Bits and Pieces
Midair Collisions: Knowing What the True Risks Are
Several years ago I was climbing out of Minneapolis in C-FWXD, our beautiful Cessna 206. It was a hot, bumpy afternoon and I was heading generally northwest. With the blinding sun on my left, I was attempting to get what sun protection was possible by hiding behind the vertical cabin member on the left of the windscreen.
Suddenly, out of my left sight perspective I saw the image of a large, very near, low-wing aircraft crossing my pathway from left to right. By instant instinct reaction, I threw the aircraft in a tight right downward maneuver, saving the day and scaring the dickens out of my three passengers. This was way too close for comfort. The other pilot never saw me since I was below him and on his right. I was not looking to my left in order to avoid the sun. Conditions were perfect for a potential for a midair collision.
When we hear of the recent midair collision over New York’s Hudson River involving an aircraft and a helicopter, we wonder why this could not have been avoided. When I remember my near accident, I understand the complexity of the situation and the fallacy in second-guessing such situations.
The following timely article by Alex Burton, Chief Flight Instructor, Mount Royal College, Calgary, is reprinted from the Aviation News Journal, with permission:
"Avoid" is our key concept in avoiding midair collision in a visual reference flight environment. We can only avoid what we can see and we can only see what we know how to look for.
According to data from a study carried out for the Federal Aviation Administration in 1973, researchers determined that a pilot has an 86% chance of identifying an aircraft the size of a DC-3 six miles distant if he or she is fixating on the target (1). This is not all that encouraging. There is a lot of sky to scan and we all know how very small an aircraft appears against the background of big sky.
At a distance of six miles, a distance considered close enough to give a pilot a reasonable opportunity to detect another aircraft, a large aircraft subtends a visual angle of approximately 0.015 degrees. If you want to give yourself a reality check, remember that this is referring to a large aircraft, grab a protractor and see what that looks like.
And, of course, aircraft do not remain fixed in the same location in the air.
"The key to reducing our risks when flying is to know clearly what the risks are and to know clearly what we need to do to reduce those risks."
Just to toss out some numbers to work with, if we look at two, slow general aviation aircraft, a couple of 152's for example, traveling through the sky at a leisurely 90 knots - one nautical mile each 40 seconds - we can calculate their rate of convergence quite easily.
From six miles apart, if the two aircraft are converging head on, they are approaching one another at 180 knots, one nautical mile each 20 seconds; they will cover six miles in two minutes (120~ seconds). If they are six nautical miles apart and are converging at a 90 degree angle, they will occupy the same piece of airspace in 2.83 minutes (169.7 seconds).
Remember those six miles? It was based on the 86% chance of recognizing and identifying an aircraft the size of a DC-3. It's also based on the idea that the pilot is looking in the right place at the right time. A C-152 occupies quite a bit smaller chunk of the sky and is much less likely to be recognized and identified as easily as a larger aircraft.
The aircraft that presents a risk of collision appears stationary in our windscreen. Its relative position remains constant. Remaining stationary in our visual field adds to the difficulty of detection. Our eyes are much more suited to picking out movement than recognizing fixed objects unless we are looking directly at them and they are not obscured by a complex background.
Studies show that it takes a pilot approximately 10 seconds to recognize, identify, appreciate the risk of collision with another aircraft, and take appropriate action. This takes a chunk out of our available tune. If the aircraft we are flying or converging on our position faster than a C-152, our time margins are reduced accordingly.
The key to reducing our risks when flying is to know clearly what the risks are and to know clearly what we need to do to reduce those risks.
Recent studies of midair collisions conducted by the National Transportation Safety Board (NTSB) determined that:
- Most aircraft involved in midair collisions are involved in recreational flying.
- Most midair collisions occurred in VFR weather conditions during weekend, daylight hours.
- Most midair collisions occurred at or near uncontrolled airports at altitudes below 1000'.
- Pilots of all experience levels from first solo to 20,000-hour veterans were involved in midair collisions.
- Flight instructors were on board in 37% of midair collisions.
- Most collisions occur in daylight with visibility greater than 3 miles.
So, what can we usefully do to reduce the risk of midair collision? There are several, very good ideas we can incorporate into our behaviours to minimize the risk factors. Here are nine very good ideas:
- Be familiar with the layout of your aircraft and its systems. Pilots who are comfortable and familiar with their aircraft know where things are and do not have to use excess time and energy with their eyes in the cockpit looking for things. In a NASA-funded study, researchers Colvin Kurt and his colleagues found that some experienced pilots tested spent as much as 68% of their time with their eyes inside the cockpit. As workload increased, tune spent looking inside also increased. When your eyes are inside the cockpit, your opportunities for recognizing other aircraft in your area are considerably diminished.
- Plan your flight ahead and be organized with your navigation and informational materials. The less time you spend referring to materials inside the cockpit, the more time you have available to spend with your eyes outside scanning for potential threats.
- Keep your aircraft's windows clean and unobstructed. This seems fairly obvious, but how many people conscientiously clean their windows before each flight?
How many casually lay maps, flight computers, or other objects on the dash? Dead bugs, water stains or dirt on the windscreen can be a significant contributor to the phenomenon known as Empty Field Myopia; your eyes will tend to use the focal points on the windscreen rather than more distant targets (2). You effectively become myopic or nearsighted and aircraft in your area will may be overlooked.
- Wear sunglasses, as required; do not use opaque sun visors. We all know the effect of glare and the limitations it puts on our ability to see. Using opaque visors can cut down a significant portion of the visible sky and result in failure to recognize a potential collision threat.
- Always look carefully before manoeuvring your aircraft. Remember all the blind spots—high wing/low wing blockage of view, doorposts, above, below? Whether you are about to turn, climb, or descend, a good lookout is an excellent plan.
- Develop an organized and efficient scanning system and make use of it. Know and understand the limitations of your visual abilities and the blind spots of your particular aircraft. You won't normally find collision threats inside the cockpit; plan to keep your eyes outside engaged in scanning at least 3 times as long as your internal scan. Instructing your passengers in scanning techniques and encouraging them to report sightings can add an extra degree of safety.
- Monitor the appropriate frequency for your flight area and make certain you understand what other pilots are saying when they report their positions. Report your own position accurately, as necessary. Be very certain you hear and understand communications from ATC; if you don't understand a transmission for your aircraft, don't be afraid to ask ATC to "Say again".
- If you are flying VFR, remain VFR. Don't even think about playing about with a little cloud or two. Your visibility and your see-ability will be drastically reduced.
- Be very alert when in high traffic areas. Approaching or departing an aerodrome, particularly an uncontrolled aerodrome, puts you in a significantly more vulnerable situation with relation to midair collisions: more traffic, limited space, convergence on a particular location and altitude.
Midair collisions are not healthy. They are not fun. They should be avoided.
As summer shines down on us and the skies become more crowded, our need to be careful, considerate, and vigilant increases. Appreciating the truth of a risk factor and taking the steps available to minimize that risk can only result in a safer and more enjoyable flight experience.
As David St. George, MCFI, said, "Since more than 80 percent of accidents are pilot error, I argue that the most important airspace is between the pilot's ears."
1. Harris, J.L. (1973). Visual aspects of air collision. Visual Search, National Academy of Sciences, Washington, D.C.
2. For a more detailed description of Empty Field Myopia see TP 12863E, Human Factors for Aviation Basic Handbook, pp. 70-71
Alex Burton is a Class I Instructor and a Pilot Examiner. He is Chief Flight Instructor at Mount Royal College in Calgary, Alberta.