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.