Avoiding Mid-Air Collisions: Flightdeck View
Debris from a business jet vs. general aviation aircraft mid-air collision rained down on a neighborhood in Marietta, Georgia, on April 4, 1998. A Cessna 525 departed under instrument flight rules from DeKalb-Peachtree Airport (KPDK), received vectors, and was initiating a climb on course. The 525 was in radio contact with terminal approach control and the pilot's acknowledgment of the climb clearance was interrupted by the collision. A Cessna 172 had departed Mathis Airport (GA27), near Cumming, Georgia, located just outside the 30-mi. Mode C veil airspace of a terminal airport, and proceeded southwest to inspect power lines for the Georgia Power Co. Both airplanes were operating in visual flight conditions. The Cessna 525 and Cessna 172 collided at about 3,400 ft. MSL on converging courses, with the 525 heading north and the 172 heading southwest. The collision occurred as the 172 was approaching the Class D airspace of a military tower, and the pilot was initiating radio contact with that tower. The terminal approach controller in contact with the 525 stated that he did not observe the primary target of the 172, and conflict alert software was not installed. The 172 did not display a transponder signal and the transponder switch was subsequently found in the “Off” position.
Further limiting the ability of pilots to see and avoid is the design of flightdeck windows. Most cockpits severely limit the field of view available to the pilot. Obstructions to vision can include window posts, instrument panels, eyeglass rims, windscreen bug splatter, windscreen imperfections, sun visors, wings and front seat occupants. Obstructions will not only mask some of the view completely, but will result in certain areas of the outside world being visible to only one eye, making it less likely to be detected. The eye has a built-in 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 (such as by a window post), 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. The blind spot covers a visual angle of about 5 deg. horizontal, which is roughly 18 meters at a distance of 200 meters, or enough to obscure a small airplane. A second undesirable effect of a window post or similar obstruction is that it can act as a focal trap for the eyes, drawing the point of focus inward, resulting not only in blurred vision but distorted size and distance perception.
The design of a cockpit window is anything but simple. It bakes on heat-soaked ramps in excess of 130F, then you blast off and within 20 min. it protects you from the sub-zero temperatures of cruise flight in the stratosphere. It keeps your face from battling 500+ kt. winds, and it endures the pressure differential to maintain adequate pressurization in your aircraft. A properly designed window needs to shed rain sufficiently so that you can still see the runway on an approach through weather, and, of course, the windshield needs an anti-icing or deicing system to keep it from accumulating ice.
Mechanical engineering teams that specialize in window design are paying attention to these design challenges without putting a special emphasis on providing an optimum external viewing environment for a pilot to maximize “see and avoid.” There is an interesting exception to this. The design team for the original Cessna Citation put a priority on the sizing of the cockpit windows for this very purpose. Their market analysis predicted that the businessmen buying their aircraft would often fly into uncontrolled airports where it would be possible for gliders and aged taildraggers without electrical systems (and hence without radios) to operate. Hence the design team specifically included those extra-large windows to enhance the ability of the pilot to have an expanded field of view as compared with the much smaller cockpit windows of their competitors. These design features were key to recognizing Cessna in 1985 with the prestigious Collier Trophy from the U.S. National Aeronautic Association for the outstanding safety record of the Citation aircraft.
The NTSB’s investigation into the Georgia accident examined the ability of these pilots to see and avoid the other aircraft in a formal process often called a “Cockpit Visibility Study.” This is a methodical and painstaking process to determine approximately how each aircraft would appear in a pilot’s field of view from his approximate sitting position in the seat and at the pilot’s eye position. This process will also take into account the varying pitch, roll and yaw angles of each aircraft. Position and orientation information for both airplanes was estimated based on an analysis of the radar data, combined with models of each airplane’s aerodynamic performance. The Cockpit Visibility Study indicated that from a fixed-eye position the Cessna 172 was essentially hidden behind the aircraft structure of the 525 for the 125 sec. before impact. The 172 would be seen only by shifting the pilot's eye position. The 525 was viewable in the left lower section of the 172's windscreen.
The NTSB determined the probable cause(s) of this accident were the failure of both pilots to see and avoid conflicting traffic, and the failure of the 172 pilot to operate the transponder as required by current regulations. Factors were the controller's failure to observe the traffic conflict, the lack of radar conflict alert capability, and the training emphasis on maximum autopilot usage with the autopilot controller placed at the rear of the cockpit center-mounted pedestal.
One of the factors highlighted by this accident common to business aircraft is the convergence speed. In this accident, the converging speed was 300 kt. Detecting a target at jet speeds leaves a very small, if any, window of opportunity to “see and avoid.” When two aircraft are on a collision course, the time for detection becomes of prime importance, especially if the closure speed is high. Let’s put this into jet-speed terms: for example, a jet that is descending at roughly 400 kt. ground speed. At a closure rate of 400 kt., the jet would cover 1.39 nm in 12.5 sec. Let’s say that the general aviation aircraft is on a roughly perpendicular flight path, presenting more surface area to be detected and making it easier to see. Both the visual angle and its rate of change remain very small until imminent impact. Just 7 sec. to impact a 40-ft.-long aircraft would subtend only a 0.5-deg. angle, which is still very small.
W. Graham, in a research paper titled “Separation of Air Traffic by Visual Means: An Estimate of the Effectiveness of the See-and-Avoid Doctrine,” published in 1970, stated, “As speed increases, the effectiveness of ‘see and avoid’ greatly decreases. It is estimated that see and avoid prevents 97% of possible collisions at closing speeds of between 101 and 199 kt. but only 47% when the closing speed is greater than 400 kt.”
Vulnerabilities Waiting for “Pop-Up” Clearance
When operating from uncontrolled airports, as we often do in business aviation, there is the added inconvenience involved with getting an IFR clearance. Sometimes a remote communications outlet will allow us to directly talk with an ATC facility to get our clearance prior to takeoff. When that isn’t available it is a common practice to “pick up the clearance once airborne” while remaining in VFR conditions. This practice of picking up your clearance while airborne leaves you without the services of ATC. Additionally, it takes time and attention for the radar controller to acquire your IFR clearance and transmit this over the radar frequency.
This vulnerability led to a collision on Sept. 11, 1992, in clear, fine weather at 2,100 ft. AGL, 2 mi. north of Greenwood Municipal Airport (KHFY), Indianapolis. A Mitsubishi MU-2 had taken off from Runway 36 and had begun a climbing turn to the east toward Columbus, Ohio, its destination, while a Piper PA-32, which had earlier departed from Terry, Indiana, was flying south. The PA-32 was operating under VFR, with “flight following,” and had just been advised that it was 3 mi. north of Greenwood, while the MU-2 had just contacted ATC after departure for IFR clearance to Columbus. All persons on board the MU-2 (a pilot and four passengers) were killed. The occupants included four “prominent Indiana businessmen.” Of the three persons on board the PA-32, one (the pilot) was killed, and the two passengers were seriously injured.
The NTSB determined the probable cause(s) of this accident were the inherent limitations of the see-and-avoid concept of separation of aircraft operating under visual flight rules, which precluded the pilots of the MU-2 and the PA-32 from recognizing a collision hazard and taking actions to avoid the mid-air collision. Contributing to the cause of the accident was the failure of the MU-2 pilot to use all the air traffic control services available by not activating his instrument flight rules flight plan before takeoff. Also contributing to the cause of the accident was the failure of both pilots to follow recommended traffic pattern procedures, as recommended in the Airman's Information Manual, for airport arrivals and departures.
Visual Scanning Is Not Sufficient
On an average day, a flight crew deals with high cockpit workload while “running the gauntlet” through busy airspace. The vast majority of us no longer have 20/20 eyesight, and normal cockpit duties require us to refocus from overhead panels to hazy conditions through crazed cockpit windows, while trying to keep track of ATC’s communications with six other aircraft.
According to Dr. Craig Morris of the Bureau of Transportation Statistics, “While visual scanning is necessary to prevent mid-air collisions, it is not sufficient…. Potential mitigation strategies include reliable altitude encoding transponders activated at all times in all aircraft, and affordable and reliable collision avoidance technologies in all general aviation aircraft, as the NTSB recommended in 1987.” Whether it is a general aviation version of TCAS, or the mandatory implementation of ADS-B for every aircraft utilizing the national airspace, or some system currently sitting on the drawing board, the Safety Board’s recommendation in 1987 was right on the mark.
Incidentally, while this article has focused on the prevention of mid-air collisions between two piloted aircraft, I would be woefully remiss to avoid mention of the soon-to-be prevalence of unmanned aerial vehicles in the national airspace.
This nation was able to put men on the moon 52 years ago, back when engineers were still using slide rules to make calculations. It is unfathomable that in today’s era we are still depending on the old and fallible “Mark VIII eyeball” to avoid mid-air collisions. The number of mid-air collisions isn’t going to change markedly until we get serious about implementing these remedies.