One year to the day that Comair Flight 3272 took off from Cincinnati and crashed in Michigan, killing all 26 passengers and three crew members, authorities have been unable to say with any certainty why it happened.
Icy weather conditions are one focus of the investigation, but the National Transportation Safety Board (NTSB) said this week a final report won't be ready for about six months, possibly longer.
Several leading crash experts say the cause may never be known. Complicating the investigation is the lack of pilot reference on the cockpit voice recorder to problems. In fact, for all but the final 38 seconds, everything appeared to be going smoothly.
''I thought there certainly would be an indication, something, from the voice cockpit recorder,'' Ohio State aviation Professor Richard Jensen said Thursday. ''But if there was no mention of trouble, it might be difficult to get an answer to.''
Despite the mystery surrounding the exact cause, the crash prompted recommendations last May by the NTSB that all airlines using the Embraer EMB-120:
- Increase minimum air speed in icy conditions.
- Install automated ice-detection and alerting systems.
Comair has complied with those recommendations and is phasing out its EMB-120s, replacing the 30-seat prop planes with 50-seat jets at a cost of $18 million each, airline spokeswoman Meghan Glynn said. The phaseout began prior to the Flight 3272 crash. The ice-detection systems cost $18,000 each and are now on every Embraer that Comair flies.
The Cincinnati-based airline typically conducts 650 flights per day, and no crashes have occurred at Comair since - approximately 237,250 flights.Comair has 56 jets and 37 Embraers.
The crucial piece
Flight 3272, an Embraer EMB-120 turboprop plane, left Cincinnati - Northern Kentucky International Airport at 2:51 p.m. on Jan. 9, 1997, bound for Detroit.
Thirty-eight seconds before the crash, air-traffic controllers at Detroit Metro Airport instructed the pilot, Capt. Dann Carlsen, to make a left turn to prepare for landing. He did, tilting the plane's left wing down. The engaged autopilot attempted to correct the pilot-commanded turn by leveling it back to the right.
But the plane's left tilt continued.
Within a 16-second span, the plane's speed dropped from 189 mph to about 167 mph. At 3:54.24, the plane experienced ''a rapid upset in roll and pitch.''
''Whatever caused that rapid upset is the crucial piece of information,'' said Chuck Eastlake, an aeronautical engineering professor at Embry-Riddle University in Daytona Beach, Fla., and a leading authority on crash investigations. Mr. Eastlake is a Hyde Park native.
The autopilot disengaged and a stall warning activated. Within two seconds, the plane rolled from 40 degrees left bank to 146 degrees left bank.
''What causes it to get that way,'' Mr. Eastlake theorized, ''also might have prevented the pilot from recovering from it.''
In a nose dive, the plane plummeted 4,000 feet into a snow-covered field in Raisinville, Mich., 18 miles from its destination, Runway 3R.
''A huge part of the diagnosis of the cause is examination of the wreckage,'' Mr. Eastlake said. ''When an airplane is so severely damaged like Comair, an awful lot of the clues are damaged beyond use. . . . I'm certain ice was a contributing factor, but the cause, we might never know.''
Smaller commuter planes, often referred to as ''puddle-jumpers'' for their short routes, are more susceptible to icing, primarily because they fly at lower altitudes and at slower speeds. Comair has not altered the routes its Embraers fly.
NTSB spokesman Paul Schlamm emphasized this week that icy weather was a focus, but not the focus, of the investigation. The NTSB continues testing with engineering simulators at wind tunnel laboratories at NASA, he said, and craft performance tests also are continuing at Embraer, the plane's manufacturer. Mr. Schlamm declined to elaborate on specific tests and their findings.
He said no public hearing will be held, as was the case with larger airline crashes such as the 1994 crash of a USAir jet near Pittsburgh, which killed all 132 aboard.
The NTSB in September met with researchers at the National Center for Atmospheric Research (NCAR) in Boulder, Colo., but a second round of tests confirmed, but did not advance, preliminary findings that ice was present. The tests also revealed that weather patterns at 4,000 feet were conducive to the presence of a little-known icing condition called ''super-cooled large drops.'' Those factors include shallow clouds. Also, drizzle was reported at ground level the day the Comair flight crashed.
An oddity
Super-cooled large drops are an atmospheric oddity. They are liquid despite being as cold as minus 15 degrees. The drops, likened to freezing drizzle, are suspended, though not necessarily stationary, in clouds. When an object such as a wing moves through the cloud, the drops can immediately freeze on the surface of the craft, weighing down the plane before inflatable de-icing boots could break them off. They also might affect the plane's propellers.
''It certainly looks like icing was there, a lot of pieces of information show that,'' said Ben Bernstein, an atmospheric scientist at NCAR who headed the tests on Flight 3272. ''(But) We still can't get a handle on these large drops. The conditions were right for it, but we could never prove it was definitely present.''
Even if so, was it enough to prompt the crash, or a series of events that led to it?
Investigators examined whether ice could have built up enough to alter the smooth air flow over the wing, which gives the plane its lift.
Mr. Bernstein confirmed that super-cooled large drops were present at the altitude from which American Eagle Flight 4284 fell from the sky over Roselawn, Ind., on Halloween night 1994. The pilot apparently did not notice the icing, and when the autopilot was turned off, the plane plunged.
Without sufficient lift, a plane enters stall speed. Stall does not require a loss of engine power. Researchers think that ice might have weighed down the Comair Flight 3272 plane so that a higher speed was required to avoid stall, something of which the pilot might have been unaware.
Investigators say 167 mph, the speed that prompted activation of the stall warning indicator, would not result in stall or near-stall speed in an EMB-120 under normal weather conditions.
But under icy conditions, it might, experts generally agree. In a May 21 letter from NTSB Chairman Jim Hall to Barry Valentine, acting administrator of the Federal Aviation Administration, obtained by the Enquirer, Mr. Hall detailed all the items for which anti-ice equipment was activated. They included the windshield and propellers.
However, he wrote, ''There is no evidence from the CVR, FDR (flight data recorder), performance of the aircraft, or aircraft wreckage to determine if the flight crew activated the de-icing boots.'' The de-icing boots are inflatable ''balloons'' on the leading edge of each wing that break off accumulated ice.
Enquirer coverage of the crash of Comair Flight 3272
