Originally Posted by
Tu.114
A few words on autofeather systems, again from a DH8 perspective (that comes with the same PW120 series engines the ATR uses as well).
Generally, they are only armed for takeoff, as this is when an unfeathered propeller will have the most performance impact (and yes, an engine failure during go around will consequently by default require manual feathering). Arming conditions are (roughly) the following:
- Selection by flight crew
- Both power levers above a certain angle
- Both torques above a certain threshold.
That fulfilled, the system will feather a propeller in case of one engines torque dropping below a defined threshold for a defined time; on the Dash 8-400 it was 25% for 3 seconds IIRC (it has been a while). This will trigger:
- a feather command to the affected propeller control unit, also involving the alternate feather pump,
- most importantly, a cutout signal to the other, good engine to inhibit autofeathering on that side.
So even if autofeather should fire by error on one engine, it would take additional system malfunctions for it to affect both engines at once. Even in the previously quoted case of the Taiwanese ATR, I understand that the malfunction affected only one of the engines and the systemic safety barriers worked, keeping the fault manageable in principle.
If indeed both propellers ended up feathered on the accident flight, this would most likely not be due to autofeather imho. Rather, some flight crew action would have to have been involved to achieve this. The other ways of feathering a propeller would be pushing the associated alternate feather push button (activating an oil pump drawing engine oil from a dedicated reservoir to force the prop into feather) or pulling the condition lever back to low RPM, lifting it out of the reached detent and further pulling it to start/feather position. But none of this seems appropriate in final approach; as has been mentioned, the condition levers are either (type and situation dependent) left in low RPM or pushed forward into max RPM position.
The reason that the DHC 8 autofeather was switched off after climb power was set, was due to malfunctions in the dual channel system that produced intermittent uptrim as the system got transient engine fail indications. My understanding is that it was intended to be on during flight but SOP changed after the fault was found, which I think went unchanged?
What is SOP on the ATR?