Fuel Controls Are Smarter Than Pilots

[SASless]

Had a chat the other night about digital fuel controls and the possibilities modern electronics provides that never existed before. A comment was made about the ability for some digital fuel controls to determine when the need for Maximum engine power (to the point of destroying an engine) outweighed obeying normal engine limits or OEI limits as normally used.

That seems a bit amazing to me and when I did some research....I found there exists a US patent for such a system.

That begs some questions....which aircraft/engines have that ability? How do we as pilots know the fuel control has usurped our command authority and gone to this "supercontingency power setting" and how does this affect our maintenance costs, standards, defect reporting and such?

From my experience teaching in a Simulator....I know some pilots will quite happily fly into the ground because they refuse to demand performance from the aircraft/engine that exceeds "normal" limitations.

Thus it appears the engineers can design a fuel control that is smarter than some pilots in that regard. I just wonder how the electron brained thing knows when to pull its palace coup and pull the guts out of an engine?

[212man]

I think we have touched on this before and it is an area that seems to lead to many misconceptions.

In fact, FADEC controlled engines do not limit engine power any more than a normal Hydro/Mechanical FCU (or Pneumatic/mechanical). Think about it: how many topping checks have you done in your career? What does a topping check demonstrate? Come the day you wish to avoid hiting the ground and want to pull the collective under your armpit, how much power will a conventionally governed engine give you? The 2.5 minute rating, and no more! Unless you revert to manual control, but I think that's a little far fetched and not really within the scope of the argument.

FADECs try and respect gear box and engine limitations as best they can. They do, though, recognise conditions where the pilot is obviously demanding more power than is normally acceptable (Nr drooping, rate of change of collective pitch etc) and will then normally revert to 'blow away' power (Sikorsky expression). This is usually the single engine limits being available when twin engine. This is exactly the same as a normally governed engine. Actually, it is probably beter; FADECs' precise fuel scheduling and modern materials use have allowed higher ratings called super contingency (normally a 30 second limit) to be used which offer more power than a standard 2 or 2.5 minute limit.

Another misconception is that the time limits are applied by the FADEC; they are not! If you want to pull the 30 second rating for 3 minutes, you can. You will be warned in some way (caption or flashing light etc) but the FADEC will not overide you.

Of course, we could revert to good old fashioned designs like the 212 where the FCU starts to bleed P3 air off to limit the torque at 104.3% to stop you damaging the gear box (regardless of what damage is done to the airframe and your little pink bodies!)

I think this topic shold be included in the 'Urban Myths' that Nick started a while ago. FADECs do NOT overide the pilot in any way more limiting than a conventional governor.:ok:

[Hilife]

SASless, I cannot say that this applies across the board, but as I understood it, a modern quality FADEC works by monitoring a multitude of inputs and one of these is ‘rate of change’ of the input by the crew. In normal use, the FADEC should protect the engines from overtemps/overtorques/overspeeds etc.

However, in the event that a life threatening situation occurs and you snatch the collective up and hook it on to the hat rack, the FADEC software senses the deliberate and very high ‘rate of change’ and the software should override all limitations and provide the maximum emergency power that the engine is capable of providing, even if this results in buggering the engine.

[ATN]

If the collective is smoothly and gently pulled up to the ceiling, is that detected as a high rate of change ? If not then what happens ? Does the metering valve open no more than max N1/torque allowed for the air density conditions with ensuing RRPM decay or does it try to maintain RRPM at any cost ?
I understand that limits are calculated only but can be overriden and recorded.
Hitting the 30s limit (OEI HI) on the Arriel 2C means engine change.

ATN

[Shawn Coyle]

I hate to disagree with 212man, but some Digital Engine Controls will limit the amount of power.
It depends more on the airframe manufacturer than the engine manufacturer, it seems. One engine fitted to several airframes has a torque limiter on one machine (110% per engine when both are working) but not on another. The torque limiter in this case merely limits fuel flow. For engines with 30 second ratings, most DEC will also limit the fuel flow when the first 30" limit is reached. One manufacturer put in a limit over-ride to go above the 30" limit on a one-time save-your-life basis.
Some hydro-mechanical fuel controls have limiting on N1, so this is not a completely new idea. What is new is that TOT and torque can now become limiting items as well.
It would be nice to have a 'soft' limit at takeoff power, so you could pull to that value without having to sort out which of several limits it was, and when reached, it drooped the rotor RPM as it prevented you exceeding the limit.
Since the computer records everything, what I would like to see is some logic that says "If we limit at takeoff power and droop the rotor, and the pilot is still pulling up on the collective when the rotor RPM reaches the minimum power on rotor speed, drop all the takeoff limits and give him what you can safely give him, but record it all". At least you might probably be there to defend yourself if you had to use the power.

[Hilife]

ATN

Firstly, not all FADEC software logic is the same from manufacturer to manufacturer and for the point of argument, let’s forget twist throttle systems and stick to a modern top end FADEC system that uses position transducers.

In normal operation, if the collective is smoothly and gently (low rate of collective pitch change CLP) pulled up then the FADEC would schedule fuel flow through the FCU accordingly whilst governing NR.

If the collective pitch change (CLP) is slowly increased still further (even to the ceiling if that were possible), then the FADEC will continue to provide regulated fuel flow until a limitation is reached, i.e. Ng / ITT / Tq, at which point the FADEC software and hardware protection systems should prevent exceedence of any of these limitations by reducing or even stopping fuel flow through the FCU metering valve.

Note that the FADEC should allow transients above these limits during normal accel/decel operation.

Forgetting OEI operation, If say the pilot having reached max Tq in an emergency kept pulling up on the collective, I believe that some FADEC systems will sense this additional demand (High rate of collective pitch change CLP) for power and override the limitations set into its logic and keep delivering.

The theory being that if you are that desperate for additional power, safe management of the engine is not an immediate priority.

[SASless]

The theory being that if you are that desperate for additional power, safe management of the engine is not an immediate priority.


My signature is "Limitations are for normal operations, Stupid!"

[paco]

When I did my 407 training I was told that originally the FADEC's way of dealing with excessive fuel flow was to cut the engine - that's about as limiting as it can get! Fortunately, they seem to have kicked the programmer's butt

Phil