Aviation safety issues and actions
Interim Recommendation issued to: SAAB Scania As
|Date issued:||18 December 1998|
|Safety action status:|
|Background:||Why this Interim Recommendation was developed|
The Bureau of Air Safety Investigation recommends that Saab
Aircraft AB note the circumstances of this occurrence and alert
SF340 operators accordingly.
The Bureau of Air Safety Investigation simultaneously issues related interim recommendations IR980269, IR980270, IR980271, IR980272 and IR90273 to Kendell Airlines, Hazelton Airlines, Macair, the Civil Aviation Safety Authority and Saab Aircraft AB respectively.
|Date issued:||18 February 1999|
|Response from:||SAAB Scania As|
|Action status:||Closed - Accepted|
Please find enclosed SAAB Aircraft AB response to your Air Safety Interim Recommendation IR980273. These AOM operation bulletin (one for 340A, 340B) have been released and will be distributed to the operator in the near future.
OPERATIONS BULLETIN NO. 56
Artificial Stall Warning in Icing Conditions
Applicable to all Saab 340 aircraft.
NOTE: With Mod. No. 2650 (ice speed system) installed and selected, the stall warning triggering level is changed to give stall warning at a higher speed.
To highlight the fact that the design of the artificial stall warning system does not always provide a stall warn before stall is encountered if there is ice on the wing-
Even a small amount of ice on the wing will reduce the lifting capability and increase the stall speed of the aircraft. The aircraft will stall at a lower angle of attack than for the normal clean (free of ice) case. Most artificial stall warning systems are designed to give an artificial stall warning (shaker and aural warning) and subsequent pusher at preset angles of attack for a clean wing. In the case of the SAAB 340 the artificial stall warning will activate approximately 8 knots before stall with a clean wing. The stall warning system has one trigger level, which is designed for a clean wing. This means that with ice on the wing, the aircraft may stall at an angle attack which is lower than the preset warning angle of attack and stall may be encountered before the artificial stall warning is activated.
With reference to the above, it is essential that the crew is aware of the adverse effects of ice on the aircraft The operational speeds shall be increased according to AFM (section 5) and AOM (section 2711) if ice is observed on the aircraft or if it is not certain there is no ice on the aircraft. The amount of ice allowed to build up shall be kept at a minimum.