737ng n1

[D O Guerrero]

In the climb today I noticed that the FMC was commanding an N1 of 100.1% which set me thinking about N1 limitations. On looking at the manual, I noticed that the N1 limitation overall for the engine is something like 105%. Furthermore I noticed that there were several inches of play left in the thrust levers when 100.1% was set. Presumably pushing them all the way forward to the stops would take the engines to the red line limitation?
Could someone explain why the limit isn't 100%?! To me, it doesn't seem to make sense that you can go above 100% ever, but I'm sure someone with a more encyclopaedic knowledge of these things can explain better?

[misd-agin]

Other a/c or engines have N1 limits of 117.5%.

So the old statement "they're giving 110%" means that they're actually not trying as hard as they could.

[D O Guerrero]

Thanks - but do you know why?

[GarageYears]

My understanding is that the 100% RPM value for an engine is the point at which the engine produces the ground level rated thrust and will do so all day if asked.

For those engines that allow in excess of 100% there will be some time limit defined by the engine manufacturer at which the engine may be run (beyond which damage may occur), which will produce some greater thrust value.

For example I believe the takeoff thrust setting for a 737NG is 101.5% N1.

- GY

P.S. There are certainly others who regularly contribute to this forum with more knowledge related to this topic area.

[lomapaseo]

In an attempt to KISS (others may add)

There are performance N1 limitation values based on fan aero and mechanical N1 limitation values based on stress. The installer knows the relationships. Depending on the conditions of the day you can operate above the 100% gage reading to achieve maximum permitted thrust (like somebody said 105% or even 117% depnding on the engine specs. However mechanically you will be in do-do if you go much above 105% of permitted redline speed (in the manuals). The first thing that gets you into trouble with N1 is running into unsafe vibration modes e.g. the National airlines DC10 CF6 fan. The second thing that gets you into trouble is rotor disk growth and blade wear and the last thing would be rotor burst (extremely unlikely given that it would take higher than 130% of rediline.)

[nick14]

I have a feeling that its something to do with pressure.

If the engine is pressure rated (ie certified to a max pressure) then the max N1 setting is different for different pressure altitudes. At SLP it could be 100% but at 10,000' it could be 102%, 20,000 104% etc.

The FMC can calculate all this and so sets its limit (red line on the gauges) if im not mistaken firewalling the thrust levers will give you to the red line at any altitude/phase of flight.

Im not sure you can overstress the engines with the EEC's in normal mode as there is a protection system on the -800

Please do correct me as I want to learn too!

edited to add the EEC's comment

[misd-agin]

Limits -

CF6 - 117.0
CF6 - 117.5 different model CF6
RR - 108.4

737 - red line

Maybe a RR or GE engineer can explain the details. I can't.

[Avenger]

There ıs an explanatıon the 100 percent N1 etc of thıs ın the flıght testıng sectıon, probaby more easy readıng

[D O Guerrero]

Thanks Avenger...

http://www.pprune.org/flight-testing...at-100-n1.html

[Green Guard]

Engines are NOT limited ONLY by N1

but by lower limit of the next 3:

N1

Temp like EGT etc...

and Burner Pressure, given by EPR etc...