Topic Author
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Joined: Sun Jul 16, 2000 3:44 am

N1 Greater Than 100

Thu Dec 28, 2000 2:54 am

Correct me if I'm wrong but N1 can exceed 100, right? What about N2? And can someone please explain WHY it can exceed 100? Thanks.
CX Flyboy
Posts: 6113
Joined: Sun Dec 26, 1999 6:10 pm

RE: N1 Greater Than 100

Thu Dec 28, 2000 3:01 am

We operate 747-400s with RB211-524Gs, and our N1 is certified to 110.5% during MAX takeoff, 109.5 continuous, 111.5 MAX overspeed.

N2s 107.7% during start, MAX continuous 105% and MAX overspeed 109.5.

The RB211 is triple spooled, and our engines have a MAX overspeed N3 of 101.5%, so yes, they can exceed 100.

I am not sure of the certification ratings and why they can exceed 100%. Better ask an engineer!!!
Posts: 220
Joined: Sat Oct 21, 2000 4:57 am

RE: N1 Greater Than 100

Thu Dec 28, 2000 5:24 am

On the DC-9 we have JT8D-9's and our N1 limit is 100.1%. N2 limit is 100%

RE: N1 Greater Than 100

Thu Dec 28, 2000 5:52 am

I think that the 2 gentlemen above have pointed out GAUGE RPM values to you, although these are calibrated with the actual engine rotational speed, it doesnt mean that the engine is turning 10% faster than it should.

CX flyboy, do you use rotational speeds for primary power settings on the RB211? I always thought that it was an EPR engine.
Posts: 3140
Joined: Fri Jan 21, 2000 11:51 am

RE: N1 Greater Than 100

Thu Dec 28, 2000 8:59 am

>Correct me if I'm wrong but N1 can exceed 100, right?


>What about N2?


>And can someone please explain WHY it can exceed 100? Thanks.

One possible reason is an engine modification without changing or recalibrating the rpm guages. Or a simple limitation change is all that may have been needed to "uprate" an engine.

*NO CARRIER* -- A Naval Aviator's worst nightmare!

RE: N1 Greater Than 100

Fri Dec 29, 2000 12:45 am

(Hi Andy!)

A turbine engine is designed for reliability, it's
makers even warranty their life. So it's sorta
like a car you wanna keep, you don't want to
abuse it.

The complete engine assembly is limited by
it's weakest part. Consider that some parts
are "worn" by excessive heat, some by RPM,
some by pressure and others by lubrication

While 100 percent is normally a number we
associate with "everything", it's not the case
here. A turbine engine's 100 percent is a target,
a designed level to ensure relaibility.
N1 can usually spin faster than we run it
normally, but in doing so it could overpressure
engine sections behind it, exceed needed
lubrication, destroy bearings and maybe loose
blades. Each piece and engine section has
limits and each are protected when we target
the designed engine assembly speed.

Turbine engines are normally monitored,
watching and noting max temps, rpms, fuel
flows , etc. In airline service it's either an
automatic ACARS broadcast or a manual
(pilot) report.

A Maint. center monitors the parameters
and makes engine replacements decisions based
on time or parameters showing possible wear
or stress well in advance of failure.

Now, this summary is obviously a simplication
of numerious very involved areas and is only
meant to convey the basic concept, I didn't
want to write a book. (grin)

So, in closing... it's like a sports car, you've
got a tachometer. It shows engine redline,
you "know" it'll go beyond. Just not for long!

CX Flyboy
Posts: 6113
Joined: Sun Dec 26, 1999 6:10 pm

RE: N1 Greater Than 100

Mon Jan 01, 2001 12:50 am


We do use EPRs for thrust settings, but nevertheless, we have a set of figures/limits for the other engine parameters such as N1 as well. I just stated those to help answer the question!

Colin Parker.

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