Beech C-99 accident at San Luis Obispo, California, 24 Aug 1984 (N112SM)

Accident narrative

On August 24, 1984, at about 1117:38 Pacific daylight time, a Wings West Airlines Beech C-99 and an Aesthetec, Inc. Rockwell Commander 112TC collided in midair near San Luis Obispo, California. The weather was clear. The Beech C-99 had just departed San Luis Obispo County Airport and was climbing on a westbound heading, while the Rockwell Commander was descending toward the airport on an eastbound track. The airplanes collided head-on at an altitude of about 3,400 feet. All 17 people aboard the two aircraft were killed.

### The flight

Wings West Flight 628 was a regularly scheduled commuter flight from Los Angeles to San Francisco, with en route stops at Santa Maria and San Luis Obispo. It departed San Luis Obispo at 1116 with 13 passengers and two pilots on board. The 28-year-old captain had 4,110 flight hours, including 873 in the Beech C-99. The 46-year-old first officer had 6,194 flight hours, with 62 in the Beech C-99.

The Rockwell Commander, N112SM, departed Paso Robles, California, around 1052 on a training flight. Aboard were a 58-year-old instructor pilot with 4,857.6 flight hours and a 42-year-old student pilot with about 2,450 hours. No flight plan was filed, nor was one required.

### Sequence of events

San Luis Obispo County Airport was an uncontrolled airport. Flight 628 departed runway 29 in visual meteorological conditions, intending to request an instrument flight rules (IFR) clearance from the Los Angeles Air Route Traffic Control Center (ARTCC) once airborne. The pilot of another airplane overheard Flight 628 announce on the UNICOM frequency that it was "departing runway two-nine straight out."

At 1116, Flight 628 called the Los Angeles ARTCC. The radar controller was busy sequencing two other airplanes and answered at 1116:40. At 1116:51, Flight 628 reported climbing through 2,700 feet. The controller assigned a discrete transponder code. At 1117:16, the controller reported radar contact six miles northwest of the airport and asked for the flight's altitude. Flight 628 replied, "Three thousand one hundred, climbing."

At 1117:23, the controller cleared Flight 628 to San Francisco, instructing it to climb and maintain 7,000 feet. Flight 628 began reading back the clearance at 1117:32.

Meanwhile, the Rockwell Commander was flying toward San Luis Obispo. About 1115, the UNICOM operator at the airport recalled hearing an airplane report "Inbound, approaching Dobra" (an approach fix 6.1 nautical miles from the runway). The Board concluded this transmission was made by the Rockwell Commander.

At 1117:38, the two airplanes collided about 8 nautical miles west-northwest of the airport. At 1118:40, the radar controller noticed he had lost Flight 628's transponder signal and attempted to call the flight, but received no response.

### What the investigation found

The wreckage of both airplanes fell into an open field. The Rockwell Commander struck the ground in an almost vertical attitude. The Beech C-99 struck the ground horizontally and was largely destroyed by a postimpact fire.

Damage to the aircraft, including paint marks and a propeller slash on the Rockwell's right wing, indicated that although the collision was virtually head-on, the Rockwell was in a slight descending right turn at impact. Radar data also indicated the Rockwell had turned slightly to the right just before the collision. There was no evidence that either pilot performed an evasive maneuver.

The Board analyzed the visibility from both cockpits. Both airplanes were painted predominantly white, which would have washed out color contrast on a bright day. The head-on closure rate of about 544 feet per second meant there was little relative movement of the aircraft across the windshields. The Board concluded that the conditions were such that the pilots' ability to see and avoid each other was quite marginal.

The investigation examined the air traffic control radar data. The Rockwell Commander's transponder returns were processed by the computer and provided to the controller's display. The Board concluded the Rockwell's radar return had been displayed on the scope, but the controllers did not observe it. The Board believed the developmental radar controller's attention was focused on entering Flight 628 into the IFR system and monitoring other IFR traffic, resulting in a "tunneling" of attention. Furthermore, he had only about 28 seconds to evaluate the relative positions of the two airplanes and issue an advisory.

The Board also examined the pilots' use of radio communications. Wings West procedures required pilots to keep one radio tuned to the company frequency at all times. Consequently, Flight 628's crew switched their other radio from the UNICOM frequency to the ARTCC frequency before 1116. Because they were no longer monitoring UNICOM, they most likely did not hear the Rockwell Commander's inbound radio call.

Similarly, the Rockwell Commander pilot did not contact the Los Angeles ARTCC to advise them of his practice instrument approach. Had he done so, the controller would have known the Rockwell was flying directly toward Flight 628 and could have issued a safety advisory.

### Probable cause

The National Transportation Safety Board determined that the probable cause of this accident was "the failure of the pilots of both aircraft to follow the recommended communications and traffic advisory practices for uncontrolled airports contained in the Airman's Information Manual to alert each other to their presence and to enhance the controller's ability to provide timely traffic advisories."

The Board found that underlying the accident were the physiological limitations of human vision and reaction time, as well as the short time available to the controller to detect and appraise radar data and to issue a safety advisory. Contributing to the accident was the Wings West Airlines policy which required its pilots to tune one radio to the company frequency at all times.