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
EFFECTIVITY
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.
REASON
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-
BACKGROUND
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.
PROCEDURE
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.
ABNORMAL/EMERGENCY PROCEDURES
No change.
