What's a 'PX' switch?

[martinlest2 58]

Googling only gave me the vaguest idea.. What does a PX switch do on a turboprop (I encountered it in an (FS9!) Embraer 110)? How/when is it used/switched on/off?

 

(And a 'PY' switch?).

 

Thanks.

[Morrigan 90]

I can't give you a detailed response, but Py and Px are indicators of the air pressures coming from the compressor. Px is lower, Py upper side of compressor's bellow. By changing the pressures you can move the bellow up and down which would correlate to the fuel flow. For example, if Py increases the bellow moves downwards with the torque tube which would result in increased fuel flow. When Py pressure is decreased, the fuel flow would also decrease.

As far as I know, those measurements are connected to the Flight Control Unit to govern work of the turbines and prevent them from overspeeding.

Now to your actual question - I can only assume the Px/Py switches somehow correlate to that fuel system, but I have no idea how.

Unless of course, I am completely wrong on that matter

[Morrigan 90]

Too late to edit - I obviously meant Fuel Control Unit (not flight).

Anyhow, as a bonus - link to the AAIB's incident report involving the Py pressure. Look at the last paragraph of the repor.

https://assets.digital.cabinet-office.gov.uk/media/5422ed29ed915d1371000155/Embraer_EMB-110_P1_Bandeirante__G-OCSZ_12-95.pdf

 

And again, I might be completely wrong here 

[martinlest2 58]

Thanks - no, you could be right. But if so, I don't see why there is a switch in the cockpit to turn the Px on/off? (There are other features in the EMB-110 cockpit I have never seen before, like the 'Inertial Separators', though I managed to trace some information online about that).

 

EDIT: Are, I think I see. The Px switch is placed next to the pitot switch, so reading the last paragraph of the report you linked to, it makes sense. The Px is apparently a kind of heater to stop icing in the servo.

[Morrigan 90]

You mean next to the pitot heating switch? If so, it's likely it's a heating swtich to prevent icing in the fuel governing system. 

 

As for inertial separators, that's somewhat another anti-ice feature. It's working on pretty basic premise of having an additional outlet that would take care of disposal of anything with relatively high mass from the intake, before it actually gets to the engine. I think all PT6 engines have simialr feature, and PT6s are extremely popular (used not only in Embrarer 110 bu talso in PC-6/12, C208, B200, B1900 and much, much more. I think even some twin otters have those). 

[martinlest2 58]

Yes, that's what I understood of the inertial separators. :smile: