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Old 23rd Apr 2012, 16:33   #1 (permalink)
Join Date: Apr 2008
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Posts: 1,285
Saab 340 incident report clarification needed

Just finished reading a runway excursion report and I need abit of clarification. It says one of the propellers auto-coarsened at touchdown due to an intentional power lever split for a crosswind landing.

Is auto-coarsening an autofeather situation?

Autocoarsen Activation and Its Timing
(1) Based on the descriptions in 2.13.2 ‡@, it is considered probable that the actual angles of the power levers between 11:26:13 and 11:26:14 met the autocoarsen activation condition (PLA > 64 degrees).

(2) During the period between 11:26:13 and 11:26:14, the left engine torque was below 50% while the right engine torque was above 50%. At 11:26:13, the torque differential between the engines was 29.1% although at 11:26:14 it was 23.2%, a level slightly below the autocoarsen activation threshold. It is therefore considered probable that the torque differential met the autocoarsen activation condition in this 11:26:13 to 11:26:14 period.

(3) In the period between 11:26:12 and 11:26:14, the right engine P3 rose above 150 psi while the left engine P3 remained below 145 psi. It is therefore considered probable that the P3 sensor switch trip threshold (i.e., 145 psi maximum) for pressure rising condition described in 2.12 (2) was met.

As described above, it is considered highly probable that around the period between 11:26:13 and 11:26:14, the Aircraftfs left engine met the autocoarsen activation conditions in HIGH POWER mode. This is also supported by a sharp rise in torque and a drop in propeller speed of the left engine in addition to a sound suggesting decreased propeller speed, which were recorded for this period by the DFDR and CVR, respectively. It is therefore considered highly probable that the left propeller was brought to the coarsen pitch at that timing.
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