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Interim Recommendation issued to: Luftfartsverket

Recommendation details
Output No: IR19990075
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 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.

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

IR990072

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.

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.

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: 07 July 1999
Response from: Luftfartsverket
Action status: Monitor
Response text:

We have studied the BASI air safety interim recommendations as notified to us in ref letter. Before deciding upon any mandatory continued airworthiness action, if any, we have addressed a number of questions to Saab Aircraft AB for their consideration and reply to us before Aug 2, 1999. Copy of our letter dated June 15, 1999 is enclosed for your information.

Please regard this information as an interim reply to your recommendations.
Further actions will depend upon Saab reply and our final position will not be available within 60 days as requested but hopefully late August.

SAAB 340. Request for investigation of modifications or procedures to reduce the probability of stall in icing conditions

Ref.: BASI, Australia, "Release for Air Safety Interim Recommendation IR990072, IR990073, IR990074, IR990075 and IR990076".

LFV have studied the referenced recommendations, which you also have been informed of. Although that SAAB 340 has been correctly certificated according to the requirements and their interpretations, applicable at the time, the concerns of BASI seems well founded. It is evident from many other cases that pilots may not follow correct procedures during high work load, when they are subjected to unusual situations, when procedures in or characteristics of another aircraft they have been flying earlier are slightly different, when they are complacent, etc. It seems obvious that the pilots involved in the Australian incident (to VH-LPI on Nov 11, 1998) did not follow the AFM Normal Procedure: If it is not certain there is no ice accumulation on the aircraft; or if ice accumulation is observed on the aircraft, maintain an airspeed .... not less than 1.4 times the stall speed in any configuration." In this case the pilots apparently knew they were in icing conditions.

Normally we are hesitant to require retroactive actions on one type of aircraft that should also apply to other aircraft of similar design. Such actions should be coordinated within JAA and possibly with other authorities. We understand that such actions in the icing area already are ongoing through the Ice Protection Harmonization Working Group, IPHWG, with participation of JAA, FAA, Transport Canada, and industry representatives.

However, notwithstanding that we believe that an action normally should be coordinated within JAA to include all aircraft of similar design and that the SAAB 340 AFM procedures in icing conditions recently have been changed (i.e. a lesser amount of ice accreted before turning on the de-icing boots, and to consider using continuous mode to reduce pilot work load), we hereby request Saab to provide the answers to the following questions. Our request is based on the fact that at least one modification that possibly might reduce the likelihood of a repeat occurrence of the Australian incident already is available as an option, i.e. the "ice speed button".

1. What modifications to Saab 340 or additional procedure changes in order to reduce the probability of an icing incident similar to that encountered by VH-LPI, taking into consideration pilot work load and possible pilot situation misjudgement, could be rather easily retrofitted to the aircraft? The "ice speed button" is assumed to be one of them.
2. Do Saab believe that any of the studied modifications or procedure changes might reduce the probability of an icing incident similar to that encountered by VH-LPI? Could the modifications or procedure changes still result in the same or other unwanted effects? If so, what is the conclusion?
3. What modification or procedure change, if any, of those practicable, would according to Saab constitute the highest degree of improved safety, and the highest cost-effectiveness respectively? Cost of retrofit to Saab and operators?

Further correspondence
Date issued: 19 August 1999
Response from: Luftfartsverket
Response status: Open
Response text:

I hope you received my letter dated June 29, 1999 with copy of a letter to Saab dated June 15, 1999. We have now received Saab answer to our questions and are in the process of reviewing them together with our Flight Operations section.

You have also received Saab comments on recommendation IR990072. LFV supports in principle these Saab comments. (Letter dated 3 Aug 1999). Our goal is to take a decision with regard to the line of action mid-September.

Further correspondence
Date issued: 24 September 1999
Response from: Luftfartsverket
Response status: Closed - Not Accepted
Response text:

BASI Air safety interim recommendation IR9900765 has now been considered by LFV and we have reached the following position.

Although the "ice speed button", if used, probably would have contributed to preventing the stall incident on 11 Nov 1998, a mandatory requirement of implementation of this modification, as recommended in IR990076, is not supported by LFV. There are operational drawbacks as shown in Saab Aircraft AB letter 72DSS0957 to BASI, dated 3 Aug 1999. This may result in the crew not using the button in icing conditions assumed to be light. Then, if they do not keep the AFM normal procedure in mind ("If it is not certain there is no ice accumulation on the aircraft, or if ice accumulation is observed on the aircraft, maintain an airspeed of not less than VREF + 10 knots for landing and 1.4 times the stall speed in any configuration."), this may still result in stall incidents.

As you know, FAA NPRM 99-NM-148-AD proposes activation of the deicing boots at first sign of ice build up on the airplane. This eliminates the need for the crew to make judgements on when to activate the boots and at the same time reduces the amount of ice on the aircraft. This will reduce the likelihood of non-observed speed reductions due to drag increase and stall without warning. This in combination with the already existing AFM text cited above, and pilot understanding of the reasons behind, would significantly reduce the probability of future stall incidents in icing conditions with the SAAB 340, we believe. The ice speed button, although also an acceptable approach, does not appear to be the ultimate remedy. Basic pilot knowledge and skills will always need to be applied.

Increased stall speed due to airframe icing is intrinsic to all aircraft. Any retroactive requirements that would significantly increase safety for all types of aircraft in icing conditions should therefore be coordinated world wide and not just be applied to one type. This is another reason we do not support the "ice speed button" as a retroactive requirement for the SAAB 340. On the other hand, we welcome the FAA NPRM, which appears to solve a problem for all modem aircraft with de-icing boots with limited cost impact, mainly higher maintenance costs due to more frequent use of the boots.

A change implementing the FAA proposal, based on the new knowledge that ice bridging does not occur on modem type boots, will be introduced in all LFV Approved Airplane Flight Manuals (AFM) and in the manufacturers Airplane Operation Manuals shortly. On the FAA side this change will most probably be addressed as an Airworthiness Directive which in the U.S legal system mandates the procedural change. Under the Swedish regulations operators are required to apply the latest version of the AFM.

 
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Last update 01 April 2011