View Full Version : A320 Engine question

25th Oct 2008, 20:08
Sorry for the stupid question, but there is a switch on the Bus ovhd panel to revert to N1 mode. What exactly is this for?

Is it when the FADEC can't measure EPR? If so - how would that happen? Why would that happen?


Tight Slot
25th Oct 2008, 21:11


Two modes of power setting, EPR and N1 modes are used for engine control depending on the availability of air data inputs to the FADEC which are used for rating computation.

Two sources of air data are used by the FADEC :

- either engine data : P0 static pressure
P2 total air pressure
T2 total air temperature
- or ADIRS 1 or 2 data : Ps equal to engine P0
Pt equal to engine P2
Tt equal to engine T2

Thrust computation for both engines is mainly based on one ADIRS data. This ensures engine thrust symmetry.

A comparison between ADIRS 1, ADIRS 2 and engine data is made by the FADEC to determine the source to be validated and the mode to be used.


EPR mode is the normal mode to control the thrust.

The required EPR is set by controlling the fuel flow.

The FADEC computes the command EPR as a function of :

Thrust Lever Angle (TLA)


Mach number

Air data (static pressure, total air pressure/temperature)

Service bleed.

Note : During reverse operation, the thrust is controlled as a function of N1.


In the event of no EPR available the affected FADEC will automatically revert to N1 mode.

At the reversion to N1 mode, an equivalent thrust to that achieved in EPR mode is provided until a thrust lever position change.

Autothrust control is no more available. ALPHA FLOOR protection is lost.

In case of dispatch in N1 mode, flex take-off is not available. ALPHA FLOOR protection is lost.

Depending on the failure case leading to EPR mode loss the FADEC will revert to either rated or unrated N1 mode.


An automatic reversion to rated N1 mode occurs when :

engine P2 and/or P5 are not available,

or, engine P2 lower than ADIRS Pt.

The FADEC will compute an EPR COMMAND depending on TLA, then convert it into a N1 COMMAND as a function of Mach.

The rated N1 mode can also be manually selected through the ENG N1 MODE pushbutton on the overhead panel.


An automatic reversion to unrated N1 mode occurs when :

engine P2 and ADIRS 1 + 2 Pt are not available,

or, engine T2 and ADIRS 1 + 2 Tt are not available,

or, engine P0 and ADIRS 1 + 2 Ps are not available.

The N1 is defined as a function of TLA and altitude and is limited by the FADEC to either the smaller of N1 max or N1 redline (if T2 is available) or N1 redline (if T2 is not available).

The N1 rating limit, N1 TLA and N1 max indications on ECAM E/WD are lost.



25th Oct 2008, 22:28
gawad help us if this morphs into a, what's better EPR or N1 mode? slagging match:}

25th Oct 2008, 23:18
Utterly fabulous answer Tighty! Thank you for that.

Could you just give me a scenario where you might need to use the pushbutton? Reversion to N1 mode is obviously automatic - so why use the p/b?


Tight Slot
26th Oct 2008, 00:48
er yeah, when the EPR signals fail... pant pant.... shall I check me FCOM's again? :)

Down Three Greens
26th Oct 2008, 12:22
When you get one engine in N1 mode...you may want the other too! Also you could get an intermittent failure. Selecting it keeps the engine in N1 mode.


28th Oct 2008, 20:23
Are we talking CFM or IAE here?

One is "controlled" on EPR and the other on N1!

2nd Nov 2008, 09:15
If not used carefully EPR can give erroneous indications - i.e. leading the pilot to think he has a certain amount of power/thrust wehn he hasn't. Remember the 737 accident in Washington. The engine inlet probe was probably blocked and thus the EPR gauge over-read.

A good way to have the best of both worlds is to check both EPR and N1 especially during takeoff. It only tgakes a second or two and a "realistic" N1 should be seen to compare with the indicated EPR. This would probably have prevented the Washington accident.

Be careful with EPR - it can lie. On the other hand, engine speed is a good indicator of thrust/power.

2nd Nov 2008, 09:21
What a great post, I forgot all about that little button. FCOM for me next time to LCA!


2nd Nov 2008, 14:51
... On the other hand, engine speed is a good indicator of thrust/power.

... until a bird damages the fan

2nd Nov 2008, 17:37
Or the LP turbine is eaten by tigers...

St. Ex
19th Dec 2015, 05:41
This was a HIL item on my flight today and the related MEL 73-20-04A specifies to switch on the N1 mode PBs on the overhead panel as part of the pre flight operational procedures (o).

21st Dec 2015, 10:23
As an aside, you lose about 10 tonnes of take-off performance on an A321 in N1 mode.