Air Force investigators now believe that Yugoslavia jury-rigged its system of radars and communications to shoot down an F-117 stealth fighter two weeks ago, and as part of an accelerated air campaign Pentagon officials are sending forces that could help avoid a similar incident.
Military officials said that as a result of the inquiry they now believe that a combination of low-tech tactics, quick learning and improvisation came together in one brilliant moment to shoot down the premier attack jet in America's arsenal.
The culprit, they believe, is an SA-3 surface-to-air missile. But it probably was not used in the normal fashion, with its operators relying on their own local radars to detect the target, which leaves them vulnerable to NATO fighters that can home in on the radar beams.
Instead, Air Force and other military officials now believe that spotters in Serbia, and perhaps in Bosnia and along the Montenegrin coast, may have patched together enough quick glimpses of the warplane from scattered radars to track the elusive aircraft, however briefly, and to fire a missile at it from a battery near Belgrade.
They reached their conclusions, which are still under review, after extensive interviews with the rescued pilot, a review of technical clues and an analysis of how the Serbian air defenses have been operating.
"We think whoever did this won the lotto that night," said one senior American officer.
NATO commanders are already applying the lessons learned from this incident, the only time a manned aircraft has been hit by enemy fire during the Balkan campaign.
As part of the 82-plane buildup of American air forces in the region announced Saturday, the Pentagon said it would add 24 additional Air Force F-16 fighters armed with specialized anti-radar missiles, along with six radar-jamming Navy EA-6B Prowlers.
NATO wants those aircraft primarily to protect additional non-stealth warplanes on their bombing runs. But those kinds of planes will also be attacking the very network that most likely brought down the F-117.
And NATO has increasingly struck at communications nodes of the kind that allow widely dispersed Serb radar stations to link together as they track allied planes. This week the alliance destroyed a telephone switching center in Pristina and a microwave relay station, also in Kosovo, even though that caused civilian damage that NATO targeters say they usually take pains to avoid.
The Serbs have used their long-range radar sparingly in order to avoid an attack by NATO fighters.
Still, NATO and Pentagon officials have such respect for Yugoslavia's air defenses that low-flying attack planes like the A-10 have flown only a handful of strike missions.
But shooting down an F-117 defied the odds. It was a longshot, low-tech solution to a challenge posed by one of the most sophisticated warplanes in the world, Pentagon officials said. In the end, it may have been tactics aided by luck, and not technology, that brought down the F-117 and gave Belgrade a propaganda windfall.
In addition to its radar-absorbing skin and radar-scattering angles, the Nighthawk, as the F-117 is called, typically flies a zig-zag pattern to avoid tracking.
The F-117 is not invisible, but barely visible to most radars. It is most vulnerable when turning suddenly at low altitudes, which can reflect radar beams to receivers, or when opening its bomb bay door.
What American military officials now suspect is that Serbian spotters, perhaps starting with spies in Italy watching the F-117's take off, were able to determine a rough schedule of how long it took the planes to cross the Adriatic, and how long to fly to Belgrade.
Knowing those things, Serbian radar operators would have a better sense of when and where to watch. Once the aircraft's radar reflection crossed their screens, they would alert the operator down the line. The downed F-117 had already dropped at least one of its laser-guided, 2,000-pound bombs near Belgrade before it was hit, so that was another clue.
Analysts are unsure now whether Serbian gunners were able to integrate this net of far-flung radars to feed the location of the plane to a missile launcher, or to cue up the SA-3's own radar so the operator had only to briefly flip it on to track and fire.
Since the stealth plane went down, other planes on bombing runs have recorded instances when the Serbian forces lobbed surface-to-air missiles at them without radar guidance. This is clearly less reliable, but it lets the Serbian missile sites avoid counterstrikes by NATO's fighters.
"Given the limited air space over there and the sophisticated air defense system they have," the senior American officer said, "there's always the possibility of a kill."
Officially, the Pentagon says the cause of what downed the plane is still under investigation. But military officials familiar with the inquiry say this account is emerging as the most likely explanation of what caused the first-ever loss of an F-117 in combat, and the only manned aircraft felled so far in the aerial bombardment of Yugoslavia.
After the crash, the Air Force assembled a secret team of experts in air defenses, stealth technology and other technical areas, and dispatched them under strict security to Langley Air Force Base in Virginia to figure out what happened.
Gen. Michael E. Ryan, the Air Force Chief of Staff, recently reviewed a draft of the team's report and sent it back for answers to additional questions, a senior Pentagon official said. The final report is expected to be forwarded soon to General Ryan and Gen. Henry H. Shelton, the Chairman of the Joint Chiefs of Staff.
The Pentagon spokesman, Kenneth H. Bacon, refused Saturday to comment on any detail of the F-117 downing to avoid tipping off the Yugoslav military. "The Yugoslav air defense system is both sophisticated and adaptable," Mr. Bacon said in an interview. "The Yugoslav forces are attempting to make rapid adjustments in the way they use their air defenses to shoot down U.S. planes while protecting their radars and missiles from allied attack."
Here is the account that experts now say is the most likely scenario.
The F-117's, operating out of the Aviano air base in northern Italy, typically were flying missions at medium altitude, between 15,000 feet and 25,000 feet. That is close enough to drop their payloads with near pinpoint accuracy, but out of reach of most of Yugoslavia's antiaircraft batteries and surface-to-air missiles.
At about 8:45 P.M. in the Balkans on March 27, the fourth night of the air war, an F-117 was heading back to base after dropping at least one of its 2,000-pound, laser-guided bombs on a target near heavily defended Belgrade. Bad weather, which had forced scores of missions to be canceled, did not hinder the aircraft, two senior Air Force officers said.
Suddenly, with little or no warning, an SA-3 missile streaking at three times the speed of sound and guided by an improvised network of Serbian radars, exploded in a blast of fragments a few feet from the bat-winged plane, slamming it into an uncontrolled dive.
The antiaircraft missiles have warheads with 130 pounds of high explosives that are designed to detonate when the projectile gets within about 20 feet of its target.
Alarms in the cockpit of the F-117 usually warn a pilot when a SAM radar is homing in, or painting, his plane. But the Yugoslav military so fears the antiradiation missiles that NATO warplanes launch at the radars, it has largely turned them off.
It is unclear whether the pilot got any warning of the missile bearing in on him. If he did, it was probably too late. Stunned by the explosion, the veteran pilot struggled against what he said approached pressure five times the force of gravity to yank the handles below his seat to eject from the crippled warplane.
"The one fragment of this whole event I can't remember is pulling the handles," the pilot said in an account the Air Force released this week, withholding his name at the aviator's request. "God took my hands and pulled."
Seven hours later, a daring commando team snatched the downed pilot from the hiding place to which he parachuted behind enemy lines, and whisked him back to allied hands, first in Bosnia, then in Italy.
With the wreckage of the $43 million plane now in Yugoslav hands, some questions may never be answered satisfactorily. And, of course, there are competing theories of what happened:
¶That a barrage of ground fire or an SA-6 missile, not an SA-3, scored a lucky hit. All would have produced what look like bullet holes in the wing wreckage that was shown on Serbian television.
¶That the pilot, frustrated by dense clouds, took a chance and flew his jet-black plane under 15,000 feet, only to be silhouetted against the dull gray evening sky by an eerie glow from ground fire reflecting off the cloud ceiling.
¶Or that a bomb-bay door stuck open, making the F-117 vulnerable to radar.
But officials closest to the investigation have discounted or ruled out those theories.
A malfunction seems unlikely, industry officials say, because the plane's manufacturer, Lockheed Martin, and its fabled Skunk Works development center in Burbank, Calif., have not yet been consulted by the Pentagon. Lockheed referred all questions to the Air Force.
Some Air Force officers are also upset that NATO did not bomb the wreckage to prevent the plane's secrets from falling into hostile hands. Pentagon officials have tried to discount this fear, saying the F-117's technology is more than 20 years old, and less sophisticated than that of its larger cousin, the B-2 bomber.
Some experts say it appears that the F-117 may have glided on auto-pilot after the pilot ejected, and made a more or less controlled landing, pancaking into a field. That may be why such large portions of the plane, including its wing, were visible. If the Nighthawk had landed nose first at high speed, it would have been a smoldering ruin, those officials say.
In any event, Air Force and NATO commanders do not appear overly concerned. After the downing, commanders issued stern verbal warnings to F-117 pilots to keep alert. But Gen. Wesley K. Clark, the Supreme Commander of NATO forces, requested, and was granted, an additional 13 F-117's.
Saturday, the Pentagon showed a combat video of a strike earlier this week, when the military said an F-117 scored a direct hit on a surface-to-air missile site, destroying it.
"When flown in its operating envelope, above 15,000 feet and outside of huge acquisition radars, the F-117 is still difficult, if not impossible, to see on a SAM radar until it's too late to do anything about it," said Gen. John Michael Loh, who was the Air Force Vice Chief of Staff during the 1991 Persian Gulf war.
Indeed, the fighter has assumed an almost mystical reputation since the war, when F-117's flew more than 1,250 missions, including all the riskiest runs over downtown Baghdad. None of the Nighthawks suffered even a scratch.
And in the skies over Yugoslavia, the F-117's have even more help than they did in the gulf. The plane's angular design and composite skin are meant to deflect or absorb searching radar beams, and offset its relative lack of speed (it is subsonic) and maneuverability.
Besides the antiradiation missiles, the Air Force is for the first time using a new, advanced radar-jamming system. Unlike conventional electronic jammers, which radar operators can detect, the new technology remotely injects signals into the radar that are not evident to the operator on the ground, and can hinder the missile's accuracy.
For these reasons, senior Pentagon officials say Yugoslav forces are essentially using a point-and-shoot method for firing missiles now, rather than risk turning on their radars.
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