It seems that there are a lot of pilots out there that believe in the "Big Sky Technique." They think the amount of airspace out there is so wide and vast, and that they are so small, that the chances of hitting another aircraft is too small to worry about. And yet history begs to differ.
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.
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.
Everything here is from the references shown below, with a few comments in an alternate color.
Figure: Midair Collisions, United States, (www.seeandavoid.org)
If you are facing another airplane "beak to beak," how long will it take to impact, based on how much of that airplane you can see?
Figure: Time to impact large/fast "beak to beak" with small/slow on downwind, (Cessna 172 Photo From Chris Parker)
The classic "beak to beak" midair collision scenario came from the Air Force in the seventies and featured an F-4 Phantom II heading right at you, the closure speed was on the order of 900 knots and you didn't stand a chance. Well let's make that more realistic so it applies to something you might actually see. Say, for example, you are flying an average business jet which normally flies 200 knots on downwind. (That's as fast as you are allowed to fly in most traffic patterns, after all.) You are at a non-towered airport and the airport directory says right-hand traffic is the norm. Now let's say a Cessna 172 pilot thinks it should be left hand traffic and also thinks flying at your pattern altitude is the thing to do. Now the two of you are flying "beak to beak," one at 200 knots and the other at 100 knots. This 300 knot closure speed is a third what the Air Force scenario used. So does that give you three times the time to impact?
It does indeed. But that still isn't enough time. As you will see below, it usually takes 12 seconds to acquire an aircraft visually (provided you know where to look), make a decision, and then to take evasive action. In our visual aid, above, if you are heading "beak to beak" and are not already maneuvering to avoid a collision, it is probably too late.
Where did these numbers come from? See: In Case You Were Wondering, below.
The first powered flight took place on December 17, 1903 and the first midair was just seven years later.
We all intuitively understand that flying is a three-dimensional activity but we tend to only look straight ahead when clearing for other traffic. The first midair collision was the case of an airplane descending steeply into another. From this we should understand our eyes need to be where the airplane is headed and where other airplanes are likely to approach.
Many novice instrument pilots joke that "I Follow Roads" is what the non-instrument pilot will do. But in the early days of aviation, that was indeed the normal practice. The first midair of an airliner took place over a railroad track that was the common route between Croydon Airport, London and Le Bourget Airport, Paris. But the need to standardize rules between nations was not fully understood until this mishap.
The biggest limitation of the "See and Avoid" technique is the requirement to see. Perhaps a systems review of the "Mark One Eyeball" is in order.
Photo: Human eye, section view, (ZStardust, public domain)
[Flight Safety Digest] The optic nerve is joined to the eye in the retina at a point called the optic disk. Because the optic nerve contains no light-sensitive receptor cells, it is considered “blind” and renders the optic disk blind, as well — creating the area commonly referred to as the “blind spot.” Normally, the blind spot is between five degrees and 10 degrees wide. The small size of the blind spot may make it sound insignificant, but it is enough to allow an aircraft to disappear from view, often before the eyes have detected it.
[Limitations of the See-and-Avoid Principle, ¶2.5.1] The eye has an inbuilt blind-spot at the point where the optic nerve exits the eyeball. Under normal conditions of binocular vision the blind spot is not a problem as the area of the visual field falling on the blind spot of one eye will still be visible to the other eye. However, if the view from one eye is obstructed (for example by a window post), then objects in the blind spot of the remaining eye will be invisible. Bearing in mind that an aircraft on a collision course appears stationary in the visual field, the blind spot could potentially mask a conflicting aircraft.
Ideally our binocular vision can help make up for this blind spot but so can moving the direction of our scans.
The shape of the eye and its position in our skulls limits the outward angles at which the eye can take in visual information. But just because an object is within this field of view doesn't mean it will be seen. Further, the angle itself decreases with age.
[Limitations of the See-and-Avoid Principle, ¶2.2] The average person has a field of vision of around 190 degrees, although field of vision varies from person to person and is generally greater for females than males (Leibowitz 1973). The field of vision begins to contract after about age 35.
[Collision Avoidance, pg. 7] Although our eyes accept light rays from an arc of nearly 200°, they are limited to a relatively narrow area (approximately 10 – 15°) in which they can actually focus on and classify an object. Anything perceived on the periphery must be brought into that narrow field to be identified.
[Flight Safety Digest] Visual acuity is greatest for objects that are directly in front of the eye. But the fovea is a mere two degrees wide, which results in a very narrow high-acuity detection area and leaves as much as 178 degrees of the detection area in the realm of peripheral vision. This is one reason that we often tend to spot traffic or obstacles out of the “corner” of our eye.
In daylight, you need to look right at an object to see it; at night, you need to look slightly askance.
Figure: Variation of Visual Acuity, (Limitations of the See-and-Avoid Principle Figure 5)
[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.
The greatest threat comes from those stationary targets, they are headed right for you. If you want to see them, you must focus on something at a distance:
[Collision Avoidance, pg. 7] When the eye has nothing to specifically focus on, which happens at very high altitudes, but also at lower levels on vague, colourless days above a haze or cloud layer with no distinct horizon, people experience something known as ‘empty-field myopia’, and opposing traffic entering the visual field is just not seen.
[Flight Safety Digest] In the absence of a visual stimulus (for example, empty airspace), the muscles in the eye relax, preventing the lens from focusing. This creates a problem for a pilot who is attempting to scan for traffic in a clear, featureless sky. Because the eye cannot properly focus on empty space, it remains in a state of unfocused, or blurred, vision. This phenomenon, known as “empty-field myopia,” hinders effective search and detection.
[AC 90-48D, ¶4.2.2.] The human eyes tend to focus somewhere, even in a featureless sky. If there is nothing specific on which to focus, your eyes revert to a relaxed intermediate focal distance (10 to 30 feet). This means that you are looking without actually seeing anything, which is dangerous. In order to be most effective, the pilot should shift glances and refocus at intervals. Most pilots do this in the process of scanning the instrument panel, but it is also important to focus outside to set up the visual system for effective target acquisition.
There is some dispute on just how far away the eyes tend to focus, if not given anything to focus on. But everyone agrees they do not focus far away enough.
[Collision Avoidance, pg. 7] One inherent problem with the eye is the time required for ‘accommodation’ or refocusing. Our eyes automatically accommodate for near and far objects, but the change from something up close, like a dark instrument panel, to a bright landmark or aircraft several miles away, takes one to two seconds. That can be a long time when you consider that you need 10 seconds to avoid a mid-air collision.
[AC 90-48D, ¶4.2.3.] Pilots should also realize that their eyes may require several seconds to refocus when switching views between items in the cockpit and distant objects.
You need to give your eyes some time to focus; when switching from inside to outside, allow several seconds on the outside task.
[Collision Avoidance, pg. 11] Glancing out and moving your eyes around without stopping to focus on anything is practically useless; so is staring out into one spot for long periods of time.
[Collision Avoidance, pg. 8] 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.
[AC 90-48D, ¶4.2.1] Research has shown that the average person has a reaction time of 12.5 seconds. This means that a small or high-speed object could pose a serious threat if some other means of detection other than see and avoid were not utilized, as it would take too long to react to avoid a collision.
|Become Aware of Collision Course||5.0|
|Decision to Turn Left or Right||4.0|
|Aircraft Lag Time||2.0|
The key to knowing when to look outside is to understand where possible conflicts exist.
Because the fovea is primarily a daytime instrument, at night it becomes a second blind spot.
[AC 90-48D, ¶4.2.6.] Visual search at night depends almost entirely on peripheral vision. This is due in part to the night blind spot that involves an area between 5 and 10 degrees wide in the center of the visual field. By looking approximately 10 degrees below, above, or to either side of an object, “off center” viewing can compensate for this night blind spot.
Figure: Anatomy of the Eye, (Aeromedical Training for Flight Personnel, Figure 8-4)
Figure: Night Blind Spot, (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 a somewhat ironic twist of fate, Croydon Airport, London, introduced the first air traffic control in 1921. It was just a year later that an airplane departing Croydon collided with an airplane destined for Croydon. That midair occurred while both airplanes were en route and illustrated the need for standardized procedures for aircraft while en route. We can credit that incident for the introduction of airways, albeit airways often marked out by ground markers.
Photo: Transcontinental Air Mail Route Beacon 37A, St. George, Utah, (dppowell)
The U.S. Post Office began coast-to-coast air mail service in 1920, including a series of bright yellow concrete arrows every ten miles. These were eventually replaced by tall steel towers with a rotating beacon.
Photo: TIBA Instructions on Middle East South Asia Chart, 14 Nov 2013, (Jeppesen Airways Manual)
Early air traffic control relied on operators calling in flight plans and an air traffic control service ensuring the route was free of conflict. The system relied on pilots following standardized procedures and did not always have a component actively following the progress of each flight.
There are still air traffic control systems that exist today without active flight following. Many of these rely on pilots broadcasting their positions and intentions to nearby air traffic.
More about this: Traffic Information Broadcast by Aircraft.
Photo: Radar approach control, Aviano, Italy, (USAF Photo)
Radar control is a game changer in air traffic control, but it isn't perfect, as the following examples show:
En route air traffic control did exist over the United States in 1956, but it was nothing like what we have today. Controllers tracked aircraft flying at relatively high altitudes and did attempt to deconflict traffic. But "see and avoid" ruled the skies when in visual conditions. But, as the accident report says, "seeing other aircraft in flight is difficult."
Just because you are in controlled airspace doesn't mean you are safe. Pilots make mistakes; a pilot operating his or her aircraft within all procedural rules isn't always protected from those who don't. It can be argued that this mishap illustrated the need for TCAS.
[AC 90-48D, ¶4.5.1] For improved safety and to aid in collision avoidance, the following safety equipment is recommended:
Any kind of light is better than no light and a flashing light is even better. Pulse lights that alternate tend to attract one's eyes.
An Airborne Collision and Avoidance System (ACAS), which is also known as a Traffic Alert and Collision Avoidance System (TCAS), can be a lifesaver. But your TCAS will only spot other aircraft with TCAS that are operating. Many aircraft still do not have TCAS and in some parts of the world it may be common practice to turn the transponder off:
[AC 91-70B, ¶E.5.4] You must ensure the operation of your transponders, even when you are outside radar coverage, in order to enable Traffic Alert and Collision Avoidance System (TCAS)-equipped aircraft to identify conflicting traffic.
More about this: Traffic Alert and Collision Avoidance System (TCAS).
An Automatic Dependent Surveillance Broadcast (ADS-B) system is a satellite-based system that allows aircraft to broadcast accurate position and flight path data (ADS-B Out) and receive that data (ADS-B In). Where a TCAS equipped aircraft can receive and display approximate position data from other TCAS equipped aircraft, an ADS-B In equipped aircraft can receive and display very accurate position data from an ADS-B Out equipped aircraft.
There are many operational advantages for ADS-B In aircraft, especially once ADS-B Out equippage becomes mandatory. (This will happen in 2020 in the United States.) There are no areas of the world, as of 2016, that will mandate ADS-B In equippage.
[AC 90-48D, ¶3.1] From January 2009 through December 2013, a total of 42 midair collisions occurred in the United States. During this same time period, there were 461 reported NMACs. Statistics indicate that the majority of these midair collisions and NMACs occurred in good weather and during daylight hours.
Even in a modern era, it seems having an air traffic control system, radar control, or even TCAS is not enough.
In some parts of the world without radar equipped air traffic control, you are clearing for traffic with your eyes and your ears. Being in controlled airspace will not prevent collision with an airplane that is at the wrong altitude (as with this case) or the wrong route. TCAS could have prevented this mishap, but vigilance might have too, had the C-141 crew heard the Luftwaffe crew's position report.
Both aircraft had TCAS, but they were not using the same procedures. The ICAO has since standardized TCAS resolution advisory procedures that would have prevented this midair. But wasn't of any help in the next example . . .
Just because you are a highly trained domestic pilot mean you are safe when operating internationally. In this case the TCAS and air traffic control didn't protect the two airplanes. But this mishap could have been prevented by improved procedural knowledge on the part of the U.S. pilots.
One of the many misconceptions among novice and experienced instrument pilots is that when IFR, you are protected. "See and Avoid," as a procedure, is required by regulation:
[14 CFR 91, ¶91.113(b)] When weather conditions permit, regardless of whether an operation is conducted under instrument flight rules or visual flight rules, vigilance shall be maintained by each person operating an aircraft so as to see and avoid other aircraft.
But even if you think TCAS, ADS-B, and your filed flight plan will protect you against IFR-on-IFR traffic, you are sharing the airspace with more than just other IFR traffic. I've always had an intuitive feeling my greatest risk of a midair collision was from VFR pilots flying in the IFR environment; but I attributed that to my natural paranoia. I've recently heard from a reader who spends a lot of time helping experienced pilots take delivery of airplanes after recent maintenance or that have just been purchased. Here are just two of the things he's seen these pilots do once they have leveled off at a legal VFR altitude, just 500' away from your IFR altitude and certainly through the airspace you are climbing or descending through:
While many pilots consider any radio chatter between other aircraft and air traffic control to be nothing more than noise they should tune out, experienced pilots realize all of that chatter is an excellent way of building situational awareness.
[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.
Flying a high-speed jet in a visual traffic pattern is more challenging than many of us realize. If you add a mix of aircraft at lower altitude and lower speeds, you have what a fighter pilot would call DACT, or "Dissimilar Aircraft Combat Training." Combat? Yes, thinking about the VFR traffic pattern as a combat arena might be the right mindset.
The NTSB pinned the blame for this midair on the tower controller and limitations in the "see and avoid" concept. All that is true. But employing a few fighter pilot techniques could have prevented it.
[AC 90-48D, ¶4.3.1.] Prior to taxiing onto a runway or landing area for takeoff, scan the approach areas for possible landing traffic by maneuvering the aircraft to provide a clear view of such areas. It is important that this be accomplished even though a taxi or takeoff clearance has been received.
[AC 90-48D, ¶4.3.1.] During climbs and descents in flight conditions which permit visual detection of other traffic, execute gentle banks left and right at a frequency which permits continuous visual scanning of the airspace about them.
[Collision Avoidance, pg. 11] In normal flight, most collision threats will come from an area within 60° left and right of your flight path. However, do not forget the rest of the sky around you. You should also scan at least 10° above and below the projected flight path of your aircraft, because collision threats may be climbing from below or descending from above.
[Collision Avoidance, pg. 11] Concentrate your search on the areas most critical to you at any given time. In the circuit especially, always look out before you turn and make sure your path is clear. Look out for traffic making an improper entry into the circuit.
[Collision Avoidance, pg. 11] During that very critical final approach stage, do not fix your eyes on the point of touchdown, but scan all around. Another pilot may be aiming for the same point!
[AC 90-48D, ¶4.3.1.] Execute appropriate clearing procedures before all turns, abnormal maneuvers, or acrobatics.
You can improve your situational awareness by listening to all radio conversations on your assigned ATC frequency as well as the guard frequency.
You should always include TCAS or ADS-B In (if available) in your primary instrument scan. If possible, you should superimpose these onto your primary navigation display. The NTSB tested pilot abilities to sight traffic with and without this kind of advanced notice in their report of the 1986 midair collision of Aeromexico 498 and a Piper PA-28. The results were dramatic.
[NTSB Aircraft Accident Report, PB87-910409, pgs. 14 - 15]
Figure: Probability of seeing the other aircraft as a function of time, (NTSB Aircraft Accident Report, Figure 3)
Figure: The effect of TCAS-type alert, (NTSB Aircraft Accident Report, Figure 4)
In 1991, the Australian Transport Safety Bureau (ATSB) published a research report titled “Limitations of the See-and-Avoid Principle.” The report discusses the role of the see-and-avoid concept in preventing collisions and some of its inherent limitations:
Think of this as "defensive driving" for aviation: the more predictable your flight path, the less likely someone is to try to occupy the same space at the same time. This means flying published routes, flying standard traffic patterns, using hemispheric altitudes, and generally flying "by the book."
[AC 90-48D, ¶4.3.1.]Following the AIM, chapter 4, Air Traffic Control, section 3, execute pattern entries and departures for the runway in use appropriate to the airport configuration and information depicted.
Figure: Traffic pattern operations, single runway, (AIM, Figure 4-3-2)
Perhaps the highest threat arena for many higher speed jets is the traffic pattern at a nontowered airport. Keep the visual pattern entry in mind, you may have airplanes entering downwind from a 45° angle. A common technique is for a light aircraft to cross overhead the airport at a "higher" altitude in preparation for a descent into the light aircraft pattern, which is normally at 1,000 feet AGL. If your jet pattern is at 1,500 feet AGL (as it usually is), the "higher" altitude could be where you are.
[AC 90-48D, ¶4.2.4.] Effective scanning is accomplished with a series of short, regularly spaced eye movements that bring successive areas of the sky into the central visual field. Each movement should not exceed 10 degrees, and each area should be observed for at least 1 second to enable detection.
[Collision Avoidance, pg. 6]
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:
[AC 90-48D, ¶4.2.5.] Peripheral vision can be most useful in spotting collision threats from other aircraft. Each time a scan is stopped and the eyes are refocused, the peripheral vision takes on more importance because it is through this element that movement is detected. Apparent movement is almost always the first perception of a collision threat, and probably the most important, because it is the discovery of a threat that triggers the events leading to proper evasive action. It is essential to remember, however, that if another aircraft appears to have no relative motion, it is likely to be on a collision course with you. If the other aircraft shows no lateral or vertical motion, but is increasing in size, take immediate evasive action.
[Collision Avoidance, pg. 7] Most importantly, the eye is vulnerable to the vagaries of the mind. We can ‘see’ and identify only what the mind permits us to see.
Our brains tend to be stronger than our eyes and can often fool us into seeing empty space because that is what we expect. It may be helpful to think there is an airplane out there even if you suspect there is not.
As we get older our eyes will naturally lose acuity and the ability to accommodate quickly. It may become natural to squint because narrowing one's eye lids will improve the center focus. (If you wear glasses you can prove this by attempting to read a distant road sign that is just out of focus without your glasses. Attempt to read the sign through the narrow hole formed by your index finger curled into a circle.) The problem with squinting, however, is it reduces the total amount of information. If you need glasses to see without squinting, you should wear glasses.
[Collision Avoidance, pg. 7] To accept what we see, we need to receive cues from both eyes (binocular vision). If an object is visible to only one eye, but hidden from the other by a windshield post or other obstruction, the total image is blurred and not always acceptable to the mind. Therefore, it is essential that pilots move their heads when scanning around obstructions.
[Limitations of the See-and-Avoid Principle, ¶2.3.3.] Glare occurs when unwanted light enters the eye. Glare can come directly from the light source or can take the form of veiling glare, reflected from crazing or dirt on the windscreen. Direct glare is a particular problem when it occurs close to the target object such as when an aircraft appears near the sun. It has been claimed that glare which is half as intense as the general illumination can produce a 42 per cent reduction in visual effectiveness when it is 40 degrees from the line of sight. When the glare source is 5 degrees from the line of sight, visual effectiveness is reduced by 84 per cent (Hawkins 1987). In general, older pilots will be more sensitive to glare.
Sun glasses that reduce glare can greatly improve your odds of spotting threats.
[Collision Avoidance, pg. 7] The eye, and consequently vision, is vulnerable to many things including dust, fatigue, emotion, germs, fallen eyelashes, age, optical illusions, and the effect certain medications. In flight, vision is influenced by atmospheric conditions, glare, lighting, windshield deterioration and distortion, aircraft design, cabin temperature, oxygen supply (particularly at night), acceleration forces and so forth.
The prototypical drawing of vision angle versus closure time comes from an Air Force drawing in the seventies showing a "beak to beak" engagement with an F-4 Phantom II and was made famous by a 1986 issue of "Aviation Safety Digest." All that is well and good but you are unlikely to find yourself head-on with an Air Force fighter from the seventies. So I've recalculated everything base on flying toward a Cessna 172 flying on downwind at 100 knots while you are doing 200 knots for a closure speed of 300 knots.
Figure: Fast/Large jet "beak to beak" with a Slow/Small light aircraft, (Eddie's notes)
Using a Cessna 172 as the slow/small aircraft, which has a wingspan of 34 feet, we see our overtake speed is:
So, assuming we are view the monitor from 24 inches and it draws the image of the Cessna accurately, our time to impact while overtaking the airplane is as follows:
|Distance to Aircraft||Wing Span as Seen on Screen||Time to Impact|
|163 feet||5 inches||0.32 seconds|
|204 feet||4 inches||0.40 seconds|
|272 feet||3 inches||0.54 seconds|
|408 feet||2 inches||0.81 seconds|
|816 feet||1 inch||1.61 seconds|
Thanks to Simon Hedderman for help with the math!
Advisory Circular 90-48D, Pilot's Role in Collision Avoidance, 4/19/16, U.S. Department of Transportation
Advisory Circular 91-70B, Oceanic and International Operations, 10/4/16, U.S. Department of Transportation
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.
Collision Avoidance Must Go Beyond "See and Avoid" to "Search and Detect", Flight Safety Digest, May 1997
Jeppesen Airway Manuals
Limitations of the See-and-Avoid Principle, Australian Transport Safety Bureau, April 1991.
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
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