Collision Avoidance System Almost Causes Collision

Air Safety Week, Oct 18, 1999

The report of a near mid-air collision between a cargo jet and an airliner could not have come at a worse time for the Independent Pilots Association (IPA). The union representing cargo pilots with United Parcel Service [UPS] sent a letter Oct. 8th to Jim Hall, chairman of the National Transportation Safety Board (NTSB), supporting the Board's recommendation (A-99-55/56) that the requirement for TCAS (Traffic alert and Collision Avoidance System) should be extended to include not only passenger jets but cargo aircraft as well. The IPA wrote to dispute a letter to Hall from the Cargo Airline Association (CAA), which is promoting ADS-B as a potential collision-avoidance system that would make the retrofit of TCAS in cargo planes unnecessary. IPA believes a "belt and suspenders approach" would offer the best of both.

The IPA letter was sent just days before a front-page article appeared in the Oct. 12th Wall Street Journal newspaper, which outlined a chilling case where a TCAS-equipped Korean Air Lines B747-200 cargo jet with a crew of 3 came within 600 feet or less of a British Airways B747-400 carrying 419 passengers and crew last June 28 in a remote area over China. In this case, the KAL jet was the "guilty" party.

Highlights of the case are as follows:

* The Korean jet was flying west from Seoul to Tashkent. The BA jet was flying east from London to Hong Kong.

* Both were TCAS-equipped. The BA jet was at 33,500 feet. The KAL freighter was at 31,500 feet.

* However, the TCAS in the KAL jet had been improperly installed, and the system thought it was 2,400 ft. higher than it actually was. As a consequence, the KAL crew was advised to climb by the plane's TCAS, rather than to descend. The crew responded to this improper advisory, which took the KAL jet closer rather than further away from the oncoming BA jet. The two aircraft flashed past each other with a mere 600 feet to spare.

The near-catastrophe prompted Britain's Civil Aviation Authority (CAA) to issue an emergency airworthiness directive Aug. 28 requiring operators to inspect the installation of their TCAS systems and to report the results within 45 days. A failure of one of 11 wires that are part of the shaft encoder, known as the Gillham interface, could lead to single-bit error, which occurred in testing, that would correspond precisely to an altitude error of 2,400 feet. It also turns out that a connector pin had not made contact between the two air data computers in the airplane (one of which feeds altitude data to the TCAS). As a result, the air data computer connected to the TCAS could have been sending a wrong altitude. Normally, if the two computers vary by more than 600 ft., the TCAS shuts down automatically. "We think the shaft encoder provided erroneous altitude information and the comparator didn't catch it," said an AlliedSignal official. The company manufactures TCAS equipment. As a consequence, the KAL pilot was not alerted to the possibility of faulty altitude data in the airplane's TCAS.

A CAA official said reports of the required installation inspections of British aircraft are still dribbling in, but "all have been satisfactory. There are no reports of the same problem in the UK fleet."

The AlliedSignal official said the company has been conducting tests, and found that by placing a greater than normal electrical load on the air data computer, "they got it to fail in a similar way" as most likely occurred on the KAL jet. AlliedSignal plans to issue a service bulletin in the next two weeks asking operators to check the entire circuit. Sources say the FAA will issue an airworthiness directive making the checks mandatory for U.S. aircraft, following the CAA action by some 3 months.

There is a relevant footnote to this near miss. The airplanes may have been saved by marginally inaccurate navigation. Had they been using the Global Positioning System (GPS), where the probable navigation error is contained within the dimensions of the airframe, they might have collided. Indeed, about 12 months ago the Flight Safety Committee of the International Federation of Air Line Pilots Associations (IFALPA), recommended, that "...all aviation GPS...contain a small embedded offset to the right that will protect against this increased risk of head-on collision." IFALPA cautioned, "If two aircraft are flying on reciprocal tracks using GPS, and they do not have adequate vertical separation, they will almost assuredly collide."

IPA On The Complementary Roles of TCAS and ADS-B

"TCAS is like an automobile seatbelt. ADS-B might be compared to the air bag. The seat belt is a proven system that has saved countless lives. The air bag is new technology that can also save lives and was supposed to replace the seat belts. It has turned out though that both systems (seatbelts and air bags) are best if they are both installed. ADS-B can enhance and augment TCAS as it has proven in tests where Honeywell Hybrid TCAS/ADS-B units have depicted targets as far as 150 miles away."

Source: IPA letter to NTSB of Oct. 8