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Aviation Accident

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NTSB Identification: ERA15MA259A
14 CFR Part 91: General Aviation
Accident occurred Tuesday, July 07, 2015 in Moncks Corner, SC
Probable Cause Approval Date: 11/15/2016
Aircraft: CESSNA 150M, registration: N3601V
Injuries: 2 Fatal, 1 Minor.

NTSB investigators traveled in support of this investigation and used data obtained from various sources to prepare this aircraft accident report.

The pilot of the F-16, who was operating on an instrument flight rules (IFR) flight plan, was in contact with air traffic control (ATC) and was provided radar vectors for a practice instrument approach to Charleston Air Force Base/International Airport (CHS), Charleston, South Carolina; the F-16 descended to an altitude of about 1,600 ft mean sea level as instructed by the air traffic controller. Shortly thereafter, the Cessna departed under visual flight rules (VFR) from a nearby nontowered airport; the Cessna pilot was not in contact with ATC, nor was he required to be, and had not requested traffic advisory (flight-following) services. As the Cessna continued its departure climb, the airplanes converged to within about 3.5 nautical miles (nm) laterally and 400 ft vertically, triggering a conflict alert (CA) on the controller's radar display and an aural alarm. About 3 seconds later, the air traffic controller issued a traffic advisory notifying the F-16 pilot of the position, distance, and indicated altitude of the radar target that corresponded to the Cessna, stating that the aircraft type was unknown. When the F-16 pilot replied that he was looking for the traffic, the controller issued a conditional instruction to the F-16 pilot to turn left if he did not see the airplane. The F-16 pilot did not see the airplane and responded, asking "confirm two miles?" The controller responded, "if you don't have that traffic in sight turn left heading 180 immediately." As the controller began this transmission, the F-16 pilot initiated a standard rate (approximately) left turn using the autopilot so that he could continue to visually search for the traffic; however, the airplanes continued to converge and eventually collided about 40 seconds after the controller's traffic advisory notifying the F-16 pilot of traffic. (Figure 1 in the factual report for this accident shows the calculated flight tracks for the Cessna and F-16.)

Air Traffic Controller and F-16 Pilot Performance

During postaccident interviews, the controller reported that when she observed the Cessna's target on her radar display as it departed, she thought that the airplane would remain within its local traffic pattern, which was not the case. Therefore, it was not until the airplanes were within about 3.5 nm and 400 vertical ft of one another that the controller notified the F-16 pilot of the presence of the traffic by issuing the traffic advisory, which was about 3 seconds after the ATC radar CA alarmed. (Federal Aviation Administration [FAA] Order 7110.65, Air Traffic Control, paragraph 2-1-21, "Traffic Advisories," states, in part, that a controller should "Unless an aircraft is operating within Class A airspace or omission is requested by the pilot, issue traffic advisories to all aircraft (IFR or VFR) on your frequency when, in your judgment, their proximity may diminish to less than the applicable separation minima. Where no separation minima applies, such as for VFR aircraft outside of Class B/Class C airspace, or a TRSA [terminal radar service area], issue traffic advisories to those aircraft on your frequency when in your judgment their proximity warrants it. …")

When the controller issued the traffic advisory, about 40 seconds before the eventual collision, the F-16 and the Cessna had a closure rate of about 300 knots. If the F-16 pilot had reported the Cessna in sight after the controller's traffic advisory, the controller likely would have directed the F-16 pilot to maintain visual separation, which is a common controller technique to separate aircraft. While the controller tried to ensure separation between the airplanes, her attempt at establishing visual separation at so close a range and with the airplanes converging at such a high rate of speed left few options if visual separation could not be obtained.

The options available to the controller when issuing instructions to the F-16 pilot to avoid the conflict included a turn, climb, some combination thereof, or not issuing an instruction at all. (An instruction to descend was not an option because the F-16 was already at the minimum vectoring altitude for the area.) The controller indicated in a postaccident interview that she chose not to instruct the F-16 to climb because the altitude indicated for the Cessna's radar target was unconfirmed (the Cessna pilot had not contacted ATC). An element informing the controller's decision-making as to which instruction to provide was likely the flow of other traffic into the airport at that time. Arriving aircraft, including the accident F-16, were being sequenced to runway 15 via the final approach course extending from the approach end of the runway. Given the traffic flow, the left turn instruction to the F-16 would have kept the airplane on a heading closer toward, rather than farther from, its destination and would have made returning the F-16 to the intended final approach course much easier. However, the controller's instruction to the F-16 pilot to turn left required the F-16's path to cross in front of the Cessna. Although this decision was not contrary to FAA guidance for air traffic controllers, it was the least conservative decision, as it was most dependent on the F-16 pilot's timely action for its success.

Further, the controller issued the instruction to turn left if the F-16 pilot did not have the Cessna in sight. The F-16 pilot responded to the controller's conditional instruction with a question ("confirm two miles?") that indicated confusion about the distance of the traffic. The F-16 pilot's attempt to visually acquire the Cessna per the controller's conditional instruction likely resulted in a slight delay in his beginning the turn. The collision likely would have been avoided had the F-16 pilot initiated the left turn, as ATC instructed, when he realized that he did not have the traffic in sight. About 7 seconds elapsed between the beginning of the controller's first conditional instruction to turn and the beginning of her subsequent conditional instruction to the F-16 pilot to turn "immediately." Analysis of the radio transmission recordings and the F-16's flight recorder data showed that, as the controller was making the subsequent conditional instruction, the F-16 pilot began turning to the left, which pointed his aircraft toward the Cessna.

Due to the closure rate, the close proximity of the two airplanes, and human cognitive limitations, the controller did not recover from her ineffective visual separation plan, which placed the airplanes in closer proximity to each other, and switch to an alternative method of separation. The controller's best course of action would have been to instruct the F-16 pilot to turn before the airplanes came into close proximity with each other and preferably in a direction that did not cross in front of the Cessna's path.

In postaccident interviews, the controller stated that when she issued the command to the F-16 pilot to turn left "immediately," she expected that the F-16 pilot would perform a high performance maneuver and that she believed that fighter airplanes could "turn on a dime." The FAA's Aeronautical Information Manual (AIM) Pilot-Controller Glossary defines "immediately" as a term used by ATC or pilots "when such action compliance is required to avoid an imminent situation." Further, the AIM states that controllers should use the term "immediately" to "impress urgency of an imminent situation" and that "expeditious compliance by the pilot is expected and necessary for safety." As described above, the F-16 pilot did not meet her expectation that the turn be conducted at a greater-than-standard rate.

The controller's expectation of the F-16 pilot's performance was based on her assumption that a fighter airplane would perform a high performance turn to the heading; however, this expectation of performance was not clearly communicated. Based on the controller's instructions and the actions of the F-16 pilot in response, it is clear that the term "immediately" held different expectations for both parties. Although the controller's use of the term "immediately" was in keeping with FAA guidance, further clarification of her expectation, such as directing the pilot to "expedite the turn," would have removed any ambiguity.

See-and-Avoid Concept

According to 14 Code of Federal Regulations 91.113, "Right-of-Way Rules," "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." The concept that pilots are primarily responsible for collision avoidance was similarly stressed in US Air Force training documents. In addition, FAA Advisory Circular (AC) 90-48C, "Pilots' Role in Collision Avoidance," which was in effect at the time of the accident, stated that the see-and-avoid concept requires vigilance at all times by each pilot, regardless of whether the flight is conducted under IFR or VFR. (AC 90-48D replaced AC 90-48C in 2016 and contains the same statement.)

The see-and-avoid concept relies on a pilot to look through the cockpit windows, identify other aircraft, decide if any aircraft are collision threats, and, if necessary, take the appropriate action to avert a collision. There are inherent limitations of this concept, including limitations of the human visual and information processing systems, pilot tasks that compete with the requirement to scan for traffic, the limited field of view from the cockpit, and environmental factors that could diminish the visibility of other aircraft.

Factors Impacting the Pilots' Ability to Detect Other Traffic

The collision occurred in a relatively low-density air traffic environment in visual meteorological conditions (VMC). The Cessna was equipped with an operating transponder and single communication radio but was not equipped with any technologies in the cockpit that display or alert of traffic conflicts, such as traffic advisory systems, traffic alert and collision avoidance systems, or automatic dependent surveillance-broadcast systems. The Cessna had departed from a nontowered airport and was still in close proximity to the airport when the collision occurred. The Cessna pilot had not requested or received flight-following services from ATC at the time of the collision, nor was he required to do so. Based on his proximity to the departure airport, it is reasonable to expect that the Cessna pilot likely was monitoring that airport's common traffic advisory frequency (CTAF) for awareness of airplanes in the vicinity of the airport, as recommended by the FAA's AIM. Based on statements from the Cessna pilot's flight instructor and from his logbook entries, which both cited past experience communicating with ATC, it is also reasonable to assume that had the collision not occurred, the pilot likely would have contacted ATC at some point during the flight to request flight-following services.

Due to the Cessna's lack of technologies in the cockpit that display or alert of traffic conflicts and the pilot's lack of contact with ATC, his ability to detect other traffic in the area was limited to the see-and-avoid concept. While not required, had the Cessna been equipped with a second communication radio, the pilot could have used it to contact ATC while still monitoring the departure airport's CTAF. Had the Cessna pilot contacted ATC after departing and received ATC services, the controller would have had verification of the Cessna's altitude readout and its route of flight, which would have helped her decision-making process. The controller also could have provided the Cessna pilot awareness of the F-16.

The F-16 was operating under IFR in VMC. The F-16 pilot's ability to detect other traffic was limited to the see-and-avoid concept, supplemented with ATC traffic advisories. While the F-16 pilot could use the airplane's tactical radar system to enhance his awareness of air traffic, it was designed to acquire fast-moving enemy aircraft rather than slow-moving, small aircraft and was thus unable to effectively detect the Cessna. (The radar system did detect a target 20 miles away, which is likely what led the F-16 pilot to question the location of the traffic that the controller had indicated was 2 miles away.) The F-16 was not otherwise equipped with any technologies in the cockpit that display or alert of traffic conflicts. The F-16 pilot did eventually visually acquire the Cessna but only when the airplanes were within about 430 ft of one another, about 1 second before the accident.

A factor that can affect the visibility of traffic in VMC is sun glare, which can prevent a pilot from detecting another aircraft when it is close to the position of the sun in the sky. For the F-16 pilot, the sun would have been behind and to his left as the airplanes approached one another. Although the Cessna pilot would have been heading toward the sun, the sun's calculated position would likely have been above a point obstructed by the Cessna's cabin roof and would not have been visible to the Cessna pilot. Thus, sun glare was not a factor in this accident.

Aircraft Performance and Cockpit Visibility Study

Our aircraft performance and cockpit visibility study showed that, as the accident airplanes were on converging courses, they each would have appeared as small, stationary, or slow-moving objects to the pilots. Given the physiological limitations of vision, both pilots would have had difficulty detecting the other airplane. Specifically, the study showed that the Cessna would have appeared as a relatively small object through the F-16's canopy, slowly moving from the center of the transparent heads-up display (HUD) to the left of the HUD. As the F-16 started the left turn as instructed by the air traffic controller, the Cessna moved back toward the center of the HUD and then off to its right side, where it may have been obscured by the right structural post of the HUD. It was not visible again until about 2 seconds before the collision. (Figures 3a and 5a in the factual report for this accident show the simulated cockpit visibility from the F-16 at 1100:18 and 1100:56, respectively.) The F-16 pilot reported that before the controller alerted him to the presence of traffic, he was actively searching for traffic both visually and using the airplane's targeting radar. He reported that after the controller advised him of traffic, he was looking "aggressively" to find it. By the time he was able to visually acquire the Cessna, it was too late to avert the collision.

Our investigation could not determine to what extent the Cessna pilot was actively conducting a visual scan for other aircraft. Our aircraft performance and cockpit visibility study showed that the F-16 would have remained as a relatively small and slow-moving object out the Cessna's left window (between the Cessna's 9 and 10 o'clock positions) until less than 5 seconds before the collision. Given the speed of the F-16, the Cessna pilot likely would not have had adequate time to recognize and avoid the impending collision.

Cockpit Display of Traffic Information

Although the Cessna and F-16 pilots were responsible for seeing and avoiding each other, our aircraft performance and cockpit visibility study showed that, due to the physiological limitations of vision and the relative positions of the airplanes, both pilots would have had difficulty detecting the other airplane. Research indicates that any mechanism to augment and focus pilots' visual searches can enhance their ability to visually acquire traffic. (AC 90-48D highlights aircraft systems and technologies available to improve safety and aid in collision avoidance, and our report regarding a midair collision over the Hudson River [AAR-10/05] states that "traffic advisory systems can provide pilots with additional information to facilitate pilot efforts to maintain awareness of and visual contact with nearby aircraft to reduce the likelihood of a collision. …") One such method to focus a pilot's attention and visual scan is through the use of cockpit displays and aural alerts of potential traffic conflicts. Several technologies can provide this type of alerting by passively observing and/or actively querying traffic. While the accident airplanes were not equipped with these types of systems, their presence in one or both cockpits might have changed the outcome of the event. (The images from our in-cockpit traffic display simulation are representative of the minimum operations specifications contained in RTCA document DO-317B, Minimum Operational Performance Standards for Aircraft Surveillance Applications System [dated June 17, 2014], but do not duplicate the implementation or presentation of any particular operational display exactly; the actual images presented to a pilot depend on the range scale and background graphics selected by the pilot.)

Because the Cessna pilot was not in contact with ATC and was relying solely on the see-and-avoid concept, an indication of approaching traffic might have allowed him to visually acquire the F-16 and take action to avoid it. While most systems are limited to aiding pilots in their visual acquisition of a target and do not provide resolution advisories (specific maneuvering instructions intended to avoid the collision), the augmentation of a pilot's situational awareness might allow the pilot to change the flightpath in anticipation of a conflict and, thus, avoid airplanes coming in close proximity to one another. The Cessna pilot might have noted the presence of the F-16 and its level altitude of about 1,600 ft as he continued his departure climb. With this information, the Cessna pilot might have arrested his airplane's climb as he began a visual search, thus creating an additional vertical buffer between his airplane and the approaching F-16.

While the F-16 pilot's visual search was augmented by the controller's traffic advisory, a successful outcome would have depended upon the pilot's visual acquisition of the target airplane in time to take evasive action. Our in-cockpit traffic display simulation showed that the F-16 pilot might have first observed the Cessna when it was about 15 nm away, or nearly 3 minutes before the collision. As the F-16 closed to within 6 nm of the Cessna, or slightly more than 1 minute before the collision, the conflict might have become even more apparent to the pilot showing that not only were the airplanes in close proximity laterally but also that they were only separated vertically by 600 ft. As the F-16 pilot was beginning his left turn as instructed by ATC, the presence of the Cessna would have been aurally annunciated, and its traffic symbol would have changed from a cyan color to a yellow color. The information presented on the in-cockpit traffic display would have clearly indicated that the airplanes were on a collision course that might not be resolved by a left turn and that the vertical separation between the airplanes had decreased to 300 ft.

Consequently, an in-cockpit traffic display could have helped the F-16 pilot recognize the potential for a collision in advance of the controller's instruction to turn left. The earlier warning also could have provided him additional time to conduct his visual search for the Cessna and potentially take other preemptive action to avoid the collision. Had the F-16 been equipped with a system that was able to provide the pilot with resolution advisories, the F-16 pilot could have taken action in response to that alarm to avoid the collision, even without acquiring the Cessna visually.

Postaccident Actions

In November 2016, we issued safety recommendations to the FAA and Midwest Air Traffic Control, Robinson Aviation, and Serco (companies that operate federal contract towers) to (1) brief all air traffic controllers and their supervisors on the ATC errors in this midair collision and one that occurred on August 16, 2015, near San Diego, California; and (2) include these midair collisions as examples in instructor-led initial and recurrent training for air traffic controllers on controller judgment, vigilance, and/or safety awareness.

In November 2016, we also issued a safety alert titled "Prevent Midair Collisions: Don't Depend on Vision Alone" to inform pilots of the benefits of using technologies that provide traffic displays or alerts in the cockpit to help separate safely. (In May 2015 [revised in December 2015], we issued a safety alert titled "See and Be Seen: Your Life Depends on It" regarding the importance of maintaining adequate visual lookout.)

After the accident, the Cessna's departure airport engaged in several outreach efforts (including posting midair collision avoidance materials locally and having outreach meetings with pilots) to raise awareness regarding midair collisions and encourage contact with ATC. The airport also updated its chart supplement to note the presence of military and other traffic arriving at and departing from CHS.

The National Transportation Safety Board determines the probable cause(s) of this accident as follows:
  • The approach controller's failure to provide an appropriate resolution to the conflict between the F-16 and the Cessna. Contributing to the accident were the inherent limitations of the see-and-avoid concept, resulting in both pilots' inability to take evasive action in time to avert the collision.