737NG: CLB-2 higher than TO N1?
 

Is it possible to get a thrust increase if you do a 22K max assumed temp TO and you have selected CLB-2? I believe I have seen a thrust increase in situations with a light aircraft and very high thrust reductions for take off. We can use 25 % thrust reduction for TO and CLB-2 gives approx 20 pct thrust reduction, which leads me to think that it is possible. Please provide me with a link or reference if you have any inputs. Thanks

No it is not possible.
An increase can be seen if the greater CLM or CLM-1 is (manually) selected on N1 Limits page.
When passing the altitude (aal) preprogrammed (1500') or inserted in FMC T/O Page 2, having used the derate for takeoff, thrust mode will become N1 and selected CLM will be commanded.

Would you be able to find the relevant documentation to back that up? I'm aware that by selecting a higher thrust after take off, thrust would naturally increase. Say you are doing a 25 pct thrust reduction on a light ac. Since your CLB-2 is now only a 20 pct reduction it would be an increase in thrust. If the 20 pct reduction is calculated from 26k when airborne (?), let's instead imagine a 26K TO with max assumed temp and CLB-1 selected. We would now again have 25 pct reduction and only 10 pct reduction in climb thrust. Again an increase in thrust. Is there a feature in the aircraft that prohibits a climb thrust higher than than take off thrust, when a reduced climb thrust is selected on the N1 page before TO? If so when is this feature described.

There's no written reference material that backs this up to my knowledge, but I can assert from experience that CLB 1 thrust would definitely be higher than a high assumed temp R-TO thrust. It's policy in my organisation to always use CLB 1 or higher.

Typical 22 K lightweight 700 assumed temp takeoffs at an OAT of 30C with assumed temp of about 68C result in an R-TO N1 of some 86%, which rams up to 90% R-CLB at thrust "reduction" height!

Haven't seen this occur at CLB 2 values though. The reason could probably be that 737 FMCs allow a max assumed temp of 70C precisely to cap it conservatively at 20% thrust reduction although the hypothetical limit is 25%.

Regularly see climb thrust higher than TO thrust...and I always comment on the pointless 5 minute limit when I do.

That limit is for the full rated takeoff thrust i.e taking off bleeds off full thrust, upto forward stop with EECs normal.

That will always be higher than even max CLB.

Indeed, but many companies treat it as a limitation for the thrust you set at takeoff and expect you to keep an eye on it, despite the fact you are taking off at 22K or whatever!

No, anything above MCT actually.

Boeing state that when selecting an assumed temp take off the CLB thrust defaulted in the FMC should be selected to avoid an increase in thrust at " thrust reduction altitude" . Our company always selects "full CLB" in the FMC regardless and yes, this does result in an increase in thrust when N1 mode is engaged. There is no need for manual intervention.

Of course it's pointless, especially when Boeing will sell you AFM Appendix for 10-minute thrust without any aircraft modification :)

But how often in the sim when you're cleaning up and ask for MCT, your friend in the other seat says "increasing thrust by xxx"?

That actually leads me on to a question - I would say a decent portion of my engine failures in the sim have been with less than full rated thrust; I can probably count the number of full 26k engine failures I've done on one hand. I know we're simulating how we fly the line generally, but occasionally we do have to do departures at reasonably light weights at 26k. I suppose the theory is that the aircraft is heavier and more performance limited so the handling won't be that different but it makes you think.

With a typical lightweight bird on a long, sea-level (or near) r/w, there's a significant excess performance available for TO. That's the reason for ATM/FLEX procedures, to reduce needless wear & tear by bringing the bird back towards a fixed thrust/mass ratio.

But there's little reason not to take advantage of full climb performance, even if that seams incongruous because climb N1 may be higher than TO N1. The engineers don't complain, because the reduced TO N1 serves as a warmup for the climb cycle. Only pilots seem to object to this apparent upset in the gages.

:)

Fuel wise it would be most economical to take off and climb with as much power as avaialble. But that's having a negative effect on engine maintenance cost. So some operator use derate together wit assumed temperature to determine the T/O N1 and then select higher climb thrust to cater for the fuel.
And yes, that is producing a thrust increase when climb power is selected.

737NG: CLB-2 higher than TO N1?
 

I think everyone knows the pros and cons regarding reduced climb thrust. My question was a pure technical question concerning the 737NG. Powershanks seems to be the only one addressing it. Would you in certain circumstances see an increase in N1 with CLB-2 selected? I have been told by instructors better educated than me that it is not possible. But I believe that I have seen and the 25 pct (max assumed T) vs 20 pct ( CLB-2) suggest to me that it is possible. I was wondering if there was some sort of built-in feature which prevented N1 to increase when reduced CLB-2 was selected before departure, but I guess the conclusion is that there isn't. (This is was not a question concerning power up at thrust reduction alt as a company procedure).

It's possible and I have seen it many times. Might be due to company procedures but it does happen especially at light weights and high T/O thrust reductions.

Also it is more beneficial over the life of the engine in both fuel and maintenance to take max thrust reductions as SFC has a direct relationship to EGT. EGTs increase over the life of the engine as does the SFC hence the greater the reduction, the less the EGTs increase and the less fuel burnt.