Allegheny Airlines Flight 853
Allegheny Airlines Flight 853, a McDonnell Douglas DC-9-30, collided in mid-air with a Piper PA-28 at approximately 3,550 feet on September 9, 1969, near Fairland, Indiana. The DC-9 carried 78 passengers and 4 crew. The Piper was leased to a student pilot making a solo cross-country flight. The occupants of both aircraft were killed in the accident and the aircraft were destroyed by the collision and ground impact.
Allegheny 853 (N988VJ), a DC-9-30, was a regularly scheduled flight departing Boston, Massachusetts, for St. Louis, Missouri, with stops in Baltimore, Cincinnati and Indianapolis. The flight departed Cincinnati at 3:15pm enroute to Indianapolis. Allegheny 853, flying under Instrument Flight Rules (IFR) clearance to Indianapolis, was instructed by Indianapolis Approach Control to descend to 2500 feet after passing the Shelbyville VOR at 6000 feet. The flight was then vectored to a 280 degree heading. Meanwhile, the PA-28 (N7374J) was on a southeasterly heading operating under a filed Visual Flight Rules (VFR) flight plan which indicated a cruising altitude of 3500 feet. The PA-28 was not in communication with Air Traffic Control, was not transponder equipped, and there was no evidence it appeared as a primary radar target on the radarscope.
Eight witnesses saw the aircraft collide. They reported broken to scattered cloud cover in the area, but both aircraft were below the clouds and could be seen clearly at the time of the collision. According to the witnesses, neither aircraft attempted a collision avoidance maneuver. Wreckage analysis later concluded the PA-28's left forward side just forward of the left wing root clipped the DC-9's upper right vertical tail just below the horizontal stabilizer.
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 Federal Aviation Administration 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.
After this and similar mid-air collisions or near collisions, both the NTSB and FAA realized the inherent limitations of the "see and be seen" principle of air traffic separation in visual meteorological conditions, especially when aircraft of dissimilar speeds or cloud layers and other restrictions to visibility are involved. The following mitigating steps have since been taken:
- Transponders are now installed in most general aviation aircraft and all commercial/air-carrier aircraft, dramatically increasing radar visibility of lower and slower flying smaller aircraft, especially near atmospheric disturbances or other clutter (see Air Traffic Control Radar Beacon System and Secondary Surveillance Radar)
- Most airports with scheduled airline service now have a surrounding controlled airspace (ICAO designation Class B or Class C) for improved IFR/VFR traffic separation - all aircraft must be transponder equipped and specifically cleared by and in communication with ATC to operate within this controlled airspace
- Most commercial/air-carrier aircraft (and some general aviation) now have an airborne collision avoidance or TCAS device on board, that can detect and warn about nearby transponder-equipped traffic
- ATC radar systems now have "conflict alert" - automated ground based collision avoidance software that sounds an alarm when aircraft come within less than a minimum safe separation distance