Low-level radar system faulted in fatal crashes
Monday, October 18th 1999, 12:00 am
News On 6
WASHINGTON (AP) -- A radar system that is supposed to warn
low-flying planes of nearby obstacles was plagued with problems and
fixed nationwide only after a 1997 fatal airplane crash in Guam,
according to a published report.
In some cases, programming errors caused the Minimum
Safe-Altitude Warning system not to operate over wide areas,
including near busy airports such as those in Chicago and
Dallas-Ft. Worth. In other cases, false alarms were so numerous
that air traffic controllers put cardboard over warning speakers to
silence the noise.
The Federal Aviation Administration was warned about the trouble
after a business jet crashed outside Washington in 1994, but it did
not take decisive action to resolve it until after a Korean Air
jumbo jet slammed into a hill on approach to Guam in August 1997,
killing 228, said USA Today, which reported about the problems on
The National Transportation Safety Board, which will hold a
final hearing about the Korean Air crash on Nov. 2, is expected to
recommend that the FAA acknowledge that system problems contributed
to the accident.
Investigators have found plenty of blame for the crew: It
appears they did not follow charts outlining a step-down approach
procedure to the airport. Tapes also show there was cockpit
confusion about whether another landing system using radio waves to
outline the proper approach path was operational. It was not -- as
some crew members acknowledged on the tape.
The FAA said in response to the story that it would have had
difficulty correcting the problems until it got new computer
database-management tools in 1997. The agency has since
standardized the operation of the system, inspects it monthly and
checks each airport site with test aircraft every 18 months.
However, the agency also said the system is "a tool" and
pilots have the ultimate responsibility for maintaining the proper
"In 20/20 hindsight, we wish we could have done a better job
prior to the (Guam) accident, but now the system is being managed
in a uniform way rather than as a number of individual entities,"
said FAA spokesman Les Dorr.
The first Minimum Safe-Altitude Warning systems were installed
in 1977 in an effort to curb ground collisions, a vexing problem
known within the industry as "controlled flight into terrain."
The systems divide the area around an airport into a grid and
are programmed with the highest point in each square. When radar
detects a plane descending within 500 feet of one of those points,
an alarm sounds in the air traffic center and radar screens flash
with a sign saying "LA" or "LOW ALT." The controller is then
supposed to warn the pilot by radio.
The systems are now installed in 193 air traffic control
facilities and monitor airspace around almost every airport with
NTSB investigators blamed the 1994 crash outside Washington on
the flight crew, but they also found that an incorrect compass
heading was entered into the altitude-warning system, preventing an
The safety board warned the FAA about the problem, and the
agency replied that it had checked the remaining systems. Three
more fatal crashes followed. Each was blamed on the pilots, and in
each case the system produced alerts that were either not seen or
heard by controllers.
After the Guam crash, though, the FAA assembled a team of top
computer experts to reanalyze the system. The team soon found that
repairs were needed at nearly half of the 130 smaller airports that
had the system.
In some cases, incorrect obstacle heights were entered into the
database. In the case of the Chicago and Texas airports, errors led
computers to calculate planes were hundreds of feet higher that
they actually were, effectively blocking alarms in wide areas.
In the case of Guam, a programmer disabled the system for all
but a 1-mile band encircling the airport some 55 miles out at sea.