Friday, July 01, 2005

Fuel Control Unit

John asked me a question about the fuel control unit:
Just curious, does the PT6 engine you're describing use P3 bleed air to modulate the fuel supplied fuel control unit? Or are the power levers directly connected to the the fuel metering?

Each power lever is mechanically connected to its respective fuel control unit and the primary governor. Bleed air does not participate in fuel metering.

Jimmy Little asked

Can the engine controller on something like the PT6 automatically detect the conditions caused by the stuck-open bleed valve and issue the appropriate warning? It would seem relatively easy to correlate fuel consumption, temperature, and such to detect this, or at least warn that it's an issue.

I need to point out that this is not a computer-controlled engine. It predates the invention of the microprocessor. So yes, there is temperature detection and yes, correlation between high NG, high T5 and torque lower than selected by the power levers might well indicate a stuck open bleed air valve, but this information isn't digitised. The computer that correlates the data is the pilot's brain, hence the need for all this training. (Not to say that the folks flying the computerized airplanes don't need training: they need to know "what it's doing now.")


Jimmy Little said...

Thanks -- I forgot about that computer (I clearly don't use mine enough...). I guess I tend to think that anything that's younger than me -- like the PT6 family -- will be digital and computer-controlled. Obviously not...

John said...

The fuel control unit in the Caravan's PT6 uses P3 bleed air to modulate the flow of fuel to the engine. So if you slam the power lever forward from a low Ng setting, the fuel flow will gradually increase as P3 pressure increases. Of course, a leak in the P3 system would render the regular power lever inoperative. That's where the emergency power lever comes in - it's directly connected to the fuel metering valve.

Both the normal and the emergency power levers in the Caravan are digitially controlled - you use your digits to move them! ;-)

Aviatrix said...

Fascinating. Do you move both emergency and regular power levers together, or do you have to switch over when a failure is recognized?

And digital: I like it. I'm going to telling lots of people about the fully optical-digital control panel in our fleet.

John said...

If there is a P3 failure, the engine just spools down regardless of the power lever position. The drill is FBI - Fuel selectors on, Boost pump on, Ignition on - then start diagnosing.

If it was an engine flame-out, FBI probably cured it. If the engine is running but with low Ng, move the power lever and see if Ng changes. If Ng is low but doesn't change, you've confirmed a P3 failure and the power lever is useless and you stow it.

Next, break the "witness wire" on the emergency power lever and move it out of detent oh so slowly so you don't toast the engine. Use the emergency power lever to set torque/ITT, but realize it won't have the dampening that the regular power lever has.

Anonymous said...

Normal FCU operation on a Caravan is through the power lever setting the N1 governor, which in turn regulates the fuel metering valve via a pneumatic bellows-based rate and fuel flow scheduling system to set the desired gas generator speed (and consequently, power output). A separate emergency power lever (used only when the normal power lever isn't working because of an internal fuel control failure) directly, mechanically positions the FCU fuel metering valve, with no overspeed or acceleration-deceleration rate protection. The emergency power lever feature is only found on a few PT6A-powered aircraft types.