Go Back  PPRuNe Forums > Flight Deck Forums > Tech Log
Reload this Page >

737-800 reduce thrust take-off???

Tech Log The very best in practical technical discussion on the web

737-800 reduce thrust take-off???

Old 7th Jul 2007, 21:59
  #1 (permalink)  
Thread Starter
 
Join Date: May 2006
Location: europe
Age: 51
Posts: 15
Likes: 0
Received 0 Likes on 0 Posts
737-800 reduce thrust take-off???

when you get the load-sheet how is your reduce thrust procedures?Do u use 22K 24K derate or just 26K assume performace?And what is the differents?
Some companies use 22K or 24K derate instead of 26K assume value.Anybody tell me the differents and what is your procedures?Thanks...

Last edited by boeing738driver; 9th Jul 2007 at 21:27.
boeing738driver is offline  
Old 7th Jul 2007, 22:45
  #2 (permalink)  
 
Join Date: Dec 2001
Location: Mostly FL360-380, M0.78
Posts: 247
Likes: 0
Received 0 Likes on 0 Posts
We use 27K engines on our -800's and use the assumed method only.

One of my previous employers used one of two engine ratings 22K/20K and assumed thrust also.. so whatever gave the least engine wear we would use eg. 20k derating and 40 C assumed.
Jetavia is offline  
Old 7th Jul 2007, 22:51
  #3 (permalink)  
 
Join Date: Feb 2005
Location: flyover country USA
Age: 80
Posts: 4,579
Likes: 0
Received 0 Likes on 0 Posts
The problem with not having 20 or 22K derate is on a contaminated r/w - no flex ops, and thus higher Vmcg than if derated.
barit1 is offline  
Old 8th Jul 2007, 12:31
  #4 (permalink)  
 
Join Date: Jan 2002
Location: Brisbane, Australia
Posts: 960
Likes: 0
Received 0 Likes on 0 Posts
If I may....

I think what "barit1" is trying to say is...

The Vmcg value is different between derated and assumed temp reduced thrust takeoffs.
Basically, when derated thrust is used, the takeoff Vmcg is still calculated using maximum thrust, since the derated thrust setting may be increased during the takeoff, if considered necessary.
But when assumed temperature reduced thrust is used, that is the thrust setting used to calculate Vmcg, since (except in real emergencies) that is considered the maximum thrust setting for the takeoff.

So, as you can see, in some shorter runway situations, you can sometimes lift more weight with a/temp reduced thrust than you can with derated power, simply because of the restriction on V1 with regard to Vmcg.

Am I making any sense here?

Cheers...FD
Flight Detent is offline  
Old 8th Jul 2007, 12:49
  #5 (permalink)  
Moderator
 
Join Date: Apr 2001
Location: various places .....
Posts: 6,940
Likes: 0
Received 8 Likes on 7 Posts
.. sort of close, but no banana ... unless you do the old walnut trick ..

(a) derate is similar to unbolting the big engine .. and bolting on a little one ... the max rated (available) thrust goes down and the original engine's higher rated thrust is no longer available ... in the real world with the FMC controlling the above, one may NOT retrieve the higher rated thrust as that would invalidate all the limit thrust sums .. Vmcg relates to the limit thrust associated with the particular rating

(b) flex is a part max thrust takeoff but the limit thrust sums remain based on the rated thrust level .. ie one can push up the throttles if that is to your liking on the day ...

(c) derate and flex can be combined in which case (b) applies after (a). However, the derate thrust level should not be exceeded to avoid frights .. and Vmcg/Vmca provide big frights if the thrust goes too high .. or even if it comes up too quickly sometimes

.. but, that's OK .. you're a sailor so you must be an OK sort of bloke ..
john_tullamarine is offline  
Old 8th Jul 2007, 12:58
  #6 (permalink)  
 
Join Date: Jul 2000
Location: Thailand
Posts: 942
Likes: 0
Received 0 Likes on 0 Posts
As a general principle, operators should endevour to use the maximum degree of derate/reduction combination whenever conditions permit. (notwithstanding slippery runways, cross winds etc.)
Engine life can be greatly extended by the use of derate/reduced thrust.
It also produces less noise.
rubik101 is offline  
Old 8th Jul 2007, 13:11
  #7 (permalink)  
 
Join Date: Jan 2002
Location: germany
Posts: 114
Likes: 0
Received 0 Likes on 0 Posts
F D,

I think you are almost right, itīs exactly the other way around.

Letīs assume you derate from 27k to 24k, then the Vmcg wil be calculated for 24k. So your green bugs will show the max thrust rating, in that case 24k. Increasing thrust is therefor only allowed till you reach the bugs, where you should be anyhow, since no Ass Temp on slippery RWY. You could apply more power, but that might spoil your day if Vmcg limited.

Makes only sense if not RWY limited, e.g. short sector, light aircraft, slippery RWY. Then you rather have a directional control problem (increases with rating selected) since there is a great V1 Vr split. Using less power makes keeping the CL easier, still giving plenty of acceleration.

On a short slippery RWY with heavy weight, that is no option, either use full power, or stay and wait.

Let me know if you see it differently

Regards

Repulo
repulo is offline  
Old 8th Jul 2007, 18:32
  #8 (permalink)  
 
Join Date: Jul 2000
Location: Thailand
Posts: 942
Likes: 0
Received 0 Likes on 0 Posts
KISS. Keep it simple, somone said!
Don't be misled or sidetracked by considerations of Vmcg. During normal operations, Vmcg will not be a factor in Take-Off calculations. I assume you are considering the situation where V1 might be at or close to Vmcg after taking into account the derate/reduction calculation. Remember, V1 will never be less than Vmcg. Should you encounter an engine failure below V1 and elect to abandon the Take-off then you will fulfill all the performance requirements.
If the engine fails at or above V1 then nothing needs to be done to the operating engine to fulfill the obstacle clearance requirements for the 1st and 2nd segment climb sectors.
If obstacle clearance becomes a factor then provided you are flying at V2 plus 15 kts you will always be safe if you then advance the thrust to Go-around power. You just need a bigger bootfull of rudder.
Forget numbers, apply logic!
rubik101 is offline  
Old 8th Jul 2007, 22:48
  #9 (permalink)  
2XL
 
Join Date: Aug 2000
Location: anywhere for cash
Posts: 130
Likes: 0
Received 0 Likes on 0 Posts
So if I have a 26k engine, derate it to 24k via fmc and input an assumed temp of say 34, can someone answer this question. At what point on the take off roll or airbourne do I have the full 26k capability again?
2XL is offline  
Old 8th Jul 2007, 23:01
  #10 (permalink)  
 
Join Date: Aug 2001
Location: Winter Hill
Posts: 38
Likes: 0
Received 0 Likes on 0 Posts
On the second press of TOGA perhaps ???
Avadoo is offline  
Old 8th Jul 2007, 23:09
  #11 (permalink)  
 
Join Date: Dec 2005
Location: Istanbul
Posts: 1
Likes: 0
Received 0 Likes on 0 Posts
i've tried the second to-ga push lots of times but nothin happens at all...you'll get your 26k n1 when ever you want, just go to n1 page and delete the selected CLB (CLB-1 or CLB-2) i u dont will auto. change at 15100feet.But if u selected a 26k assume thrust will change at a earlier alt.
thycpt737 is offline  
Old 9th Jul 2007, 08:38
  #12 (permalink)  
 
Join Date: Jul 2000
Location: Thailand
Posts: 942
Likes: 0
Received 0 Likes on 0 Posts
Take the case of an engine rated at 26k, derated to 22k and reduced to 50C or any degree of derate/reduction in between.
Pressing the TOGA button for a second time will simply give you G/Around power for the selected derate/temperature combination selected in the FMC.
2XL, in answer to your specific question; To get 26k you will need to manually push the thrust levers forward to the N1 you noted earlier during your pre departure briefing. (you did make a note of the 26k N1, didn't you?)
The power will automatically and gradually revert to full rated power at FL 150 (watch it change from R/CLB to CLB as you pass 150 next time)
To regain full power earlier, as mentioned, delete the CLB 1 or CLB 2 reduction from the FMC when full CLB power will be restored.
I have to ask though, why would you want to restore full power any earlier than that in the event of a normal take off or even if you have an engine failure? In the first case all climb considerations are covered, in the second you are climbing to MSA and will, in all liklyhood, return to the field of departure.
Being given a direct routing over high terrain or to get over bad weather on departure would be two good reasons to delete the reduction but otherwise, I don't see the point.
rubik101 is offline  
Old 9th Jul 2007, 10:35
  #13 (permalink)  
 
Join Date: Jun 2000
Location: last time I looked I was still here.
Posts: 4,507
Likes: 0
Received 0 Likes on 0 Posts
Pressing the TOGA button for a second time will simply give you G/Around power for the selected derate/temperature combination selected in the FMC.

Is this true? Will it not give you full G/A N1% for the ambient conditions; i.e. the G/A N1% that you would find if you pushed N1 button on FMC? G/A is not effected by derates & assumed temps.
RAT 5 is offline  
Old 9th Jul 2007, 21:15
  #14 (permalink)  
Thread Starter
 
Join Date: May 2006
Location: europe
Age: 51
Posts: 15
Likes: 0
Received 0 Likes on 0 Posts
We have no any RWY limited.We get the load sheet and OAT is lets say 23C and we get the 22K 37C assume refer to the current load.Again in the same conditions and the same weight we get 26K 56C assume value...We get less N1 than 22K and this means more protection of the engines.

According to our company procedures; we get the load sheet,first 22K assume considered,if it's not possible than 24K assume,still not finally we goes for 26K assume...

Could you please let me know why do we both do 22 derate and assume?Is it your procedure too?

Last edited by boeing738driver; 9th Jul 2007 at 21:26.
boeing738driver is offline  
Old 10th Jul 2007, 06:54
  #15 (permalink)  
 
Join Date: Jan 2002
Location: Brisbane, Australia
Posts: 960
Likes: 0
Received 0 Likes on 0 Posts
Oh my gosh...
I just read my ravings again and you are quite right, I've quoted the effects exactly opposite to their actual correct meanings.
I apologise to all and will now refrain from ever quoting this stuff again.
No excuses, I got it wrong!
Cheers...FD...

and, jt, i'm not qf!
Flight Detent is offline  
Old 10th Jul 2007, 19:36
  #16 (permalink)  
 
Join Date: Jan 2002
Location: germany
Posts: 114
Likes: 0
Received 0 Likes on 0 Posts
Donīt worry too much F D, have been instructing on the 737 for 4 years and you donīt want to know how often I found out later that I was talking total bull....
What really separates men from boys is admitting having screwed up!
repulo is offline  
Old 11th Jul 2007, 06:52
  #17 (permalink)  
 
Join Date: Jun 2006
Location: ---
Posts: 281
Likes: 0
Received 0 Likes on 0 Posts
Engine life can be greatly extended by the use of derate/reduced thrust.
It also produces less noise.
Although you might find that your noise footprint will be greater since you have less performance.
I personally prefer the deep rumble over full thrust max rating over this "screaming" of derated and assumed temp'ed engines.
But that's just me. I realize as well it saves a fair amount of cash when reduced thrust is used..
ray cosmic is offline  
Old 12th Jul 2007, 04:07
  #18 (permalink)  
 
Join Date: Jun 2004
Location: Australia
Posts: 1,843
Likes: 0
Received 0 Likes on 0 Posts
There's a zillion considerations for choice of thrust rating as related to aircraft performance, primarily related to Vmcg, wet, contaminated runways etc., but to consider the commercial implications.....

If your engine has 3 thrust ratings, i.e. 22K, 24K or 26K, and reduced thrust (Assumed / Flex) is available for all of them, consider a day when 20K is required for the particular weight and performance scenario.

Any of the 3 ratings with an assumed temperature will satisfy the 20K requirement, e.g. 22K @ 40°, 24K @ 45°, or 26K @ 50° (I just made those numbers up) will all produce 20K thrust at the same EPR / N1 / N2 / EGT, thus there is no difference in engine wear regardless of which rating that you use with the appropriate Assumed Temperature.

Manufacturer's guarantees do not consider thrust reduction, the benefit gained here is in the hangar with reduced ACTUAL engine wear. The manufacturer ASSUMES that you do use the full value of 22K, 24K, or 26K, and does apply the guarantees applicable to the number of occasions that each rating is used.

There are therefore 2 savings -

(1) Using Thrust reduction (Assumed / Flex) reduces actual engine wear, and reduces maintainance costs, AND

(2) Using the lowest RATING possible (22, 24, or 26K, regardless of thrust reduction) increases the warranty from the manufacturer, further reducing costs with an improved guarantee.

If safety of operation calls for it, to hell with the cost, use the highest possible thrust consistent with safety but plan it in advance and work the V speeds appropriately. If you're doing a 22K Takeoff on a wet runway with lower V1 speeds, and go to the full 26K at V1 with an engine failure, you're gonna lose it! Except in the rare cases (al low weights) where V2 is governed by Vmca, there should be no problem in advancing to full thrust following engine failure after V2 has been reached.

Regards,

Old Smokey
Old Smokey is offline  
Old 12th Jul 2007, 04:48
  #19 (permalink)  
 
Join Date: Jul 2006
Location: Earth
Posts: 6
Likes: 0
Received 0 Likes on 0 Posts
There is a the consideration that, with at least the FAA, the maximum flex/thrust reduction you can have from full-rated is 25% (thrust, not N1).

So, if you compute your takeoff performance with a 22K full-rated takeoff and then apply a maximum 25% reduction, then you can have a deeper effective thrust reduction than if you used a 26K rating.

That is, (22K x 75%) = 16.5 == 36.5% reduction from 26K.
Checkers is offline  
Old 12th Jul 2007, 21:14
  #20 (permalink)  
Thread Starter
 
Join Date: May 2006
Location: europe
Age: 51
Posts: 15
Likes: 0
Received 0 Likes on 0 Posts
Thanks for the replies everyone...
boeing738driver is offline  

Thread Tools
Search this Thread

Contact Us - Archive - Advertising - Cookie Policy - Privacy Statement - Terms of Service - Do Not Sell My Personal Information

Copyright © 2022 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.