Beech U-21A accident at Independence, Missouri, 20 Jan 1987

Accident narrative

On January 20, 1987, about 1228 central standard time, a U.S. Army Beech U-21A and a Sachs Electric Company Piper PA-31-350 collided at 7,000 feet over the Lake City Army Ammunition Plant in Independence, Missouri. The collision occurred in daylight and visual meteorological conditions, with high scattered clouds and 20 miles visibility. The two pilots and one passenger aboard the U-21, and the pilot and two passengers aboard the PA-31, were fatally injured. Both airplanes were destroyed.

### The flights

The PA-31 departed Kansas City Downtown Airport at 1221 in accordance with visual flight rules (VFR), en route to St. Louis, Missouri. The pilot advised the local controller he would make a left turn to the east after departure. The airplane climbed eastbound at an average rate of 1,066 feet per minute. The onboard transponder transmitted a VFR code and mode-C altitude information. The pilot was not required to be in communication with air traffic control, and the flight did not request flight-following services.

The U-21 departed Anniston, Alabama, at 0944, en route to Sherman Army Airfield at Fort Leavenworth, Kansas. The flight was conducted under instrument flight rules (IFR). At 1221, Kansas City Air Route Traffic Control Center handed the flight off to the Kansas City Terminal Radar Approach Control (TRACON) East Radar controller. The U-21 was level at 7,000 feet.

At 1225:09, the East Radar controller advised the U-21 of traffic at its 12 o'clock position, 5 miles away. The crew reported the traffic in sight. Radar data confirmed this advisory pertained to a different aircraft, not the PA-31. There were no further radio transmissions from the U-21.

### The collision

The airplanes collided nearly head-on. Radar data indicated the PA-31 was tracking about 093 degrees true and the U-21 was tracking about 296 degrees true, yielding an approach angle of 157 degrees. The closure rate was 350 knots, or 591 feet per second.

The PA-31, which was climbing, passed in front of and rose above the cockpit of the U-21. The bottom of the PA-31 contacted the top of the U-21 windshield, severing the U-21's cabin roof, aft fuselage, and empennage. Both airplanes fell to the ground within the confines of the ammunition plant. The U-21 wreckage was largely consumed by a postcrash fire; the PA-31 wreckage did not burn.

### What the investigation found

Examination of the wreckage revealed no evidence of pre-collision failure or malfunction of the control systems or powerplants of either airplane. Both flightcrews were properly certificated and qualified.

Because the flight was in visual meteorological conditions, both pilots were required to maintain vigilance to "see and avoid" other aircraft. A cockpit visibility study showed the PA-31 would have been visible through the windshields of both U-21 pilots without obstruction. The U-21 would have been positioned near the center windshield post of the PA-31, which would have restricted the pilot to a monocular view unless he moved his head.

The Board noted the limitations of the see and avoid concept given the 350-knot closure rate. A visual acquisition model indicated that the probability of either crew seeing the other airplane was not high until the last few seconds before the collision. Because it takes a pilot approximately 12.5 seconds to recognize a collision threat and execute an evasive maneuver, the Board believed the see and avoid concept alone may not have been sufficient to avert the accident.

At the time of the collision, the TRACON East Radar position was staffed by an area supervisor providing on-the-job training to a developmental controller. The developmental controller signed on to the position at 1228, the same minute the accident occurred, immediately following a position relief briefing from the supervisor. Both controllers reported they did not see any radar information associated with aircraft in the vicinity of the U-21, and they did not issue any traffic advisories regarding the PA-31.

The TRACON's Automated Radar Terminal System (ARTS) III displayed tracked targets with a full data block (FDB) and untracked VFR targets with a limited data block (LDB). The U-21 was a tracked target; the PA-31 was an untracked VFR target. A post-accident retrack of the recorded radar data showed the LDB for the PA-31 was displayed on the scope until the collision. The Board concluded the radar target was displayed, but both controllers failed to perceive it. The Board believed the controllers were likely distracted by the position relief briefing, and that controller reliance on FDB symbology may have caused them to overlook the LDB.

The ARTS III system included a "conflict alert" subprogram that provided aural and visual warnings of impending collisions between tracked targets. Because the PA-31 was flying VFR and had not requested flight-following services, it was not tracked, and the conflict alert did not activate. The Board noted that if the PA-31 had been tracked, the system would have alerted the controllers about 40 seconds before the collision.

### Probable cause

The National Transportation Safety Board determined that the probable cause of the accident was "the failure of the radar controllers to detect the conflict and to issue traffic advisories or a safety alert to the flightcrew of the U-21; deficiencies of the see and avoid concept as a primary means of collision avoidance; and the lack of automated redundancy in the air traffic control system to provide conflict detection between participating and nonparticipating aircraft."