Jet engines, twin spool - governing
 

Hi all,

I understand that for twin spool engines, the FCU works to maintain the N2 speed to be in accord with the power lever angle (PLA).

The other night, upon encountering icing conditions in our Cessna Citation Bravo (PW530 engines), and about to activate engine anti-icing, I confidently informed my FO that, upon switching on the engine anti-icing whilst leaving the PLA untouched, the N2 will remain exactly the same, the ITT will increase, and the N1 will either increase or decrease a little.

But, upon activating the engine anti-icing, the N1 remained constant and the N2 and ITT both increased slightly. The engine synchroniser was off throughout.

Why did we not see what I had confidently predicted we would see?

I don't know that engine, but, assuming it is FADEC controlled, I think that action of selecting Anti-Ice on would tell the FADEC to increase the fuel slightly to compensate for the loss of power due to the extra bleed load. Hence N2, ITT and Fuel Flow go up to maintain the N1.

Yes. I would agree with that.
Slightly confused about the thread title - Governing.
Governing suggests that the engine is being controlled by one of the Governors ( speed or temperature or pressure) as opposed by being on FADEC control.

There's a difference between controlled and limited-by

There are many examples, but what's best for the pilot and the installation dictates what does what. To maintain safe operation (not blow up or stall its guts out) limits are selected in the control.

As covered in other threads, opening bleeds results in decreasing efficiency in the engine which in many controls causes more fuel and higher temp needed for the same thrust, assuming that no limits are exceeded

As loma notes, there are many control laws that come into play with FADEC (the old hydro-mechanical controls used fewer, more basic control laws - suitable for what was basically a mechanical computer, but the concept was similar).
At or near idle, the primary control is N2 speed and burner pressure (PS3) - and control can switch between N2 and PS3 depending on the conditions. So you could have a situation where the FADEC is happily controlling to a constant N2, you turn on anti-ice which causes PS3 to drop below the minimum for the conditions so control switches to PS3, fuel is added to get PS3 back to it's minimum limit causing N2 to increase.
At power, control is typically to a commanded N1 or EPR that is based on the throttle position. So at a consistent throttle position and ambient conditions, N1/EPR will be held constant, N2 and PS3 will be allowed to vary as necessary to maintain the N1/EPR - provided they remain with limits (e.g. redline). When you turned on anti-ice, the core suddenly needed to do more work, so fuel flow and N2 increased to provide the additional anti-ice bleed while maintaining a constant N1.
This is all for steady state stuff - there is a whole other set of control laws that come into play during accel/decel transients.

Thanks all for your contributions so far.

I forgot to include in my initial post the fact that these engines are not FADEC controlled - they have the old fashioned mechanical cable running from the power levers.

Most posters seem to be aware of this, but I should also have mentioned that, when turning on the engine anti-ice, a lot of bleed air is taken from the compressor, which without governing action, would result in decreases to engine and fan speeds. But there is governing action, of course, and it works to keep things "on speed" by boosting the fuel flow slightly.

It is this governing action that is at the heart of my question. According to the Flightsafety Pilot Training Manual, the power lever angle (PLA) determines the N2 speed (consistent with every other twin spool engine I have experience with).

So, without touching the power levers, switching on the engine anti-ice should result in the N2 remaining exactly the same, should it not? The PLA does not change, so the N2 should not change.

But it does - and the N1 stays the same. How on earth?

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Yes. Right. When I thought about what I had posted I realised I should have said limited by instead of controlled so thanks for the clarification.

I take it that AI is bled from the HP Compressor and that the engine is on temperature control.
So on that basis:
There is less air for the combustor to burn at a constant fuel flow so N2 and temperature drops by an amount.
The FCU then needs to increase fuel flow to restore temperate to the damanded figure but because the HP Spool is operating at a lower efficiency due to the bleed N2 will rise by a small amount to achieve the temperature limit.

Not quite, Buster15. The engine is not on "temperature control" but is being governed to maintain the N2 speed called for by the power lever angle (not "temperature control", but "N2 control"). This is what the Pilot Training Manual says, and is consistent with every other twin spool engine I am familiar with.

Half correct. The FCU is increasing fuel flow to bring the N2 back to "on speed" - not the temperature.

The Pilot Training Manual, logic and experience all say the N2 should stay the same (with everything else changing), but that is not what happens. The N1 stays the same and the N2 increases.

???

OK. All the engines I have been involved with have been temperature controlled.
The pilot lever angle normally demands a NH/N2 but the top limiter is JPT/TBT and NH/N2 can vary for a given JPT/TBT.

Anyway. A simple way of thinking about it is that NH/N2 equates to thrust/power demand..
When AI is selected the engine has to increase in speed slightly to maintain the same thrust/power for the same speed.
If not, the engine would produce less thrust/power for a given NH/N2.

Did you tell us what the N2 increase was. I would be surprised if it was more than 0.5%.

The PW530 isn't FADEC, the climb thrust temp N1 charts will give 630 ITT the cruise charts give 600 ITT

N2 governing is quite unusual, IMHO. N1 is much more typical. That's because in moderate and high bypass ratio engines (the PW530 is ~3.7 I believe), the majority (~75%) of the thrust comes from the fan. Hence, keeping N1 constant keeps thrust constant much better than keeping N2 constant. The behaviour you observed is certianly entirely consistent with keeping N1 constant, not N2. Hence, I question if the Manual may have been misinterpreted, though it should be clear?

The use of a hydro-mechanical fuel control system vs a FADEC should not be an issue because either can be used to control the engine in any chosen manner, a FADEC can just implement more advanced schedules easier, such as for acceleration, so that performance can be maximised without unnecessary conservatism to avoid engine stall, as an example.

Is there anything in the public domain that describes the control system? I can't find anything. I checked the EASA type certificate, but it just stated it "The PW530A and PW535A models are controlled by a hydromechanical system the PW535B, PW535E are controlled by a dual channel FADEC".

Cheers...

PS. How do you know the engine was not operating temperature limited? Was the EGT observed to be below the limit? If the enigne was on the limiter, the limit schedule would override the RPM governing (which ever spool it is).

Not all twin spool engines... For example, on the CF-6 and GENx engines, the EEC attempts to maintain N1; so fuel flow, N2, and EGT will vary with changing conditions.

Could be that the real picture is more complicated than how it's presented in the pilot training manual. Maybe they just put the N2 control as a general statement. But, for example, the previous plane I flew had hydro-mechanical N2 control up until 80% N1, and above that an electronic N1 control kicked in instead. And if it failed or was turned off (as happened to me once) the HM N2 control was active for the full thrust range.

Disclaimer - not familiar with the PW530 engine. But most older hydro controlled engines were controlled to N2 - but not necessarily "isochronous" (constant N2). The Hamilton Standard FCU's I'm familiar with (JT9D and JT8D) use something called a 'droop slope governor' - it's not constant speed with PLA, with a droop slope governor N2 changes with load. It's rather complicated to explain, but droop slope N2 governors will vary N2 with load - this has a rather desirable 'lapse rate' characteristics during Takeoff as the engine heats up (if you hold constant N2, as the engine heats up during TO and initial climb, N1 and thrust will drop - a droop slope will allow N2 to increase so N1 and thrust don't drop as much).

Intruder, the only CF6 hydro engine that controls to N1 is the CF6-80C2 (it controls to both N1 and N2 - N2 at near idle, N1 at higher power - far and away the most complex hydromechanical control I ever dealt with). All other CF6 engines use an N2 isochronous governor with burner pressure limiting - although the CF6-80A/A2 had an electronic "PMC" (Power Management Control) that 'trimmed' to an N1.

That's all a long winded way of saying that I suspect the PW530 uses a droop slope N2 governor - hence the change in N2 when you turned on anti-ice.

He already told us that the engine does not have a FADEC/EEC.

Thanks all, for some quality contributions!

Thanks, Buster15. The increase was 0.3 - 0.4%.

Thanks, Jetthrust. The engines were well below their ITT limits. There was a smallish ITT rise after switching on the anti-ice.

Thanks, tdracer. Another Citation in our fleet has FADEC and it too controls N2 at low speeds and N1 at the higher power settings. With FADEC, that is really easy to do, of course.

I have never heard of the "droop slope" governor before, but will now add that to my list of considerations for the explanation! I will try to find out about it.

Since I last posted, it has occurred to me that engine idle speed may have something to do with it. The Pilot Training Manual (PTM) says that the N2 flight idle speed, normally 49.5%, is increased to 60% when engine anti-ice is on. So, I reasoned that, for a mechanical system, the way the idle speed increase is implemented/enforced could explain things.

I spent several hours today trying to find more in the PTM and also talking with a maintenance engineer that works on these engines. The degree of detail in the PTM is very low, unfortunately, and only basic information can be found. The engineer was a bit stumped, but brought out the page from the PW530 Maintenance Manual relating to engine anti-ice. The information there was not at all consistent with that of the PTM, so now I probably have even more questions!

My next flight in this aircraft may present some more clues and opportunities. I don't know when that will be.

Thanks, again, to everybody here for their thoughtful and quality contributions.

I'd like to educate you about the droop slope governor, but the truth is I don't understand it all that well myself (hydro-mechanical isochronous I understand quite well - and could probably build one from scratch with enough time and tools - droop slope not so much).