Attentive readers know that the air intake on a PT6 is near the back of the engine, but remember, that's a description only of the engine. The engine itself is a little thing. If it were a shell, I'm not sure I could squeeze myself inside it. It only weighs twice as much as me, and it's made of steel and titanium, while I'm made of squishy girl bits. The engine is housed inside a nacelle with a great big scooplike hole in the front, underneath the propeller, and that is where the engine air enters the cowling.
It is undesirable to have snow, ice or sand entering the engine, so each nacelle is equipped with an intake deflector, a hinged screen that drops down from the top inside the scoop at an angle, to block about 70% of the intake area. The effect is to deflect any solid particles downwards. Air still enters, and comes around the corner of the deflector and enters the engine, but solids can't make the corner and rattle out through a small door in the rear of the cowling. I was going to compare it to a mass spectrometer, but analogies work better when they reference things people are familiar with, like Q-tips and bacon. Intake deflectors have nothing whatsoever to do with Q-tips or bacon.
The air can negotiate the corner around the intake deflector, but there is a loss of pressure because the ram air pressure is reduced and the airflow is disturbed by the presence of the deflector screen. Extending the deflectors reduces engine torque by a pound or two. That's only a few percent, so it's tolerable. The deflectors are required during flight in visible moisture (i.e. rain, snow, mist or clouds) while the air temperature is below five degrees. Some operations even wire the deflectors in the permanently deployed position, to avoid maintenance problems with the complicated deployment and retraction system which I am about to describe.
The cockpit control for the intake deflectors consists of a 3-position toggle switch spring loaded to the centre OFF position. When the pilot pulls the switch down to the EXTEND position, that opens a solenoid in each nacelle, allowing bleed air pressure into the actuators for the intake deflectors. The bleed air pressure pushes the deflector plates down towards the extended position. The motion of the deflector plates pulls cables to open the previously mentioned small door at the back. After three to five seconds the deflector plates reach the fully extended position and engage with spring loaded levers that lock the plates in place, with pins. The pilot can release the switch and allow the bleed air valve solenoid to deenergize.
A pair of microswitches connected in series on each nacelle activate the deflector indicators, causing them to display EXT when the deflectors are locked in the extended position and the small door is open. The indicators are blank when the door is closed. Power for the indicators, and power to operate the bleed-air solenoid comes off the right DC bus bar.
When the pilot lifts the switch to the RETRACT position the solenoid valve is opened again, extending the deflector plate far enough to disengage it from the locking mechanism. sort of like how you have to pull down on a projector screen in order to roll it up again. The release lever electrical solenoid energizes, rotating a cam and disengaging the locking pins. Full retraction of the locking pins operates yet another microswitch, which deenergizes the bleed air valve solonoid and allows retraction springs and ram air pressure to push the deflectors back to the up position, closing the rear doors at the same time.
The pressure required to extend the deflectors against the force of the springs and ram air is quite high, so NG must be above 80% for successful extension.
Some early models didn't have the door in the back: I guess they just collected a pile of ice or sand inside the nacelles. Normal deflectors use a coarse wire screen, but for desert operations a finer screen may be used.