Cessna 172M and Sabreliner midair collision on August 16, 2015, final report

NTSB Identification: WPR15MA243A

14 CFR Part 91: General Aviation

Accident occurred Sunday, August 16, 2015, in San Diego, CA

Probable Cause Approval Date: 11/15/2016

Aircraft: CESSNA 172M, registration: N1285U- NA265-60SC Sabreliner N442RM

Injuries: 5 Fatal.

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Photo NTSB

HISTORY OF FLIGHT

On August 16, 2015, about 1103 Pacific daylight time, a Cessna 172M, N1285U, and an experimental North American Rockwell NA265-60SC Sabreliner, N442RM (call sign Eagle1), collided in midair about 1 mile northeast of Brown Field Municipal Airport (SDM), San Diego, California. The pilot (and sole occupant) of N1285U and the two pilots and two mission specialists aboard Eagle1 died; both airplanes were destroyed. N1285U was registered to a private individual and operated by Plus One Flyers under the provisions of 14 Code of Federal Regulations (CFR) Part 91 as a personal flight. Eagle1 was registered to and operated by BAE Systems Technology Solutions & Services, Inc., for the US Department of Defense as a public aircraft in support of the US Navy. No flight plan was filed for N1285U, which originated from Montgomery-Gibbs Executive Airport, San Diego, California. A mission flight plan was filed for Eagle1, which originated from SDM about 0830 and was returning to SDM. Visual meteorological conditions prevailed at the time of the accident.

On the morning of the accident, the SDM airport traffic control tower (ATCT) had all control positions (local and ground control) in the tower combined with the local control position. The position was staffed by a qualified local controller (LC)/controller-in-charge (CIC) who was conducting on-the-job training with a developmental controller (LC trainee) on the local control position. The LC trainee was transmitting control instructions for all operations; however, the qualified LC was closely monitoring the LC trainee’s actions and was responsible for all activity at that position.

According to air traffic control (ATC) radar and voice communications data, the pilot of N1285U contacted the SDM ATCT at 1049:44 and requested touch-and-go maneuvers in the visual flight rules (VFR) traffic pattern. N1285U was inbound about 6 miles to the northeast of SDM, at an indicated altitude of 2,600 ft. About that time, another Cessna 172 (N6ZP) and a helicopter (N8360R) were conducting operations in the VFR traffic pattern, and a Cessna 206 Stationair (N5058U) was inbound for landing after carrying parachutists to a local drop zone about 5 nautical miles (nm) east of the field.

Between about 1049 and 1054, N6ZP and the helicopter continued to conduct approaches, N5058U landed on runway 26L, a Skybolt (N81962) reported west of SDM for landing on runway 26L, and a Cessna Citation (XALVV) reported straight in for landing on runway 26R. At 1052:57, the LC trainee cleared the pilot of N1285U for a touch-and-go on runway 26R, which the pilot acknowledged. At 1054:46, when N1285U was on final approach to the first approach to runway 26R, the pilot advised the LC trainee that he was going to go around. The LC trainee acknowledged the transmission and instructed the pilot to follow “a Cessna” (N6ZP) on the right downwind.

At 1056:31, the LC trainee advised the pilot of N1285U to expect runway 26L on the next approach, which the pilot acknowledged. At that time, three aircraft were using runway 26R (Global Express [N18WZ] was inbound for landing, N6ZP was on a right base for a touch-and-go, and XALVV was on short final) and three aircraft were using runway 26L (N1285U was turning right downwind for the touch-and-go, N81962 was on a left downwind for landing, and N8360R was conducting a touch-and-go operation).

At 1057:22, the LC trainee cleared the pilot of N1285U for a touch-and-go on runway 26L, and at 1057:27, the pilot acknowledged the clearance. At 1058:22, the LC trainee cleared the pilot of N6ZP for a touch-and-go on runway 26L. At 1058:29, the pilot of N6ZP stated, “…ah two six right cleared touch and go.” After the pilot of N1285U completed the touch-and-go on runway 26L, the pilot turned the airplane right, crossing through the departure corridor of runway 26R, and entered a right downwind for runway 26R.

At 1059:04, when Eagle1 was 9 miles west of SDM, the flight crew contacted the SDM ATCT and requested a full-stop landing. Throughout Eagle1’s cockpit voice recorder (CVR) recording, the pilot, seated in the left seat, was communicating on the radio and responding to checklists, consistent with that pilot acting as the pilot monitoring and the copilot, seated in the right seat, acting as the pilot flying. The LC trainee instructed the Eagle1 flight crew to enter a right downwind for runway 26R at or above an altitude of 2,000 ft mean sea level (msl).

At 1059:18, the pilot of N5058U reported holding short of runway 26L on taxiway C. (N5058U had landed on runway 26L at 1052:30 and was returning to runway 26L for takeoff.) The LC trainee mistakenly advised the pilot of N5058U to hold short of runway 26R. The pilot of N5058U clarified that he was holding short of 26L, and, at 1059:31, the LC trainee acknowledged the transmission. That was the last transmission from the LC trainee. At 1059:33, the qualified LC terminated the LC trainee’s training and took over control of communications due to increased traffic. The LC trainee signed off the position but remained in the tower to observe operations. From this time until the collision occurred (about 1103), the LC was controlling nine aircraft.

During the next 2 minutes, the LC made several errors that were either corrected by him or by the pilots under his control. At 1059:44, after the pilot of N6ZP completed a touch-and-go on runway 26R, he requested a right downwind departure from the area. The LC did not respond. At 1100:23, the LC instructed, “Station Air five eight uniform two six right cleared for I’m sorry two six left cleared for takeoff.” At 1100:29, the pilot of N5058U stated, “uh I’m sorry was that for five eight uniform?” The LC then cleared the pilot of N5058U for takeoff from runway 26L. At 1100:36, the LC transmitted, “helicopter six zero romeo there is a cessna, correction, Station Air just ahead they are going to the right runway base leg for two six left.” At 1100:46, the pilot of N6ZP repeated his request for departure; the LC then approved N6ZP’s departure request, and N6ZP departed the traffic pattern in a northeasterly direction. At 1100:53, the LC instructed the helicopter pilot, “helicopter six zero romeo listen up turn crosswind” before correcting the instruction 4 seconds later to “turn base.” At 1101:15, the Eagle1 CVR recorded the copilot state, “got one on the runway,” and at 1101:19, the Eagle1 CVR recorded the pilot comment, “wowww. he’s like panicking” (with an emphasis on panicking). Figure 2 shows the aircraft in the SDM traffic pattern from about 1101 until the time of the accident.

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At 1101:49, the Eagle1 CVR recorded one of the mission specialists seated outside the cockpit ask “see him right there?” At 1102:14, while on the right downwind leg (and, according to radar data, while overtaking N1285U from behind and to the left) and abeam the tower, the Eagle1 flight crew reported to the ATCT that they had traffic in sight to the left and the right of their position. Radar data indicated that N6ZP was to the left of Eagle1 and heading to the northeast, and N1285U was between Eagle1 and SDM, on a closer-in right downwind leg.

At 1102:32, the LC instructed the pilot of N6ZP, which he thought was the Cessna on right downwind, to make a right 360° turn over the airport and rejoin the downwind. Despite the fact that, at that time, N6ZP was 2.3 nm northeast of the airport and was departing the area, the pilot of N6ZP acknowledged the instruction and initiated a right turn. At the same time, Eagle1’s CVR recorded the pilot asking, “you still got the guy on the right side?”

At 1102:42, the LC instructed the Eagle1 flight crew to turn base and cleared the flight to land on runway 26R. The LC stated in the postaccident interview that after he cleared the Eagle1 flight crew to land, he looked up to ensure that Eagle1 was turning base and noticed that the Cessna on downwind (which he still thought was N6ZP) was continuing on its downwind track and had not begun the turn that he had issued. At 1102:56, the LC contacted the pilot of N6ZP, and the N6ZP pilot replied by stating that he was turning. At 1102:59, Eagle1’s CVR recorded the pilot comment “I see the shadow but I don’t see him.”

At 1103:04, the LC transmitted “November eight five uniform”; this was the first ATC transmission with N1285U in almost 6 minutes and the first communication between the LC and N1285U. At 1103:07, the pilot of N1285U acknowledged the transmission, “eight five uniform.” At 1103:08, the LC asked the pilot of N1285U if he was still on the right downwind leg. The pilot of N1285U did not respond. The LC and the LC trainee then witnessed Eagle1 and N1285U collide.

Two witnesses located on the ramp at SDM saw the two airplanes flying eastbound, to the north of SDM. The witnesses turned away momentarily, and as they turned back, they saw an explosion, followed by airplane fragments falling to the ground. Another witness located about 2 miles east-northeast of SDM saw both airplanes at the same altitude, on intersecting flightpaths. That witness reported that the smaller airplane was flying away from the airport and that the larger airplane was flying toward the airport and descending. He noted that neither airplane appeared to make any corrective action before the collision and stated that after the collision, the smaller airplane broke apart, while the larger airplane lost a wing, nosed down, and impacted the ground.

The LC stated in a postaccident interview that the traffic level was “light and not complex” at the beginning of the training session. He stated that he noticed the traffic volume and complexity became “moderate” when the LC trainee was under instruction, which prompted the LC to terminate training and take over communications. He reported that, at that time, he had four issues to resolve, one of which was the potential conflict between Eagle1 and the Cessna on the right. He indicated that he saw Eagle1 on a midfield right downwind leg when the pilot of Eagle1 reported that he was “abeam and had the traffic to the left and right in sight.” The LC stated that, at that time, Eagle1 was flanked by two Cessnas. Although the Cessna on the right of Eagle1 was N1285U, the LC believed that the Cessna on the close-in right downwind was N6ZP; therefore, he instructed the pilot of N6ZP to make a right 360° turn to rejoin the midfield downwind. He stated that he felt the turn would resolve the conflict with Eagle1 and that the right turn would help the Cessna avoid Eagle1’s wake turbulence. When the pilot of N6ZP acknowledged the turn, the LC believed that the pilot of the Cessna to the right of Eagle1 had received the instructions and that the potential conflict with Eagle1 would be resolved. The LC then instructed Eagle1 to turn base and cleared the flight crew to land on runway 26R.

The LC stated that after he cleared the Eagle1 flight crew to land, he looked up to ensure that Eagle1 was turning as instructed. When the LC noticed that the Cessna to the right of Eagle1 had not started the right 360° turn, he began to query the pilot of N6ZP and then the pilot of N1285U. At that point, he witnessed the collision.

The LC also indicated in the postaccident interview that controllers have personal limits about how many airplanes they could handle and that he could handle four aircraft on runway 26R and three aircraft on runway 26L. When the LC was asked what caused him to realize that the Cessna was N1285U and not N6ZP, he said it dawned on him that he had a right downwind departure, and through the process of elimination, it could not have been anyone else. The LC trainee stated in a postaccident interview that when the Cessna on the right did not start the right turn, he suggested to the LC that the intended aircraft may have been N1285U. The LC indicated that, in retrospect, he should have issued a traffic alert; however, the moment he realized that Eagle1 was turning into N1285U, it was too late to help. Figure 3 shows the calculated flight tracks of Eagle1 and N1285U. Figure 4 shows the aircraft under SDM ATCT control from 1049 until the time of the collision.

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PERSONNEL INFORMATION

N1285U Pilot

The pilot, age 60, held a private pilot certificate for airplane single-engine land issued on December 2, 1997. His most recent Federal Aviation Administration (FAA) third-class medical certificate was issued on November 20, 2014, with limitations stating that he must wear corrective lenses for near and distant vision. The pilot’s logbooks revealed that he had accumulated about 277 total flight hours, including 9.7 hours in the last 6 months.

Eagle1 Pilot (Pilot Monitoring)

The pilot, age 41, held an airline transport pilot certificate issued on April 1, 2011, and a flight instructor certificate issued on November 8, 2008 (most recent renewal on November 25, 2014). He held instructor ratings for airplane multiengine, single-engine, single-engine instrument, and glider. His most recent FAA first-class medical certificate was issued on April 30, 2015, with no limitations. According to BAE, the pilot had about 4,480 total flight hours. In the 90 days before the accident, he logged 18 hours in airplanes, including 4 hours in the accident airplane make and model. His most recent flight review was completed on April 13, 2015. The pilot was seated in the left seat and was acting as the pilot monitoring.

Eagle1 Copilot (Pilot Flying)

The copilot, age 66, held an airline transport pilot certificate issued on March 8, 2005, and a flight instructor certificate issued on October 20, 2009. The copilot held ratings for airplane multiengine and single-engine land. His most recent FAA first-class medical certificate was issued on January 12, 2015, with the limitation that he must wear corrective lenses. According to BAE, the copilot had about 7,150 total flight hours, and his most recent flight review was completed on April 13, 2015. The copilot was seated in the right seat and was acting as the pilot flying.

Local Controller/Controller-in-Charge

The local controller at the time of the accident, age 59, was a certified professional controller and CIC. He had 37 years of ATC experience: 5 years in the US Air Force, 24 years with the FAA, and 8 years with his current employer. He was qualified in all positions in the SDM ATCT on September 18, 2014, and was certified as an SDM CIC on September 19, 2014. He was designated as an on-the-job training instructor on February 10, 2015. His most recent recurrent training was completed on July 31, 2015, and included, but was not limited to, the topics of runway separation, visual separation, limited aviation weather reporting station (LAWRS), and opposite direction operations. His most recent FAA second-class medical certificate was issued on September 23, 2014, with the limitation that he must wear corrective lenses. He indicated in a postaccident interview that he was in compliance with the limitation at the time of the accident.

Local Control Trainee

The LC trainee, age 27, was qualified on ground and flight data control positions on June 25, 2015. He completed local controller classroom training on June 22, 2015, and started on-the-job training on the local control position on June 27, 2015. His most recent recurrent training was completed on July 31, 2015, and included, but was not limited to, the topics of runway separation, visual separation, LAWRS, and opposite direction operations. His most recent FAA second-class medical certificate was issued on April 28, 2015, with no limitations.

AIRCRAFT INFORMATION

N1285U Airplane
The white- and yellow-colored Cessna 172M was a high-wing, four-seat airplane manufactured in 1976 and powered by a Lycoming O-320-D2G engine rated at 160 horsepower, installed under RAM Aircraft Modifications supplemental type certificate SA2375SW. The airplane had a gross weight of 2,300 lbs. The most recent annual inspection was conducted on July 15, 2015. At the time of inspection, the airplane had a total time of 9,848.1 flight hours. It was equipped with a rotating beacon light, anticollision strobe lights, navigation position lights, a landing light, and a taxi light. The operational status of each lighting system at the time of the accident could not be determined. N1285U was not equipped with a traffic advisory system (TAS), traffic alert and collision avoidance system (TCAS), or automatic dependent surveillance-broadcast (ADS-B) equipment or displays.
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Photo (C) Chris Kennedy

Eagle1

The white-colored North American Rockwell NA265-60SC Sabreliner was a low-wing, five-seat airplane manufactured in 1974 and powered by two Pratt and Whitney JT12A-8 turbojet engines, each rated at 3,000 lbs of thrust. The accident airplane was operating with an experimental airworthiness certificate because it had been modified with an external test pod attached to the lower side of the airplane aft of the nose landing gear. The airplane had a maximum gross weight of 22,900 lbs. According to the maintenance records, the most recent annual inspection was conducted on July 20, 2015. At the time of inspection, the airplane had a total time of 13,418 flight hours. The Sabreliner was equipped with a Fairchild GA-100 CVR with 30 minutes of analog audio on a continuous loop tape in a four-channel format. It was equipped with anticollision lights on the vertical tail and under the fuselage just forward of the main wheel well, wing ice inspection lights, strobe and position lights on the tail cone and each wing tip, and landing-taxi lights forward of the nose landing gear. The operational status of each lighting system at the time of the accident could not be determined. The Sabreliner was not equipped with a TAS, TCAS, or ADS-B equipment or displays.

sabre

Photo (C) Wojtek Kmiecik

METEOROLOGICAL INFORMATION

The 1053 SDM automated weather observation included wind from 310º at 6 knots, visibility 10 statute miles, clear skies, temperature 33º C, dew point 19º C, and an altimeter setting of 29.87 inches of mercury.

AIRPORT INFORMATION

SDM is located about 14 nm southeast of San Diego on the Otay Mesa at an elevation of 526 ft msl. The rising terrain associated with the Otay Mountain peaks begins about 2 miles east-northeast of SDM, and the highest terrain, at an elevation of 3,566 ft msl, is located about 8 miles east of SDM. Designated skydiving areas are located at SDM and at a second location about 3 miles east of SDM (see figure 5).

SDM has two parallel runways. Runway 8L/26R measures about 7,972 ft long and 150 ft wide, and runway 8R/26L measures about 3,180 ft long and 75 ft wide. Although the published traffic pattern for 26R is right traffic, it is common in west operations for controllers to use a right traffic pattern for both runways 26R and 26L due to the proximity of Tijuana Airport, Tijuana, Mexico, to the south of SDM. Some helicopter traffic is assigned to use a left traffic pattern for runways 26L and 26R. The published VFR pattern altitude at SDM is 1,526 ft for runway 8L/26R and 1,126 ft for runway 8R/26L. SDM operates within class D airspace, which includes the airspace extending upward from the surface to and including 3,000 ft msl within a 2.6-mile radius of SDM. (These dimensions are nonstandard; the normal radius is around 5 miles.)

The SDM ATCT is a nonapproach control federal contract tower, operated and staffed by a private company. Local controllers at nonapproach control towers must devote the majority of their time to visually scanning the runways and local area. The SDM ATCT employed five controllers; at the time of the accident, the ATCT was operating and had three controllers in the facility, which was the normal staffing schedule for that day and time. Both accident airplanes were operating under VFR in the class D airspace and were communicating with and being provided ATC services by SDM ATCT personnel. After the accident, on August 26, 2015, the SDM ATCT issued a corrective action plan regarding inconsistencies in how controllers were issuing traffic advisories and safety alerts. The plan required controllers to review FAA JO 7110.65V, Air Traffic Control, paragraphs 2-1-6 and 2-1-21, as refresher training before working an operational position.

FLIGHT RECORDERS

Cockpit Voice Recorder

The CVR was recovered from the Eagle1 wreckage and forwarded to the National Transportation Safety Board (NTSB) vehicle recorders laboratory in Washington, DC, for readout. The CVR had 30 minutes of analog audio on a continuous loop tape in a four-channel format: one channel for each of the two pilot stations, one channel for the cockpit observer station, and one channel for the cockpit area microphone (CAM). The magnetic tape was retrieved from within the crash-protected case and was successfully downloaded.

The quality of the CVR audio information was degraded due to the erase mechanism not completely erasing the previous recordings, especially on the CAM channel. Timing on the transcript was established by correlating the CVR events to the common events recorded by SDM ATC. The CVR recording started at 1032:28 and ended at 1103:10. Due to the poor quality of the CVR recording, the SDM ATC transcript was used in conjunction with the CVR recording to clarify the flight crew’s radio transmissions.

WRECKAGE AND IMPACT INFORMATION

The wreckage was located in a large open area about 1 1/2 miles northeast of SDM and consisted of two primary debris fields, one for each airplane.

N1285U Airplane

N1285U’s debris field, which was about 1,200 ft long and aligned on a magnetic heading of 055º, contained some components and fragments from Eagle1 and was located about 400 ft northeast of the Eagle1 debris field. The N1285U main wreckage contained the engine, propeller, and part of the main cabin. The main cabin wreckage consisted of parts of the floor, seats, and cabin structure. The engine remained partially attached to the firewall and exhibited impact damage to its left side, revealing cylinder components. The propeller assembly was found separated from the engine and partially buried in a small crater. The propeller was heavily gouged in multiple directions, and one blade had aft bending.

The left wing remained attached to a portion of the cabin roof and came to rest inverted. The wing and roof section had thermal damage. The flap and aileron remained attached to the wing. The inboard portion of the leading edge of the left wing displayed impact damage and red transfer marks. The fuselage and right wing were highly fragmented and spread throughout the debris field.

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Photo NTSB

Eagle1Most of the Eagle1 wreckage was contained within a radius of about 100 ft; no parts from N1285U were located within that radius. The Eagle1 main wreckage was on a magnetic heading of 060º and consisted of the cabin area, left wing, empennage, both engines, and the externally mounted test pod. The forward cabin area came to rest on its upper left side and was crushed. The remaining cabin area was crushed and had thermal damage. The left wing came to rest on its trailing edge, supported at an angle by the landing gear. Both engines were found near the tail section and displayed crush damage. The test pod and internal equipment had impact and thermal damage.

The Eagle1 right wing was found on a road near the N1285U debris field, about 400 ft north of the Eagle1 main wreckage. A power transmission line near the wing’s location was separated during the accident. The wing displayed leading-edge damage from near the tip to the separation point from the inboard portion of the wing. About 4 ft of the inboard wing was separated and recovered with the main wreckage. A 5-ft section of leading edge, from the stall fence inboard, displayed leading-edge damage revealing the internal surfaces of the wing. The lower surface of the wing displayed metallic impact marks and paint transfer marks.

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Follow-up Examination

Detailed examination of the wreckage from both airplanes was conducted at a secure facility several days after the accident. The right wing of Eagle1 was positioned with the N1285U wreckage, and investigators conducted an examination searching for contact evidence between the airplanes. The Eagle1 right wing had impact marks consistent with the impact of N1285U’s engine. Specifically, the spacing of the impact marks, on the inboard lower surface of the Eagle1 right wing were consistent with the spacing of the N1285U engine crankcase upper studs, flanges, and engine lifting eye. The angle of the marks relative to the Eagle1’s longitudinal axis was about 30º and indicates that this was the convergence angle between the airplanes. The damage on the N1285U crankcase upper studs, flanges, and engine lifting eye was consistent with impact from its left side and with the computed convergence angle.

In addition, the conformity of the Cessna fuselage, wing strut, and wing spar damage to the Eagle1 wing shape indicates that the Eagle1 right wing impacted the left side of the Cessna. The evidence is consistent with the longitudinal axes of the two airplanes being approximately perpendicular to one another at the time of impact, with Eagle1 approaching the Cessna from the left, and with the Eagle1 right wing below the Cessna left wing.

The reconstruction of the airplanes’ flightpaths, based on radar data, is described in the NTSB’s Aircraft Performance and Cockpit Visibility Study for this accident. The collision geometry resulting from the trajectory reconstruction is consistent with the collision geometry indicated by the wreckage examination.

MEDICAL AND PATHOLOGICAL INFORMATION

The FAA’s Civil Aerospace Medical Institute performed toxicology testing on tissue specimens from the three pilots. The specimens tested negative for ethanol and major drugs of abuse.The LC and LC trainee on duty at the time of the accident tested negative for drugs and alcohol.

ANALYSIS

The Cessna 172 (N1285U) was conducting touch-and-go landings at Brown Field Municipal Airport (SDM), San Diego, California, and the experimental North American Rockwell NA265-60SC Sabreliner (N442RM, call sign Eagle1) was returning to SDM from a mission flight. SDM has two parallel runways, 8R/26L and 8L/26R; it is common in west operations for controllers to use a right traffic pattern for both runways 26R and 26L due to the proximity of Tijuana Airport, Tijuana, Mexico, to the south of SDM. On the morning of the accident, the air traffic control tower (ATCT) at SDM had both control positions (local and ground control) in the tower combined at the local control position, which was staffed by a local controller (LC)/controller-in-charge, who was conducting on-the-job training with a developmental controller (LC trainee). The LC trainee was transmitting control instructions for all operations; however, the LC was monitoring the LC trainee’s actions and was responsible for all activity at that position.

About 13 minutes before the accident, the N1285U pilot contacted the ATCT and requested touch-and-go landings in the visual flight rules (VFR) traffic pattern. About that time, another Cessna 172 (N6ZP) and a helicopter (N8360R) were conducting operations in the VFR traffic pattern, and a Cessna 206 Stationair (N5058U) was inbound for landing. Over the next 5 minutes, traffic increased, with two additional aircraft inbound for landing. (Figure 1 in the factual report for this accident shows the aircraft in the SDM traffic pattern about 8 minutes before the accident.)

The LC trainee cleared the N1285U pilot for a touch-and-go on runway 26R; the pilot acknowledged the clearance and then advised the LC trainee that he was going to go around. The LC trainee advised the N1285U pilot to expect runway 26L on the next approach. At that time, three aircraft were using runway 26R (Global Express [N18WZ] was inbound for landing, N6ZP was on a right base for a touch-and-go, and a Cessna Citation [XALVV] was on short final) and three aircraft were using runway 26L (N1285U was turning right downwind for the touch-and-go, a Skybolt [N81962] was on a left downwind for landing, and N8360R was conducting a touch-and-go landing). After N1285U completed the touch-and-go on runway 26L, the pilot entered a right downwind for runway 26R.

Meanwhile, Eagle1 was 9 miles west of the airport and requested a full-stop landing; the LC trainee instructed the Eagle1 flight crew to enter a right downwind for runway 26R at or above an altitude of 2,000 ft mean sea level. At this time, about 3 minutes before the accident, the qualified LC terminated the LC trainee’s training and took over control of radio communications. From this time until the collision occurred, the LC was controlling nine aircraft. (Figure 2 and Figure 4 in the factual report for this accident show the total number of aircraft under ATCT control shortly before the accident.)

During the next 2 minutes, the LC made several errors. For example, after N6ZP completed a touch-and-go on runway 26R, the pilot requested a right downwind departure from the area, which the LC initially failed to acknowledge. The LC also instructed the N5058U pilot, who had been holding short of runway 26L, that he was cleared for takeoff from runway 26R. Both errors were corrected. In addition, the LC instructed the helicopter pilot to “listen up. turn crosswind” before correcting the instruction 4 seconds later to “turn base.” (Figure 2 in the factual report for this accident shows the aircraft in the traffic pattern about 2 minutes before the accident.)

About 1 minute before the collision, the Eagle1 flight crew reported on downwind midfield and stated that they had traffic to the left and right in sight. At that time, N1285U was to Eagle1’s right, between Eagle1 and the tower, and established on a right downwind about 500 ft below Eagle1’s position. N6ZP was about 1 mile forward and to the left of Eagle1, heading northeast and departing the area. Mistakenly identifying the Cessna to the right of Eagle1 as N6ZP, the LC instructed the N6ZP pilot to make a right 360° turn to rejoin the downwind when, in fact, N1285U was the airplane to the right of Eagle1. (The LC stated in a postaccident interview that he thought the turn would resolve the conflict with Eagle1 and would help the Cessna avoid Eagle1’s wake turbulence.) The N6ZP pilot acknowledged the LC’s instruction and began turning; N1285U continued its approach to runway 26R.

However, the LC never visually confirmed that the Cessna to Eagle1’s right (N1285U) was making the 360° turn. Ten seconds later, the LC instructed the Eagle1 flight crew to turn base and land on runway 26R, which put the accident airplanes on a collision course. The LC looked to ensure that Eagle1 was turning as instructed and noticed that the Cessna on the right downwind (which he still mistakenly identified as N6ZP) had not begun the 360° turn that he had issued. The LC called the N6ZP pilot, and the pilot responded that he was turning. In the first communication between the LC and the N1285U pilot (and the first between the controllers in the ATCT and that airplane’s pilot in almost 6 minutes), the LC transmitted the call sign of N1285U, which the pilot acknowledged. N1285U and Eagle1 collided as the LC tried to verify N1285U’s position.

A postaccident examination of both airplanes did not reveal any mechanical anomalies that would have prevented the airplanes from maneuvering to avoid an impact.

Local Controller Actions

In a postaccident interview, the LC stated that his personal limit for handling aircraft was four aircraft on runway 26R plus three aircraft on runway 26L (for a total of seven). From the time the LC took over local control communications from the LC trainee (3 minutes before the accident) until the time of the collision, the LC was in control of nine aircraft. Thus, the LC had exceeded his own stated workload limit. Research indicates that the cognitive effects of increasing workload may include memory deficits; distraction; narrowing of attention; decreased situational awareness; and increased errors, such as readback errors or giving instructions to the wrong aircraft. (Mica Endsley and Mark Rodgers’s 1997 report, Distribution of Attention, Situation Awareness, and Workload in a Passive Air Traffic Control Task: Implications for Operational Errors and Automation [FAA Report No. DOT/FAA/AM-97/13], details the cognitive effects of increasing workload.) To resolve the increasing workload, the LC had two options. He could have directed traffic away from SDM or split the local control/ground control positions, but he did neither. The LC trainee was qualified to work the ground control position, and the SDM ATCT had three controllers in the facility, which was the normal staffing schedule for that day and time.

As a result of the high workload, the LC made several errors after taking over the position from the LC trainee, including not responding promptly to a departure request from the N6ZP pilot and incorrectly instructing a helicopter pilot to turn to crosswind before correcting the instruction to turn base. The LC also did not provide traffic and/or sequence information with the instructions for the N6ZP pilot to turn 360° right. If the LC had done so, the N6ZP pilot might have reminded the controller that he was departing the airspace or requested clarification per 14 Code of Federal Regulations (CFR) 91.123(a), “Compliance with ATC [Air Traffic Control] Clearances and Instructions.” In addition, if the Eagle1 flight crew had heard their aircraft called as traffic to another aircraft, it may have helped their visual search or prompted them to seek more information about the location of the conflicting traffic. The LC’s stress amid the high workload was evidenced in his “listen up. turn crosswind” instruction to the helicopter pilot, after which the Eagle1 cockpit voice recorder (CVR) recorded the pilot comment, “wowww. he’s like panicking” (with an emphasis on “panicking”).

Most importantly, the LC misidentified N1285U as N6ZP and did not ensure that the Cessna to the right of Eagle1 was performing the 360° turn before issuing the turn instruction to Eagle1. Although the N6ZP pilot had already requested a departure from the area and the LC had approved the departure request, the LC still believed that N6ZP was to the right of Eagle1, which indicates that the LC lacked a full and accurate mental model of the situation once he took over communications from the LC trainee. The LC trainee stated in a postaccident interview that when the Cessna on the right did not start the right turn, he suggested to the LC that the intended aircraft may have been N1285U. The high workload due to the increased traffic likely contributed to the LC’s incomplete situational awareness.

In a postaccident interview, the LC reported that, at the time that he took over for the LC trainee, he had four issues to resolve, one of which was the potential conflict between Eagle1 and the Cessna on the right. Thus, he was aware of the potential conflict between two aircraft, even though he did not have the accurate mental picture of which Cessna was which. The LC explained that the acknowledgement from the N6ZP pilot of the right 360° turn to rejoin the downwind indicated to him that the intended Cessna pilot to Eagle1’s right had received and acknowledged his instructions. Had he looked up to ensure that the control instructions that he provided to the Cessna on the right were being performed, he would have noticed that the Cessna to the right of Eagle1 was not turning and likely would not have issued the conflicting turn instruction to Eagle1.

Further, Federal Aviation Administration (FAA) Order 7110.65, paragraph 2-1-6, “Safety Alerts,” states, in part, that a controller should “issue a safety alert to an aircraft if you are aware the aircraft is in a position/altitude that, in your judgment, places it in unsafe proximity to terrain, obstructions, or other aircraft…” About 14 seconds elapsed between the LC’s base turn instruction and landing clearance for Eagle1 and his call to the N6ZP pilot to ask about the right 360° turn instruction. When the LC saw that the airplanes were in unsafe proximity to each other, his priority should have been to separate the aircraft by issuing a safety alert to the Eagle1 flight crew (such as “TRAFFIC ALERT, Eagle1, to your right and below at pattern altitude, advise you climb immediately”). However, instead of issuing a safety alert to the Eagle1 flight crew, he separately called each Cessna pilot to verify their call signs and positions, which demonstrated narrowing of attention, another indication of the LC’s stress due to high workload. If the LC had issued a safety alert to the Eagle1 flight crew as soon as he looked up after clearing Eagle1 to land and noticed that the Cessna to the right of Eagle1 was not turning, the Eagle1 pilots may have been able to take action to avoid N1285U. After the accident, on August 26, 2015, the SDM ATCT issued a Corrective Action Plan regarding inconsistencies in how controllers were issuing traffic advisories and safety alerts. The plan required controllers to review FAA Order 7110.65V, Air Traffic Control, paragraphs 2-1-6 and 2-1-21, as refresher training before working an operational position.

See-and-Avoid Concept

According to 14 CFR 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.” 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 instrument flight rules 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.

A review of the ATCT and Eagle1 CVR transcripts revealed that during the entire time that the Eagle1 flight crew was on the ATCT local control frequency, there were no communications to or from N1285U. According to the CVR transcript, the Eagle1 pilots were aware of other traffic in the area and were actively looking for it; they had multiple airplanes in sight while on the downwind leg, and the pilot stated “I see the shadow but I don’t see him” shortly before the accident. (Review of available data indicated that it was most likely the shadow of N1285U.)

Aircraft Performance and Cockpit Visibility Study

NTSB aircraft performance and cockpit visibility study determined that once Eagle1 began the turn to base leg, Eagle1 would have been largely obscured from the N1285U pilot’s field of view but that N1285U should have remained in the Eagle1 pilots’ field of view until about 4 seconds before the collision. (Figures 8a and 8b in the factual report for this accident show the simulated cockpit visibility from the Eagle1 copilot’s seat and the N1285U pilot’s seat at 1102:34, respectively.) Although the Eagle1 copilot would have had a better viewing position (in the right seat) to detect N1285U than the pilot, he was the pilot flying; thus, his attention would have been divided among multiple tasks, including configuring, operating, and maneuvering the airplane for approach and landing, as well as scanning for traffic. (Throughout Eagle1’s CVR recording, the pilot, seated in the left seat, was communicating on the radio and responding to checklists, consistent with that pilot acting as the pilot monitoring and the copilot, seated in the right seat, acting as the pilot flying.) N1285U’s lack of relative motion in the Eagle1 pilots’ field of view, combined with the fact that N1285U was below their horizon and, therefore, against the visual clutter of the background terrain, significantly decreased N1285U’s visual conspicuity to the Eagle1 pilots.

It is likely that, as N1285U neared the end of the downwind leg (after Eagle1 overtook N1285U from behind and to the left), the pilot was anticipating his turn to the base leg and that his primary external visual scan was to the right, toward the airport, instead of to the left where Eagle1 was. Although the pilot may have had some cues of Eagle1’s relative positioning in the pattern based on his monitoring of the ATCT communications, the challenge remained of detecting the airplane visually while maneuvering in the pattern.

Cockpit Display of Traffic Information

Although the N1285U and Eagle1 pilots were responsible for seeing and avoiding the other aircraft in the traffic pattern, our aircraft performance and cockpit visibility study revealed that their fields of view were limited and partially obscured at times. Research indicates that any mechanism to augment and focus a pilot’s visual search 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. The accident airplanes were not equipped with these types of systems, but 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.)

While Eagle1 remained obscured from the N1285U pilot’s field of view during Eagle1’s downwind-to-base turn, N1285U remained in the Eagle1 (right seat) copilot’s field of view for the majority of the 3 minutes preceding the accident. Even though both Eagle1 pilots were aware of and actively looking for traffic in the pattern, they still failed to see and avoid colliding with N1285U, which underscores the shortcomings in the see-and-avoid concept. An in-cockpit traffic display would have shown the Eagle1 pilots all of the traffic at the airport about the time of their initial call to the ATCT, and, about 2 minutes later, the Eagle1 pilots would have received an aural alert; the display would have shown N1285U’s target change from a cyan color to a yellow color positioned between Eagle1 and the airport. About 1 1/2 minutes later, the Eagle1 pilots would have received another aural alert. The N1285U pilot would also have received an aural alert several seconds before impact, which may not have given him enough time to take evasive action. While most systems are limited to aiding pilots in their visual acquisition of a target and cannot provide resolution advisories (specific maneuvering instructions intended to avoid the collision), a cockpit indication of traffic would likely have heightened the pilots’ situational awareness and possibly alerted them of the need to change their flightpaths to resolve the conflict.

Postaccident Actions

In November 2016, NTSB 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 July 7, 2015, near Moncks Corner, South Carolina; 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, NTSB also issued a safety alert titled “Prevent Midair Collisions: Don’t Depend on Vision Alone” (See NTSB Issues Safety Alert to Pilots on Midair Collision Prevention. November 2016) 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], NTSB issued a safety alert titled “See and Be Seen: Your Life Depends on It” regarding the importance of maintaining adequate visual lookout. (See: See and Be Seen: Your Life Depends on It. NTSB Safety Alert 045 May 2015)

PROBABLE CAUSE

The National Transportation Safety Board determines the probable cause(s) of this accident as follows:

  • The local controller’s (LC) failure to properly identify the aircraft in the pattern and to ensure control instructions provided to the intended Cessna on downwind were being performed before turning Eagle1 into its path for landing. Contributing to the LC’s actions was his incomplete situational awareness when he took over communications from the LC trainee due to the high workload at the time of the accident. Contributing to the accident were the inherent limitations of the see-and-avoid concept, resulting in the inability of the pilots involved to take evasive action in time to avert the collision.

REFERENCES

Excerpted from

  1. NTSB WPR15MA243A layout
  2. NTSB WPR15MA243A full narrative
  3. NTSB youtube channel

Further reading

  1. Cessna 150M and a Lockheed Martin F-16CM midair collision. Final report
  2. NTSB Issues Safety Alert to Pilots on Midair Collision Prevention. November 2016
  3. See and Be Seen: Your Life Depends on It. NTSB Safety Alert 045 May 2015

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minime2By Laura Duque-Arrubla, a medical doctor with postgraduate studies in Aviation Medicine, Human Factors and Aviation Safety. In the aviation field since 1988, Human Factors instructor since 1994. Follow me on facebook Living Safely with Human Error and twitter@dralaurita. Human Factors information almost every day 

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