A Trans World Airways Boeing 727 was descending through clouds to join an instrument approach to Washington Dulles International Airport on Dec. 1, 1974, when the radio altimeter warning horn sounded the first of two warnings, alerting the crew that they were 100 ft above the ground. Six seconds later, the warning horn sounded again, followed immediately by the 727’s crash into the western slope of Mount Weather, 25 nm (46 km) west of Dulles.
The crash killed all 92 passengers and crew, destroyed the airplane, and prompted calls for the development not only of terrain awareness and warning systems (TAWS) but also of voluntary, confidential safety reporting systems and changes in air traffic control (ATC) communications.
A private TWA Flight 514 remembrance ceremony is scheduled for Sunday, Dec. 1, in nearby Bluemont, Virginia, which was home to the emergency operations center for the crash recovery effort. The Bluemont Citizens Association, which organized the ceremony, expects about 200 relatives of crash victims, plus more than 50 airline personnel, first responders, representatives of the government, and military personnel to attend.1
In its final report on the Mount Weather crash, issued in November 1975, the U.S. National Transportation Safety Board (NTSB) said the probable cause was the flight crew’s decision to descend to a lower altitude (1,800 ft) before the airplane reached the approach segment where that altitude applied.2
“The crew’s decision to descend was a result of inadequacies and lack of clarity in the air traffic control procedures which led to a misunderstanding on the part of the pilots and of the controllers regarding each other’s responsibilities during operations in a terminal area under instrument meteorological conditions,” the report said. “Nevertheless, the examination of the plan view of the approach chart should have disclosed to the captain that a minimum altitude of 1,800 feet was not a safe altitude.”
The report said that factors contributing to the accident included the failure of the U.S. Federal Aviation Administration (FAA) to “take timely action to resolve the confusion and misinterpretation of air traffic terminology, although the agency had been aware of the problem for several years”; the issuance of an approach clearance when the airplane was 44 nm (81 km) from Dulles “on an unpublished route without clearly defined minimum altitudes”; and the “inadequate depiction of altitude restrictions” on the profile view of the approach chart for the VOR/DME (VHF omnidirectional range/distance measuring equipment) approach to Dulles’s Runway 12.
Even before the report was issued, the FAA had directed that all air carriers be equipped with a ground-proximity warning system (GPWS, a form of TAWS), that an incident reporting system be established “to identify unsafe operating conditions in order that they can be corrected before an accident occurs,” and that requirements be established for pilot and controller actions associated with clearances for nonprecision instrument approaches.
Terrain Awareness
GPWS, developed in the 1970s, was an early version of TAWS, designed to help prevent controlled flight into terrain (CFIT) accidents. Although GPWS was credited with substantial reductions in the number of CFIT accidents, its effectiveness was limited because it relied on a radio altimeter to measure an aircraft’s distance from terrain and to calculate terrain closure rates. This dependence meant that, in situations involving rising terrain, pilots had little time for avoidance.3
In the 1997, Honeywell’s Enhanced Ground-Proximity Warning System (EGPWS) became available; it addressed the short-warning concern by using GPS as the source for aircraft position information and matching that with a regularly updated terrain/obstacle database. In the first five years after its introduction, Honeywell said that it documented 20 incidents in which the system saved an airliner from certain disaster.4
Today, multiple manufacturers produce similar systems, which the International Civil Aviation Organization refers to generically as “TAWS.”
“TAWS is one of the key contributors to the dramatic decline in fatal aviation accidents in recent years and to aviation’s steadily improving safety record,” Flight Safety Foundation President Hassan Shahidi said. “Over the years, TAWS has prevented countless accidents and saved countless more lives.”
Voluntary Reporting
About five months after the accident, the FAA issued Advisory Circular 0046, “Aviation Safety Reporting Program,” which the agency said would invite pilots and air traffic controllers, among others, “to report discrepancies or deficiencies noted in the system to the FAA.” The plan was to use those reports to identify potential safety problems before they could contribute to an accident.
Some airlines had implemented such programs before the accident, but in the years that followed, scores of mandatory and voluntary safety reporting systems were established by government agencies, airlines, and manufacturers. Today, safety management systems require a formal process for data collection and analysis. Voluntary nonpunitive reporting systems — such as the Air Traffic Safety Action Program and the National Aeronautics and Space Administration’s Aviation Safety Reporting System in the United States and Eurocontrol’s Voluntary Air Traffic Management Incident Reporting in Europe — receive and analyze confidential information aimed at helping identify unsafe conditions with the goal of correcting them before they cause an accident.
ATC Communications
The report noted that “imprecise terminology, unresolved differences of opinion, and unnoticed changes in the definitions and procedures” contributed to the flight crew’s erroneous assumption that, because they had been cleared to land, they could safely descend to 1,800 ft even though the airplane had not yet arrived at the segment that prescribed that altitude.
“[T]his accident resulted from a combination of conditions which included a lack of understanding between the controller and the pilot as to which air traffic control criteria were being applied to the flight while it was operating in instrument meteorological conditions in the terminal area,” the report said. “Neither the pilot nor the controller understood what the other was thinking or planning when the approach clearance was issued.”
The report said the Mount Weather accident demonstrated the need for a lexicon and procedures manual to be shared by pilots and controllers.
After the accident, the FAA revised ATC procedures to “provide for the issuance of altitude restrictions during nonprecision instrument approaches” and revised its own regulations to clarify “pilot responsibilities and actions after receiving a clearance for a nonprecision approach.”
Image: MahDev20 / Shutterstock
Notes
- Brehm, Brian. “Ceremony Will Recognize 50th Anniversary of TWA Flight 514 Crash,” The Winchester (Virginia) Star. Nov. 19, 2024.
- NTSB. Report No NTSB-AAR-75-16, “Trans World Airlines, Inc., Boeing 727-231, N54328, Berryville, Virginia; December 1, 1974.” Published Nov. 26, 1975.
- “Terrain Awareness and Waning Systems (TAWS).” SKYbrary. https://skybrary.aero/articles/terrain-avoidance-and-warning-system-taws
- Phillips, Don. “Air System Helps Avert Jet Crashes.” The Washington Post. May 19, 2002.
