What is the nature of feathering in an ATR type aircraft?

Question

If the engines fail for some reason midflight, does the ATR aircraft automatically feather the affected engine? In rare cases if both engines fail, does the automatic feathering happen for both propellers? If automatic feathering happens (if there’s an automatic behaviour), is the pilot notified in any way?

Answer 1

Depends when the power loss happens and the status of the opposite engine. Airplanes like ATRs and Dash 8s have Autofeather Systems, but they are only active during takeoff and climb and are inactive during cruise.

When the system is armed for takeoff, Autofeather happens when one engine's torque is below some threshold, say 20%-25%, and the other engine is above some threshold, say 50%. If the torque is lost, a solenoid valve in the propeller control system opens to dump fine pitch oil pressure in the prop of the low torque engine and force it to the feather position.

Once the system is disarmed during the climbout, an engine failure will not trigger Autofeather and the crew has to select START/FEATHER on the propeller control levers to feather the props. If that doesn't work, there is normally a backup "Alternate Feather" system that powers up a dedicated electric oil pump in each engine (powered off the battery) to drive the prop to feather even if there is no oil pressure remaining.

Autofeather systems apply another risk level, that of feathering spontaneously when you don't want them to, to the airplane, so they are only armed for the most critical performance case, takeoff and initial climb. The rest of the time, you want that possibility excluded because an engine failure without immediate feathering is not critical to flight, and the crew following its normal procedures to secure the failed engine will feather the engine the propeller then.

If both engines fail at the same time, Autofeather, even if armed, won't activate because you need the power differential between the two engines, one high and one low, for it to trigger when armed. In any case, the double engine failure case is not accounted for in the design, the probability of simultaneous internal mechanical failures being too low to have to be accounted for in the mathematically derived risk analysis that drives system architecture and design (actual double engine failure cases are almost always due to an external cause, like fuel exhaustion or some weather phenomenon).

Answer 2

Short answer: If the ATPCS is armed, it can auto-feather upto one engine. Both engines will never be auto-feathered.

(Disclaimer: This answer is based on the ATR 72 FCOM. That being said, the systems on ATR 42 would work in a similar way.)

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ATPCS

In case of engine failure, the Automatic Takeoff Power Control System feathers the propeller of failed engine and increases power on the remaining engine; doing so is crucial in order to continue the takeoff.

However, this system only works during the takeoff and initial climb. The following are the arming conditions for ATPCS:

• Power management selector at TO detent
• ATPCS push button - ON
• Both power levels above 49°
• Both torques above 46%.

Provided that all arming conditions are met, the arming of ATPCS will be denoted by an $\small \color{green} {ARM}$ light on the engine control panel.

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Engine failure during takeoff

The ATPCS provides facility to perform takeoffs with a derated power setting of 90% (100% RPM and 90% torque). If an engine fails with ATPCS armed, the following changes will take place:

On ground: An uptrim will be immediately applied on the remaining engine, which increases it's power by 10% (upto a maximum of 100%). Following a delay of 2.15 seconds, the auto-feather sequence will initiate, which includes the following items:

• A valve lift solenoid will be energized, which enables the flow of feathering oil

• The electric feathering pump will be energized

• The failed engine will be feathered

• Auto-feather will be inhibited on the remaining engine

• The ATPCS $\small \color{green} {ARM}$ light extinguishes, indicating the completion of ATPCS sequence.

In flight: The feathering sequence remains same; the only difference is that 10% uptrim will not be applied in flight.

Reason for 2.15 second delay: This is in case the takeoff is rejected - the windmilling propeller will assist in slowing down the aircraft. The ATPCS does not know if the aircraft has/hasn't exceeded $V_1$, so it will always apply this delay - even in flight. If the power levers are retarded during this time period, the ATPCS will automatically disarm.

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Engine failure after takeoff

During the initial climb-out, the ATPCS remains armed, and so the failed engine will feather normally. The uptrim will not be applied however, and so the pilot must manually move the power levers to the ramp.

However, soon after takeoff (at the acceleration altitude), the power management selector will be set to CLB detent, which disarms the ATPCS. Beyond this point, should an engine failure occur at any point in flight, the auto-feather will not occur, and the propeller of the failed engine will be windmilling.

In fact, the governer of the failed engine fine-out the propeller in an attempt to maintain the selected RPM, which will further increase the drag. In this case, the pilot must manually move the appropriate condition lever to FTR - to feather the propeller, and then to FUEL S.O. - to stop fuel supply to the failed engine.

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Duel engine failure

With ATPCS armed: There is a possibility that one engine will auto-feather - that is, if both engines do not fail simultaneously. Provided that the torque of one engine drops below 18% while that of the other is still above 46%, the ATPCS will feather the propeller with lower torque. And again, this is possible only during takeoff and initial climb - when the ATPCS is armed.

If both engines were to fail simultaneously, none will auto-feather, since the ATPCS would automatically disarm as soon as both torques drop below 46%. The pilot must manually feather the props by moving condition levers to FTR detent.

With ATPCS disarmed: Now the auto-feather is completely disabled - none of the engines can auto-feather. Should one or both engines fail, the pilot must manually feather them.

is the pilot notified in any way?

The notification that feathering is complete is that the RPM of affected engine will rapidly drop down to zero. This is also indicated by an "auto-feather" text below the RPM indicator on the Engine Warning Display (EWD). A similar notification will be given for uptrim, displayed above the torque indicator.

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 _{Note: Please refer to ATR 72 FCOM for more details on ATPCS and other ATR systems.}