After careful consideration Saab Aircraft AB has the following
comments to your Air Safety Interim Recommendation IR990072
relating to the Saab SF340A stall incident 11 November 1998.
COMMENTS ON FACTUAL INFORMATION AND ANALYSIS
The comments are divided in different subjects to avoid repetitive
comments on different parts of the text in IR990072. The following
comments apply:
Experienced Stall warning in the occurrence summary
In "FACTUAL INFORMATION", the last paragraph in "occurrence" states
that "The crew received very little warning of the impending stall.
Only the autopilot disconnect and the severe vibration indicated
that a stall might have been about to occur." Also in "ANALYSIS",
the second paragraph states that "... it appears that the only
warning the crew received of the impending stall was the
disconnection of the autopilot, and this occurred less than a
second before the aircraft actually stalled....".
As detailed in the Saab document 72ADS4196 (the DFDR analysis)
there were several natural warnings of the impending stall. The
stall buffet started slightly more than 6 seconds prior to the
stall and 5 kts above the aerodynamic stalling speed. There was
also a significant increase in pitch attitude prior to the stall,
while the aircraft was still in level flight. This increase in
angle of attack occurred with relatively high rate, more than one
degree per second, and should have been noticed. The buffet and the
sudden increase in angle of attack are both typical behaviours for
an impending stall situation for any conventional aircraft.
Finally, the speed was decreasing to values well below the minimum
recommended speeds for holding which in itself challenges the stall
margin.
Crew actions
In this particular occurrence the crew was entering the holding
pattern with a too low speed which also was notified by the Captain
and started to be corrected by the First Officer by applying more
power than what was initially estimated to be required to keep the
holding speed of 154 KIAS.
It must be remembered and emphasized that according to common
practices and procedures during flight in IMC conditions with any
aircraft, one of the two pilots should always monitor the flight
instruments and especially speed and attitudes. Since the speed
continued to be reduced during the turn after entering the holding
pattern with as much as 18 kts below the holding speed, this must
be questioned for this particular occurrence.
In "FACTUAL INFORMATION", in the second paragraph of "Occurrence
Summary" the IR990072 states that "The crew reported that they had
previously operated the aircraft with more than that amount of ice
without problems." Provided that the aircraft is flying at speed
higher than the published minimum operating speed in icing
conditions, and that the de-icing system is used as intended, such
a statement is of course true.
In "FACTUAL INFORMATION", in the "Action of flight crew", the
IR990072 states that "The crew were operating the aircraft, as
would any normal crew, in the knowledge that the stall warning and
protection system would afford them the necessary margin above the
stall, should it occur."
According to information from the specific operator of the
occurrence aircraft, the fact that the stall warning margins are
only applicable to a clean wing is well known and fully understood.
Also that being in icing conditions with ice accretion on the wings
results in higher stall speeds, is well known within the operator
organisation. Hence, the statement that a normal crew is using the
stall warning as a kind of stall margin protection seems odd and
not correct.
Also there are always minimum speeds detailed in the AFM and AOM
for different segments and different conditions. These minimum
speeds (like 1.4 Vs for icing) should be monitored by any crew,
which is a natural part of any normal flight training as well as
standard practices and procedures.
Ice accretion
The IR990072 states the following. "The crew involved in the
occurrence at Eildon Weir were not aware of the amount of ice
accretion on the aircraft, as they were following the guidance in
the aircraft flight manual. This guidance stated that there should
be at least half an inch of ice built up before the activation of
the wing de-icing boots."
The guidance at the time for the occurrence, in both the airplane
flight manual as well as the aircraft operations manual, states
that the boot de-icing system should be operated when the ice has
accumulated to approximately 1/2 inch thickness on the leading
edges. Both manuals also clearly define that all detailed minimum
speeds and aircraft performance for ice accretion assumes 1/2 inch
of ice accumulation on all protected surfaces.
The IR990072 continues with "The manual also stated that the crew
should use ice accretion on the windscreen wiper as the method for
determining this amount."
The following is detailed in the airplane flight manual regarding
the ice accumulation during flight: "Monitor the accumulation of
ice. The windshield wiper arms give a visual cue of icing, although
airframe ice can be present without any build up on the wiper
arms." Hence, the windshield wiper arm is never used for estimating
the ice thickness on the wing leading edges as indicated in
IR990072. Also the wiper arm is a cue of ice build up, but never
the sole cue. The wing leading edges must always be monitored as
well.
Certification regulations
Regarding the certification basis the IR990072 details requirements
in FAR 25 for Stall Warning and Operation in Icing conditions. The
certification regulations applicable for a certain aircraft type is
the regulations at the time for establishment of the certification
basis for the particular type. For the Saab 340, JAR 25 Change 7
and FAR 25 Amendment 42 was used to form the certification basis.
The applicable FAR requirements were also adopted for certification
in Australia with some differences not related to operation in
icing conditions.
The JAR 25 Change 7 as well as FAR 25 Amendment 42 details the
requirements for aircraft performance and flying qualities for
clean aircraft conditions. The interpretation of the FAR/JAR 25
paragraphs 101 to 255 are applicable for clean conditions only. A
separate stall warning system was not required to fulfill the
intentions and to show compliance with paragraph FAR 25.1419. This
is also seen in other aircraft types developed prior to or at the
same time as the Saab 340. Despite it was formally not required,
Saab decided to publish minimum operating speeds in icing
conditions in the AFM as well as AOM in order to create natural
awareness of ice accretion effects on the stalling speeds among the
Saab operators.
As informed earlier to BASI, for the Saab 340 there are different
certification regulations between Transport Canada and the rest of
the world in the aspect of operation in icing conditions. To fully
meet the Canadian requirements stated in Transport Canada
Airworthiness Manual Advisory (AMA) 525/5-X, the stall warning and
stick pusher systems are modified in aircraft operating on the
Canadian register. The supplementary certification according to the
Canadian regulations was reviewed according to standard practices
by both LFV and Transport Canada.
As a conclusion, the certification process of the Saab 340 has been
adequately fulfilling all the intentions with the FAR and JAR
regulations applicable at the time for the type certificate.
Ice detection systems
The IR990072 states that if this particular aircraft would have
been equipped with the Saab ice detector option, the crew would
have been assisted in assessing the level of ice accretion on the
aircraft. The optional ice detector is related to the engine anti
ice system and warnings from the ice detector is surpressed by
selecting ENG A/1 ON as was the case in this particular incident.
Further the ice detector has the same viability as the windscreen
wiper, it is a cue of being in icing conditions, but not the sole
cue. Hence, you may be in icing conditions without experiencing a
warning from the ice detector.
Ice detectors which are actually measuring the amount of ice
accretion are currently not available certified for FARIJAR 25
aircraft with de-icing boots.
Related Incidents
IR990072 mentions some other incidents with the Saab 340 and
accidents with other aircraft types which are stated to be related
to this specific occurrence.
Regarding the first incident mentioned (23 September 1991), it
should be clarified that the crew was operating the aircraft in an
incorrect autopilot mode during the climb, which resulted in a
speed drop well below the minimum speed in icing conditions.
The second incident on 23 March 1994, needs the following comments.
Although the data provided to Saab for investigation was limited,
the analysis conducted showed that the airplane drag just prior to
stall was up to 70% higher than the total clean airplane drag. This
corresponds to more than 4 times the drag from 1/2 inch residual
ice on the protected parts and 3 inches of ice on the unprotected
parts. Hence the airplane was heavily iced up without any use of
the boot de-icing system by the crew and the resulting stalling
speed was significantly increased to about 129 KIAS. The
inappropriate autopilot mode used in this case reduced the speed
well below the minimum speeds for operation in icing conditions
during the climb.
The third mentioned incident (12 June 1994) relates to an icing
condition which was well outside the FAR 25 Appendix C specified
conditions. Hence, the resulting ice build-up was outside the
requirements in the icing certification regulations and this
specific incident is not related to the occurrence at Eildon
Weir.
The references to "anecdotal evidence" is found inappropriate for a
serious investigation.
The reference to the Roselawn accident with the ATR72 investigated
by NTSB is also found inappropriate as a related accident. The
ATR72 accident was of a different nature related to design
deficiencies compared to the Saab 340 occurrence as well as outside
the FAR 25 appendix C conditions used for certification.
Regarding the EMB 120 accident referenced to, there is a major
difference which is not recognized, namely that in the EMB 120
documentation (AFM and the corresponding AOM) there was no
information about minimum speeds in icing conditions. Such
information has always been provided in the Saab 340 manuals and
has been well known among the operators of the Saab 340.
Comments regarding the comparisons between this occurence at Eildon
Weir and the different NTSB recommendations after the EMB 120
accident are detailed in previous chapters.
The ice speed option for the Saab 340
As indicated in the IR990072 there is an ice speed option developed
for the Saab SF340A, 340B and B(WT) versions.
The ice speed modification developed for the Canadian operation
with some of the airliner versions of the Saab 340, is certified
towards Transport Canada only. It is currently not certified in any
other country. The functions of the ice speed modification
certified according to Transport Canada regulations are as
follows:
- The ice speed modification will decrease the angle of attack
triggering levels for stall warning and stall pusher system,
provided that the crew has selected ICE SPD on.
- The stalling speeds are developed using the most adverse ice
accretion defined by Transport Canada. The resulting minimum
operating speeds used for Canadian operation are therefore
significantly higher than what is used in the rest of the
world.
- For takeoff the lower angle of attack triggering levels are
inhibited in 6 minutes from lift off when the ICE SPD is selected
on, in order to prevent the crew from an undesired pusher
activation in an OEl takeoff. The use of ICE SPD is limited in
takeoff to a second segment procedure only with a prompt
acceleration directly to enroute climb speed (defined for icing
conditions). Consequently, other OEl takeoff procedures may be
required for icing conditions compared to non icing conditions
using the ice speed modification.
- In landing when the ICE SPD is selected on, the resulting
reference speeds are about 20 to 25 kts higher than for the clean
aircraft, which sometimes creates difficulties when landings are
made with a less critical ice accretion which is the most common
case. Also the required landing distances are significantly
longer.
Experience has shown that operational pilots dislike the ice speed
modification, because of the resulting high landing speeds and
there is a resistance to use it amongst the pilots.
SAAB POSITION RELATED TO THE INTERIM RECOMMENDATION IR990072
Saab does not agree with the interim recommendation to include the
ice speed modification worldwide to the fleet of Saab 340 based on
the above statements. The ice speed modification is available as an
option, but introduces difficulties in high landing speeds since it
is based on the most adverse ice accretion defined by the Transport
Canada for the Canadian certification. This system also requires
crew awareness of being in icing conditions since a manual
selection is required.
We are looking forward to receive your investigation results as to
why the crew allowed the airspeed to decrease 18 kts below the
value selected for the holding pattern and 9 knots below the
minimum speed in icing conditions according to the aircraft manuals
despite the buffeting and high pitch attitude prior to the stall.
With reference to your information we understand that this part is
still under investigation.
