B777 EEC functions

[baobab72]

Hi
Premetting i am not a B777 driver so go easy on me, i have a few questions regarding the engine fuel system and the EEC.
As far as i understand pressurized fuel from the engine fuel sov is delivered into the metering unit or EEC controlling the thrust output of the engine by modulating the fuel flow to the engine based on inputs from the thrust levers and based upon a number of parameters.
The EEC can operate either in the normal mode or in the alternate mode.
When operated in the normal mode the EEC modulates the ff so as to control the N1 based on inputs from the thrust levers. Besides fuel metering the EEC will also provide reference/target N1 or EPR computed by the FMC; red line based on environmental conditions, maximum N1 or EPR based on environmental conditions and corresponding to max thrust available as the thrust levers are advanced to the mechanical stop, N1 and N2 or N1, N2 and N3 overspeed protection based on the engines preventing the computed red line from being exceeded when the TL's are advanced to the mechanical stop reducing the ff to the engine and triggering an EICAS message; overboost protection and idle selection - minimum or approach.
Should any of the requied inputs become unavailable the system will revert to an alternate mode - either soft or hard.
Now what is the difference between overspeed protection and max N1/EPR?
What is exactly overboost - is it some kind of compressor stall protection?
What are the required inputs?
What functions do u loose when in the alternate modes - other then overboost protection in the hard mode, if i am not mistaken?

Many thanks

Baobab

[lomapaseo]

Now what is the difference between overspeed protection and max N1/EPR?

I seem to reacll

Two different levels above a set speed, The overspeed is a higherlevel protection that assumes the engine has likely damaged itself or about to do something really bad. It shuts down the engine to idle and locks it out from going above idle.

The later limit is a controlling limit that it tries to control below some safe demonstrated RPM

happy to hear from others

[tdracer]

Overboost is N1/EPR higher than the max rating for the ambient conditions (i.e. above Takeoff or Max Con, depending on altitude/airspeed). In Normal mode, overboost is prevented by the EEC.

In Alternate mode, the EEC doesn't have enough validated data to calculate the ratings (N1 engines) and/or can't validate EPR (EPR engines). Although the specifics change between engine types, it's possible to significantly exceed the max rating for the flight conditions in Alternate mode (i.e. overboost the engine). Overboost is primarily an economic concern - the engine will rapidly deteriorate, but it's generally not unsafe.

Overspeed means you've exceeded the redline value for a rotor speed (N1, N2, and for Rolls N3). The EEC will always protect rotor speed redlines in both Normal and Alternate mode. Aside from a momentary transient overshoot, if the rotor speed exceeds redline, something in the engine control has failed. Exceeding redline is potentially unsafe and can result in an uncontained engine failure.

Again, it varies between engine types, but all engines have some sort of backup overspeed protection to protect against potential engine control failures (e.g. control is lost of the fuel metering valve and it goes wide open). Some engines use an electronic system that, if it senses an overspeed it'll shut the engine down, others use a hydromechanical system (essentially a fancy fly-ball governor) that will bypass fuel to maintain rotor speed at redline. In most cases, the overspeed protection system only protects the high rotor (N2 or N3) - the engine manufacture has to show that low rotor will maintain structural integrity for the worst case low rotor speed with the high rotor at redline.


In general, the only difference between Normal and Alternate mode is that Alternate mode is "unrated" and overboost is possible. In some cases the engine control will use somewhat more conservative control algorithms to account for the uncertainty in ambient conditions, which can result in a slight increase in fuel burn, but the key word is "slight".

[JammedStab]

But when are you really going to overboost the engine? You are using autothrottle and the engine will be commanded to stay within limits. We had a situation a while back with a failed SAARU using the MEL. After start, as expected, both EEC's were inop. Full thrust takeoffs for both flights at light weights but no other real worries.

In this situation on the 74, when you disconnect autothrottle for a normal landing, you are very vulnerable if you do a go-around due to manually advancing thrust. But on the 777, the autothrottle is left on until the landing. So if you go around, TOGA is pressed and the throttles move forward automatically and respect the limits(except if the go-around is after touchdown or is it below 2 feet RA).

Also in alternate mode, you now have more power available to get out of a windshear or GPWS scenario if really desperate and you want to push the throttles to a higher than maximum rated power setting. I suppose a stall/low speed scenario could also be a factor.

[baobab72]

Hi
So if i understood it correctly for each environmental input the EEC computes the max thrust rating which if exceeded it will result in an overboost of the engine and corresponding to the full forward thrust lever position - is the overboost shown on the gauges?
The overspeed whereas is a mechanical limitation and it is a fixed value - 97% of N1 will always be for instance 9700rpm's - and the max speed will varies based on the thrust settings - TO, CLB, MCT and so on.
So what does the red line shows? Overspeed or overboost?

Many thanks

Baobab

[FullWings]

So what does the red line shows? Overspeed or overboost?
Overspeed. Overboost is shown by an amber line (maximum N1 or EPR), overspeed (N1, N2, N3 operating limit) is a red line.

[de facto]

But when are you really going to overboost the engine? You are using autothrottle and the engine will be commanded to stay within limits. We had a situation a while back with a failed SAARU using the MEL. After start, as expected, both EEC's were inop. Full thrust takeoffs for both flights at light weights but no other real worries.

Lets see..manual go around,AT is OFF,PF firewalls it ...go around thrust is 98% (temp/elevation) and 100 % is set.Thats an over "boost" , over thrust if you prefer.
As written above for an overspeed...n1 and or N2 briefly overshot until the EEC reduces the fuel or the overspeed governor takes over.

[baobab72]

Thanks for the replies
So is the red line a fixed value or it changes based on what cos being a mechanical value i dont think it is affected by environmental conditions? in the airplane that i fly the red line is computed by the eec amd displayed on the N1 indicator and it shows the overthrust or overboost limit while the max N1 depends on the phase of flight - for instance 96.2 for takeoff and 98.6 for MCT - CF34/3B engines
Meaning that in the 777 if i firewall the thrust levers and the MCT is 97% for instance or whatever value is reported as a limitation in the AFM, if the density altitude is low i could actually overboost the engine if it was not for the overboost protection which with the TLs in the full forward position will limit the thrust to the computed level - let us say 94.5%; on the other hand with a high density altitude if i firewall the thrust levers the overspeed limit will be reached first and the EEC will limit the rotational speed of the compressor to the mechanical limit even though it would result in less than max thrust?
Is that correct?

Many thanks for clarifying that

Baobab

[baobab72]

To correct myself:
In the acft that i fly the red line shows the mechanical limit of 98.6% therefore the overspeed limit, the N1 target is computed by the EEC based on the selection made on FCP - TO, MCT, CLB, CRZ and APR - and affected by environmental conditions, density altitude, bleed air configuration and usage of wing/cown anti-icing: when the AT is engaged the AT will command the target N1 only in takeoff/go around mode or in FLC mode adjusting the pitch to maitain the selected speed while in PITCH mode and in VS mode will adjust the thrust levers so as to maintain the bugged speed for the selected pitch.
Now i am wondering if i can overboost my engines since i have always been taught that i can if needed advance the thrust levers to the limit of 98.6% or even more for a very short time.
The N2 target can be exceeded without any time limit.
What kind of indication would i have if i overboost the engine?
Many thanks

Baobab

[FullWings]

What aircraft do you normally fly?

The 777 is thrust-protected, i.e. you can firewall the thrust levers at any stage of flight and it will give you maximum rated thrust.

If the EEC(s) are in alternate mode, this protection is not available and it is possible to exceed maximum rated thrust under certain atmospheric conditions and thrust lever angles. Overspeed protection is still active.

[Amadis of Gaul]

What aircraft do you normally fly?



Sounds like he's on the CRJ-100/200 (98.6% N1 limit).

[baobab72]

The chally 605 with the same engines as the crj.

Baobab

[lomapaseo]

[ quote] What kind of indication would i have if i overboost the engine? [/quote]

Typical engine temp and pressures as well as N1, N2, N3 possibly on the high side

Trust the EEC and only worry if the EGT exceeds redline for a length of time.

[de facto]

Yes the EEC protects against overspeed,doesnt mean firewalling like your life would depend it,an overspeed would not occurr.
Max thrust by firewalling with eec ON would result in over thrust.
As written above not an issue..but best not do it unless dictated by few conditions(windshear/terrain).
I believe CFM on the 737NG have been tested to mac temp plus 40 deg C for 5 mins without need for maintenance.
Nevertheless,care of your engines and theyll care of you.

[8che]

lomapaseo do you actually fly the 777 ? I hope not.


Baobab72 just to clear some of this up,


If the EEC's are in alternate mode you do not have overboost protection any more. If you advance the throttles past the limit you get an EICAS ENG LIMIT PROT (L/R) message to inform you. There will be no yellow line on the engine N1 hence the need for the message. That's the only effect and autothrottle still works.
You must retard the thrust lever before selecting alternate mode. Note both engines must be the same so if one goes soft you must select alternate on both (but retard first).


The difference between max rate thrust N1/EPR and overspeed N1/N2 ? Well max thrust is simply an N1/EPR number that takes into account internal engine design limits including core temperature, pressure limits and fan speed. That one number reflects the most limiting of the 3 and takes into account outside environmental conditions. Overspeed protection is just physically protecting the actual moving parts (N1/N2) from going too fast. For this you get EICAS ENG RPM LIMITED (L/R) and the EEC's will simply limit N1/N2 from going too fast, there will be no immediate shutdown/idle or any previous rubbish you have been told by lomapaseo.
Next time you have a look at a GE90 777 engine look at the tail pipe. You will see a number of square pop out panels formed around the pipe. These pop out when ever the pressure is too great ie an Overboost !

[lomapaseo]

8che

omapaseo do you actually fly the 777 ? I hope not.

It's not important whether I fly anything or not. It's only important to any pilot that the EEC protects the engine from bad things 99.98% of the time.

The OP asked a specific question and this thread is developing technical explanations. Your vote is truly noted. Others I'm sure will follow

[baobab72]

Hi
Thanks for the explanation.
Two questions just to dissipate any doubt: the red line stays the same cos it is related to centrifugal force and thus to the rotational velocity of the compressor while the amber index changes and it is computed by the EEC. During alternate mode overspeed protection is provided, overboost not if the thrust levers are firewalled, thus the lack of the amber mark.
What is the difference between soft and hard alternate modes?
Many thanks

Baobab

[8che]

You got it....all correct ! The difference between hard and soft mode is how it was selected. Its described as soft if the EEC automatically reverted to alternate with no pilot input. Its hard if the pilot selected the alternate mode manually. In soft at a fixed thrust lever position thrust does not change. In hard at a fixed thrust lever position thrust may change to set commanded N1 when the alternate button is pressed. Hence move the lever back first to prevent possible overboost.

[tdracer]

Alternate mode always provides equal or greater EPR/N1 to Normal mode at the same throttle position.
When the EEC automatically "reverts" to Alternate mode ("Soft" Alternate), it will latch in any difference between the Normal and Alternate mode EPR/N1 Command for that throttle position (sometimes referred to as 'bumpless reversion'). When the pilot selects "Hard" Alternate via the flight deck switch, any latched in difference will be immediately removed. That latched in difference can be anywhere from zero to over 10% N1 (depending on the exact conditions when the reversion occurred), so selecting Hard Alternate can result in a sudden, large increase in thrust.


BTW, rotor speed redlines are fixed, certified values - they don't change with flight conditions. Some engines do have a short time limit 'transient' redline that is a bit higher than the steady state redline value (and it's common for the takeoff EGT redline to a little higher - limited to 5 or 10 minutes). That's all part of the engine cert basis and documented in the engine model's Type Certificate Data Sheet (TCDS).

[FullWings]

During alternate mode overspeed protection is provided, overboost not if the thrust levers are firewalled, thus the lack of the amber mark.
I think the amber line (maximum) is still displayed on the N1 dial, the difference is that the EEC will allow you to exceed it. I assume this is because you don’t want thrust limited to a lower than necessary value if the rated thrust calculation has gone wrong.

[NSEU]

Overspeed protection is just physically protecting the actual moving parts (N1/N2) from going too fast. For this you get EICAS ENG RPM LIMITED (L/R) and the EEC's will simply limit N1/N2 from going too fast, there will be no immediate shutdown/idle or any previous rubbish you have been told by lomapaseo.

From the Boeing Maintenance Manual regarding the RR Trent Engine:

"If the engine N1, N2 or N3 speed goes to the redline, the EEC modulates the fuel flow. This prevents the engine from exceeding the N1, N2, and N3 redlines.

The EEC (also) monitors the LP turbine for an overspeed condition. If the low pressure shaft breaks, the EEC closes the HP fuel shutoff valve.

Overspeed Protection Unit (OPU)

The OPU prevents a LP or IP shaft overspeed. The OPU monitors the N1 and N2 shaft speed sensors. If an FMU failure causes an overspeed condition, the OPU supplies a signal to the FMU that closes the HP fuel shutoff valve."

The manual adds that the OPU shutdown occurs at about 10% above redline.

I recall that US regulations imposed additional restrictions on RR engines for overspeed. There was also an overspeed shutdown on the 747-400's RB211.

One man's rubbish is another man's truth :ok:

Rgds
NSEU

(Edited for typos)

[JammedStab]

You must retard the thrust lever before selecting alternate mode. Note both engines must be the same so if one goes soft you must select alternate on both (but retard first).


Just to clarify this statement to make sure its intent is not misinterpreted and I'm sure you meant it this way.

If the thrust lever is already at a relatively low power setting, no need to retard prior to selecting hard alternate. Would hate to see someone at a mid altitude with a low power setting at their flap maneuver speed want to retard the throttles further.

The checklist says, "retard to mid-position".

[8che]

NSEU,


I was referring to the GE90 engine (where the rubbish applies). However you may be confusing two separate issues.

Just to clarify the overspeed protection message (GE90) EICAS ENG RPM LIMITED is a one line drill from the QRH and simply states "engine thrust is at the N1 or N2 red line limit". There will be no engine shutdown and the QRH does not ask you to shutdown.

Having checked the RR Trent QRH it is exactly the same. I think you may be confusing over speed with internal failure. The first line of your quote states the same protection for N1/2/3 with EEC adjusting fuel flow. The second part talks about internal failure dealt with by a separate OPU which is a rather different issue. I suspect EEC and OPU are two different things. We are talking only about EEC control on this thread.

As a good example have a look at the AIIB report into the incident with G-YMMP. A RR Trent machine.

As you say... ones mans rubbish !

[NSEU]

I think you may be confusing over speed with internal failure. The first line of your quote states the same protection for N1/2/3 with EEC adjusting fuel flow. The second part talks about internal failure dealt with by a separate OPU which is a rather different issue. I suspect EEC and OPU are two different things. We are talking only about EEC control on this thread.

Perhaps we have different interpretations of overspeed protection?

Anyway, the manual does say that the Trent EEC is capable of closing the HP Valve as well as the OPU in response to "overspeed". The Trent EEC has an overspeed system known as LPTOS. The OPU is only used when the EEC fails to protect the engine with it's normal control and it's LPTOS, and the engine experiences (quote) "severe" overspeed.

Yes, for overspeed protection (mentioned in my manuals) to activate, there has to be a failure of some sort (internal or external to the control system... sensors, EEC, FMU, VSV, shaft, etc).

[tdracer]

I think the amber line (maximum) is still displayed on the N1 dial, the difference is that the EEC will allow you to exceed it. I assume this is because you don’t want thrust limited to a lower than necessary value if the rated thrust calculation has gone wrong.

In Normal mode, the amber line (N1 or EPR Max) is provided by the EEC. In Alternate mode, it comes from the FMC since EEC is now in an 'unrated' mode (if the FMC has detected unreliable air data, the amber line will blank entirely).
One semi-common problem is Ice Crystal Icing (ICI). ICI has a tendency to ice up heated temperature probes (aircraft TAT and/or engine inlet T12) - causing the sensor to read ~0 deg C. At high altitudes, that can be well above cornerpoint temperature resulting in a thrust cutback on all engines. On EPR engines, the procedure is to manually select 'Hard' Alternate mode and set thrust manually in order to maintain altitude/airspeed. N1 engines are not susceptible to ICI T12 corruption because the inlet probe is unheated, however it may be necessary to disconnect the A/T and set thrust manually.

Alternate mode is always N1 control, even on EPR engines (since unreliable EPR is a primary reason for going to Alternate mode). If the EEC can still validate EPR (e.g. the failure was on the other engine) EPR may still be displayed, but the EEC is controlling to N1.