Board Suspects Icing... But Could Not Determine For Certain

Investigators with the National Transportation Safety Board hate unanswered questions... and the probable cause report on the May 2007 downing of a Cessna Citation 550, issued this week, ends with a big one. What caused the aircraft to lose control?

As ANN reported, pilot Allan Hamilton and passenger John Wauterlek were killed when their bizjet impacted a field east of Dillon Airport (DLN) on May 3, 2007. The aircraft was on a VOR approach to land at the airport when it turned off course, and descended quickly.

"The last transmission from the pilot was an acknowledgment of the cancellation of radar service and an instruction to switch to the airport advisory frequency," reads the NTSB's summary. "At that time, the airplane was at a mode C reported altitude of 14,000 feet. The airplane maintained a steady descent rate for the next minute and leveled off at 13,000 feet. About 2 1/2 minutes later, the airplane began a turn to the right to head outbound for the procedure turn on the approach and descended to 12,900 feet. The approach procedure specified a minimum altitude of 8,200 feet in the procedure turn. The airplane lost 1,600 feet in the next 10 seconds, and this was the last radar contact."

Observed conditions at the time of the accident lead to one possible theory. A storm hung over the area at the time of the crash, and freezing levels were reported along the aircraft's projected flight path.

"Airmen’s Meteorological Information (AIRMET) ZULU Update 2 was in effect for an area that included the accident site," the NTSB states. "It noted that the freezing level was between 4,000 and 10,000 feet. It stated that there was the potential for moderate icing conditions from the freezing level to 20,000 feet."

The NTSB notes there were two witnesses to the accident. "A witness working in his office at the airport heard a loud engine noise, and then a "plop" noise. He said that the engine noise was loud, then softer, and then loud again. He heard it for 3 to 5 seconds," the Board notes. "Another witness saw an airplane below the cloud bases that was turning to the right with a nose low pitch attitude of about 75 to 80 degrees. It made six to seven turns before it disappeared from sight behind terrain, and the radius of the turn got tighter as the airplane descended."

The aircraft then impacted terrain at a steep nose-down angle, and caught fire. Investigators found the plane's landing gear was extended, with partial landing flaps deployed. The aircraft's TKS anti-icing system had been activated prior to impact, and oily fluid consistent with the properties of deice solution was found along the leading edges of the wing and empennage assemblies.

The accident aircraft (type shown below) was on a regular maintenance program, and no mechanical anomalies were found. Examiners with Pratt & Whitney determined both JT15D-4B turbofans were making "significant" power at the time of impact. Autopsies of both victims did not show any sign of carbon monoxide poisoning, or drugs.

While most indications of an icing-related accident seem to be present, the Board could not determine for certain that icing caused the plane to crash. A cockpit voice recording may have helped shed added light on the accident... but investigators found the CVR had "failed at a point prior to the accident events."

That left the Board to determine the accident was the result of "in-flight loss of control for undetermined reasons"... a ruling no one is likely very satisfied with.

FMI: Read The Probable Cause Report