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The Bombardier Dash-8 Q400 FCOM says (emphasis mine):

The autofeather system provides automatically initiated propeller feathering following an engine failure during take-off, and power uptrim of the operating engine (Figure 12.22-5). Autofeather is selected on for take off only, using the AUTOFEATHER switchlight on the engine instrument panel.

(source: smartcockpit.com)

It only says autofeather should be used for take off only, but fails to explain why. I would assume autofeather would also be useful when landing. The uptrim (automatic power increase for remaining engine) might not, but it is independent anyway:

Uptrim is triggered (regardless of autofeather selection) [...]

Why should autofeather not be selected during landing?

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3 Answers 3

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It's because the critical case that autofeather caters to is engine failure right after V1, on takeoff, where the normal identify/feather drill uses up precious time. An engine failure on a missed approach or go-around starts well above the low energy state of an engine failure after V1 and there is much more time and energy margin to do the normal feathering procedure.

The system is quite sensitive and there are risks associated with having the autofeather system armed and "ready to fire" you might say. Mainly, having it put a prop in feather when you don't want it to. Or even BOTH props in feather. That risk outweighs the benefit of having it armed outside of the take-off case.

It's a bit like having a policy of taking a firearm off SAFE only when there is a critical need to able to fire instantly when clearing a house.

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    $\begingroup$ Great answer for turboprops in general, but is there any reason why some aircraft (e.g. King Air) recommend arming autofeather for landing, and some don't? $\endgroup$
    – Bianfable
    Commented Dec 20, 2018 at 10:32
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    $\begingroup$ Afraid not. It could be that the go-around performance for that particular airplane tips the risk balance in favor of having it available. $\endgroup$
    – John K
    Commented Dec 20, 2018 at 12:55
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There is no point to use autofeather when there is no great benefit of that because:

Multiplying the phases where you rely on it, is multiplying the risk to forget it armed, and this feature being forgotten has been a source of incidents that could have turned into dramatic accidents.

Please refer to the following very interesting Website:

https://assets.publishing.service.gov.uk/media/5422ed34e5274a1317000179/dft_avsafety_pdf_500839.pdf

You may read:

The commander's decision to delay the selection of climb power was the initiating event in the sequence which started with the failure of the first officer to disarm the autofeather systemand allowed a train of events to occur which could have had more serious consequences. It is clear that had the standard practice of 'autofeather disarm - select climb power' occurred at the normal point, this incident would not have happened

Clearly the objective of limiting the use of this feature, is to reduce human factors consequences, while keeping its advantage in the most critical phase.

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It could also be that Bombardier decided it wasn’t worth the cost of certification. An engine failure immediately after or during go around was unlikely, just like moments after V1. However, the certifying agencies do not give them any choice for the later.

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