History of Ground Proximity Warning
Trans World Airlines Flight 514, registration N54328, was a Boeing 727-231 en route from Indianapolis and Columbus, Ohio, to Washington Dulles International that crashed into Mount Weather, Virginia, on December 1, 1974. All 85 passengers and 7 crew members were killed.
The flight was originally destined for Washington National Airport. However, the plane diverted to Dulles when high crosswinds, east at 28 knots and gusting to 49 knots, prevented safe operations on the main north-south runway at Washington National. The flight was being vectored for a non-precision instrument approach to runway 12 at Dulles. Air traffic controllers cleared the flight down to 7,000 feet before clearing them for the approach while not on a published segment.
The jetliner began a descent to 1,800 feet shown on the first checkpoint for the published approach. The voice recorder indicated there was some confusion in the cockpit over whether they were still under a radar controlled approach segment which would allow them to descend safely. After reaching 1,800 feet there were some 100-to-200-foot altitude deviations which the flight crew discussed as encountering heavy downdrafts and reduced visibility in snow. The plane impacted the west slope of Mount Weather at 1,670 feet at approximately 230 knots (260 mph). The wreckage was contained within an area about 900 feet long and 200 feet wide. The evidence of first impact was trees whose tops were cut off about 70 feet feet above the ground. The elevation at the base of the trees was 1,650 feet. The wreckage path was oriented along a line 118 degrees magnetic. Calculations indicated that the left wing went down about 6 degrees as the aircraft passed through the trees and the aircraft was descending at an angle of about 1 degree. After about 500 feet of travel through the trees, the aircraft struck a rock outcropping at an elevation of about 1,675 feet. Numerous heavy components of the aircraft were thrown forward of the outcropping. Numerous intense post-impact fires were located, and extinguished. The mountain's summit is at 1,754 feet.
The accident investigation board was split in its decision as to whether the flight crew or Air Traffic Control were responsible. The majority absolved the controllers as the plane was not on a published approach segment. The dissenting opinion was that the flight had been radar vectored. Terminology between pilots and controllers differed without either group being aware of the discrepancy. It was common practice at the time for controllers to release a flight to its own navigation with "Cleared for the Approach," and flight crews commonly believed that was also authorization to descend to the altitude at which the final segment of the approach began. No clear indication had been given by controllers to Flight 514 that they were no longer on a radar vector segment and therefore responsible for their own navigation. Procedures were clarified after this accident. Controllers now state, "Maintain (specified altitude) until established on a portion of the approach," and pilots now understand that previously assigned altitudes prevail until an altitude change is authorized on the published approach segment the aircraft is currently flying.
At the time of the accident there were two (2) companies that had, on the shelf, GPWS (Ground Proximity Warning System) that used the radio altimeter as the primary source of input information.
The two companies were Sundstrand Corporation and Allied Signal. Both companies eventually were merged into Hamilton Standard and Honeywell Corporation. Each GPWS system was somewhat similar, with one costing $5000 and the other $10,000. One gave a warning sound with a "Whoop Whoop, Pull Up" and the other a "Whoop Whoop, Terrain" aural warning.
No one single person invented GPWS regardless of what Aviation Week, Flying Magazine, and Wikipedia reported. The idea was conceived by two different companies and engineers simultaneously. Both companies lobbied Washington for the equipment and GPWS was mandated for the airlines.
Since that time CFIT (Controlled Flight Into Terrain) has not been solved and there are now over one hundred 'near misses' since GPWS where the aircraft has just missed impacting the terrain because the GPWS warning allowed the aircraft to descend below the Minimum Safe Altitude. Since GPWS does not protect the aircraft on final approach, there have been numerous CIFT accidents while on final approach. The latest US accident was Asiana Airlines approach to San Francisco. The onboard GPWS gave no warning that the aircraft was too low with the ILS glide slope out of commission. AMSADD would have immediately alerted the crew they were too low. AMSADD monitors all final approaches, regardless of either non precision or precision approaches.
NASA is now testing a new type of GPWS that has similar displays to the present system in use. See YouTube video: https://www.youtube.com/watch?v=Erz4cdRNWZI This adds credence that even NASA thinks the present GPWS is inadequate! However, once again using the NASA system, the MSA is ignored.