Allegheny Airlines Flight 853 was a regularly scheduled flight departing Boston for Baltimore, Cincinnati, Indianapolis, and St. Louis. Captain James Elrod (47) and First Officer William Heckendorn (26) were at the controls. Elrod was a seasoned veteran with more than 23,800 flight hours.[1] The flight left Cincinnati at 3:15 pm en route to Indianapolis. They were flying under Instrument Flight Rules (IFR) clearance to Indianapolis, and Approach Control instructed them to descend to 2,500 feet (760 m) after passing the Shelbyville VOR at 6,000 feet (1,800 m). The flight was then vectored to a 280 degree heading.[1]
Meanwhile, the private Piper PA-28 piloted by Robert Carey (34) was on a southeasterly heading. It was operating under a filed visual flight rules (VFR) flight plan which indicated a cruising altitude of 3,500 feet (1,100 m). It was not in communication with air traffic control and was not equipped with a transponder,[2] and there was no evidence that it appeared as a primary radar target on the radarscope.[1]
The two aircraft converged at a relative speed of 350 mph (300 kn; 560 km/h). The initial point of impact was at the top front right section of the DC-9's vertical stabilizer, just underneath the horizontal stabilizer. On the Piper, the impact point was just forward of the left wing root.[3] The impact severed the entire tail assembly of the DC-9, which inverted and plowed into a soybean field at an approximate speed of 400 mph (350 kn; 640 km/h) about 100 yards (300 ft; 91 m) north of the Shady Acres mobile home park.[3]
Probable cause
The National Transportation Safety Board released the following probable cause in a report adopted July 15, 1970:[1]
The Board determines the probable cause of this accident to be the deficiencies in the collision avoidance capability of the Air Traffic Control system of the FAA in a terminal area wherein there was mixed instrument flight rules (IFR) and visual flight rules (VFR) traffic. The deficiencies included the inadequacy of the see-and-avoid concept under the circumstances of this case; the technical limitations of radar in detecting all aircraft; and the absence of Federal Aviation Regulations which would provide a system of adequate separation of mixed VFR and IFR traffic in terminal areas.
Legacy
The NTSB and FAA realized the inherent limitations of the "see and be seen" principle of air traffic separation in visual meteorological conditions, especially involving aircraft of dissimilar speeds or cloud layers and other restrictions to visibility.[1] Over a period of years, following similar incidents and taking advantage of technological advances, the two agencies drove a number of corrective steps for the aviation industry, including:
Most airports with scheduled airline service now have a surrounding controlled airspace (ICAO designation Class B or Class C) for improved IFR and VFR traffic separation; all aircraft must be transponder-equipped and in communication with air traffic control to operate within this controlled airspace[5]
Most commercial and air-carrier aircraft now have an airborne collision avoidance or TCAS device on board that can detect and warn about nearby transponder-equipped traffic[4]
ATC radar systems now have "conflict alert"—automated ground-based collision avoidance software that sounds an alarm when aircraft come within a minimum safe separation distance[6]
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