Stormy weather and shortcomings of the radio beacons at the Croatian airport where Commerce Secretary Ronald Brown’s flight was headed have raised questions about whether the Air Force plane should have been flying in such conditions.
With military and civilian investigators combing the crash site about 1.8 miles from the Dubrovnik airport, officials declined to speculate on the exact cause of Wednesday’s crash, which claimed all 35 lives.
But U.S. Ambassador to Croatia Peter Galbraith said two things were clear. “First, the weather yesterday, as the plane flew in, was terrible,” he said at a news conference near the crash site. “People in Dubrovnik say it was the worst storm in decades. Secondly, the plane was not where it should have been.”
Rather than approaching the airport from the Adriatic coast, Galbraith said, the plane appeared to have traveled along a valley northeast of the airport before crashing into a 2,300-foot hillside.
The Boeing T-43A, the military version of the 737 passenger jet, usually serves as a flying classroom, used to train navigators for a wide range of military planes. The plane that crashed Wednesday was one of two T-43As configured to carry passengers, and had been used to carry Defense Secretary William J. Perry and First Lady Hillary Clinton during recent visits to the Balkans.
While investigators’ efforts are likely to be hampered because the plane was not equipped with a “black box” flight data recorder, attention quickly focused on the instruments used to guide the plane to the runway.
“It was a classic sort of an accident that good instrumentation should be able to prevent,” Perry told reporters Thursday as he flew back to Washington from a visit to Egypt.
Because of the bad weather, Brown’s plane was making an instrument approach, relying on radio beacons on the ground to guide it to the runway. While the plane carries other navigation equipment, it used this method to home in on the runway - described by military pilots as a “non-precision approach” - because Croatia’s Cilipi airport does not have a more advanced system.
To use the beacon system, a plane’s radio receiver is tuned to a particular frequency, which can pick up signals from the runway. According to standardized international flight procedures, pilots heading for the Cilipi airport a few miles south of Dubrovnik are to set their radio to 318 kilohertz when they are 5 to 10 miles from the airport.
At that point, the plane should pick up a signal coming from the first of two ground-based beacons. At Cilipi, the beacon sends out the Morse code for the letters K-L-P. Once that signal is received, the radio is reset to another frequency to pick up a second signal 10 miles away. Lining up the two signals, pilots are guided to the runway.
“You know immediately - you either get it or you don’t get it,” said Bob Flocke, spokesman for the Air Line Pilots Association.
On Wednesday’s flight, Air Force officials said the plane was off course during its final approach. One possible explanation is that the beacons have a history of malfunctioning during bad weather.
According to a Federal Aviation Administration pilot’s manual, the beacons are “subject to disturbances that may result in erroneous bearing information.”
Those disturbances, caused by lightning, precipitation and other factors, are well known to Air Force Lt. Col. Tom Perry, who uses the planes to train navigators at Randolph Air Force Base in San Antonio.
“It’s interference between the ground and the airplane,” he said. “The radio needle on the instruments in the airplane could point in the wrong direction, or it could spin around.”
While the malfunctions could be obvious to an experienced pilot, they could also take forms that are harder to detect, Lt. Col. Perry said.
For example, the instrument could give a reading that was just 10 degrees off, he said. “You would not necessarily know that you were receiving unreliable information.”
The beacons, which rely on 50-year old technology, are “very task intensive” for crews operating the plane, the Air Force instructor added.
At a Pentagon briefing Thursday, Air Force Lt. Gen. Howell Estes said the beacon approach has “been around for a while … but it’s still a very valid approach.”