Air Carrier on approach to MDW with C90 experienced a NMAC with an unreported general aviation aircraft descending from 6;000 to 4;000.
Synopsis
Air Carrier on approach to MDW with C90 experienced a NMAC with an unreported general aviation aircraft descending from 6;000 to 4;000.
Narrative
We had been talking to an Approach Controller prior to CGT inbound to MDW. Our position relative to ORD was the 139 degree bearing at 39 DME. Our aircraft had been cleared to descend from 6;000 FT to 4;000 FT on a heading of 290 and to intercept the MDW Runway 31C localizer. I was the non flying pilot. The localizer frequency was not being displayed on the Primary Flight Display. As I began to insert the correct frequency and course into the localizer section of the NAV/RAD page of the FMS; the First Officer made an exclamation and began to take evasive action; first by disconnecting the autopilot and pushing the nose over. I looked up to see a Piper Saratoga pass overhead from left to right. The aircraft passed over us at approximately 100 FT. First Officer recovered and continued a normal descent to 4;000 FT. We intercepted the 31C localizer and landed without incident. There were no reports of trauma or injury to passengers or other flight crew members. ATC did not give any traffic advisories prior to the incident. Suggestions; ATC was unaware of traffic in our area and failed to see any other targets as we reported the event. Our TCAS system failed to give a TA or RA.
Second reporter narrative
We had been given a clearance to descend from 6;000 FT to 4;000 FT on a 290 heading to join the 31C localizer at MDW. We were in a fairly shallow 500 FT-1;000 FT/minute descent when I looked down and away to retrieve and put on my sunglasses. At approximately 4;500 FT; I looked up to see what seemed like a windscreen full of Piper. He was at our level or slightly higher; and passing from our left to right; on an east-northeasterly heading. If your were to hold out your hand at arms length; the other aircraft would have been nearly the size of your fist. I immediately shouted 'aircraft!' disconnected the autopilot; and increased our rate of descent so as to pass under the Piper. As I did so; I glanced at the TCAS; which showed a blue diamond right over us at 100 FT and diverging. As I leveled off at 4;000 FT and re-engaged the autopilot; the Captain advised ATC what had happened; but they claimed to see no other aircraft in our vicinity. Thereafter we proceeded to MDW for a normal approach and landing. Suggestions; this incident reminded me again that despite all the technology on our aircraft and in ATC; that when operating in VMC; the last best defense against a midair collision is; just as our instructors taught us many years ago; to see and avoid with the good old; standard issue; 'Mk. I Eyeball.' On our aircraft; I am convinced that our TCAS was working properly; as it gave us several timely TA's throughout the day; both before and after this incident. Instead; it seems to me that the only cause which would explain the lack of both ATC and TCAS warning in this case; is an intermittent or otherwise malfunctioning transponder on the other aircraft. But regardless of the cause; you can be sure that in the future; I'll be spending much more time 'heads-up' while operating in or near the terminal area.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.