January 1, 2000
By Bruce Landsberg
Midair collisions are one of the few types of accidents where seconds of inattention can lead to disaster. Everything is normal and then, in a split second, one or both aircraft may be out of control and spinning earthward. Fortunately, these collisions are relatively rare. In 1998 there were 14. Small details play significant roles in the accident chain, and in the case below, a transponder figures prominently.
On April 4, 1998, at about 10:34 a.m. Eastern Standard Time (EST), a Cessna 525 (CitationJet or CJ) and a Cessna 172 collided over a residential area in Marietta, Georgia. The CJ had departed from the Dekalb-Peachtree Airport at about 10:30 on an instrument flight plan to Harrisburg, Pennsylvania. According to radar data, the 172 departed Mathis Airfield, near Cumming, Georgia, at about 10:25. The 172 was on a power line patrol and did not file a flight plan.
Visual meteorological conditions prevailed at the time of the accident. The visibility at nearby reporting stations ranged from seven to 10 miles, with broken to overcast skies varying from 2,800 to 3,100 feet agl. Radar information showed that the collision occurred at 3,425 feet msl. This was approximately 2,400 feet agl or just about 500 feet below the clouds.
After takeoff, ATC assigned the CJ 3,000 feet and a heading of 280 degrees. At 10:33:01, ATC cleared the jet to a heading of 360 degrees with a climb clearance to 14,000 feet. According to radar data, the CJ had just vacated 3,000 feet msl and was passing 3,400 feet msl when a primary target merged with the radar target of the jet. The CJ pilot's acknowledgment of the climb clearance was interrupted at about 10:34.
The Atlanta ATC radar showed a primary target (radar energy reflected from the skin of the airplane without electronic enhancement; i.e., no transponder) beginning at 10:25:07. The target appeared approximately two miles southwest of Mathis Airport and traveled to the southwest, intersecting the flight path of the CJ at the collision site.
At about 10:34, the 172 contacted the Dobbins Air Force Base Tower and identified itself as Seven-Whiskey-Delta. The controller responded, "Seven-Whiskey-Delta, Dobbins Tower." The aircraft replied, "Good morning, sir, Seven-Whiskey-Delta, Cessna One-Seventy...." The controller stated, "Seven-Whiskey-Delta, you were cut out." No further transmissions were received from the airplane, and the controller was unable to reestablish contact.
A reconstruction of the accident sequence based upon radar data showed that the CJ was tracking north, while the primary target that merged with the CJ target exhibited a southerly track with an approximate closure rate of 300 knots. A study was done to determine how long each pilot might have had the other aircraft in sight, based on radar data. The data plots indicated that the 172 was either behind the CJ's center windshield post or may have been visible to the right of the center windshield post for 35 seconds prior to collision. The plots indicate that the CJ would have been visible to the 172 pilot in the lower left quadrant of the pilot's windshield somewhere be-tween 35 to five seconds before impact. The 172 was in level flight and collided with the climbing jet at an angle of about 52 degrees to the left of head-on.
The CJ came to rest inverted in the rear of a residence, and the 172 crashed inverted about a half-mile away. The commercial pilot of the 172, the airline transport pilot of the CJ, and the CJ's three passengers were killed.
The wreckage of the CJ was scattered over an area of approximately 1.5 square miles. The horizontal stabilizer, elevators, and the top one-quarter of the vertical stabilizer separated from the airplane. Black tire marks were found on the vertical stabilizer leading edge. A small five-inch-long section, separated from the outboard end of the left horizontal stabilizer leading edge, was crushed aft, forming a tight three-inch-diameter U shape. This section was among the first debris located on the wreckage path. Additionally, blue paint that was similar to the blue paint of the 172 was found on the leading edge of the horizontal stabilizer. All of the fractures to the empennage exhibited overload failures.
The 172 wreckage was also scattered over an area of approximately 1.5 square miles. Witnesses observed the aircraft hit trees before reaching the ground. The wreckage burned in a post-impact fire. There was evidence of metal scrape marks on the underside of the tail cone. The nose landing gear, which was found with the main wreckage, exhibited fire damage. The upper strut housing evidenced an indentation between five and seven inches below the upper end of the strut. The indentation on the outboard left horizontal stabilizer leading edge of the CJ matched the indentation on the nose landing gear strut housing of the 172.
The CJ pilot held a type rating for single-pilot operation of the jet with 1,825 hours' total flight time and 86 hours in the CitationJet. He had learned to fly in 1994 and moved rapidly through a series of high-performance aircraft, including a Beech 60 Duke and a Beech 90-series King Air. The pilot's instructor, who had trained him from the beginning and attended FlightSafety International CJ training with him, described the pilot as highly qualified, competent, and detail-oriented. The pilot had enrolled in a CJ proficiency course that required attendance every three months.
The pilot of the 172 held commercial pilot and flight instructor certificates, as well as a mechanic's certificate with airframe and powerplant ratings. He had logged almost 14,000 hours and in August 1997 had completed a flight review in the accident aircraft. FAA records indicated that the pilot's certificate had been suspended on three occasions. In 1987, he received a 30-day suspension for a violation involving minimum safe altitudes. He received another 45-day suspension in 1990 regarding an ATC clearance for operations in Class B airspace. In 1993, a 60-day suspension was given for violation of various sections of FAR Parts 23 and 91 that dealt with miscellaneous markings and placards on the airplane, airman certificate requirements, careless operation, and airworthiness of the airplane.
The 172 pilot's power line patrol contract required that the pilot adhere to FAA regulations on certification of aircraft, special permits, etc., and that the surveillance aircraft must have an observer on board at all times. The observer's job was to look at the lines while the pilot flew the airplane. According to the power company maintenance manager, who maintains the lines, if a pilot were discovered conducting surveillance flights without an observer, he would be counseled first and possibly terminated from employment if the action were repeated. The pilot's power line patrol records, however, showed multiple flights that did not reflect an observer, or "self" was recorded in the observer block.
The accident investigation team determined that the 172's transponder function switch was in the Off position. The unit was returned to the manufacturer for testing. The ATC code switch wafers were burned but readable. The wafers and the function switch were compared to an undamaged unit that corresponded to a VFR code of 1200 and Off.
The pilot's son, who had flown often with his father, stated that because of the type of flying involved, his father would leave the transponder in the Off position during normal flight until he approached airspace in which he needed the transponder. At that point, he would turn on the transponder before entering the airspace. According to FAR 91.215, the Cessna 172 was required to have an operating transponder while flying within the "Mode-C veil" extending 30 nautical miles from the Atlanta Hartsfield International Airport — and the accident occurred within this veil.
During interviews, the air traffic controllers stated that they did not observe the primary target associated with the 172 and therefore did not provide advisories to the CJ pilot. As mentioned earlier, the 172 was not in contact with ATC despite a call just before the collision occurred. In visual conditions, ATC will call observed traffic on a time-permitting basis to aircraft for which it is providing radar service, but that is not guaranteed. The controller's handbook states, "Additional services are provided to the extent possible contingent only upon the controller's capability to fit them into the performance of higher-priority duties and on the basis of limitations of the radar, volume of traffic, frequency congestion, and controller workload."
The workload in a single-pilot cockpit, regardless of aircraft type, can be significant. Even momentary and necessary distractions such as tuning a radio or programming the autopilot may mean less time for looking outside. The CJ pilot's instructor reported that the FlightSafety training course recommended the use of the autopilot as soon as possible after takeoff, to reduce the workload (wheels up, flaps up, autopilot on). According to the instructor, to initiate a climb in the CJ with the autopilot engaged involves looking down, locating and turning the altitude select knob, checking the readout on the electronic attitude primary flight display, and then rotating the pitch wheel to begin the climb. A pilot familiar with the aircraft can do this by feel, since the controls are differentiated by feel and the EADI altitude readout is just below the glareshield. The distraction, by design, is minimal to enhance single-pilot operation. FlightSafety also recommended that the aircraft recognition lights be illuminated below FL180. It is not clear from the accident report whether they were in use.
When on a collision course, the target aircraft with which you are converging will appear to remain stationary. It just "blossoms" suddenly in the windshield, assuming the pilot is in a position to see it at all. A closure speed of 300 kt equates to five miles per minute and allows little time to identify a threat and react.
This accident chain involved failure to follow procedures requiring an observer, high closure speeds, operation without a transponder, not using VFR flight following, and aircraft structure that may have blocked the other airplane from the pilot's vision. An aircraft operating without a transponder in high-density airspace may not only be illegal, but it may also deprive the ATC system of an opportunity to spot a threat that the pilots involved could not. Talking and squawking will help when scanning alone may not do the job.
See also the index of "Safety Pilot" articles, organized by subject. Bruce Landsberg is executive director of the AOPA Air Safety Foundation.
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