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Derate vs ATM

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Derate vs ATM

Old 14th May 2018, 12:00
  #41 (permalink)  
 
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I have remarked that as you use higher power rating, longer is your Engine inop go distance but shorter is your ASDR and All engine go distance (normal because your reach a certain speed faster)

However note this about the Engine inop go distance, for same conditions:
at 26K full (the strongest) it is 2625 m, at 24K ATM 30C (the weakest in our example) it is 2165 m

I know it is not linear and we can't interpolate because V1 varies same as the time to reach V1 HOWEVER it should be logically between these distances.
That's why I said 3051 m is just not logical.
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Old 14th May 2018, 13:04
  #42 (permalink)  
 
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Is the assumed temperature limit assuming that full thrust is applied on the remaining engine for the OEI go case?
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Old 14th May 2018, 13:46
  #43 (permalink)  
PAG
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Alex, FCTM just says:

With DERATE (ie 24K)
"If an engine failure occurs during takeoff, any thrust increase could result in loss of directional control"

with ATM (ie 26K 42C)
"If more thrust is needed during ATM takeoff following an engine failure, thrust on the operating engine may be increased to full rated thrust by manually advancing thrust levers. This is because Takeoff speeds consider VMCg and VMCa at the full rated takeoff thrust."

with DERATE and ATM (ie 24K 32C)
"If an engine failure occurs during takeoff, any thrust increase beyond the fixed derate limit could result in loss of directional control and should not be accomplished unless in the captain's opinion, terrain contact is imminent."
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Old 14th May 2018, 19:38
  #44 (permalink)  
 
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The problem in this case of OP is that the POLICY for V1 is not known to the pilot.
There are various policies that can be set for different configurations, eg FULL v ATM can be different.
Without having access to the config file your question cannot easily be answered.

As suspected by other authors, my guess is that the policy is set to use the V1 min for FULL thrust departures, but not for ATM.
Regarding Balanced Filed Length performance of FMC, beware that this does NOT include obstacle clearance calculations, hence OPT may be different as it does take these into account, and others for that matter...

As often is the case, john_tullamarine is the guy to ask for the details and has explained various things very well.
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Old 18th May 2018, 10:47
  #45 (permalink)  
PAG
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Hi everyone one, here is the answer from Performance Department, I will update my first topic:

"With full 24K there is a range of V1 options between v1min and v1max that will satisfy minimum regulatory margins (obstacle clearance, 2nd and 4th segment climb, screen height etc.) The driver for the engine inop go distance is the v1min, in this case 94kts. This means that aircraft will accelerate to 94kts (relatively quickly) and can continue with a longer engine out ground roll to Vr. On the other hand the driver for the ASD is the V1max.

The V1 selection is impacted by the weight and the thrust. With a lower thrust set we are no longer presented with a range of V1 speeds but with V1 speed that will just satisfy both the go case and stop case. (balanced field). So in answer to your question the OEIGD 24K is greater than the OEIGD R24K due to lower V1min.
Ryanair’s policy on V1 selection is OPTIMUM. Optimum gives the return on weight when factoring in available clearways and stopway. For OPT to choose the optimum V1 there needs to be a range of V1 speeds possible"
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Old 19th May 2018, 01:16
  #46 (permalink)  
 
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Originally Posted by Alex Whittingham View Post
Is the assumed temperature limit assuming that full thrust is applied on the remaining engine for the OEI go case?
The Boeing ATM is based on how the aircraft would perform at a given temperature. Say the aircraft weights 200 tonnes. Looking on a chart we'll say the maximum temperature we can depart from runway XX is 50 degrees. Looking at another chart the engine will produce xx% power at 50 degrees. Therefore weighing 200 tonnes we can set that xx% power and meet all the performance requirements. As PAG said, with the ATM only, the crew can increase the thrust but it is not required to do so to meet the performance standards.
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Old 19th May 2018, 10:12
  #47 (permalink)  
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with the ATM only, the crew can increase the thrust but it is not required to do so to meet the performance standards.

Not only is increasing the thrust setting not required (other than, perhaps, in situations of impending ground contact), it is potentially hazardous unless you have practiced the sequence well in the sim. A thrust increase puts a very high cognitive and hand-eye-leg co-ordination workload on the manipulating pilot. Very easy to get into PIOs and, depending on the speed schedule and how good the engine is (ie potential for a thrust overswing) the potential is there for a lateral loss of control.

Many years ago, I was involved with a post fatal investigation where the most likely precipitator to the departure leading to ground impact was just this situation ...

If you REALLY must increase thrust, please do it slowly and steadily so that you have a better chance to keep the feet dancing in time with the music.
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Old 27th Nov 2019, 06:22
  #48 (permalink)  
 
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Hi there

Can someone please explain why a fixed derate e.g. TO2 MAX would provide a higher thrust setting than TO1 with an assumed temperature. Or possibly point me in the direction of a document explaining this methodology?

Many thanks in advance

VG
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