Jump to Content

Interim Recommendation issued to: Saab Aircraft AB

Recommendation details
Output No: IR19990072
Date issued: 03 June 1999
Safety action status:
Background: Why this Interim Recommendation was developed

Output text

The Bureau of Air Safety Investigation recommends that Saab Aircraft AB fit the ice-speed modification of the stall warning system to the worldwide fleet of Saab 340 aircraft, as a matter of priority.

As a result of the investigation into this occurrence, the Bureau simultaneously issues the following interim recommendations:

IR990073

The Bureau of Air Safety Investigation recommends that the Federal Aviation Administration note the circumstances surrounding this occurrence, and note the fact that the Bureau shares a number of concerns regarding aircraft certification procedures, particularly those involving flight in known icing conditions.

IR990074

The Bureau of Air Safety Investigation recommends that the Joint Airworthiness Authorities note the circumstances surrounding this occurrence, and note the fact that the Bureau shares a number of concerns regarding aircraft certification procedures, particularly those involving flight in known icing conditions.

IR990075

The Bureau of Air Safety Investigation recommends that Luftfartsverket note the circumstances surrounding this occurrence, and note the fact that the Bureau shares a number of concerns regarding aircraft certification procedures, particularly those involving flight in known icing conditions. The Bureau also recommends that Luftfartsverket as the initial certifying agency of the Saab 340 aircraft, review the certification aspects of the aircraft's stall warning system, particularly in icing conditions.

IR990076

The Bureau of Air Safety Investigation recommends that the Civil Aviation Safety Authority examine the circumstances surrounding this occurrence and take whatever steps it considers necessary to ensure the safety of the Saab 340 fleet operating within Australia.

Initial response
Date issued: 06 August 1999
Response from: Saab Aircraft AB
Action status: Monitor
Response text:

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.

Further correspondence
Date issued: 08 April 2005
Response from: Saab Aircraft AB
Response status: Closed - Accepted
Response text:

ATSB Comment: The following response relates to R20030183 but is equally applicable to IR19990072.

A series of meetings were held between the manufacturer, CASA, the Australian operators of the aircraft type and the ATSB in May 2004. Saab undertook to review the issues surrounding this and other icing incidents to the Saab 340 aircraft. The operators also undertook to review their procedures relating to flight in icing conditions.

Saab met with the ATSB and CASA in March 2005 and outlined their development of an ice advisory system for fitment to Saab 340 aircraft.

The system consisted of an ice detector and an advisory light that would illuminate and warn the crew if ice was present. They also planned to visit the Australian operators to brief them on the ice detection system and Saab's winter operation training program.

 
Share this page Comment
Last update 01 April 2011