Figure: Tunnel Vision, from SafetySense, ¶3.

Eddie Sez:

The year before I started Air Force pilot training, my base experienced a midair between a low wing T-37 and a high wing light airplane. The two aircraft never saw each other, all aboard the smaller aircraft were killed. In that atmosphere I was taught the need to "keep your head on a swivel, keep your eyes outside." Over the next few years we lost a few more airplanes and while most of the military accepted that "you have to accept a few losses in a big operation," we continued to attack the problem with heads on swivels and eyes out of the cockpit.

In 1986, a collision between Aeromexico 498 & Piper N4891F changed all that. The 14 CFR 121 world adopted TCAS and enforcement of airspace rules around busy airports became stricter. In the military we were still using our heads and our eyes. Then the very next year we lost C-141 65-9405 & Luftwaffe 74 over the skies of Africa. The Air Force realized it needed TCAS too.

Officially, we called it "clearing." I think most civilians call it "look out doctrine." I've always called it "the big sky theory," because you were betting your life on the hope the sky is big enough for everyone. But it isn't and you need to realize just how limited your ability to see a moving target is. If you have no choice but to fly into a hornet's nest, light the airplane up, fly a predictable route, use the radio, and, yes, head on a swivel and eyes out of the cockpit.

What follows are notes from a few technical references, their scanning techniques, my scanning techniques, and a case study from that collision between Aeromexico 498 & Piper N4891F.

Visual scanning

Situational Awareness

It may seem counter intuitive, but the first step to improving your visual scanning is to improve your listening skills. Your Situational Awareness depends on your ability to hear what is going on around your airplane on the radio. In some parts of the world it could be your only formal method of traffic separation, see Traffic Information Broadcast by Aircraft (TIBA). But even in a strict IFR environment, listening to other traffic can help you understand if a threat is headed your way.


While there have been a few very notable cases of mid-air collisions when both aircraft were under IFR control, (see C-141 65-9405 & Luftwaffe 74 and Mishaps / DHL 611 & Baskirie Avialinii 2937), your highest threat comes from aircraft not under IFR control.

[U.K. Safety Leaflet ¶1.c.] A study of over two hundred reports of mid-air collisions in the US and Canada showed that they can occur in all phases of flight and at all altitudes. However, nearly all mid-air collisions occur in daylight and in excellent visual meteorological conditions, mostly at lower altitudes where most VFR flying is carried out. Because of the concentration of aircraft close to aerodromes, most collisions occurred near aerodromes when one or both aircraft were descending or climbing, and often within the circuit pattern. Although some aircraft were operating as Instrument Flight Rules (IFR) flights, most were VFR.

[Collision Avoidance, pg. 8] Motion or contrast is needed to attract the eyes’ attention, and the field of vision limitation can be compounded by the fact that at a distance an aircraft on a steady collision course will appear to be motionless. The aircraft will remain in a seemingly stationary position, without appearing to move or to grow in size, for a relatively long time, and then suddenly bloom into a huge mass, almost filling up one of the windows.


If you are scanning for an aircraft you know is below, at, or above your altitude, it would be helpful to know where that is in relation to your perception of level in your cockpit. Most cockpit windows tend to bend light downward, so at your level actually looks to be below you. You can get a sense of this in your hangar by picking an object on the wall you know is the same height as your eyes while seated, and then looking at the same point from the cockpit. The next time ATC calls out traffic from a distance, see how its perspective changes as it comes nearer.

Limitations of your eyes

[Collision Avoidance, pg. 7]

Day versus night vision

Figure: Anatomy of the Eye, from Aeromedical Training for Flight Personnel, Figure 8-4.

[Aeromedical Training for Flight Personnel, ¶ 8-16] Rods and Cones. The retinal rod and cone cells are so named because of their shape. The cone cells are used principally for day or high-intensity light vision (viewing periods or conditions). The rods are used for night or low-intensity light vision (viewing periods or conditions). Some of the characteristics of day and night vision are due to the distribution pattern of rods and cones on the retina. The center of the retina, the fovea, contains a very high concentration of cone cells but no rod cells. The concentration of rod cells begins to increase toward the periphery of the retina.

Figure: Night Blind Spot, from Aeromedical Training for Flight Personnel, Figure 8-9.

[Aeromedical Training for Flight Personnel, ¶ 8-26] The night blind spot occurs when the fovea becomes inactive under low-level light conditions. The night blind spot involves an area from 5 to 10 degrees wide in the center of the visual field. If an object is viewed directly at night, it may not be seen because of the night blind spot; if the object is detected, it will fade away when stared at for longer than two seconds. The size of the night blind spot increases as the distance between the eyes and the object increases.

In the daytime, looking for an object straight ahead is effective because the center of your eyes, the cones, are attuned to doing just that. But at night they are not as good as the rods, around the periphery. So at night you should look slightly askance where you think the target actually is.

Effective scan patterns

[Collision Avoidance, pg. 11]

Most of the references advocate a side-to-side or front-to-back scanning pattern which I suppose is okay. What I grew up using, from day one in a very busy hornets nest of T-37, T-38, and F-5 aircraft all sharing the same air patch, was a bit different:

Target acquisition performance test

The NTSB included test results of pilot ability to sight traffic in their report of the 1986 midair collision of Aeromexico 498 and a Piper PA-28.

[NTSB Aircraft Accident Report, PB87-910409, pgs. 14 - 15]

The ability of pilots to sight other airplanes was evaluated during two test programs conducted by the Lincoln Laboratories of the Massachusetts Institute of Technology (MIT). These tests were part of a general research project and were not conducted as a result of this accident. In addition to counting the number of times that these pilots either acquired or failed to acquire an intruder airplane visually, the tests determined the distance at which the targets were acquired.

Figure: Probability of seeing the other aircraft as a function of time, from NTSB Aircraft Accident Report, Figure 3.

One test evaluated pilot performance during unalerted search. The tests were conducted during a series of triangular round robin flights from Hanscom Field, Massachusetts, using two VORTACS near, but not inside, the Boston TCA as waypoints. The subject pilots were not alerted that there would be intruder aircraft or that scanning behavior was the focus of the study. Each leg was flown at a different altitude and the pilot was required to perform his own navigation and answer various questions asked by a the evaluator during the flight. The planned angles of the intercepts were head-on, 90, and 135’, and the intercepts were predominantly from the left (the pilot’s side of the airplane). Data were obtained for 64 unalerted encounters. Visual acquisition was achieved in 36 encounters (56 percent of the total), and the median acquisition range for these 36 encounters was .99 nmi. The greatest range of visual acquisition was 2.9 nmi.

Figure: The effect of TCAS-type alert, from NTSB Aircraft Accident Report, Figure 4.

The other test program evaluated the performance of pilots who had been alerted to the presence of an intruder airplane. Data for 66 encounters were collected during the testing of the TCAS II. The subject pilots were aware that intercepts would be conducted and they received traffic advisories on a TCAS II cathode ray tube (CRT) display. The subject pilots acquired the intruder visually in 57 of the 66 encounters (86 percent of the total). In five of the nine failures, the failure was partially due to the pilot’s response to a TCAS resolution advisory. The median range of the visual acquisitions was 1.4 nmi.

The performance of the pilots was used to provide data for a mathematical model of visual acquisition. This model is based on the experimental observation that the probability of visual acquisition in any instant of time is proportional to the product of the angular size of the visual target and its contrast with its background. The cumulative probability of visual acquisition is obtained by integrating the probabilities for each instant as the target approaches.

The data cited herein were developed by a project leader on the Air Traffic Control Division, Lincoln Laboratories, MIT, who had conducted research on human visual performance and flight testing of collision avoidance systems. At the Safety Board’s request, the project leader constructed Probability of Visual Acquisition Graphs based on the extrapolation of pertinent data contained in the facts and circumstances of the collision between the Piper PA-28 and flight 498 with the data described above. (See figures 3 and 4.) The graphs are based on the closure rate between flight 498 and the Piper and on the results achieved by pilots having an unobstructed view of the intruder. The graphs do not account for such limiting factors as cockpit structure and the visibility that the airplanes might be positioned so that they can be seen with only one eye e However, the information in this report is of significance in that it provides a baseline for further evaluation.

Book Notes

Portions of this page can be found in the book Flight Lessons 1: Basic Flight, Chapter 30.


Aeromedical Training for Flight Personnel, Department of the Army Field Manual 3-04.301, 29 September 2000

Collision Avoidance, Methods to Avoid the Risk, European Aviation Safety Agency (EASA), Safety Promotion Leaflet, GA-1, Jan 2010.

NTSB Aircraft Accident Report, PB87-910409, Collision of Aeronaves de Mexico, S.A., McDonnell Douglas DC-9-32, XA-JED and Piper PA-28-181, N4891F Cerritos, California, August 31, 1986

United Kingdom Civil Aviation Authority Safety Sense Leaflet 13, Collision Avoidance, February 2010