Determines Pilot Failed To Maintain Airspeed, Stalled At Low
The NTSB has released its probable cause report from an
accident in April, 2009, in which the sole occupant of a Wisconsin
Department of Natural Resources aircraft was fatally injured while
conducting an aerial inspection of a wildfire. The probable cause
was determined to be the pilot's failure to maintain adequate
airspeed which resulted in an aerodynamic stall at a low
HISTORY OF FLIGHT
On April 8, 2009, at 1439 CDT, a Cessna 337C, N2489S, operated by
the Wisconsin Department of Natural Resources (DNR), impacted an
agricultural field while responding to a ground fire. Visual
meteorological conditions prevailed at the time of the accident.
The airplane was being operated as a public-use aircraft. The
pilot, who was the sole occupant, was fatally injured. The flight
responded to the fire by departing from Necedah Airport, Necedah,
Wisconsin, at 1411.
The Rack Township Fire Department responded to a ground fire
that started from a burn barrel located at a residence north of the
agricultural field where the airplane impacted. A wooded area
separated the residence and the agricultural field. The Fire
Department Deputy Chief stated that when he arrived on scene, the
Cessna 337C, was flying above the area at an altitude that was
“several thousand” feet above ground level and in the
“stratosphere.” The Deputy Chief then walked through
the wooded area to a point that was about 100 feet north of the
main wreckage. From his vantage point, he saw the airplane circle
three times while descending. He did not hear a decrease in engine
noise during the airplane’s descent from “several
thousand feet.” During the third circle, he saw the airplane
fly south at an altitude that the Deputy Chief “seemed
low” and was about 100 feet above the trees. About “15
seconds” before the accident, the airplane continued in a
southerly direction and was simultaneously banking in a left 30-45
degree roll attitude. The airplane was about 150 yards from the
tree line when it made its turn to the north. The airplane
continued to descend and roll out to an almost wings level attitude
and was on a northerly heading approaching the tree line. The
Deputy Chief stated that the airplane was now at an altitude of
about 35 feet above the ground. He then heard the airplane engine
noise increase about “5 seconds” prior to the left wing
dropping and it seemed to him that the airplane was descending. The
airplane then impacted the ground. The Deputy Chief stated that
this was the third DNR airplane the he has seen responding to a
fire and the other airplanes flew “way up there” and
were more than 100 feet in altitude when they were responding and
did not fly lower. He stated that the landing gear was retracted,
and there were no lights illuminated on the Cessna 337C.
The Pittsville Fire Department Chief stated that he saw the
Cessna 337C at an altitude of about 1 ½ - 2 times the height
of the trees. The airplane descended at a “sharp angle”
during the third time the airplane was circling. He stated that the
engine was running.
The pilot held an airline transport pilot certificate with an
airplane multiengine airplane rating and commercial pilot
privileges with an airplane single-engine land rating. He also held
a flight instructor certificate with airplane single-engine,
airplane multiengine, and instrument airplane ratings. As of April
8, 2009, he accumulated a total flight time of 4,739.1 hours.
The pilot served as a DNR pilot of Cessna 182 and 185 airplanes
prior to receiving a Cessna 337 initial flight check dated April 3,
2009, which was 1.0 hours in duration that originated from Wittman
Regional Airport (OSH), Oshkosh, Wisconsin (A pilot logbook entry
indicates a local flight from OSH on March 27, 2009, that was 1.0
hours in duration). Since the initial flight check, the pilot
accumulated an additional 3.1 hours of flight time in the accident
airplane during two flights on April, 7, 2009, and one flight on
April 8, 2009.
Federal Aviation Administration (FAA) records for the pilot show
no reported incidents, accidents, or enforcement actions. On
February 25, 2009, the pilot was issued a first class airman
medical certificate with no limitations.
The 1967 Cessna 337C was operating as a public-use
aircraft as of January 7, 2009, at a Hobbs time of 205.3 hours. The
airplane was also used by the DNR to conduct aerial observations.
The airplane was powered by two Continental IO-360-CB engines. The
front engine was serial number 236534 and the rear engine was
serial number 236536.
The airframe and engines received their last annual inspections
on September 15, 2008, at an aircraft total time of 2,530.0 hours
and a Hobbs time of 192.0 hours.
The airplane was equipped with Horton STOL kits under
supplemental type certificates (STC) SA937CE (Installation of
leading edge cuffs, stall fences, droop wing tips, and vortex
generators on rear engine lower cowling) and SA2821CE (Installation
of wing tips).
The airplane was equipped with BAS, Inc. shoulder harness and
harness system under STC SA2067NM.
WRECKAGE AND IMPACT INFORMATION
The main wreckage, which consisted of the fuselage, empennage,
wings, and both engines were located at 44 degrees 28.705 minutes
North and 90 degrees 16.690 minutes West at an elevation of 1,094
feet on a fallow wheat field with north/south rows. The fuselage
was resting in an upright position with a tail to nose heading of
about 240 degrees.
The left wing displayed wrinkling along the wing’s
spanwise axis with greater relative damage than the right wing.
Both wing were attached to the fuselage but were fractured through
at the wing strut attachment. The forward and aft wing spar
attachment points were fractured. The fractures exhibited
deformation and surface features consistent with overload. All of
the wing fuel caps were in place.
The stall fence on the right wing was attached to the wing and
the left wing stall fence was separated. The left stall fence was
deformed and found laying about 15 feet behind the main
The wing flap jackscrew was extended 3.1 inches, which equates
to 10 degrees of flap extension. The cockpit flap control switch
was in the 10-degree flap position.
Flight control continuity from the control surfaces to both
cockpit flight controls was confirmed. Engine and propeller control
continuity from the cockpit to the fuel servo and propeller
governor was confirmed.
The landing gear was in the retracted position.
The front engine propeller was separated from the hub, and the
propeller hub bolts were separated. One blade of the propeller was
separated through a fracture that exhibited a surface consistent
with overload. The propeller also displayed bending and twisting
along the span. The rear engine propeller was attached to the
engine and displayed twisting along one blade.
Both engines were rotated at their propeller hubs by hand after
removal of the top spark plugs. During rotation, air was noted to
be drawn in and expelled from each of the top cylinder spark plug
The front and rear engine magneto to engine timing could not be
verified due to impact damage. Both magnetos on each engine were
rotated by hand and a spark from each lead was noted.
The fuel pumps from both engines were operated using an electric
drill with 100 low lead aviation fuel. Fuel was poured into the
inlet port of each pump and was noted to discharge from the output
port while moving the mixture control lever from the full rich
mixture to the idle cutoff positions. Fuel ceased to flow when the
mixture control was placed into the idle cut off position at the
fuel pump. Both fuel pump couplings were intact.
An undetermined amount of oil drained from a hole in the oil pan
of the front engine. An undetermined amount of engine oil was
drained from the rear engine. The front engine did not have an oil
screen but was equipped with an oil filter, which did not have
obstruction or debris. The rear engine oil screen was unobstructed
and did not contain debris.
The fuel manifold valve screens and fuel injectors for both
engines were unobstructed.
First responders reported smelling fuel when they responded to
the accident. During recovery, fuel spilled from the right
An autopsy of the pilot was conducted by the Wood County
Coroner on April 10, 2009. The autopsy’s final anatomic
diagnosis reported multiple blunt force injuries, no significant
disease identified, no ethanol detected, and negative drug screen
results. The FAA’s Final Forensic Toxicology Fatal Accident
report of the pilot was negative for all substances tested.
TEST AND RESEARCH
The accident airplane and two other airplanes operated by
the DNR, N185NR and N6991H, were equipped with Horton STOL kit
installations. The airplanes did not have any additional labeling,
placards, or airspeed indicator markings relating to changes to
aircraft performance. Also, there are no STC supplements available
for the airplanes’ flight manuals with quantitative data
showing changes in aircraft performance.
A Cessna 337 A/C Familiarization questionnaire completed by the
accident pilot lists the airplanes V-speeds, which include VS0 (VS0
is the minimum steady flight speed with flaps extended) and VS1
(VS1 is the stall speed with flaps and landing gear retracted).
These speeds are handwritten as: “VS0 66” and
“VS1 75”. The airplane flight manual recovered from the
wreckage lists for 0 degrees of bank, power off stall speed, with
the landing gear down, and full flaps as 67 mph, calibrated
airspeed (CAS) and the landing gear and flaps retracted stall speed
was listed as 78 mph, CAS.
According to the DNR representative for the investigation, who
was also the pilot that conducted the accident pilot’s Cessna
337C initial flight check, aircraft are flown as single-pilot
operations in aerial observations. While observing, pilots would
fly at an airspeed near stall with an intermittent stall warning
and visual reference was made outside of the cockpit and not at the
airspeed indicator. He said that flying was by aircraft feel more
than by reference to instruments. He stated that the airspeed and
configuration for observation flights in the Cessna 337C are 95 mph
indicated airspeed with 10 degrees of flaps. There is no minimum
altitude for flight operations. He stated that the DNR has a low
altitude waiver issued by the FAA’s Milwaukee Flight
Standards District Office to conducted flights at altitudes below
those required by Federal Aviation Regulations. Following review of
his statement, the DNR representative later stated that his
statements may not reflect those procedures used in other DNR
operations and/or by pilots.
The low altitude waiver is applicable to Parts 91.119(b), (c)
and 91.313(3). Part 91.119 Minimum safe altitudes states:
Except when necessary for takeoff or landing, no person may operate
an aircraft below the following altitudes:
- (a) Anywhere. An altitude allowing, if a power unit fails, an
emergency landing without undue hazard to persons or property on
- (b) Over congested areas. Over any congested area of a city,
town, or settlement, or over any open air assembly of persons, an
altitude of 1,000 feet above the highest obstacle within a
horizontal radius of 2,000 feet of the aircraft.
- (c) Over other than congested areas. An altitude of 500 feet
above the surface, except over open water or sparsely populated
areas. In those cases, the aircraft may not be operated closer than
500 feet to any person, vessel, vehicle, or structure.