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Fadec Details

Sun Nov 03, 2002 4:23 pm

Ok I have a brief understanding of FADEC, but in a previous post I learnt that Qantas has only one of their 763ERs with FADEC and the rest without. I wondered what the differences (in the engine or in the flight deck) are between FADEC and non-FADEC planes.
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RE: Fadec Details

Sun Nov 03, 2002 10:15 pm

The analogy I would use when comparing FADEC and non-FADEC airplanes is to compare fly-by-wire with non-fly-by-wire aircraft. All FADEC really is fly-by-wire for engines. Instead of a control cable running from the throttle quad you have a wire bundle.
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RE: Fadec Details

Sun Nov 03, 2002 10:53 pm

The only thing that is noticeable in the flightdeck from a FADEC controlled engine to a non-FADEC is that the throttles travel ALOT smoother.
On the engine it is a whole different story. There are no cables to the fuel control. Everything is done with electrics. Fuel Metering Valve is operated electrically & the fuel shutoff valve inside the fuel control. Flightdeck indications are pretty similar.

RE: Fadec Details

Mon Nov 04, 2002 1:11 am

I may be way out of my league here, the last time I had anything to do with airliner engines was in the late Jurassic when I "helped out" flying B727's. However, I have flown bizjets with FADEC equipped engines. My comments are based on my bizjet experiences and from a pilot's, not mechanic's point of view.

Prior to FADEC, engine power management was a lot more complicated. You would set the appropriate thrust settings using EPR or N1 settings depending upon the engine. These settings would be obtained from either "Tab Data" or "Power Charts". There always seemed to be a bit of "fine tuning" required to get the power just where you wanted it. I remember setting takeoff power in the 727, with a matched set of engines it wasn't to bad, just 2 EPRs to consider - one for the pod engines and one for the center engine. If you happened to have an airplane with a mismatched #1 or #3 engine then you could have three different EPR targets to work with. One for the #1 engine, one for the #3 engine and yet another one for the #2 engine. It wasn't a big deal and you got used to it.

As the engine control sophistication increased the workload decreased. The FADEC now computes the appropriate power setting for each phase of flight. It's kind of like Ron Popeil's roteisseries (You know, the ones that you always see advertised on TV.) "You just set it and forget it." (I'm sorry about that.) For takeoff you place the power levers in the takeoff detent and the computers do all of the rest. Same thing for climb and cruise. We look up the power settings to verify what the computer is doing, but for all intents and purposes, it just a matter of selecting the appropriate detent and hanging on.

(Now that I think about it, the jet engine / rotisserie analogy isn't that bad. They both rotate, they both get real hot, and if you put a bird in either one of them it will get cooked.)

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RE: Fadec Details

Mon Nov 04, 2002 1:39 am

FADEC is not just "FBW for engines". The "Full Authority" term means that you have full control of the engine configuration, not just fuel scheduling. You can vary the angle of the guide vanes throughout the engine to optimize efficiency. You can also automatically regulate maximum operating parameters. When you set an engine power, you don't have to continually tweak it to avoide exceedances and maintain power.

The major advantage that FADEC engines have is that they are able to use the full power range of the engine. This allows you to install huge engines that are capable of ripping the wings off on the ground, but are derated. As altitude is gained, fuel and power parameters and engine configuration can be automatically modified to keep trimming the engine power to keep it constant.

So, instead of constantly losing power as you climb, you maintain power up to a relatively high altitude. ie: an engine rated at 6000 HP at ISA can maintain 4000 HP from sea level to 25,000 feet (For example).

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RE: Fadec Details

Tue Nov 05, 2002 4:25 pm

Nice analogy Jetguy!

FBW for engines, yeah... are there any differences in the actual engine or is is just the computer that controls it?
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RE: Fadec Details

Tue Nov 05, 2002 10:02 pm

You can vary the angle of the guide vanes throughout the engine to optimize efficiency - this is not peculiar to FADEC engines even the agricultural JT9 did that

The major advantage that FADEC engines have is that they are able to use the full power range of the engine. This allows you to install huge engines that are capable of ripping the wings off on the ground, but are derated - Wrong, an engine derated to a particular thrust a SL will also be limited to that thrust at 40 000Ft.

RE: Fadec Details

Tue Nov 05, 2002 11:39 pm

You techno-types may eat me for lunch for this post – this is a very technical subject and I don’t want to over simplify it but; hey, that’s never stopped me before.

The differences between “Flat-Rating” and “De-Rating” engines can be confusing. De-rating an engine means arbitrarily assigning a lesser output to an engine than it was designed to have. There are many ways to achieve the power output reduction – mechanically (for example a different fuel computer) or “on paper” (charts and graphs which limit output). As VC-10 stated, “an engine derated to a particular thrust a SL will also be limited to that thrust at 40 000Ft." In other words, when they take away the power, you don’t get it back. There are many reasons to install de-rated engines. One of the major reasons is that they “loaf” – they’re operating at a certain reduced percentage of their design capability. This usually makes for increased engine life. Back about 30+ years ago, a race team (I’ve forgotten the driver’s name - I want to say it was A.J. Foyt) installed a P&W PT-6 in an Indy car and proceeded to eat everyone’s lunch. If it weren’t for some bad luck, he would have won the Indy 500 the first time out with it. The following year the race officials forced them to de-rate the engines – by adding restrictor plates to the air inlet – to the point that they lost all of their previous advantage. That’s de-rating.

Now, for flat-rating…
Generally speaking, it must be remembered that (in very simplistic terms) turbine engines are not supercharged, but rather normally aspirated - in other words, they lose power with altitude just like a Cessna 152. The percentage of N1 (on most turbofans) or EPR (many turbojets) required to obtain the engine's full rated thrust will vary significantly depending upon airport elevation and outside air temperature. For example, on the aircraft that I fly, on a cool day at a sea level airport the engines will develop their maximum rated thrust with an N1 somewhere in the upper 80's say for example 88.7%. Go to a higher elevation airport on a warm day and the N1 will be higher, for example 93.4%. (As I type this, I'm looking at the Static Takeoff Thrust Setting Chart for our aircraft. Depending on the airport elevation and outside air temperature, the N1 settings vary from a low of 84.2% to a high of 96.1%.) These numbers will, of course, vary from engine to engine, but you get my point. On most older generation engines, the flight crew is required to come up with a takeoff power setting from a set of charts or tables. In later generation engines with DEECs (Digital Electronic Engine Controllers) or FADECs, the pilots only have to set the power levers into the takeoff detent and monitor things while the computer takes care of the rest.

Just to make things a bit more interesting, some aircraft have larger engines installed than they were designed for. These engines are "Flat-Rated" back down to what the airframe was designed to handle. In other words, say for example, an airplane was designed to use 10,000 LB thrust engines, the aircraft designers might specify 12,000 LB thrust engines and limit their thrust to 10,000 LBS. Why would they want to do this? Simple, remember that turbine engines are "normally aspirated" and start loosing power the moment they start to climb. By using a larger engine, the aircraft can operate at higher altitudes or temperatures before it runs out of power. The engine never produces more than the “airframe-rated” thrust (in this example 10,000 lbs), it’s just able to do it to a higher altitude.

(Gee, I hope I said all of this right - it seems awkward. Oh well, there are others who are better qualified than me to clarify this.)

(On many turbojet aircraft, flight crews are also allowed to perform Reduced Power Takeoffs. This method would also result in reduced N1 or EPR indications. There are several reasons why a flight crew might want to do this and a couple of methods that they can use to do it. I don't have the time to go into this in any detail now; but if you're interested I recommend that you search the archives, I'm sure this topic has been discussed before.)

Turboprop engines are similar, only instead of N1 or EPR, they usually measure their power output in Percent Torque. For those guys it's a bit simpler, they simply advance the power levers until the engines reach either their torque limit or their temperature limit. Typically, with flat-rated engines, they will "torque" out at lower altitudes, then as the aircraft climbs higher they "temp" out as the max operating temperatures become limiting.

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RE: Fadec Details

Wed Nov 06, 2002 2:58 am


Good post. I should have used the term "flat-rated" in my explaination.
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RE: Fadec Details

Wed Nov 06, 2002 7:07 pm

Thanks for your replies - very interesting
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RE: Fadec Details

Thu Nov 07, 2002 12:37 am

Further, you don't need FADEC to derate or flat-rate an engine (even though it does make the job easier, can't recall any non-FADEC aircraft which allows derating at the moment. Admittedly, the number of types I know to such a level of detail is very limited) - it is perfectly doable with just about any old fuel scheduler.

FADEC does take out a lot of the things you have to worry about in engines though. It looks at your PLA, goes "hey, he wants 86% power!" and does whatever it takes to give you 86% power. Push the power levers as fast as you like without worrying about if the engine keeps up. Push the power levers to the stop and you get 100% power - with traditional fuel scheduling, you had to find the right PLA yourself (anyone think of an exception?). No more staggering of power levers to even out the thrust among the engines and no more badly rigged power quadrants (FADEC also makes the always appreciated job of rigging the power levers superfluous).

FADEC can also make more efficient use of variable guide vanes and all those other nifty features jet engines have these days to increase efficiency, compensating for a larger number of variables than hydro-mechanical schedulers could ever aim for. It is, basically, a large step towards care-free engine management. You just tell your engine what you want it to do and it does it, to the best of its ability. Hot start, hung start, pumping - the little FADEC gremlins take care of it and you never need to worry about the internal workings of the engine (although most pilots will still worry - after all, the engines are quite important parts of the vessel they have strapped to their backsides).

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RE: Fadec Details

Thu Nov 07, 2002 2:02 am

Flat rating is the designed maximum thrust the engine will develop, up to a specified ambient temperature.

Usually you see, "flat rated to so much thrust to this temperature."

As ambient temperature increases, the N1 will increase, thus maintaining the maximum rated thrust. And as N1 rises, with it, EGT.

When ambient temperature increases to and beyond a specified value, EGT will remain constant and N1 will come down and with it, thrust.

EGT is the primary limiting factor. Theoretically, thrust can be maintained beyond the specified flat rating temp, but that would mean further increasing N1 speed and thus EGT beyond designed limits.

The flat in flat rating comes from a graph representing engine thrust.
A flat (horizontal) line represents the thrust until intersects the flat rated temperature than begins its slope downwards.
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