Flex to temp vs. derated thrust

[Guest pez_man]

Hi!Just got a little curious when i read the following on b737.org.uk:"Derate is a semi-permanent engine fix, used to reduce the maximum thrust available for instance down to 18.5k from 20k on -3/5/700's. It is also used to equalise the thrust where B2 & C1 engines are mixed on the same airframe. When an engine is derated the extra thrust is not available to the pilot. Derating is accessible through the FMC on TAKEOFF REF 2/2 or N1 LIMIT (NG's) but this is prohibited by most operators. The derates (TO-1 & TO-2) can be 10 to 20%. It follows that an engine may be derated 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."So apparenty this way of limiting the thrust is mostly prohibited and flex to temp is the only way to go? Does any kind soul out there have some sort of generic tables for the flex to temp?rgrds fredrik granfors

[Mike S KPDX 597]

The 'derate' use is not used very often. The explanation you quoted is actually a very good one. I understand that DTO inputs are very rare and explict on the dispatch form.In almost all cases a 'flex temp' (airbus term) or 'assumed temp' (others) are used vs a DTO. Furthur, a real life 747 capt (Tarmac) indicated that when DTO is actually used it is done in conjuction with a 'assumed temp' to acheive a very specific setting.I dont think generic tables exist. This type of table is specifically made for a specfic runway at a specific airport for a specific engine type. I can tell you that a common 'assumed temp' range for the NG is 33-45 degrees.

[Guest pez_man]

Yeah, I pretty much guessed that some generic tables don't exist, still 33-45 should work fine as a ballpark figure, thanks a lot! :-wavergrds fredrik

[Guest Darren Howie]

Generic tables do not exist in the ral world for reduced power take offs.Every runway at every airport has a table produced by an Airlines flight ops section which outlines a max wt vs temp to determine max uplift of all runways the airline uses.One way you can do it in FS9 is to work backwards through the take off tables to determine the maximum temp at which you can get airborne from the runway you are going to use.ie if you are taking off from KLAX rwy 24L and its say 11000' long and your TO weight is 140000lb simply find the max temp at which you can take off from that rwy at 140000lb.That is your max reduce power take off temp.Remember to allow for correct flap setting and wind.Darren Howie

[Guest pez_man]

That's a pretty clever idea! Thanks a lot!! :-) :-)rgrds fredrik

[Guest Darren Howie]

No worries FredrikJust also remember that flight ops regulated take off weight tables(RTOW's) allow for obstacles on the departure path as well as runway length.Something worth considering if departing from say SEQU or somewhere with obstacle problems.SeeyaDarren