Doubt on 737NG: N1 Red and ECC norm/altn modes

[Doz]

Good day everyone!

Studying for a Type Rating I have some doubts about this topic.

The OM says that:

--> "The N1 Red Line indicates the N1% RPM operating limit"

--> "The EEC provides N1 and N2 redline overspeed protection in both normal and alternate modes."


The question is: if the EEC operates in ALTN mode, why does the N1 become RED if I put the throttles full forward? Doesn't the EEC protect against overspeed in both normal and alternates modes?

The only explanation is that the N1 Red Line does not indicate the maximum number of RPM, but the Maximum Certified Thrust which then yes, in ALTN can be exceeded... yet the OM says "N1 RPM operating limit".



What am I doing wrong?



Thank you very much for your time and support!


Ciao

Teo

[Fursty Ferret]

Operating limit vs "bits of fan blade escaping the cowling" limit perhaps?

[tdracer]

Disclaimer - I'm not very familiar with the specifics of the 737NG (or any 737 model for that matter). But this is how it works on other Boeing models with EICAS:
In Normal mode, the EEC will protect the max N1 (or EPR) rating for the ambient conditions (with a little 'head room' to account for air data sensor differences). Without some sort of failure, it's not possible to significantly overboost the engine in Normal mode.
In Alternate mode, thrust rating protection is lost - it IS possible to overboost the engine - by a rather large amount (again, depending on the ambient conditions). The EEC will still protect N1 (and N2) redlines, so without some sort of secondary failure you can't exceed rotor speed redlines (EGT is a different matter). This introduces a risk that a pilot may have a brain fade - forgetting that they are in Alternate mode - and advance the thrust levers full forward resulting in a massive overboost. On EICAS equipped Boeings, there is a Alert level message that will come up (aural alert and text) if N1 Command is set 2% (or more) above the Max N1 as computed by the aircraft (FMC or FMC type function).
What I'm guessing is that - on the NG - lacking EICAS, they can't put up the message, so they simply turn the N1 gauge red.
But that's just an educated guess...

[BraceBrace]

The question is: if the EEC operates in ALTN mode, why does the N1 become RED if I put the throttles full forward? Doesn't the EEC protect against overspeed in both normal and alternates modes?

If I understand correctly you are mixing 2 limitations. N1 and N2 redline overspeed protection is a protection against structural limits (how fast can my rotor rotate?). It does not verify max EGT which is also a fixed limitation of the engine.

In normal mode, EEC calculates the N1 limit that is associated with the approved thrust limit (full rated or derated). This is a different N1, and the value depends on ambient conditions. Different ambient temp and/or pressure give a different N1 value indicating the related rating. This is important as the engines are regulated using N1 (unlike EPR ie) so the EEC continuously updates this N1 reference. When the EEC switches to soft or hard alternate mode, this realtime calculation is lost and hence the N1 is not automatically adapted. So you might end up boosting the engine, but the calculated N1 limit might be wrong as well. Note that the parameter to regulate the engine is N1, only N1.

The EEC will never allow you to push beyond the structural N1 AND N2 limit (but not EGT).