Problems with REDUCED THRUST TAKEOFF on B738

[B738HT]

Hi everyone,
First of all, sorry for my poor English. I met a problem when I set the CDU for take-off thrust management last week.

Here is the conditions: Field elevation:628ft
OAT:12C
GW:62.4ton
I checked the performance table, found the ASSUMED TEMP for reduced thrust take-off is 28C.

After I input the 28C, CDU indicated "INVALID ENTRY". Finally we took off with full power.

So, could that "28C" setting break the 25% limit?

Can you guys explain me why? Thanks a lot.

[Denti]

As far as i know ATM of 30 degrees or below is not used by the 737 FMC. Some versions allow the crew to enter those temperatures but do not change N1 values, others simply display the message you encountered.

[Jwscud]

Remember, the engines are flat rated up to ISA+15, so any assumed temperature below that (ISA at your elevation is around 13.4°) will not give any thrust reduction.

[de facto]

Next time open your fcom and look for MIN ASS temp,you carry these books for something no?
The 25 % limit is for MAX ASS.
If the fmc would accept an ASS temperature below the MiN ass it would indeed calculate a n1 that is HIGheR than the engine rating/oat/field elevation based max thrust so you would over thrust the engine.
Technical reason explained on above post.

[BARKINGMAD]

C'mon folks, easy does it!!!!!

The location, professed age and admitted language difficulty may have something to do with this query.

What, if anything, might the training staff have said, or more likely NOT said, relating to the 30 degree figure?

Let's try not to be so censorious, lofty, arrogant nor pompous when responding to questions such as this.

We all had to start somewhere, and the overconfident attitudes displayed in these fora bode ill for the presumed CRM of the respondents!! :=

[framer]

Heh heh well said BM.
On another note....man it must have been a short runway!

[de facto]

On another note....man it must have been a short runway!

Framer... I suggest you open your fcom as well if you dont understand what a mini assum temp is.

What, if anything, might the training staff have said, or more likely NOT said, relating to the 30 degree figure?

I also suggest you save your breath ,add your knowledge on the subject or if you still at your age/experience havent come across this,i suggest next time you fly,open and read your fcom as well.

[BARKINGMAD]

"I also suggest you save your breath ,add your knowledge on the subject or if you still at your age/experience havent come across this,i suggest next time you fly,open and read your fcom as well."

After 3 years on "classics" and 9 years on NGs, I believe I came across it far too long ago to make an issue of it like you are, de facto.

It is my perception that the OP is low on experience and knowledge and working in a foreign language, so he/she might prefer to fly with someone who would try to explain it patiently rather than receive advice to go and read a set of books which in my 19k+ hours experience are amongst the WORST technical publications on aviation it has been my dubious privilege to fight my way through in the quest for knowledge.

If these turgid tomes were as good as they could be, there would be no call for books such as Boone's guide or the Bulfer series.

"What the manual says is dandy, but if you are a trainer, you should be teaching what actually happens - we fly aeroplanes, not manuals. " This quote is from AluminiumShuffler discussing NGs exotic qualities in VNAV, so I like to think I'm not the only one on this frequency?!

Tin hat on and await incoming fire.............................:hmm:

[de facto]

It is my perception that the OP is low on experience and knowledge and working in a foreign language, so he/she might prefer to fly with someone who would try to explain it patiently rather than receive advice to go and read a set of books which in my 19k+ hours experience are amongst the WORST technical publications on aviation it has been my dubious privilege to fight my way through in the quest for knowledge.

It is one page which is a table under the name of MINIMUM ASSUMED.
It is not difficult ,you do not need to read all fcoms to understand..its a basic table..
Now if you never bothered opening them well you would have never realised that there was something such as MINIMUM ass temp.

You make my argument that even you with 19000 hours never read or even opened this fcom page hence my initial advice to the OP to open his manual and READ.
I DID explain this min ass temp by the way in my post #4 and the reason behind it (not to overthrust).
But you two seemed more interested in my rethoric than the content of it.

So ill try to make simple: your fmc which has the same info as your FCOM does,it will not allow you to select at an airport at SL ,an assumed temp of less than 30 C as your engine is rated at ISA+15c which would end up being a selected N1 higher than the MAX thrust of your engine rating in these conditions.
For example:you select 28 c ass temp when the minimum is 30c,if the fmc were allowed to calculate the resultant N1 it would give you for example 90 %N1 when if you were to take off with full thrust(ie no assumed) ,the resultant would be 89.5%N1.
If you take off at 2000 ft PA..your min assum will be a few degrees less.

You understand now?

But again a picture is worth a thousand words so open your FCOM,find where the calculating of assumed temp is and you will see it.

I spend a lot of time teaching and believe me when the fo opens his fcom to see the info he will remember it much better,because he will now know its there!
If he then doesnt understand,i will explain as i just did in my initial reply.

[framer]

Lol I stand by my light hearted statement, that must have been one short runway :)

[framer]

Pps defacto, do you tone down the attitude when teaching in person as opposed to online, or do you lead with agression and assumptions as you do here?

[de facto]

I am a total tyran:ok:

[Denti]

There are actually people that do not calculate their assumed temperature via a manual anymore. Those might not have access to said table as everything is done via an EFB anyway which is sometimes stupid enough to suggest a temperature that actually cannot be put into the FMC. And of course, boeing offers quite different FCOM versions to boot which doesn't help either. It is not as standardized yet as the airbus world, however usually much better presented (those airbus FCOMs suck).

[de facto]

I know Denti.
As you are aware it is possible to calculate your assumed temp based with fcom tables eventhough we dont do it as we normally have ass temp charts to refer to,and some airlines have the electronic version.
Never seen or used one before though.

And to the OP:this winter when you fly from cold northern china and you try to input an ass of 28c (even if your runway is 12000ft long )it will not work either.

[keith williams]

It sounds like a case of very lazy software writing.

A message reading something like "assumed temperatures below 30C are not permitted at this altitude" would be far more helpful.

Or better still a nice seductive sounding (female) voice saying "I'm sorry Captain but you have selected an assumed temperature that is below the minimum limit. Would you like me to select the minimum limits?"

[framer]

We've been using the electronic flight bag for about three years now.The Boeing software we're using hasn't got the same fat layers added as our old paper manuals and as a result the de rates are greater than what we were used to, 60 degree assumed temps are common now. That took a bit of getting used to.

[barit1]

This appears to be a case of simply following a procedure, vs. gaining an understanding of how and why the system functions.

There are AV programs which describe Assumed Temperature Method, and if these videos are employed, the answers become self-evident. (I looked on YouTube but did not find any. Any ideas?)

[Denti]

Following procedure instead of knowing why seems to be a trend in this industry. Not nice, not good, but sadly true. Especially in areas of the world where either crew throughput is huge or airlines are growing like crazy, which basically amounts to the same thing.

FMC software is not a piece of consumer software and therefore doesn't have the same layer of explanations, assistants and what not, it assumes, probably wrongly, that the crew knows why some things are not possible.

@de facto, if you want to see how EFB performance works and you own an iPad simply download the Boeing OPT app which is for free and play around with it. The example aircraft is not a 737 so it gets some getting used to the weights, but the principle becomes quite clear fast.

[keith williams]

FMC software is not a piece of consumer software and therefore doesn't have the same layer of explanations, assistants and what not

The failure to provide useful messages isn't limited to FMC software. It has been a problem since software was first invented. In the early days it could be justified on the basis of limited memory availability. But with things such as pocket calculators now having more memory than Apollo spacecraft, that excuse is no longer plausible.

Whether we like it or not modern pilots are learning less and less about the fundamentals of their trade, so the software needs to be improved to compensate for this fact. Having meaningful error/fault messages would go some way to reducing the frequency of pilot finding themselves in the "What is it doing now?" situations.

[barit1]

Boeing presentation on ATM / Flex TO (http://www.scribd.com/doc/41746337/Reduced-Thrust-Takeoff)