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N1 creeping up in climb 737NG

Posted: Thu Sep 02, 2021 11:03 am
by r6russian
Hi everyone, I have a question that came to mind from flying the PMDG 737NGX in FSX and comparing it to Aerosoft airbus X. Its well known that PMDG is pretty much top of the line simulation, borderline real world practice, otherwise I'd start this thread in the flight sim section of the forum

My question is, why do the engines slowly throttle up in the 737 as you climb higher? Just to throw some numbers out there, lets say my takeoff thrust at 24k derate is 92% N1, and climb thrust CLB-1 is 88% N1 . but as I climb higher, my N1 creeps higher and higher to where by the time Im getting close to my crusing flght level, I could be spinning 101% N1 before i finally level off and throttle back to mid 80s N1.

My aerososoft AirbusX doesnt do this, it pretty much stays at the same N1 (or EPR on a V2500) all the way up thru the climb til i get to the cruise FL, then it throttles back to cruise thrust

I know there are a few real world 737NG pilots on here, Im just wondering if the progressive throttle up in the climb is a real thing or a flight sim glitch. I sat ahead of the engines on one of my 737NG flights and after we throttled up from takeoff to climb thrust, it sounded like it stayed put til we hit cruise flight level and throttled back, not steady increased thrust as we climbed.

Just wondering if its something PMDG had to accept to get around coding in a circa 2006 software (fsx) or if its what actually happens in a 737NG, and if it does, why? Does slowly creeping the engines up get the plane up to cruise FL faster to where fuel savings from being up higher make up for more engine wear? And if thats the case, why doesnt Airbus do the same, vs a set climb thrust all the way to the top?

Re: N1 creeping up in climb 737NG

Posted: Thu Sep 02, 2021 1:01 pm
by Avgeek21
I fly the 737NG/MAX and yes as you climb the climb power slowly creeps up and slightly throttle down once you reach cruise level. Climb power is normally not more than 95%. As an example...I took off yesterday on a MAX with 77%N1. During climb it slowly creeps up to normal climb power based on weight, weather and cost index in the FMC. Cruise %N1 was around 85-87% I believe. NG’s will be a bit higher as a general rule but the behavior is the same.

Any takeoff derate climb will match full climb power around 15,000ft.

Re: N1 creeping up in climb 737NG

Posted: Thu Sep 02, 2021 1:08 pm
by Redbellyguppy
That’s how it works in real life.

Re: N1 creeping up in climb 737NG

Posted: Thu Sep 02, 2021 1:28 pm
by Starlionblue
Same on Airbus. If you use a climb derate it washes out with increasing altitude.

There's not much point in maintaining a derate an engine once you're above a certain altitude. You need almost all the available thrust in the thinning air, so any derate would be tiny anyway.

Re: N1 creeping up in climb 737NG

Posted: Thu Sep 02, 2021 8:28 pm
by AABusDrvr
Here are some real world numbers from one of my last NG flights.

22K takeoff (sorry, didn’t write down the OAT or what our derate temp was)

93.8% N1

At 5,000’ CLB-1 was 91.1%
10K. 93.6%. ISA + 15
18K. 96.9%. +18
30K. 99.0%. +15
36K. 100.7%. +13
Right before we leveled off at 37K 101.3% and ISA +10.

Re: N1 creeping up in climb 737NG

Posted: Fri Sep 03, 2021 9:15 am
by r6russian
Fascinating. Thanks for the answers guys, sounds like PMDG got it right and it wasnt just me overthinking.

Guess my confusion was, directly relating N1 percentage with engine wear. Outside of my aviation hobby, Im a car guy, so the picture i was painting in my mind was 101% N1 was equivalent to bouncing off the rev limiter in your car for a lengthy period of time, obviously dangerous to your engine, and definitely borrowing against its lifespan by stretching timing chains, wearing out the oil pump, etc.

Turbines obviously work completely different from reciprocating piston engines, and as long as the bearings dont get pissed from the RPMs theyre being asked to spin, and fan/compressor/turbine blade tips dont go too far into supersonic and lose efficiency, it does make sense to spin faster and faster in thinning air as you climb higher to try and maintain the climb thrust you programmed for. I was just looking at this from a wrong frame of reference, thinking that one programs an N1 speed thru climb vs programming an amount of thrust to be delivered thru climb

Thanks guys!

Re: N1 creeping up in climb 737NG

Posted: Fri Sep 03, 2021 12:07 pm
by Starlionblue
r6russian wrote:
Fascinating. Thanks for the answers guys, sounds like PMDG got it right and it wasnt just me overthinking.

Guess my confusion was, directly relating N1 percentage with engine wear. Outside of my aviation hobby, Im a car guy, so the picture i was painting in my mind was 101% N1 was equivalent to bouncing off the rev limiter in your car for a lengthy period of time, obviously dangerous to your engine, and definitely borrowing against its lifespan by stretching timing chains, wearing out the oil pump, etc.

Turbines obviously work completely different from reciprocating piston engines, and as long as the bearings dont get pissed from the RPMs theyre being asked to spin, and fan/compressor/turbine blade tips dont go too far into supersonic and lose efficiency, it does make sense to spin faster and faster in thinning air as you climb higher to try and maintain the climb thrust you programmed for. I was just looking at this from a wrong frame of reference, thinking that one programs an N1 speed thru climb vs programming an amount of thrust to be delivered thru climb

Thanks guys!


Lower temperature in the engine decreases engine wear. Hence derates.

Don't be fooled by the actual percentages, however. 101% means 101% of the design phase specified thrust. It doesn't mean there is no margin. The engine might be able to go to 112%. This might be a decade after the design specs were stated.

Similarly, the Space Shuttle Main Engines used (if memory serves) 107% thrust as "full thrust). They weren't "over-revving". They simply had more thrust than the design phase specified thrust.

Re: N1 creeping up in climb 737NG

Posted: Fri Sep 03, 2021 4:41 pm
by LH707330
Starlionblue wrote:
r6russian wrote:
Fascinating. Thanks for the answers guys, sounds like PMDG got it right and it wasnt just me overthinking.

Guess my confusion was, directly relating N1 percentage with engine wear. Outside of my aviation hobby, Im a car guy, so the picture i was painting in my mind was 101% N1 was equivalent to bouncing off the rev limiter in your car for a lengthy period of time, obviously dangerous to your engine, and definitely borrowing against its lifespan by stretching timing chains, wearing out the oil pump, etc.

Turbines obviously work completely different from reciprocating piston engines, and as long as the bearings dont get pissed from the RPMs theyre being asked to spin, and fan/compressor/turbine blade tips dont go too far into supersonic and lose efficiency, it does make sense to spin faster and faster in thinning air as you climb higher to try and maintain the climb thrust you programmed for. I was just looking at this from a wrong frame of reference, thinking that one programs an N1 speed thru climb vs programming an amount of thrust to be delivered thru climb

Thanks guys!


Lower temperature in the engine decreases engine wear. Hence derates.

Don't be fooled by the actual percentages, however. 101% means 101% of the design phase specified thrust. It doesn't mean there is no margin. The engine might be able to go to 112%. This might be a decade after the design specs were stated.

Similarly, the Space Shuttle Main Engines used (if memory serves) 107% thrust as "full thrust). They weren't "over-revving". They simply had more thrust than the design phase specified thrust.

Another thing to add on the temperatures: warm, dense sea-level air, when compressed 50x gets a lot warmer than cooler, less-dense air at 15k or 25k feet. So the derates on takeoff help keep you below the magic number, then as you climb, the atmosphere itself gives you more margin, so you can spin the engine faster and still stay under the heat limit.

Re: N1 creeping up in climb 737NG

Posted: Fri Sep 03, 2021 9:18 pm
by BoeingGuy
Avgeek21 wrote:
I fly the 737NG/MAX and yes as you climb the climb power slowly creeps up and slightly throttle down once you reach cruise level. Climb power is normally not more than 95%. As an example...I took off yesterday on a MAX with 77%N1. During climb it slowly creeps up to normal climb power based on weight, weather and cost index in the FMC. Cruise %N1 was around 85-87% I believe. NG’s will be a bit higher as a general rule but the behavior is the same.

Any takeoff derate climb will match full climb power around 15,000ft.


On other Boeing models the climb derate will washout linearly between 10,000 and 12,000 feet. So will be at full un-derated climb thrust at 12,000 feet. There is an option to extend the derate washout from between 10,000 and 30,000 feet to reduce engine wear but increase fuel burn.

Does the 737 derate washout go from 10,000 to 15,000 feet? I don’t know that model as well.

Re: N1 creeping up in climb 737NG

Posted: Sun Sep 05, 2021 10:46 pm
by bigb
The 747-8F does that as well.