747 EEC in Alternate Mode
 

According to a technical video I am watching, when in Alternate if there is a 5% overspeed sensed, the engine goes into a min fuel condition("looks like a sub-idle stall"). Isn't that kind of dangerous if you were in alternate and had a bit of an overspeed, for example during a go-around.

From the QRH: i.e. you can overboost the engines.

No FADEC?
Your EEC is RR-powered.

But by how much. I believe that overboost is different than overspeed of N1 or N2. In other words. I think that one can be overboosted but within rotational speed limits.

But as the throttle is moved forward while in alternate do the N1 and N2 speeds continuously increase and therefore if you move the throttle forward too much such as in a poorly executed addition of go-around thrust, you will have 4 engines at idle.

Jammedstab, the explanation is simple. Don't confuse overboost with overspeed.

It has nothing to do with alternate mode (other than the fact that you need to be in alternate mode to significantly overboost the engine).
I'm assuming you're talking PW4000 (GE and Rolls have different overspeed protection). If everything is working normally, you should NEVER exceed N1 or N2 redline - the EEC is programed to protect those limits. So if you do exceed redline by more than a fraction of a percent, it means something's broke and the EEC can't control the engine. In order to protect structural limits, if the EEC senses an N2 overspeed greater than a threshold (I don't recall the specifics but 5% sounds about right) it trips the overspeed protection system which forces the fuel metering valve to the min flow stop (which is sub idle except at very high altitudes).

"Overboost" is a very inaccurate term for what you fellas are describing. In fact, not applicable to jet engines. You can "exceed the trust limit" or "overthrust". Too much air is exiting the engine.

Overboost is applicable to a turbocharger or compressor, i.e. too high pressure air is being forced into the engine.

The vast difference is that exceeding a thrust limit will likely not cause any damage to the engine, as long as it is done within safe parameters for the engine (like N1, N2 and EGT). Rather is has to do with the airframe, certified limits for engine mounts etc.

Overboosting a turbo/supercharged piston engine, will however, most like cause the engine to fail.

Hence, the need to differentiate, as most people connote "overboosting" as a dangerous condition for a jet engine, based on their knowledge of the expression in piston engine circumstances.

Hard to argue with a word like connate.

Are you the spelling police?

Overboost is a term used by engine companies (well, GE / CFMI for one) to describe exceeding the N1 or EPR value for the ambient flight condition. NOT the same as, and likely much lower, than redline overspeed limits.

The manufacturer may require some sort of inspection after an overboost, likely done on-wing.

If it's the case that engine manufacturers use the expression "overboost" for exceeding a rotational value, then expression is used even more wrong by the posters above, as they use it to describe an exceedance of the engine thrust.

Since, the term is technically inaccurate, it would be better not to use it in technical discussions at all (no matter what one or more engine manufacturers do).

Cosmo

While "Overboost" may not be a technically accurate term for exceeding the max rated thrust, I've been working turbofan propulsion systems for over 37 years, and it's the commonly accepted term for exceeding the max thrust rating (EPR or N1) for the conditions. It's used by Pratt, GE and RR, Boeing propulsion engineers, the FAA specialists, and every pilot I've ever talked to about the phenomena. As barit1 notes, a large 'overboost' may require an inspection. And 'overboost' isn't all that inaccurate since, at least at sea level, a large overboost will exceed the design pressures in the compressor.

It is not the same as 'overspeed' which generally means a redline exceedance.

From what I understand from the above, you are contradicting yourself... Can you put it into other words?

I mean, your can exceed max thrust without exceeding N1 or EPR. Plus from what I read above, you are saying that a thrust exceedance is equal to exceeding N1 or EPR. That would in my opinion be the definition of an overspeed ("redline exceedance").

Sure. Max thrust for ambient conditions may be produced at 96% N1 or 102% N1, or something in between (or slightly more or less). "Red line" N1 is a fixed value for an engine, regardless of ambient conditions, such as 117.5% for the GE CF6-80C2 or 111.4% for the PW4056.

Therefore you can "overboost" the engine without exceeding "red line" N1.

Re-read 2 times... got what you mean now.

Still, I stand on my view... people think that an "overboost" is dangerous. The engine doesn't blow up due to a thrust limit exceedance.

Found an old thread on the subject too (so you see there are more people of my view), with a funny anecdote in it:

http://www.pprune.org/tech-log/28540...rboosting.html

@Intruder, technically it's still an "thrust limit exceedance", not an overboost. Tdracer's argument that it "might" exceed limit in the compressor is something that we can't know. Probably it is wrong too, I read somewhere that the CFM56 was run at 140+% during testing. Probably there is a much wider margin for the compressor than what we are able to exceed with the engine on the wing.

Thanks. I am GE CF-6. I guess that this is a protection that happens whether the EEC is in normal or alternate.

Cosmo, don't confuse absolute limits with 'certified limits'. Redline is a certified limit. However the engine manufacture has to demonstrate that the engine won't structurally fail until (IIRC) at least 25% over redline. That's what GE was doing when the ran the CFM to 140%. Similar limits apply to pressures. Turbine engines seldom 'blow up' - in no small part because these absolute limits are so much higher than what the engine should ever see. But they do occasionally blow up, and engine failures of any type become significantly more likely if the "thrust limits are exceeded". There was a 747 freighter crash not too long ago where an engine failed at B1 - the crew responded by advancing the throttles, overboosting - er - exceeding thrust limits on the remaining three - at which time a second engine promptly failed. They were unable to make it back to the airport and crashed. It doesn't matter if you call it "overboost" or 'exceeding thrust limits' - it's not something a pilot should do.


JammedStab - the PW4000 uses an electronic overspeed (which effectively shuts the engine down when tripped), CF6-80C2 has a hydromechanical overspeed - basically a "fly ball governor". The EEC/FADEC will still protect rotor speed redlines, but if something breaks such that the engine exceeds N2 redline by ~1%, the overspeed governor will start bypassing fuel back to the fuel pump to maintain N2 at ~ redline+1%. It won't shut it down. The RB211-524 on the 747-400 works similarly to GE, except of course using N3 instead of N2.
I was involved when we first went to FADEC, and there were a lot of people that didn't think we should use electronic overspeed protections - if we couldn't trust the electronics in the EEC to protect the engine, how could we trust the electronics in the overspeed protection to protect it :=.
However as we gained experience with FADEC, must of that resistance has faded. GE was the last holdout for mechanical overspeed protection - using it on the CFM56-7 (737NG) and the GE90. But the GEnx has electronic overspeed protection.

Tdracer, let me give an example.

If you have a 737-700 with 22k engines the EEC will give you 24k if you advance thust levers to the forward stop. The same engine is capable of producing 27k on a 737-800. That 27k was run up to 140% during testing.

Should you have a bad day, with the EEC failed and having to do a GPWS pull up, there is a good chance the Max Rated 22k and Max Certified 24k will be exceeded. Was the engine overboosted (as in a fed with a critical high pressure in the compressor)? Or would it be more accurate to say the "thrust limit was exceeded"?

We are not talking about whether a pilot should deliberately exceed the limits or not (in the case of imminent terrain contact, he probably should though ;) ).

Just want to add, that it is not that I want to come across pigheaded. I understand your arguments and accept that in some circles, the wording "overboost" may be accepted and unambiguous (though not technically correct).

But if you write it in a technical discussion on a board like this, with world wide audience, people will understand it completely different, which we already saw in this thread and the one I linked to...

Overspeed is the major concern regarding rotating parts. Exceeding redline by specific amounts can get you into trouble with part resonances and major failure conditions where blades come unfixed.

Overboost can get you into trouble by altering the cycle to the point where non-controlling temperatures (like in the aft parts of the compressor as well as the forward stages of the turbine) cause internal damage which must be assessed in a forced maintenance action.

Regardless of possible confusions between what different manufacturers use in their wordings, just follow the manual for the product.