Cessna Aircraft Company 172G, VH-RPI

1. The aircraft was being operated in weather conditions conducive
   to engine carburettor icing.
2. The aircraft maximum takeoff weight limitation was exceeded for
   the flight.
3. The aircraft loaded moment envelope limitation was exceeded for
   the flight.
4. The aircraft departed controlled flight at a height above the
   ground from which aerodynamic stall recovery would have been
   unlikely.

Loss of engine power under normal loading conditions causes the
aircraft nose to pitch downward for aerodynamic stall recovery
because of the aircraft designed forward centre of gravity (C of
G). Loading in a tail-heavy direction or rearward C of G condition
has a most serious effect upon longitudinal stability, affecting
the aircraft's ability to readily recover from stalls and spins. As
the C of G moves rearward, a less stable condition occurs, which
decreases the ability of the aircraft to right itself after
manoeuvring or after disturbances by gusts. The aircraft as loaded
had not exceeded the rearward C of G limitation. Although the
rearmost C of G limit had not been exceeded, the location of the C
of G just 25.4 mm ahead of that limit, meant that the aircraft
exhibited a rearward C of G condition.

The aircraft exceeded both the aircraft manufacturer's Maximum
Takeoff Weight (MTOW) limitations and the aircraft loaded moment
envelope. Exceeding the aircraft MTOW limitation may adversely
affect flight characteristics. CAAP advisory number Number
235-1(1), advised pilots against using standard weights and
recommended weighing occupants and baggage in order to prevent
exceeding those limitations.

The examination of the engine and carburettor revealed no
evidence of a preimpact failure or anomaly. The aircraft was most
likely being operated with the carburettor heat set to the OFF
position as indicated by the position of the heat lever at the air
box on the engine. With the aircraft operating in weather
conditions conducive to carburettor icing, it may have begun losing
power. The onset of carburettor icing may have been insidious, as
the pilot may not have noted the deterioration in engine RPM. As
the aircraft was not equipped with a carburettor ice detection
system, the pilot was not afforded any warning of the potential for
carburettor icing. Without this warning, if the engine performance
deteriorated, the pilot most likely would not have been able to
apply carburettor heat in time for it to take effect sufficiently
to regain full power. With the rearward C of G condition present,
the pilot may not have been able to pitch the nose of the aircraft
downward as required for aerodynamic stall recovery. However, in
any case, the stall was most likely unrecoverable because of the
low height above the ground. Weather conditions encountered on
previous flights to and from the island may not have been
sufficient to produce carburettor icing or may have been masked by
the constant speed propeller.

The possibility also exists that wind shear and turbulence in
the area, in combination with the adverse flight characteristics
resulting from exceeding the aircraft's loaded moment envelope
limitation, could have degraded the aircraft's controllability and
resulted in the aircraft's departure from controlled flight.

While several possibilities exist as discussed in this analysis,
the investigation could not conclusively determine the reason for
the excessive nose-up pitch and departure from controlled
flight.

The pilot of the Cessna 172G aircraft was conducting a series of
charter flights between the Trefoil Island Aircraft Landing Area
(ALA) and the Smithton, Tasmania aerodrome. Witnesses stated that
the aircraft, with the pilot and three passengers on board, took
off from the island ALA runway 28 on a west-south-westerly track at
approximately 1745 hours EsuT, on the third return flight of the
afternoon. Witnesses reported that the aircraft turned to the left
on a southerly heading while climbing, followed by a left turn to
the east. They reported that following the turn to the east, and
after it had overflown the buildings on the island at approximately
200 feet above ground level, the nose of the aircraft pitched up
abruptly to an angle of 30-40 degrees. According to the witnesses,
following the nose-up pitch, the aircraft rolled abruptly to the
left, lost altitude and descended from their line of sight. The
witnesses heard the impact of the aircraft and ran to render
assistance. The aircraft was destroyed by impact forces and all
four occupants received fatal injuries.

Wreckage information

The wreckage of the aircraft was oriented on a heading of 191
degrees magnetic, indicating that it had rotated through about 270
degrees during the descent. The aircraft impacted the ground wings
level, with a nose-down angle of approximately 39 degrees, on a
downward sloping hill of approximately the same angle. There were
no indications that the aircraft was in a spin at the time of
impact. The cabin roof had separated at the rear attachment and
both wing struts had separated. The forward cabin area had
collapsed, with the tail section and the tail cone buckled and bent
partially forward. Wreckage evidence indicated a high rate of
vertical deceleration, in excess of 24 g (acceleration due to earth
gravity, international standard value being 9.80665 metres per
second squared, assumed at standard sea level), with indications of
little forward airspeed.

The propeller/crankshaft assembly had separated behind the
radius of the crankshaft flange. The fracture surface displayed
evidence of a unidirectional bending overload failure, indicating
low engine RPM at the time of the fracture. Examination of the
propeller spinner and propeller blades confirmed low engine RPM at
impact. The carburettor heat lever at the air box on the engine was
noted to be in the OFF position. The aircraft was fitted with an
elevator trim that allowed the pilot to minimise load forces on the
elevator, depending on the position of the centre of gravity (C of
G), airspeed and power settings. The aircraft's elevator trim
system was found in the slightly nose down from the TAKEOFF TRIM or
neutral position. The seats and seat rails incurred substantial
damage, but the pilot's seat end stop was still located intact on
the seat rail. There were no indications of a bird strike on the
aircraft.

Aircraft information

A 100 hourly inspection was completed on 17 February 2003 at
9,663.6 hours total time airframe (TTAF) with no major anomalies
noted. A 50 hourly engine inspection was completed on 12 March 2003
at 9,713.6 hours TTAF and 1,085.2 hours engine time since overhaul
(TSO), with no anomalies noted. At the time of the accident, the
aircraft had accumulated 9,718.4 hours TTAF. The maintenance
release listed no outstanding discrepancies for the aircraft and
was current and valid.

Nothing was found during the investigation to suggest a
mechanical failure of any part of the aircraft that could have
contributed to the accident.

Engine information and examination

Supplemental Type Certificate number SA807CE was incorporated in
1977 with the installation of a 180 horsepower Lycoming model
O-360-A1A engine and a Hartzell constant speed propeller, replacing
the 145 horsepower Continental model O-300C engine and fixed pitch
propeller. At the time of the accident, the engine, serial number
L21971-36A, had accumulated 1,090 hours TSO. A technical
disassembly and inspection of the engine and carburettor was
completed at an independent maintenance facility under Australian
Transport Safety Bureau (ATSB) supervision. The disassembly and
examination did not reveal any evidence of pre-impact internal
component failure or anomaly.

Pilot information

The pilot held a valid commercial pilot (aeroplane) licence and
Class 1 medical certificate at the time of the accident. The
pilot's last flight review was completed on 6 January 2003.
Post-mortem and toxicological examination did not identify any
factor that may have impaired the pilot's ability to operate the
aircraft safely.

Meteorological Information

Documents recovered at the accident site included an Airservices
Australia on-line weather forecast briefing for the area and for
King Island and the Smithton aerodrome. The forecast was dated 14
March 2003 and the time noted was 1046 hours. Wind listed on the
briefing for the 2,000 ft level was forecast as variable at 15 kts.
The forecast also noted a south-westerly stream with a slow moving
trough with drizzle and locally broken low cloud. King Island was
located 107 km to the north of Trefoil Island. The King Island
meteorological report noted light drizzle, south-south-westerly
wind at 15 knots and a temperature/dewpoint spread of 5 degrees
C.

A series of wind generators was located on the Tasmanian
mainland at Cape Grim, approximately 5 km to the southwest of
Trefoil Island. This facility periodically monitored and logged
weather conditions. Documented information obtained from that
facility indicated that the weather conditions at 1700 hours were:
air temperature 15.4 degrees C; dew point 11.4 degrees C; relative
humidity 77 percent; and wind from the southwest at 27 kts with
gusts to 30 kts. Information documented at 1800 hours recorded: the
air temperature 15.1 degrees C; dew point 10.6 degrees C; relative
humidity 74 percent; and wind from the southwest at 28 kts, gusting
to 29 kts. Relative humidity recorded from 1500 hours to 1700 hours
(the estimated time of the first two return flights) was recorded
as 77-79 percent. The wind recorded at that time was from the
southwest and varied from 29 to 30 kts with the temperature/dew
point spread from 4.0 to 5.6 degrees C. Witnesses stated that the
accident occurred at 1750 hours. Last light on the day was about
2011 hours.

A pilot familiar with the area reported that a south-westerly
wind often caused orographic lifting (when air is forced upwards by
a barrier of mountains or hills) moving heavily laden moist air
into the flight path of an aircraft departing from the island.

Carburettor and engine induction system
icing

A search of the ATSB occurrence database indicated a total of
eight carburettor or engine induction system icing related
accidents since May 1994. These accidents resulted in two
fatalities. One aircraft was severely damaged and three aircraft
were destroyed. An article on the US Federal Aviation
Administration (FAA) website, reprinted from Vintage Airplane
Magazine and dated November 1994 stated: `According to the National
Transportation Safety Board, carburettor ice was involved in over
360 accidents in the past five years. These figures do not include
the unreported off-airport landings and incidents caused by icing.
The results were 40 deaths, 160 injuries, 47 aircraft destroyed and
313 aircraft severely damaged.' Several of these accidents noted
suspected carburettor icing at high power settings. Float-type
carburettors, such at that used in the occurrence engine, are most
susceptible to this event. Evidence of carburettor icing is highly
perishable and dissipates rapidly.

When carburettor ice forms, it can obstruct the smooth flow of
the air/fuel mixture, which results in a reduction of engine RPM,
power, and an associated loss of airspeed and altitude. FAA
Advisory Circular AC 20-113 provided information pertinent to
aircraft engine induction system icing. It noted:

`c. Fuel Vaporization Ice- This icing condition usually occurs
in conjunction with throttle icing. It is most prevalent with
conventional float type carburettors, and to a lesser degree with
pressure carburettors when the air/fuel mixture reaches a freezing
temperature as a result of the cooling of the mixture during the
expansion process that takes place between the carburettor and the
engine manifold.'

The circular also noted that vaporisation icing may occur, when
a relative humidity of 50 percent or higher is present, at
temperatures from 0 degrees C to as high as 37.7 degrees C. It also
stated that in general, when the temperature/dewpoint spread
reaches 6.6 degrees C or less and a relative humidity of 50 percent
or higher, there is a potential for icing.

The values from the weather observations for 1800 hours at Cape
Grim were plotted on a carburettor icing probability chart. The
temperature/dewpoint spread was 4.5 degrees C. The plot was located
in the area of the chart labelled `serious icing- any power
setting'.

Mitigating the effect of carburettor icing involves pilot action
to apply full carburettor heat (the ON position), which initially
causes a further loss of power (perhaps as much as 15 percent). The
air heated by the exhaust is directed into the engine induction
system, which results in a richer fuel/air mixture and additional
power loss. A delay of 30 seconds up to several minutes may be
expected until normal engine power returns. The circular
recommended the use of carburettor heat briefly (particularly with
float-type carburettors), immediately before takeoff if the
relative humidity was above 50 percent and the temperature below 21
degrees C, to remove any ice which may have accumulated during taxi
and pre-flight engine checks.

The engine manufacturer recommended that carburettor heat should
not be used for takeoff as it was not necessary and it may cause
detonation and possible engine damage. The aircraft manufacturer
recommended a check of the system before takeoff, and that
carburettor heat be placed ON in the event of an engine failure,
other than immediately following takeoff. The Operations Manual
noted that an unexplained loss in engine speed could be caused by
carburettor icing or air intake filter ice and cautioned to watch
for signs of icing and apply carburettor heat as required. The
section titled `Engine Failure After Take-off (under 700 feet)' did
not mention the use of carburettor heat.

One characteristic of the onset of carburettor, or induction
system icing, on an engine fitted with a fixed pitch propeller, is
the gradual deterioration of the engine RPM. Aircraft engines
equipped with constant speed propellers, such as the accident
aircraft, compensate for the gradual RPM deterioration by
decreasing propeller pitch to maintain a given RPM.

The previous owner of the aircraft reported experiencing an
engine power loss (with the 180 horsepower engine fitted), while on
approach to land several years earlier. That event was believed to
have been due to carburettor icing, as no mechanical anomaly was
discovered. The previous owner further reported that the onset of
the power loss was immediate, with little time to react.

Optional equipment for the Cessna 172 model aircraft included a
Carburettor Ice Detector system. This system utilised an optical
probe in the carburettor throat, which is so sensitive that it can
detect `frost' up to five minutes before ice begins to form, giving
the pilot time to take corrective action. Examination of the
carburettor revealed that an optical probe was not fitted.

Aircraft performance

Reports from witnesses indicated that the aircraft took off from
the island ALA runway 28 and that take-off performance was
apparently acceptable. According to the Aircraft Flight Manual, the
maximum permissible crosswind component for takeoff and landing was
15 kts. The Operations Manual stated, `Pilots will not take-off or
land a Company aircraft when the crosswind component exceeds that
specified in the relevant Aircraft Flight Manual.' Using the
weather information noted previously, the crosswind component
during takeoff was calculated by the ATSB to be in excess of 15
kts.

Aircraft fuel

The aircraft fuel selector was found in the BOTH position (both
tanks feeding the supply line to the engine) as required for
takeoff. Damage to the aircraft fuel tanks precluded establishing
the exact fuel state of the aircraft at the time of impact. There
was a strong smell of fuel in the area of the crash site. A fuel
sample was removed from the right wing tank and sent for analysis
by a National Association of Testing Authority approved laboratory.
The laboratory confirmed that the fuel sample was Avgas 100, which
was the correct grade and specification for the engine and no
anomalies were noted.

An examination of load sheets used on previous flights to the
off-shore islands was completed. These sheets confirmed that the
pilot had previously adhered to the Operations Manual policy of
maintaining a maximum of 120 L total fuel for flights to off-shore
islands of 40 minutes or less, to avoid aircraft structural stress
during ground operations. The flight from Smithton to Trefoil
Island was approximately 12 minutes. The investigation team
generated a flight plan using this fuel information, weather data,
witness statements and fuel consumption estimates for the aircraft.
The ATSB calculated that the aircraft had approximately 90 L of
fuel at the time of takeoff on the accident flight.

Aircraft loading

The Operations Manual stated that `Pilots shall prepare a
passenger list/manifest and leave it for retention at the aerodrome
of departure on all Charter Flights. Pilots will also prepare and
leave passenger list/manifest prior to departing the off-shore
islands.' The manual specified that the passenger/manifest sheets
were to be left `inside the tractor shed' on Trefoil Island. It
also stated that if no scales for weighing were available, portable
scales were to be carried for use by the company pilots. It further
stated that company pilots were to ensure that the aircraft was
loaded strictly within the weight and balance limitations.

No passenger/manifest sheet for the accident flight was
recovered from the accident site, the island ALA area, or from the
operator's Smithton facility. A witness stated that the pilot did
not leave the immediate vicinity of the aircraft and did not leave
any documents behind prior to the accident flight. No portable
weighing scales were recovered from the accident site. When
interviewed, passengers from previous flights to and from the
off-shore islands reported that they were not weighed, and that the
pilot had rarely asked for their body weights. These same
passengers reported that the pilot personally loaded all baggage
into the aircraft baggage compartment, but did not weigh it.
Witnesses who observed the pilot loading the baggage compartment
prior to the accident flight also reported that he personally
loaded the baggage, but did not weigh it.

The FAA Type Certificate Data Sheet (TCDS) for the aircraft
noted the maximum permissible baggage compartment load limitation
was 120 pounds (54 kg). Numerous items, such as tools and personal
equipment, were located in the immediate area of the wreckage. When
weighed these items totalled 108.2 kg. Of that amount, 19.7 kg
included items not normally kept in the baggage compartment, but in
the main cabin area. That indicated a total baggage compartment
load of 88.5 kg at the time of takeoff, 34.5 kg in excess of the
maximum weight limitation for the aircraft.

The TCDS further noted that the Maximum Takeoff Weight (MTOW)
for the aircraft was 2,300 pounds (1,043 kg). The MTOW is the
maximum allowable weight at the start of the takeoff run. The fuel
estimated to be on board at the time of takeoff was 90 L. ATSB and
aircraft manufacturer's calculations indicated an aircraft takeoff
weight of 1,123 kg (2,475.7 pounds), signifying a takeoff weight in
excess of the maximum limitation by 79.6 kg. The calculations also
indicated that the MTOW would have been exceeded even with all fuel
removed.

Advisory material

The Australian Civil Aviation Safety Authority, Civil Aviation
Advisory Publication (CAAP) Number 235-1(1), Standard Passenger and
Baggage Weights, recommended the following:

`11. Because the probability of overloading a small aircraft is
high if standard weights are used, the use of standard weights in
aircraft with less than seven seats is inadvisable. Load
calculations for these aircraft should be made using actual weights
arrived at by weighing all occupants and baggage.'

Aircraft balance and centre of gravity

Aircraft balance refers to the location of the C of G, along the
longitudinal and lateral axis. In order to assure predictable
aircraft control, the aircraft manufacturer established limitations
along the longitudinal axis at fuselage stations measured in
inches, in relation to a reference point or datum (located at the
forward face of the engine compartment firewall). The C of G
limitation for operation of the aircraft in the normal category at
maximum Takeoff Weight of 2,300 pounds (1,043 kg) was a forward
limit of 38.5 inches (977.9 mm) aft of the datum and rearward limit
of 47.3 inches (1201 mm) aft of the datum. Aircraft longitudinal C
of G was calculated by dividing the total moment of the empty
aircraft and on-board items (weight multiplied by the fuselage
station) by the total weight of the empty aircraft and items. The
location of the C of G at the estimated takeoff weight of 1,123 kg
(2,475.7 pounds) was approximately 46.3 inches (1176 mm) aft of the
datum.

US FAA AC 91-23A Pilot's Weight and Balance Handbook
(superseded) stated, `C.G. limits may be expressed graphically in
the aircraft weight and balance reports by means of an index
envelope. The envelope defines the forward and aft limits and also
the maximum weight limit in terms of index units.' The index
envelope for the Cessna 172G was referred to as the loaded aircraft
moment envelope. Flight with a total moment outside of this
envelope was not recommended. When plotted, the calculated loaded
aircraft moment at takeoff was outside the C of G moment envelope,
exceeding the aircraft manufacturer's recommendation.

The aircraft manufacturer advised that with an aircraft loading
condition as plotted, if rising terrain and strong winds combined
to create significant vertical shear, the risk of loss of control
of the aircraft would be increased, even if anticipated by the
pilot. The manufacturer further reported that the nose pitch-up and
subsequent departure from controlled flight as witnessed were
consistent with an aircraft that was flown exceeding the loaded
aircraft moment envelope limitation. The estimated aerodynamic
stall speed and other aircraft performance figures at the
calculated aircraft takeoff weight were not available from the
manufacturer, as the aircraft was being operated outside the
certified moment envelope.