aeroman444
Topic Author
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### Double Derate - 737

I know what a derate is, but what is the double derate function on 737 NG aircraft? Is it a set derate plus an assumed temp derate?

Thanks

pilotaydin
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### RE: Double Derate - 737

it's not a double derate.....

we have derate and assumed thrust...
derate is when we physically select the max thrust we can use from the engine, thus limiting the max N1 value, and saving engine life (egt decreases)

but sometimes you can also add an assumed temp to your derate, which basically fools the a/c into thinking it's carrying the maximum load of a certain temp, so the higher the temp, the less the load....

for example, we can take off at 79 tons on a 15 C day.....but let's say we only have 60 tons takeoff weight...so we know that the power setting we have can lift off 79 tons, but why use an extra 19 tons worth of energy, so what we do is find the temperature that would correspond to 60 tons, in other words, at what temperature is 60 tons the maximum we can lift? that would be something around 32 C.....so we tell the aircraft that it's 32C outside and it reduces the N1 for takeoff to whatever lower value is needed to lift 60 tons, it's a lil backwards when you think about it, but it makes sense if you re read the example....
The only time there is too much fuel onboard, is when you're on fire!

Jetlagged
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### RE: Double Derate - 737

 Quoting Pilotaydin (Reply 1):it's not a double derate.....

Assumed temperature is a derate by another name. So it's a derated derate at the very least.
The glass isn't half empty, or half full, it's twice as big as it needs to be.

speedracer1407
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### RE: Double Derate - 737

 Quoting Pilotaydin (Reply 1):derate is when we physically select the max thrust we can use from the engine, thus limiting the max N1 value, and saving engine life (egt decreases)

I'm confused about this part. I always thought that "assumed temp" was a method of "derate," not something separate.

I'm curious about how this works.
Dassault Mercure: the plane that has Boeing and Airbus shaking in their boots.

AAR90
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### RE: Double Derate - 737

"Double-Derate", "Derate", "Assumed Temp Derate"...yes, it can get very confusing. Here is the best non-technical description I have ever heard.

737NG aircraft come with the option to purchase "different engines." Physically all the same, but the buyer can purchase the option to have the pilots manually select the "different" engines: 22k thrust, 24k thrust, 26k thrust and (at least on AA 738s) "27k Bump" thrust. The pilots manually select the desired "engine" for each takeoff.

Once the desired engine is selected, the pilots now have the option of using a de-rated thrust setting FOR THAT ENGINE. So for example at DFW (13,000' runways) I will often have 24k thrust engine selected, then derate that (24k engine) using a 54 degree Assumed Temp. to give the final engine thrust settings (often less than 90% N1). If the situation prohibits a derated thrust takeoff, I simply delete the Assumed Temp and take off at "Max Thrust" (24k in this example). Even thought the physical engine is capable of more thrust, all calcualtions and performance is based upon a 24k thrust engine (because that is what I set using the FMS).

Confused? Just remember that the buyer has the option to pay for one engine (22k, 24k, 26k) or for any variation of engines/derates and Boeing will configure the software/hardware to whatever the buyer purchased.
*NO CARRIER* -- A Naval Aviator's worst nightmare!

speedracer1407
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Joined: Fri Dec 24, 2004 4:19 pm

### RE: Double Derate - 737

 Quoting AAR90 (Reply 4):737NG aircraft come with the option to purchase "different engines." Physically all the same, but the buyer can purchase the option to have the pilots manually select the "different" engines: 22k thrust, 24k thrust, 26k thrust and (at least on AA 738s) "27k Bump" thrust. The pilots manually select the desired "engine" for each takeoff.

Interesting. I had no idea. After all the threads in the past couple years about Derate/assume temp/flex thrust etc, I would have thought this would have come up.

I'm still confused about one thing: you say that the airline buys a certain thrust rating, which I was aware of. But you also say that the pilot "selects" the thrust rating for each mission. Do only the pilots of 737s that are equipped with the maximum thrust rated (26-27lb) CFMs get to "choose" which rating to use? If an airline purchases 22Klb engines for their 737s, do they eliminate the "derate" procedure, and only use "assume temp?"
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Jetlagged
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### RE: Double Derate - 737

All Boeings have fixed derate 1 and 2 available as well as assumed temperature derate. The fixed derates are just that, assumed temperature gives more flexibility. On GE engines, the fixed derates are a percentage decrement of rated thrust, something like -4% and -10%. On the older P&W engines, the fixed derates represented a lower thrust engine model, so for a JT9D-7R4G2, derate 1 might be equivalent to JT9D-7Q thrust, derate 2 JT9D-7A for example. I'm not sure how P&W fixed derates are calculated on newer engines.

I would imagine if the airline has not purchased the higher thrust limit, they can't uprate the engine to the higher thrust.

Airbus only use Flex thrust for derating, assumed temperature method, no fixed derates.
The glass isn't half empty, or half full, it's twice as big as it needs to be.

PGNCS
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### RE: Double Derate - 737

This has been around for a while. The MD-90 also has a pilot selectable thrust setting (25K or 28K), and some Airbus models also have the bump. Different operators use different procedures, and they can get rather unwieldy from time to time. Some of our early 767s didn't have the software to allow assumed temperature operations, so derate 1 or 2 was required for those aircraft, while the others in the fleet used assumed temperature, which is much more flexible. To my knowledge now that the oldest 767s are gone (I no longer fly the 757/767 domestic aircraft) they now all can take assumed temperature derates.

pilotaydin
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### RE: Double Derate - 737

 Quoting Jetlagged (Reply 2):Assumed temperature is a derate by another name. So it's a derated derate at the very least.

yes and no, once derated, then you just limit the thrust, not the max thrust...do it's a derate and an assume, a double derate would mean i could not go to the first derate power setting in case something went wrong during my assume.....   but i get your point
The only time there is too much fuel onboard, is when you're on fire!

zeke
Posts: 11449
Joined: Thu Dec 14, 2006 1:42 pm

### RE: Double Derate - 737

 Quoting Aeroman444 (Thread starter):I know what a derate is, but what is the double derate function on 737 NG aircraft? Is it a set derate plus an assumed temp derate?

I have not flown the 737, but I can give you some info on the CFM56 which I have some experience with.

We need to look at three things, they are the thrust set airline selects for the engines based upon its loads and routes, and the difference between reduced thrust and de rate.

First is what the routes are, if your flying long haul routes from either hot or high airports, chances are you will be "performance limited" over that route where you will have to offload payload so that you can meet the regulated takeoff performance if you were unlucky to have an engine failure. An airline may after balancing up the increased maintenance costs, and reduced time on wing for an engine, increase the thrust for that engine to be able to lift more payload. This increase of thrust is done by programming the engine by the mechanics on the ground, it costs money to get the software and manuals to make the engine have a higher thrust setting, this is a "hard derate" and does not change during flight. So now we have an aircraft that can just about lift all its payload off even on the worst sectors.

Next sector for that aircraft is a very short sector, and it is not very hot, so we don't need all the thrust for the long sector above, so the pilots can set a "soft derate" by inputting a derate percentage into the FMC before takeoff, that derate might be 10%, so you are going to get 90% of what the hard derate trust setting is, this is a soft derate that maybe only applicable for the takeoff, or takeoff and initial climb.

But even with this "soft derate" you maybe able to still meet all the single engine performance requirements by some margin, but not enough to go to the next step of 20% "soft derate", the next option is for what is known as a "reduced thrust takeoff", commonly know as "assumed temperature" on the Boeing or FLEX on the Airbus. This is not a derate per say, the engine is still capable of making maximum thrust, whereas with derates, the derate thrust level becomes the engine thrust limit in terms of TOGA, i.e. if you had an assumed temp and derate of 10%, the maximum thrust you should get is 90% of the hard derate.

To quote FAA AC 25-13
(4)(c) Reduced takeoff thrust, for an aeroplane, is a takeoff thrust less than the takeoff (or derated takeoff) thrust. The aeroplane takeoff performance and thrust setting are established by approved simple methods, such as adjustments, or by corrections to the takeoff thrust setting and performance." In this case, "the thrust for takeoff is not considered as a takeoff operating limit.",

this can be compared to a derate "(4)(b) Derated takeoff thrust, for an aeroplane, is a takeoff thrust less than the maximum takeoff thrust, for which exists in the AFM a set of separate and independent takeoff limitations and performance data that complies with all requirements of Part 25." In this case, "the thrust for takeoff is considered as a normal takeoff operating limit."

So with the assumed temperature we just "fool" the engine that we only need the thrust of a higher elevation airport by artificially telling it you are at a higher temperature (higher temp is the same as higher altitude in terms of reducing air density), this is again done via the FMC.

To keep it simple, a lot of short haul aircraft do not have the option of combining soft derates with assumed temperature, normally its on long haul aircraft that are occasionally used for short to medium haul flights, I am fairly sure the 777 allows you to derate and have an assumed thrust at the same time.

 Quoting Jetlagged (Reply 2):Assumed temperature is a derate by another name. So it's a derated derate at the very least.

That is not correct, "Assumed temperature" is a reduced thrust takeoff, with derates, VMCA/VMCG etc change as the thrust limit changes, they don't change with reduced thrust takeoffs because the thrust limit does not chanage.

 Quoting Jetlagged (Reply 6):Airbus only use Flex thrust for derating, assumed temperature method, no fixed derates.

That is not correct, on the 318-380 4/8/12/16/20/24/32/40% derates are available on the MCDU Takeoff Performance Page page if you have the DRT TO/ FLX TO prompt on the 4R, if you have just got FLEX TO, it means the company has not paid for the feature, but the aircraft is capable of it.
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AAR90
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### RE: Double Derate - 737

 Quoting Speedracer1407 (Reply 5):I'm still confused about one thing: you say that the airline buys a certain thrust rating, which I was aware of. But you also say that the pilot "selects" the thrust rating for each mission. Do only the pilots of 737s that are equipped with the maximum thrust rated (26-27lb) CFMs get to "choose" which rating to use?

I only know about the planes I have flown so I can't answer the "do only pilots of..." question. It is the first airplane I have flown where I have to first select the engine to be used, and _then_ the derate setting to be applied to that thrust setting.

 Quoting Speedracer1407 (Reply 5):If an airline purchases 22Klb engines for their 737s, do they eliminate the "derate" procedure, and only use "assume temp?"

I don't know for sure, but that is the way I have been told things work. You first select the engine to "put on the plane" (22k, 24k, 26k or 27k). That opens up the appropriate "derate" tables for that engine and you apply your derate (input an assumed temp) to that engine. There are different derate tables for each engine setting. If all you have purchased is a 22k engine, then that is the only option you will see in the cockpit. Similarly, if all your company purchased was a 26k engine, you would not see 22k and 24k options available... only 26k and (if purchased) an assumed temp derate option. It is whatever the buyer wants and the seller is willing to provide. For example:

 Quoting Jetlagged (Reply 6):All Boeings have fixed derate 1 and 2 available as well as assumed temperature derate.

I was not aware of that. AA's 757s used (the 10 years I flew them) two fixed derates only, no AT option.

 Quoting PGNCS (Reply 7):The MD-90 also has a pilot selectable thrust setting (25K or 28K),

I was not aware of that either. AA's (ex-QQ) MD90s used AT option only... there was no thrust setting option - we operated at max thrust (or AT applied to max thrust) only... 30k+ thrust engines.
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Jetlagged
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### RE: Double Derate - 737

 Quoting AAR90 (Reply 10):I was not aware of that. AA's 757s used (the 10 years I flew them) two fixed derates only, no AT option.

Interesting, I thought assumed temperature was a standard feature on the 757. You live and learn.
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Ryanair737
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### RE: Double Derate - 737

From http://www.b737.org.uk/assumedtemp.htm

A de-rate is a semi-permanent engine fix, used to reduce the maximum thrust available; for instance down to 20k from 22k on -3/700's. It is also used to equalise the thrust where B2 & C1 engines are mixed on the same airframe. When an engine is de-rated, the full (un-de-rated) thrust is no longer available because this would require changes to the EEC, HMU, fuel pump, engine ID plug and the loadable software; non of which can be done by the pilot in-flight.

A temporary form of de-rating known as a "T/O de-rate" is accessible through the FMC on TAKEOFF REF 2/2 or N1 LIMIT (NG's) but this is prohibited by some operators. The T/O de-rates (TO-1 & TO-2) can be 10 to 20%. It follows that an engine may be de-rated and also be using reduced thrust in which case you could be taking off at Full power -20% -25% = 60% of the full power of the engine - scary thought! Note that a T/O de-rate can overridden by firewalling the thrust levers; this action will give the thrust rating shown on the IDENT page.

 Quoting Aeroman444 (Thread starter):I know what a derate is, but what is the double derate function on 737 NG aircraft? Is it a set derate plus an assumed temp derate?

The de-rate option on the 737NG (if available in the airline) allows you use the assumed temp. method with a de-rate also. So for example you could choose to have a 22K de-rate and on top of that put the assumed temp. to let's say 34C to further reduce the N1%. Or you can just select the lowest de-rate you can take to lift the weight and leave it at that, it saves time and workload.

 Quoting Jetlagged (Reply 6):Airbus only use Flex thrust for derating, assumed temperature method, no fixed derates.

On the A330 you can do a fixed de-rate for the climb-out.

 Quoting AAR90 (Reply 10):I was not aware of that. AA's 757s used (the 10 years I flew them) two fixed derates only, no AT option.

That is very intresting as the RB211 on the 757 is rated at the same thrust in all operators of the aircraft.

737
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troubleshooter
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### RE: Double Derate - 737

HERE is an interesting document concerning this discussion.
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zeke
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### RE: Double Derate - 737

 Quoting Troubleshooter (Reply 13):HERE is an interesting document concerning this discussion.

That is reduced thrust, which is not the same as a derate.
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troubleshooter
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### RE: Double Derate - 737

 Quoting Zeke (Reply 14):That is reduced thrust, which is not the same as a derate.

Thanks. I know the differences. Have you read through the complete document? If not, you should do so...

First, Boeing describes the effects of a reduced thrust take-off and later in that document (beginning at page 41) they explain the derated thrust take-off method.
Additionally this document uses the B737NG as an example which was the aircraft in the original question of the topic starter.
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