747 EEC in Alternate Mode

[tdracer]

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"?

OK, intentionally being a bit anal here, but since you're pointing out inaccurate (although commonly accepted) nomenclature...
FADEC EECs don't "fail", at least not and keep the engine running. With FADEC, if the EEC is failed there is no engine control and the engine will quit. What happens is some fault (or combination of faults) causes the EEC automatically revert to Alternate mode control - which is fundamentally different than "failed". The terminology likely dates to the early 'supervisory' (aka PMC) EECs that used a conventional hydromechanical engine control - the EEC would 'trim' the hydro to prevent 'overboost' and give more linear throttle response. If the EEC failed (which it literally could do), the engine still ran fine, but without overboost/over thrust protections provided by the supervisory EEC.
I'm currently a bit over sensitive to people saying 'failed' when an EEC auto-reverts to Alternate mode as there was a very recent 767 event where both engines auto-reverted to Alternate mode in suspected Ice Crystal Icing (I haven't seen the data yet but I'm reasonably sure the inlet total pressure probes iced up). The airline 21.3 report said "both EECs failed" - which caused several people to go absolutely non-linear since "both EECs failed" would literally mean a dual engine failure. I'd taken a day off which allowed the resultant panic to spread far and wide before I came back and was able to stamp it back down.


BTW, yes I agree that if contact with terrain is imminent, I have no problem with a pilot intentionally overboosting (over thrusting) an engine. I recall concerns during the early days of FADEC that allowing the EEC to protect thrust limits could be a problem during an emergency such as wind shear. But analysis at that time concluded that the ability to overboost/over thrust was only of minor benefit in such an emergency. 25 years later I'm unaware of any accidents where the ability to overboost/over thrust the engines would have made a meaningful difference. So it sounds like that analysis was correct.

[cosmo kramer]

There is a difference between failed and failed. We can discuss semantics if you like: "The ECC failed (to do it's job in normal mode)" ... "The ECC suffered a failure (causing both engines to quit).

fail
feɪl
verb
1.
be unsuccessful in achieving one's goal.
"he failed in his attempt to secure election"
be unsuccessful in (an examination or interview).
"she failed her finals"

The goal with the EEC is for it to run in normal mode.

[JammedStab]

Originally Posted by tdracer

BTW, yes I agree that if contact with terrain is imminent, I have no problem with a pilot intentionally overboosting (over thrusting) an engine. I recall concerns during the early days of FADEC that allowing the EEC to protect thrust limits could be a problem during an emergency such as wind shear. But analysis at that time concluded that the ability to overboost/over thrust was only of minor benefit in such an emergency. 25 years later I'm unaware of any accidents where the ability to overboost/over thrust the engines would have made a meaningful difference.

Thanks TD. I posed the original question because of what you are discussing now. As you know, there have been a couple of 747's that crashed with the initiating cause being two engines on the same side having departed the aircraft. The more well known case was in Amsterdam.

While I realize that there were other contributing issues to the final loss of control, I had always wondered if this situation was encountered at low altitude(could be a birdstrike) on departure at heavy weight, one could be in a situation where they could not maintain altitude, could not reduce weight fast enough to be able to maintain altitude and the only way to stop the descent in this desperate situation would be to add more thrust from the good engines(assuming aircraft control could be maintained). Extra thrust above max allowable limits would be gained by selecting the remaining EEC's to Alternate.

When I read about engines going to idle due to overspeed, it got my attention.

Any comments?

[tdracer]

When I read about engines going to idle due to overspeed, it got my attention.

As I noted before, the FADEC EEC will protect rotor speed redlines, even in alternate mode. Only if there are faults that are affecting the EECs ability to control the engine is there any risk of a rotor speed exceedance exceedance - and if the EEC can't control the engine it doesn't much matter if it's Normal or Alternate mode operation. It's also worth noting that the EEC will not protect EGT redline except during autostart (at least on Boeing installations) so there is no risk of a cutback due to EGT. Selecting alternate mode in order to get additional thrust in an emergency will adversely affect engine life, but the risk of a cutback to idle due to an overspeed isn't a meaningful concern.

Cosmo, dispatch is allowed in Alternate mode per the MEL. Would you really accept an airplane where all the engine controls were "failed"?