WASHINGTON -- Allied planes resumed their patrols over the "no fly" zone in northern Iraq yesterday, three days after two Black Hawk helicopters were downed in a "friendly fire" accident that now appears to have been caused by multiple mistakes and failures. The flights, canceled for 24 hours, were restarted on a reduced schedule and under amended operational guidelines while an Air Force-led investigation into the cause of Thursday's fatal blunder continued.

A Pentagon spokesman said the reduced schedule was "adequate to meet our operational requirements." The patrols are designed to protect Kurds in northern Iraq from attack by Iraqi planes and helicopters.

Almost immediately after the tragedy, the U.S. European Command, which controls the operation, made an undisclosed change to the rules of engagement to make the missions safer, but all the procedures are being reviewed by the investigators.

From what is known today of Thursday's shoot-down, it appears that a series of mishaps occurred in the moments before the launching of the missile attack on the Black Hawks, which were carrying an international team, including 15 Americans, on a humanitarian mission to a Kurdish village.

Three facts have already been established:

* The F-15 pilots who shot down the U.S. Black Hawk helicopters made a colossal error in visually identifying them as Soviet-made Hind helicopters flown by the Iraqi Air Force.

* The identification system on the U.S. helicopters failed to give the appropriate "friendly" response when the F-15 pilots interrogated them electronically. This could have been due either to equipment or operator failure.

* The AWACS airborne command center, circling overhead and controlling operations in the "no-fly" zone, failed to use its electronic surveillance and communications ability effectively.

The unanswered question in all three cases is: why?

It is this triple layer of apparent failures that has Pentagon officials wondering how such a mishap could have occurred.

With sophisticated systems for identification and communication, none of the failures should have happened, and if any did, the fail-sale precautions should have corrected them.

The allied pilots are trained in target identification. The Black Hawk and Hind helicopters are far from identical, but one confusing ingredient for the two F-15 fliers was that the U.S. helicopters on Thursday's mission were fitted with external fuel tanks at each side of their fuselages. This gave them an overhead profile similar to that of the Hind, which has small auxiliary wings to carry its weapons systems.

Air Force officers in the Pentagon were incensed when most newspapers Friday carried photographs of Black Hawks without the fuel tank extensions. Significantly, at a Pentagon briefing later that day, Gen. John Shalikashvili, chairman of the Joint Chiefs of Staff, produced an overhead profile of a Black Hawk with the fuel tanks attached alongside a Hind, apparently to illustrate how the pilots could have made their mistake.

Once the pilots were sure in their own minds that they had spotted Hind helicopters violating the "no fly" zone, they were free to fire.

The investigation will determine whether they tried to warn the helicopters by radio of the impending attack, or whether they sought clearance from the AWACS, although they did not need higher approval of their attack decision, according to General Shalikashvili.

This raises another puzzling issue: What was it about the helicopters' behavior that persuaded the F-15 pilots to launch their missiles immediately? The rules of engagement for the mission have not been released, but in most cases pilots are required to check for signs of aggressive activity or intent before attacking.

When four Bosnian Serb bombers were shot down in the "no fly" zone over Bosnia earlier this year, the U.S. pilots waited until they saw a bomb drop before launching their attack. The delay may have allowed two other Serbian bombers in the intruding flight to escape, but it eliminated any doubt that the planes were hostile and aggressive.

The targeted Black Hawks were ferrying a United Nations team to meet with elders of a Kurdish village. One can only speculate that perhaps the pilots, seeing what they deemed to be two Iraqi helicopters approaching a Kurdish village, presumed they were about to launch an attack on the defenseless community.

But before launching their missiles the pilots ran a final check. They used their electronic IFF (identify friend or foe) systems to interrogate the helicopters, according to the initial report to the Pentagon.

If the systems had worked properly, they would have received a coded signal back from the Black Hawks informing them that they were "friendly." They received nothing, according to the initial report. Lack of a coded "friendly" response is taken as a sign of hostility.

This raises the possibility that either the transmitters in both helicopters were broken, wrongly coded, or switched off.

Similarly, the receivers in the F-15s might have been malfunctioning. Either way, the signal that could have averted the attack was apparently not received, and a heat-seeking Sidewinder missile and a radar-guided Amraam missile struck with pinpoint accuracy.

According to experts, the chances of double failure of the IFF transponders or erroneous coding of two systems are remote.

But it is not unusual for pilots to switch off their IFF beacons in hostile territory for self-protection to prevent enemy radar-guided missiles from being "locked onto" the transponder.

"If you are putting out a beacon that says 'Here I am,' they [the enemy] are going to get your signal," said an F-15 pilot who flew coordinated missions with helicopters in Iraq.

But the Black Hawk helicopters were 25 miles inside the "no-fly" zone with their own planes on patrol above them. No Iraqi planes have been spotted so deep inside the zone, so it is not evident why they should take such a self-protection measure. On the other hand, having been at a mission briefing the day before, they had apparent reason to believe that the other U.S. planes would know who they were, where they were and what they were doing without need to run an IFF check.

Two other possibilities suggested by the F-15 pilot: simple oversight by one or both the helicopter pilots. The pilot deemed a double mistake unlikely. But if the pilots had agreed to keep just one transponder activated to reduce electronic interference during their flights, it would take only a single mistake to silence the systems in both helicopters.

It is not known whether the Black Hawk pilots made such an single-operator arrangement for their IFF systems.

If all had gone to plan, the AWACS would have been monitoring all the movements in the "no fly" zone.

Two of the key questions are whether the helicopters were in radio contact with the AWACS and whether the AWACS knew that the helicopters were in the area.

This does not appear to be the case, because once the helicopters were shot down, it took almost half an hour for the awful truth to dawn that they were U.S. and not Iraqi planes as the F-15 pilots initially reported.