Convair 340 accident at Chicago, Illinois, 17 Jul 1955

Accident narrative

On July 17, 1955, Braniff Airways Flight 560, a Convair 340, struck an advertising sign while completing an instrument approach to the Chicago Midway Airport. The aircraft crashed through the airport boundary fence and came to rest inverted. Of the three crew members and 40 passengers aboard, the captain, the hostess, and 20 passengers received fatal injuries. The first officer and 11 passengers sustained serious injuries, and the remaining nine passengers received minor or no injuries. The aircraft was demolished by impact and fire.

**The Flight** Flight 560 was a daily scheduled flight from Dallas, Texas, to Chicago, Illinois, with stops in Oklahoma City, Wichita, and Kansas City. The crew consisted of Captain Allen R. Tobin, First Officer Orbin W. Hanks, and Hostess Mary E. Teel. Both pilots were highly experienced and familiar with the route and airport; Captain Tobin had over 15,000 flying hours, and First Officer Hanks had over 9,000.

The flight departed Dallas on schedule at 0100. At Wichita, a small carburetor intake manifold fire occurred on the left engine during start, but it was immediately extinguished without damage. At Kansas City, the left propeller feathered during the landing roll deceleration. The captain unfeathered it, and maintenance personnel found no malfunction. The aircraft was serviced with fuel and departed Kansas City at 0435 on a Visual Flight Rules (VFR) flight plan, loaded well below its maximum allowable weight.

**The Approach to Chicago** The flight climbed to 15,000 feet and later received an Instrument Flight Rules (IFR) clearance. Passengers described the en route segments as smooth and pleasant.

At 0556, Chicago Air Route Traffic Control broadcast a special weather observation indicating thin obscuration and one-half mile visibility. When asked if they could land in these conditions, the crew replied that they could. Under company minimums and the sliding scale for an Instrument Landing System (ILS) approach, a landing was permissible with one-half mile visibility.

Flight 560 contacted Chicago Midway Approach Control and was radar vectored to the outer marker for an ILS approach to runway 13R. The approach controller provided radar advisories to supplement the crew's instruments. The controller observed the flight properly aligned before the outer marker and noted that the approach was generally very good. At one and a half miles from touchdown, the controller advised the flight was "slightly left of on course, 20 feet low on the glide path." At one mile, the aircraft was "on course, on glide path." Approaching the one-half mile point, the controller noted the aircraft was 75 feet to the left, but at one-half mile, he advised "glide path good." Seeing the aircraft correcting right toward the course and in a good position to land, the controller discontinued advisories.

The aircraft's right wing struck a commercial advertising sign located at the intersection of 55th Street and Central Avenue, 1,000 feet short of the runway threshold. The right wing struck the sign 19 inches below the top while banked 11-1/2 degrees down. The wing separated upward and rearward into the right horizontal stabilizer. The aircraft rolled progressively to the right, crashed through the boundary fence, struck approach light installations, and stopped inverted on a taxiway. Airport firemen extinguished the resulting fire in less than a minute.

**What the Investigation Found** The advertising sign was mounted on a steel post just over 18 feet high. The Board noted that the top of the sign was below the ILS glide path and 122 feet left of the localizer centerline. While it was 12 feet lower than the allowable obstruction height for that position, the Board believed the accident resulted primarily from the extremely low altitude of the flight rather than the sign's height and position.

Examination of the wreckage revealed that the landing gear was down and the flaps were extended 15 degrees. Investigators found no evidence of structural failure, control malfunction, or in-flight emergency. Both engines were developing nearly equal and normal power at impact. The radio and ILS receivers were properly tuned and capable of normal operation. Investigators noted that the captain and first officer had different models of omni-mag indicators, but tests showed their indications would have been alike during the approach. A voice recording indicated that First Officer Hanks was likely flying the aircraft, but his injuries prevented him from recalling any details of the flight.

The Board determined that the flight executed the ILS approach precisely until it was one-half mile from touchdown. After passing this point, the aircraft departed the glide slope and descended rapidly, averaging at least 2,000 feet per minute.

Weather at the time included a narrow spread between temperature and dewpoint, which, combined with moist terrain and light winds, made radiation fog easily predictable. Witnesses on the ground reported that the fog in the immediate accident area was very dense, limiting visibility to near zero. This localized dense fog was unknown to the pilots or ground personnel. Ground witnesses heard the aircraft's engines sounding normal but very low, and one heard a reduction in power shortly before impact. Two passengers recalled seeing roof tops periodically through the mist immediately before the crash, and a former pilot passenger noted the rate of descent increased sharply a few seconds before the accident.

The Board believed that near the middle marker, the pilots likely established visual contact with the approach lights and proceeded visually. The Board did not believe the pilot continued below the prescribed minimum altitude without visual references, nor that he saw the heavy fog before entering it. Instead, it was believed that after adjusting for a visual landing, the flight unexpectedly encountered the dense fog and lost visual reference. During the necessary transition back to instrument flying, the Board believed the pilot experienced momentary disorientation, causing the aircraft to descend more rapidly before corrective action could be taken.

The investigation noted the importance of precise weather reporting in the normal breakout area of an ILS approach. The Board endorsed a U.S. Weather Bureau program to install "end-of-the-runway" electronic equipment, such as ceilometers and transmissometers, to measure conditions in this area.

**Probable Cause** The Board determined that the probable cause of this accident was "momentary disorientation caused by the loss of visual reference during the final visual phase of the approach resulting in an increased rate of descent at an altitude too low to effect recovery."