Factual Information
On the morning of 18 June 2004, a Saab 340 aircraft, registered
VH-KEQ, with a crew of three and 31 passengers, was being operated
on a regular public transport flight from Albury to Melbourne, Vic.
The pilot in command (PIC) had levelled the aircraft at flight
level (FL) 120 (12,000 ft) with indicated air speed (IAS) and half
bank selected on the autopilot. The engine anti-ice system and
propeller and airframe de-ice systems were activated.
The PIC reported that the outside air temperature was -10 ºC,
while the IAS was 145 to 150 kts. As the PIC increased the
propeller RPM to aid with ice shedding, the IAS rapidly decreased
to 137 kts. The PIC disconnected the autopilot and initiated a
descent to 10,000 ft. During the autopilot disconnection, the stick
shaker activated for about 1 to 2 seconds. The PIC reported that
there were no autopilot miss-trim indications during the event. Ice
was still present on the aircraft radome after landing.
The stall warning system fitted to the Saab 340 consists of two
independent dual channel stall warning computers, left and right
angle of attack sensors, two stick shakers and a stick pusher. The
system provides five distinct warnings of an impending stall,
commencing with stick shaker and aural clacker, followed by
autopilot disengage, a visual warning in the form of stick pusher
initialisation lights on the instrument panel, and a stick
pusher.
The stall warning computers receive inputs from separate angle
of attack sensors that are situated on the forward section of the
fuselage, which measure airflow relative to the fuselage.
Activation of the wing de-ice system increases the angle of attack
signal by 0.4 degrees to increase the stall margin by 1 to 2 kts
when the de-ice boots are inflated.
The stall warning computer activates the stick shaker at 12.5
degrees angle of attack and the stick pusher at 19 degrees angle of
attack, with zero flap deflection and wing de-ice systems
deactivated. Activation of the stick shaker causes the autopilot to
disengage. Initiation of this warning for both pilots occurs when
either of these sensors reaches the predetermined angle of attack.
The stick pusher command requires a stall warning output from both
sensors, while one or both sensors is required for stick pusher in
the event of stall identification.
Following the incident, the data from the aircraft's flight data
recorder was downloaded and analysed by the Australian Transport
Safety Bureau (ATSB). The data indicated that from the time the
aircraft levelled at FL120, the autopilot was maintaining that
flight level by providing nose-up elevator movement and
automatically re-trimming. At the same time, the IAS was decreasing
and the angle of attack was increasing. About one minute later, the
RPM of the propellers began to increase from 1,240 RPM to 1,370
RPM. However, over the same period, torque values decreased from 73
to 65 percent and the IAS continued to decrease.
About 1 minute later, with an IAS of 134 kts, the angle of
attack reached the value required for stick shaker activation and
the autopilot was disconnected. However, because of the
disconnection, the subsequent nose-down elevator movements are
considered to have been in response to control inputs from the
crew, thus the angle of attack did not reach the value required for
operation of the stick pusher (19 degrees).
No airframe buffet was evident in the recorded lateral,
longitudinal or vertical acceleration data for this event.
Recording limitations (sampling rates and the accelerometer
frequency response) mean that light buffeting may have occurred and
not been evident in the recorded data.
The stick shaker and subsequent control inputs from the crew
were initiated before a loss of control of the aircraft.
Recorded data of the incident.
An investigation by the operator following the incident
indicated that the probable reason for the rapid decrease in IAS
was the altitude capture mode being used at the time of the
incident.
As the aircraft approaches the altitude selected on the
autopilot, the autopilot will command a capture profile and will
hold the selected altitude. The capture point is variable and it is
a function of the vertical speed. For this to occur, the autopilot
changes the mode from IAS to ALTS (altitude capture) mode, thus
giving no airspeed protection during the transition of modes.
Without an increase in engine power the airspeed will decrease if
the autopilot continues to increase the angle of attack to maintain
the captured flight level.
The flight recorder data was also forwarded to the manufacturer
to conduct further analysis to establish the reason for the stall
and ascertain if the aircraft 'behaved' according to type design.
The manufacturer commented that above 10,000 ft, the rate of climb
began to decrease and reaching 11,600 ft, the rate of climb had
reduced to almost zero. The IAS was 160 kts with propeller RPM
1,240 and engine torque of 69% and 73% on the left and right engine
respectively. Thirty five seconds after reaching 11,600 ft the
aircraft began climbing at the same time as the indicated airspeed
reduced to 150 kts. The aircraft then levelled off at 11,900 ft
before making a final altitude adjustment, reaching 12,000 (FL120)
at 145 kts, while propeller RPM remained constant at 1,240, but
then began increasing about 60 seconds later. Approximately 30
seconds after reaching FL120, the IAS began reducing, until the
aircraft entered a stall 100 seconds later. The autopilot, which
had been engaged during the climb, was disengaged at the stall
warning activation. The aircraft recovered from the stall and
descended to 10,000 ft.
The manufacturer commented that the data, illustrated two
indications of a stall. The first indication was the increase in
angle of attack with no or very small increase in the corresponding
lift coefficient. The second indication of a stall was the
hysteresis effect in the lift curve seen during the stall. As the
aircraft entered the stall and the angle of attack was reduced, the
aircraft was not able to attain the normal lift coefficients until
the angle of attack was significantly reduced. The analysis also
shows that the hysteresis effect was rather moderate, which
indicated that the stall had began to build, but was not fully
developed, that is, not all parts of the wing were stalled. It is
possible that due to the partial stall, the crew may not have
recognized it as a stall, especially if control inputs were made
simultaneously.
The manufacturer reported that the stall, which happened
approximately two and a half minutes after reaching top of climb at
FL120, was probably caused by a combination of significant, or
extreme, ice accumulation on the airframe, possibly also in
combination with run-back ice accretion on the propeller blades.
There is an indication from the analysis of the data, that ice was
accumulating on the airframe and possibly also on the propeller
blades during the final part of the climb above 10,000 ft. The
aircraft encountered an aerodynamic stall at the same time as the
stick shaker was activated and the autopilot was disconnected. The
indicated airspeed at the time of stick shaker activation was 134
kts. The aircraft sustained a moderate roll disturbance to the left
during the stall, which was corrected by the crew with moderate
opposite aileron deflection. The manufacturer estimated that when
the aircraft encountered the stall, the accumulated ice had a
combined effect corresponding to a drag increase of more than 500
drag counts, which is in the same order as the total aerodynamic
drag for an aircraft without ice accumulation.
The procedure as prescribed in the Aircraft Flight Manual - ref
3 (AFM), as well as in the Aircraft Operators Manual - ref 3 (AOM),
is to operate the de-ice boots at the first sign of ice build up
anywhere on the aircraft. It is recommended to use the continuous
mode of the de-ice boot operation. The continuous mode
automatically starts a de-ice boot cycle each 3 minutes and each
cycle takes about 30 seconds. However, ice formation on the
airframe might in some conditions be so severe that manual de-ice
boot operation will be necessary to avoid large ice build-up on the
leading edges.
The manufacturer commented that the findings from their
aerodynamic analysis show that there was significant, or even
extreme, ice accumulation on the wing leading edges as well as
other parts of the airframe. It was not possible to determine from
the recorded data, if or when the de-ice boot system was operated.
Considering the significant increase in aerodynamic drag during the
last minutes before entering the stall, the de-ice boot system was
probably not operated manually to further enhance the de-icing
capability. Had the de-ice boot system been manually and frequently
operated during the final part of the climb and during the short
cruise segment before entering the stall, ice accumulation would
most likely still have been present, but with a significantly less
amount and subsequently with less aerodynamic consequence.
According to the manufacturer, the procedures prescribed in the
AFM and AOM stated that the propeller de-ice system should be
operated in the NORM mode for temperatures between -5 ºC and -12 ºC
and MAX mode at temperatures -13 ºC or colder.
Using MAX or NORM modes at warmer temperatures than specified
may result in the ice melting, running backwards and refreezing in
the form of ridges behind the propeller boots, instead of being
shed off the blades. The so-called run back ice will cause a
drastic reduction in propeller thrust, up to about 30%.
Above -5 ºC the centrifugal self-shedding capacity is usually
enough to avoid ice build-up. Should ice build-up be severe, it is
recommended to increase propeller RPM to improve the self-shedding
capacity.
From the recorded data, the manufacturer concluded that the
aircraft reached an outside air temperature of -5 ºC when climbing
through 9,500 ft. Considering the significant loss in rate-of-climb
above approximately 10,000 ft, there is a possibility that part of
this can be attributed to loss of thrust. The recorded data also
reveals that the propeller RPM was low for a climb in icing
conditions, approximately 1,240 RPM, which might have reduced the
centrifugal self-shedding capacity. The fact that the crew, shortly
before entering the stall, increased the propellers to maximum RPM,
might indicate that the crew suspected ice formations on the
propeller blades.
It is likely that the aircraft had more ice accumulation than
the crew realised, which resulted in a degradation of aerodynamic
performance that led to a decrease of the IAS, and the subsequent
stick shaker activation. Additionally, the crew did not increase
power to compensate for the decreasing IAS as the autopilot
attempted to maintain altitude by trimming the aircraft to increase
angle of attack.