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chiswick72

Engine Out question

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I dont know if this is simulated in the 737, but say you take off with derated engines, and during climb out you experience a major engine failure, does the FMC change the derated thrust rating or do you have to manually change it through the normal method?Thanks in advance!Armen at EGLLwww.veryquiet.com


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Armen L Cholakian
PMDG Sound Engineer

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>I dont know if this is simulated in the 737, but say you take>off with derated engines, and during climb out you experience>a major engine failure, does the FMC change the derated thrust>rating or do you have to manually change it through the normal>method?>>Thanks in advance!>>>Armen at EGLL>www.veryquiet.comArmen,No it does not.By the way, the FMC derate N1, is considered a limitation for takeoff, therefore, thrust levers should not be advanced further except in an emergency. A further increase following an engine failure could result in a loss of directional control. Always use the takeoff speeds corresponding to the selected derate.Floyd


John Floyd

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By the way, the FMC derate N1, is considered a limitation for takeoff, therefore, thrust levers should not be advanced further except in an emergency. Just to clarify.... According the the Bulfer B737NG FMC Users Guide it is ok to advance the throttles (in non-emergency situations) for Sel Temp derates. The restriction applies only to the fixed derates... e.g. 26K to 24K (i.e. TO to TO-1).Hopefully, someone can explain to me why these two types of derate are different without resorting to nuclear physics and university-level mathematics :( (and why large-ish Sel Temp derates on aircraft like the 747-400 automatically shift the carets on the THR LIMIT page from TO to TO-1 or T0-2).Cheers.Ian.

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The assumed temperature method (Sel Temp) is a form of reduced thrust, and is not techically a de-rate. The engine thrust rating, both actual and figuratively, remains the same. In order to be able to use reduced thrust, the airplane performance requirements (takeoff field length, climb capability, obstacle clearance, etc.) must be met at the reduced thrust level, but the airplane controllability requirements (minimum control speed) must be met at the full thrust rating. That is why it is okay to advance the throttles when using an assumed temperature thrust reduction.For a fixed derate, you are effectively re-rating the engines to a lower thrust rating for that takeoff. It's as if little gnomes have come out and replaced your engines with lower thrust engines. By de-rating the engines, you now must meet both the airplane performance requirements and the airplane controllability requirements at the lower thrust rating. Because the airplane controllability requirements consider the thrust asymmetry caused by a sudden engine failure, the lower thrust rating results makes it easier to meet the controllability requirements (that is, the minimum control speed in order to meet the requirements would be lower). If your allowable takeoff weight was limited by controllability (minimum control speed), you may actually be able to increase your takeoff weight by using a de-rate. However, since meeting the controllability requirements was based on the lower thrust rating, you must not increase thrust beyond the de-rated level (except in an emergency).The primary benefit of assumed temperature thrust reductions is to save wear and tear on the engines. De-rates do that and also allow a reduction in the minimum control speed (which may limit your allowable takeoff speeds, and hence, your takeoff weight.) Note that you can combine de-rates and assumed temperature thrust reductions, but an assumed temperature thrust reduction cannot give more than a 25% reduction in thrust.Don S.

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Thanks for trying to make it a little more understandable, Don... I'm nearly there ;-)I assume there are completely different V speed tables for TO, TO-1, TO-2, etc.I may have to go to a 744 website to ask them to explain why a Sel Temp derate also generates a TO-1/TO-2 annunciation on the MCDU (which I would normally associate with a "fixed derate"... and different flight controllability considerations).Cheers.Ian.

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Ian,If this is too bloody obvious I apologize, ;-)Maybe you are mistaking the TO derate with the CLB derate you get when using a high SEL TEMP thrust as the NG does?8<---"A reduced climb thrust (CLB-1 or CLB-2) is automatically selected if necessary when using a derated thrust or a high assumed temperature for takeoff to avoid throttle up when going to climb power"8<---Snipped from B. Bulfer NG FMC GuideCheers,


Mats Johansson
PMDG Flight Test Dept
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>Maybe you are mistaking the TO derate with the CLB derate you get when using a high SEL TEMP thrust as the NG does?I'm relying on memory here, Mats... Lemme get back to you on this one ;-) (I haven't played with the real 744 FMC for a while).Cheers.Ian.

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Looks like I was having a "senior moment" guys... Please ignore my remarks about TO-1 & 2 appearing with Sel Temp derates. As you say, only CLB derates change (744 and 737NG).Thanks! (all makes sense now)Cheers.Ian R>

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