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Faro
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Flex Thrust, Derate & Bump

Tue Jul 21, 2009 11:59 am

What is the difference between flex and derate thrust? What is bump thrust?

Also, when were these reductions first regularly applied in line service? I imagine with their relatively low-thrust turbojets, early jet airliners like the 707 & DC-8 had to use full thrust 100% of the time.

Faro
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wilco737
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RE: Flex Thrust, Derate & Bump

Tue Jul 21, 2009 12:01 pm



Quoting Faro (Thread starter):
What is the difference between flex and derate thrust? What is bump thrust?

Derate: as the name says: you derate the engine thrust available. back on the 737 we had usually 20k, but with one button you could derate it to 18k.
Flex: you "tell" the engine that it is a lot hotter than it actually is (FLEX temperature) and the engine cannot give as much thrust in high temperature conditions than in low temperature conditions. So the engine reduces the N1 and there you go  Smile
Bump thrust? never heard of...

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Mir
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RE: Flex Thrust, Derate & Bump

Tue Jul 21, 2009 7:09 pm



Quoting WILCO737 (Reply 1):
Bump thrust? never heard of...

I think a thrust bump is something that you can do if you need the extra performance to make it off of a short or high or hot (or all three) runway. It's kind of like a derate, in that it's all software. There are limits on how often you can do it, though, due to the extra wear it puts on the engines.

-Mir
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wilco737
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RE: Flex Thrust, Derate & Bump

Tue Jul 21, 2009 7:15 pm



Quoting Mir (Reply 2):
I think a thrust bump is something that you can do if you need the extra performance to make it off of a short or high or hot (or all three) runway. It's kind of like a derate, in that it's all software

Thanks. Still never heard of it. We have a max certified thrust we can use. In case of a whindshear or GPWS encounter we can overboost the engines. Just advance the thrust levers beyond the overboost stop bar and then the engines will give all the thrust which is possible, even going beyond its limits. The FADEC will be switched off automatically and the engines need a huge inspection afterwards. But still better than flying into the ground...

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wingscrubber
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RE: Flex Thrust, Derate & Bump

Tue Jul 21, 2009 7:47 pm

I was once told by a BA maintenance engineer that their 777s have a special 'denver' FADEC setting for extra thrust for hot/high takeoff performance, maybe that's like 'bump thrust'?
Also, I do know the Citation X AE3001C engines are derated permanently for that airframe, but not on the embraer RJs.
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pilotpip
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RE: Flex Thrust, Derate & Bump

Tue Jul 21, 2009 9:30 pm



Quoting Wingscrubber (Reply 4):
Also, I do know the Citation X AE3001C engines are derated permanently for that airframe, but not on the embraer RJs.

Not true. They are available with different thrust ratings. The 145EP is lower than the LR for instance.
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glen
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RE: Flex Thrust, Derate & Bump

Wed Jul 22, 2009 11:58 am



Quoting WILCO737 (Reply 1):
Flex: you "tell" the engine that it is a lot hotter than it actually is (FLEX temperature) and the engine cannot give as much thrust in high temperature conditions than in low temperature conditions. So the engine reduces the N1 and there you go

 checkmark 
This method is used to reduce take-off thrust and thus reducing EGT at take-off.

Quoting WILCO737 (Reply 1):
Derate: as the name says: you derate the engine thrust available.

 checkmark 
This method is used to reduce overall thrust available in climb, thus reducing fuel consumption. On the A320-family we had all the same engines on A319/320/321. By pin-programming they could be rated/derated differently, according to type (A319 with lower weight was more derated). With a engine which is more derated you can keep your max. climb thrust up to higher altitudes, before your thrust decreases due to lower air-pressue.
Now on the A333 we can even derate the engines through the FMS in case of low take-off weights.

Never heard of bump thrust. But it's may be a different name for a possibility we had on the MD11. By breaking a mechanical stopbar with the throttles, you could get more thrust in case of emergency (e.g. terrain avoidance), but you would also overboost the engine in this case (it was called "overboost stop bar").
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wilco737
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RE: Flex Thrust, Derate & Bump

Wed Jul 22, 2009 1:21 pm



Quoting GLEN (Reply 6):
Never heard of bump thrust. But it's may be a different name for a possibility we had on the MD11. By breaking a mechanical stopbar with the throttles, you could get more thrust in case of emergency (e.g. terrain avoidance), but you would also overboost the engine in this case (it was called "overboost stop bar").

See reply 3 from me. That is exactly what we do nowadays on the MD11F as well  Smile

Quoting GLEN (Reply 6):
his method is used to reduce overall thrust available in climb, thus reducing fuel consumption.

On the 737 you can do it for take off as well. 20k, 18k, 16k. And for the climb there are 3 as well: Full climb thrust (CLB), reduced one (CLB-1) and reduced two (CLB-2). The FMS selects them automatically according to the FLEX temperature. The higher the FLEX temperature the higher the climb thrust reduction as well.

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glen
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RE: Flex Thrust, Derate & Bump

Wed Jul 22, 2009 2:15 pm



Quoting WILCO737 (Reply 7):
On the 737 you can do it for take off as well. 20k, 18k, 16k.

Sounds quite sophisticated.

Quoting WILCO737 (Reply 7):
The FMS selects them automatically according to the FLEX temperature. The higher the FLEX temperature the higher the climb thrust reduction as well

FLEX temperature is normally not only depending on weight but also on apt. elevation, runway length, rwy condition, meteorological conditions (wind component, QNH) While in climb these factors have no more influence. So how reasonable/reliable is this automated selection of the derate level? Or is it just depending on the weight entered into the FMS?
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longhauler
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RE: Flex Thrust, Derate & Bump

Wed Jul 22, 2009 2:28 pm

On the EMJ series of aircraft, 170/175/190/195, you can derate the engines using the FMS/takeoff dataset.

For example on the E175, using Takeoff 1, 2 or 3 settings, you are setting the max thrust to 13,800, 13000 or 11,800 respectively. Then after having set the max thrust, you can then "flex" the thrust even lower given current conditions.

Also, if anything after takeoff is encountered, like engine failure, windshear or terrain concerns, using the ATTCS (automatic take-off thrust control system) when armed by default, will "bump" thrust into a RSV (reserve) regime, allowing more thrust. This must be included in take-off performance calculations, as you can well imagine, RSV thrust in a one engine/low speed condition is not good and must be accounted for!

As aircraft get newer, they get so much more advanced. I couldn't even imagine such advances in the old B737-200 days 20 years ago!
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wilco737
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RE: Flex Thrust, Derate & Bump

Wed Jul 22, 2009 2:36 pm



Quoting GLEN (Reply 8):
So how reasonable/reliable is this automated selection of the derate level? Or is it just depending on the weight entered into the FMS?

I am only talking about the automatic selection of the climb thrust. We never used the derated take off, only the FLEX temperature to reduce the thrust.
We have a laptop with a certified performance programm which takes all that into consideration. The FMS would never select CLB-2 if you need full thrust take off. Climb thrust is always same or lower than take off thrust. No problem for the performance as the programm knows about th automatic climb thrust selectino of the FMS.

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AAR90
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RE: Flex Thrust, Derate & Bump

Wed Jul 22, 2009 3:30 pm



Quoting WILCO737 (Reply 7):
On the 737 you can do it for take off as well. 20k, 18k, 16k.

Obviously, the specific numbers are specific to the aircraft/engine/airline combination. AA 738/CFM56's utilize 22k, 24k, and 26k. First select the engine (thrust setting) to be used on this particular flight, then apply an "assumed" [FLEX] temp. to reduce engine thrust (and internal temps) even more. All about prolonging engine life and saving $$$.

Quoting GLEN (Reply 6):
Never heard of bump thrust.

AA 738's utilize a "27K Bump" setting for KSNA departures only. The CFM56 engines are actually designed for more thrust capability than that, but using higher thrust setting reduces engine life and increases costs. AA restricts use of 27K thrust setting to KSNA departures and, with the elimination of the special noise abatement procedures there for AA738s, we only use it [27K] for hot/heavy situations. Like all the rest, just software settings programmed prior to takeoff.
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wilco737
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RE: Flex Thrust, Derate & Bump

Wed Jul 22, 2009 3:35 pm



Quoting AAR90 (Reply 11):
Obviously, the specific numbers are specific to the aircraft/engine/airline combination. AA 738/CFM56's utilize 22k, 24k, and 26k. First select the engine (thrust setting) to be used on this particular flight, then apply an "assumed" [FLEX] temp. to reduce engine thrust (and internal temps) even more. All about prolonging engine life and saving $$$.

I guess you are talking about the NG? I only flew the classic (300/500) at LH. Back at HLF I flew the NG and there we had 26k (27k), 24k and 22k, just like you said.

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Goldenshield
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RE: Flex Thrust, Derate & Bump

Wed Jul 22, 2009 5:39 pm

As far as "bump" thrust, the CRJ has APR (Auxiliary Performance Reserve), which allows for more than normal thrust should you lose an engine.
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Ward86IND
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RE: Flex Thrust, Derate & Bump

Wed Jul 22, 2009 5:57 pm

www.smartcockpit.com/.../Flex_and_Derate_Takeoff_and_Climb.pdf

explains all of the above as used on the A320 series
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Mir
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RE: Flex Thrust, Derate & Bump

Wed Jul 22, 2009 7:20 pm

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Faro
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RE: Flex Thrust, Derate & Bump

Thu Jul 23, 2009 8:44 am



Quoting GLEN (Reply 6):
Quoting WILCO737 (Reply 1):
Flex: you "tell" the engine that it is a lot hotter than it actually is (FLEX temperature) and the engine cannot give as much thrust in high temperature conditions than in low temperature conditions. So the engine reduces the N1 and there you go

  
This method is used to reduce take-off thrust and thus reducing EGT at take-off.

Quoting WILCO737 (Reply 1):
Derate: as the name says: you derate the engine thrust available.

  
This method is used to reduce overall thrust available in climb, thus reducing fuel consumption.

So to summarise my understanding:

- Derate thrust is an FMS/FADEC tweak that tells the engine to limit its maximum thrust to a percentage of maximum installed thrust on an aircraft by aircraft basis;
- Flex thrust is an FMS "derate of the derate" that limits the derate thrust on a take-off by take-off basis; and
- Bump thrust is an instantaneous disabling (via the thrust levers) of the derate and flex thrust settings to restore access to maximum installed thrust in engine-out, GPWS and other emergency conditions.

Is this correct?

Faro
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wilco737
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RE: Flex Thrust, Derate & Bump

Thu Jul 23, 2009 8:54 am



Quoting Faro (Reply 16):
Derate thrust is an FMS/FADEC tweak that tells the engine to limit its maximum thrust to a percentage of maximum installed thrust on an aircraft by aircraft basis;

 checkmark 

Quoting Faro (Reply 16):
Flex thrust is an FMS "derate of the derate" that limits the derate trust on a take-off by take-off basis

Not 100%. FMS derate of the full thrust available. Sure you can use the FLEX as well after you derated the engine via the FMS as well. But usually you use the normal engine thrust rating and derate it via the FLEX temperature.

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Mir
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RE: Flex Thrust, Derate & Bump

Thu Jul 23, 2009 9:48 am



Quoting Faro (Reply 16):
Bump thrust is an instantaneous disabling (via the thrust levers) of the derate and flex thrust settings to restore access to maximum installed thrust in engine-out, GPWS and other emergency conditions.

Can be, or it can be a pre-planned takeoff procedure where necessary.

-Mir
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BALandorLivery
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RE: Flex Thrust, Derate & Bump

Thu Jul 23, 2009 11:53 am

BUMP thrust is only a feature of IAE V2500 engines.


It is not a derate or flex. It is EXTRA power.

100% power is usually TOGA but............

On V2500 engines if performance neccessitates you can select BUMP thrust to give you a little extra power that is higher than the normal TOGA power setting.

It is armed by pressing a red button found at the back of the thrust levers after both engines start only on V2500 powered aeroplanes. There is no varying the power. It is a set thrust rating.

It is not the preferred method of getting away because it really wears the engine down a lot and one BUMP take off costs roughly the same as 7 TOGA take off's

It's essentially a turbo button  Wink
 
AAR90
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RE: Flex Thrust, Derate & Bump

Thu Jul 23, 2009 4:04 pm



Quoting Balandorlivery (Reply 19):
BUMP thrust is only a feature of IAE V2500 engines.

Every airliner/engine/airline combination can be (and often are) different. ex-QQ MD90s w/V2500 engines did NOT have this BUMP feature. OTOH, every AA B738 w/CFM56 engines DO have this feature.... programmed via FMS.



Quoting Faro (Reply 16):
Derate thrust is an FMS/FADEC tweak that tells the engine to limit its maximum thrust to a percentage of maximum installed thrust on an aircraft by aircraft basis;
- Flex thrust is an FMS "derate of the derate" that limits the derate thrust on a take-off by take-off basis

Correct. Think of it this way:

1. Select the engine you want to put on the airplane for this particular takeoff. For 737NG: your options are: "22k" engine, "24k" engine, "26k" engine, or "27k BUMP" engine.

2. NOW select an Assumed Temperature (if desired) that will "derate" the thrust [flex thrust] for that particular engine on that particular takeoff. Examples: my LAX-25R takeoff was "24k MAX" from the TXY-F intersection, but if we had used the full length of the runway we were supposed to use "22k-42" -- 22k thrust engine at 42 degree assumed temp. (a much lower thrust setting).

Quoting Faro (Reply 16):
Bump thrust is an instantaneous disabling (via the thrust levers) of the derate and flex thrust settings to restore access to maximum installed thrust in engine-out, GPWS and other emergency conditions.

BUMP is a term used when the engine thrust rating is going to be set at a point higher than the official engine thrust rating the engine was ORIGINALLY certified at. For 737NG/CFM56, the original engine certification was 22k, 24k, 26k. Boeing & CFM obtained an additional 27k rating, but it is not a normal option that comes with the basic aircraft certification. The buyer has to pay for the higher thrust rating (higher purchase price, lower warranty specifications). Hence the "odd" naming convention of BUMP. But in practice, it is nothing more than a fourth engine option to put under the wing of a B737NG.

BUMP is not supposed to be a normal option available for use anytime the buyer wishes. AA's 738 fleet was purchased with the 27k Bump option, but its use is restricted to KSNA only. Use anywhere else (naturally, all takeoff engine performance is recorded) immediately voids ALL engine warranties and increases engine overhaul costs --if AA paid Boeing/CFM to perform an overhaul of that engine.
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jetlife2
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RE: Flex Thrust, Derate & Bump

Fri Jul 24, 2009 12:39 am



Quoting Balandorlivery (Reply 19):
BUMP thrust is only a feature of IAE V2500 engines

Not the case...There are many other engine/aircraft combinations with so-called "bump" ratings available. The use of this term is very inconsistent. I will confine my discussion to GE/CFM engines and others will have to speak for the other guys.

In GE/CFM engines a bump is defined as the following: "A supplemental power management of the engine, which may or may not be defined as a distinct rating, offering enhanced thrust capability in a defined range of altitude and/or ambient temperature and/or Mach number". The term "bump" originates because usually these additions are beyond cornerpoint and appear on a chart as a modification of the N1-Tamb line from a straight line to a bump (increase) and back to the original line.

For example the so-called "Denver bump" referred to in an earlier post is a modification of the GE90 power management to offer enhanced thrust in certain altitude and ambient temperature conditions. In that case it was a redefinition of the basic power management and did not result in a separate rating. In other cases the bump capability is a separate option and defined by a distinct rating, and power management that includes the bump is included in engines having that rating only. Usually for modern engines the bump can be purchased either as an original installation or retrofit option that involves normally changing only the rating plug: all power management definitions exist in the FADEC and each is enabled by the installed rating plug.

Note that there may be commercial limitations separate from technical limitations: bumps are thrust, which is what engine manufacturers sell, and sometimes they are not cheap. They may offer the capability to get the airplane out full of revenue pax and cargo, or not. So they can bring a lot of value. So a customer may (unusually) buy a limited number of bump ratings for their fleet with the agreement that they can b used anywhere in the fleet provided no more than x are in use. In this way it could appear to the casual observer that the bump has restrictions in its use - in fact these are not technical but commercial limits.

FWIW

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Faro
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RE: Flex Thrust, Derate & Bump

Fri Jul 24, 2009 9:04 am



Quoting Jetlife2 (Reply 21):
Note that there may be commercial limitations separate from technical limitations: bumps are thrust, which is what engine manufacturers sell, and sometimes they are not cheap. They may offer the capability to get the airplane out full of revenue pax and cargo, or not. So they can bring a lot of value. So a customer may (unusually) buy a limited number of bump ratings for their fleet with the agreement that they can b used anywhere in the fleet provided no more than x are in use. In this way it could appear to the casual observer that the bump has restrictions in its use - in fact these are not technical but commercial limits.

So BUMP has quite a significant commercial dimension to it. In a way it's like an authorised, limited transgression of nominal thrust limits provided against cash payment. I wonder whether the concept of BUMP originated in the R&D/production or marketing department...

Quoting AAR90 (Reply 20):
BUMP is not supposed to be a normal option available for use anytime the buyer wishes. AA's 738 fleet was purchased with the 27k Bump option, but its use is restricted to KSNA only. Use anywhere else (naturally, all takeoff engine performance is recorded) immediately voids ALL engine warranties and increases engine overhaul costs --if AA paid Boeing/CFM to perform an overhaul of that engine.

Voids all warranties! That's pretty draconian! I imagine if one uses it you outside of KSNA and didn't have an iron-clad reason for doing so it could put a nice little dent in your career...

Faro
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longhauler
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RE: Flex Thrust, Derate & Bump

Fri Jul 24, 2009 1:19 pm



Quoting Faro (Reply 22):
Voids all warranties! That's pretty draconian!

I recall many many years ago, we used to use a concept called "Black Power" on the DC-10-30ER for our YYZ-NRT flights. We tried every possible way to calculate takeoff performance without using Black Power, as each engine core was only allowed so many before maintenance functions had to be completed.

While this was long after the engine warranty expired, it was however approved by GE.

I remember that MAX EGT was 900C, and there was a warning light above each EGT guage, and every one was always illuminated on a Black Power takeoff. We used to roar across Derry Road, (just north of YYZ after taking off from 33) at a very low altitude and would see the diners at the burger joint look up in terror as we passed overhead!

Incidentally, I don't know whether "Black Power" was a MDD/GE term or whether it was a company internal term.
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