B787/777 EEC Alternate Mode RTOW

[Alan Partridge!]

787 question (but applies to 777)

Does anyone know why the RTOW in EEC Alternate mode is less that in the EEC Normal mode? The thrust would be full thrust rather than Derate/ATM and I can't figure out why the reduction. Any clever boffins out there with an idea?

[spannersatcx]

stops pilots over boosting the engine.:eek:

[tdracer]

There is more uncertainty in thrust setting in alternate mode for EPR engines (thrust at N1 is less certain than thrust at EPR), and you don't get the same thrust lapse during takeoff (GE and Rolls).

[Alan Partridge!]

Thanks Tdracer. Sounds like a plausible reason. The manuals don't go deep enough to explain. Your expertise is most welcome

[JammedStab]

stops pilots over boosting the engine.:eek:

As a note.....I have twice used alternate EEC in the 777(that is 4 takeoffs). In both cases, it was both engines as per MEL for an inoperative SAARU.

Interestingly, they were both shorthaul return trips with one last pair being only one hour flights, light weight, and a 30 knot headwind making for the outbound flight having an extremely short takeoff roll and near immediate altitude capture.

The last takeoff was also the most interesting one as just after rotation, it was noted that the EGT on one of the engines was over the limit. I don't know by how much but the entire circle was red which does create concern to say the least. It was quickly corrected by retarding the thrust a little bit. Once that was done the red disappeared but after such an event, the boxed numerical indication can be cycled between red and white by cycling the recall switch.

While overboost protection is lost in alternate mode, I had thought at the time that there was still overtemp protection for the engine but it appears that this is not the case in alternate mode. Only overspeed protection at all times and overtemp protection during start. Please correct me if this statement is wrong.

I am not sure of the price of one of these engines but obviously, there is some concern about what might have happened.

According to a discussion with maintenence later in the day, the overtemp was small enough that instead of the worst-case scenario that enters one's mind after an event like this, only a boroscope inspection was required and then only within the next few days.

It was the second time in my career that I thought I had toasted and engine(previous case was on an Allison 501) and the second time that I was fortunate that this was not the case.

Interesting and.....whew.

No smilie available for an exorbitant amount of sweating.

[tdracer]

While overboost protection is lost in alternate mode, I had thought at the time that there was still overtemp protection for the engine but it appears that this is not the case in alternate mode. Only overspeed protection at all times and overtemp protection during start. Please correct me if this statement is wrong.

I am not sure of the price of one of these engines but obviously, there is some concern about what might have happened.
Boeing design philosophy is, autostart aside, there is no automatic protection against EGT exceedances, even in Normal mode (overspeed is different - protection is always there, including a mechanical backup for N2 on the GE90). That's because we don't know why there is an EGT overtemp - it might be because you lost the other engine and you need that last bit of thrust to clear an obstacle, or because the EGT harness is shorting and the EGT indication is bad. That last part actually was a problem when we briefly did have EGT limiting on the 767 - the Pratt engines had crappy EGT harnesses and the EEC was unilaterally limiting thrust during takeoff due to erroneous EGT indications - hence the reason Boeing decided that was a bad idea.

In addition to turning the gauge red, EICAS automatically records any exceedance, it's a simple matter to pull up the max temp and time over redline. There is a handy chart in the Maintenance Manual it tells you what action is necessary based on the max temp and time.

Last I heard (which was a while ago), new GE90 engines were going for about $12 million. But an overhaul is usually 'only' a million or two :p