CHI93IA115
CHI93IA115

On March 7, 1993, between 0703 and 0712 central standard time, a loss of standard IFR separation occurred between airplanes on approach to Runway 27R at the O'Hare International Airport, Chicago, Illinois. The incident involved nine airplanes and resulted in a loss of separation between eight of the nine. The losses of separation occurred both inside and outside of the approach outer marker (OM). There were no injuries or damage reported as a result of the incident. Instrument meteorological conditions were reported as sky partially obscured, measured 300 feet overcast, 1 1/2 miles visibility with fog, and wind from 250 degrees at 10 knots. Instrument flight plans were on file for all the airplanes involved.

The nine airplanes involved in the incident were: 1) Simmons Flight 4378, a DeHavilland SHD 6, 2) United Flight 925, a Boeing 737, 3) United Flight 556, a Boeing 737, 4) United Flight 453, a Boeing 737, 5) United Flight 568, a Boeing 737, 6) Air Wisconsin Flight 148, a British Aerospace ATP, 7) United Flight 402, a Boeing 727, 8) United Flight 312, a Boeing 727, 9) Air Wisconsin Flight 014, a British Aerospace BAE 46.

Noise abatement procedures were in effect for the O'Hare International Airport until 0700 cst on the morning of the incident. The procedure called for runway 27R to be in use and for air traffic to remain above 4,000' until inside the outer marker. At the time of the incident the Terminal Radar Approach Control (TRACON) was in the process of setting up for staggered approaches to runways 27R and 27L, but had not yet begun the approaches to 27L. The East Arrival TRACON Controller was sequencing airplanes to the West Arrival Controller who was then handing the airplanes off to the Local/Tower Controller.

During interviews with both TRACON Controllers, they each stated they were not aware of the losses of separation until being informed of the errors days after they occurred. The controller working the East arrival position did state, however, that the spacing/separation was not normal for operations in IFR weather. The West Arrival Controller stated that most controllers normally use the 5 mile range setting on the scopes which results in them having to "eyeball" what a 3 mile separation is. He stated that the 2 mile range setting clutters the scope.

The Local controller was working the North and South Local controll positions at the time of the operational errors. He was responsible for arrivals on 27R and 27L, along with departures on 32L and 22L. However, the only traffic at the time of the operational errors were the arrivals on 27R. The Local controller stated during an interview that due to the reduced ground visibility, the "usually tight final" was not going to work. He informed the tower supervisor that he needed more space between airplanes on final approach. The supervisor told him to "send any of them around that you have to." The supervisor then called TRACON and informed that they needed more spacing. The Local controller stated he did receive more spacing approximately 10 to 15 minutes later.

The Air Traffic Control Manual 7210.3G, paragraph 5-72a.1 states, "A minimum of 3 miles radar separation must be provided between aircraft being vectored to and while on the same final approach course."

The losses of separation occurred as follows:

0702:23 Separation outside the OM was lost between Simmons 4378 and United 925. Closest proximity was 2.8NM/700'. No immediate remedial action was issued.

0703:23 Separation outside the OM was lost between United 925 and United 556. Closest proximity was 205NM/500'. United 925 was subsequently issued missed approach instruction.

0703:46 Separation outside the OM was lost between United 556 and United 453. Closest proximity was 2.5NM/900'. The lateral separation decreased to 2.4NM inside the OM. No immediate remedal action was issued.

0705:30 Tower Supervisor informs the TRACON Supervisor that final was too tight with the current weather.

0706:46 Separation outside the OM was lost between United 568 and Air Wisconsin 148. Closest proximity was 2.3NM/00'. The lateral separation decreased to 2.1NM inside the OM. No immediate remedial action was issued.

0707:05 Sepration outside the OM was lost between United 453 and United 568. Closest proximity was 2.8NM/900'. The lateral separation decreased inside the OM to 2.6NM. No immediate remedial action was issued.

0707:46 Separation outside the OM was lost between Air Wisconsin 148 and United 402. Closest proximity was 2.1NM/600'. United 402 was subsequently issued missed approach instructions.

0711:00 Separation outside the OM was lost between United 312 and Air Wisconsin 014. Closest proximity was 2.75NM/800'. The lateral separation decreased to 2.5NM/900' inside the OM. No immediate remedial action was issued.

0714:09 Separation inside the OM was lost between Air Wisconsin 014 and United 925. Closest proximity was 2.5NM/900'. No immediate remedial action was issued.

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