3
$\begingroup$

The title is voluntarily the opposite of this question: Why isn´t TOGA thrust always 100% N1 or N2?.

I explain to you with an example: I'm looking to the FCOM of an A320 which states that the "maximum in normal operation N1" is 100%, and "the maximum N1 permitted" is 105%. But then, by looking at the THRUST RATINGS - MAXIMUM TAKEOFF page, where there is the table with altitude on the columns and OAT on the rows, the maximum value of N1 that I can find is 94.5%. Any other N1 table (climb/go around/etc.) has clearly lower values for N1. In A320 this value is enforced by the FADEC and cannot be exceeded.

So what's the point to have a 100%-105% N1 limits if the enforced N1 limit in TOGA can be at maximum 94.5%? Why not just set 100% as the RPM value of 94.5%? Or the 94.5% can be exceeded?

Clarification: My question is not related to maximum thrust (which it doesn't correspond to maximum N1). I cannot publish the table, however it's the same of this table, but for another engine and for the maximum takeoff mode instead of maximum climb. I reformulate the question in a more direct way:

  • Why FCOM says N1 can reach 100% and 105%, if then no N1 mode allows any N1 value over 94.5%?
Improve this question
$\endgroup$
5
  • 1
    $\begingroup$ Two questions for clarification: 1) Which FCOM section states the 100/105%? N1 is rarely limiting, and the limitations section lists EGT as expected 2) Which engine variant? CFM56-5A1? (It can't be a V2500 since that uses EPR.) If it's a lower-thrust variant like the 5A1, then most probably it's because that variant is held back. $\endgroup$
    – user14897
    Feb 25, 2021 at 12:45
  • $\begingroup$ @ymb1 The 100%/105% limitation is in the Limitation section (LIM-ENG) just below the THRUST SETTING/EGT LIMITS section. The engine is one of the new engines of the neo, the PW1100G $\endgroup$
    – ocirocir
    Feb 25, 2021 at 12:58
  • $\begingroup$ Actually I've just noticed the same in another old FCOM with CFM56 engines, the 3.01.70 operating limitations section states N1 max 104%, but the N1 takeoff table never go above 100.1% $\endgroup$
    – ocirocir
    Feb 25, 2021 at 13:03
  • 1
    $\begingroup$ I posted an answer, if it's not what you're looking for, let me know. Also it would help to add the info from your comments in the question whenever you have time. Also which PW1100G version? As I wrote earlier, most probably it's a low-rating one as its thrust range is 110–160 kN depending on the version. $\endgroup$
    – user14897
    Feb 25, 2021 at 15:05
  • 1
    $\begingroup$ PW1133G-JM which should be the most powerful version, but probably you are right: it is derated by the operator configuration, like what you can do on CFM56-7B: nandang-smart.blogspot.com/2014/09/… $\endgroup$
    – ocirocir
    Feb 25, 2021 at 17:16

2 Answers 2

Reset to default
2
$\begingroup$

Why not make 94.5% read as 100%?

94.5% tells you the margin you have. It will be closer to 100% in high-thrust versions – or even higher, i.e. the limit is pushed as the thrust ratings increase. The PW1100G you're asking about, its thrust varies from 110 to 150 kN based on the installed version.

For example, the A321's 150 kN CFM56-5B3 exceeds 100% near sea level and in cool to moderate temperatures, unlike the 120 kN -5B4 (both tables below).

Note that nowadays whatever the installed takeoff N1 limit is, it's rarely limiting – meaning the engine will reach the EGT limit before it reaches the N1 (barring major failures).

takeoff thrust CFM56
Source: A321/A320 FCOM

Improve this answer
$\endgroup$
0
$\begingroup$

The thing you are missing is that for a jet engine, there is no fixed relationship between rotor speed and thrust. For a given speed, the thrust depends on altitude and air temperature.

Therefore, there are separate limits on engine operation for maximum thrust and maximum rotor speeds.

If you are at a "hot and high" airfield like Nairobi (say 30C at 6,000 feet) you will be limited by the rotor speeds. On the other hand in Reykjavik (200ft and 0C maximum in winter) you will be limited by the maximum thrust when the rotor speeds are well below the maximum.

I couldn't find the table you referred to in an on-line copy of the FCOM, so no comment about the specific data you were looking at for the A320.

Improve this answer
$\endgroup$
2
  • $\begingroup$ Yes, this is correct, but the purpose of the table is exactly to show the relation with temperature and altitude. This is an example: forums.x-plane.org/uploads/monthly_2020_04/… (not the FCOM I'm looking at, I cannot publish it, but it's the same table but for Takeoff and another engine) But I expect to see at least one value at 100% of N1 in the table of the TOGA power... $\endgroup$
    – ocirocir
    Feb 25, 2021 at 9:14
  • $\begingroup$ I clarified the question. $\endgroup$
    – ocirocir
    Feb 25, 2021 at 9:18

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .