable particles the filter catches byproducts from combustion that get into the oil?
Thanks for the article, and for any additional insights on what filters filter.
—Charles Strauss, via email
There’s an old joke about not being able to read any part numbers from the metal an oil filter catches means the engine is in great condition. If you’re seeing glitter-like material spread throughout the filter, that’s grounds for investigation. Use a magnet to check for ferrous materials. Yes, the dark particles are carbon from combustion and/or burned oil. They’re a reason we change the oil.
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