B777 EEC functions

[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

Rgds
NSEU

(Edited for typos)

[JammedStab]

Originally Posted by 8che

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]

Originally Posted by 8che

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]

Originally Posted by FullWings

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.