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Grandpa

FLEX TEMP Settings in a PSS Airbus and in a real Airbus

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Ah ha one of my favourite topics (along with Cost Index)! The basics of flex are fearsomely complex which is why they are usually done by computers rather than manually.Flex is a takeoff reduced thrust setting (it is not derated thrust which is a bit different) and is used when the runway is not contaminated by being wet or icy, when the runway is not short and the aircraft is not doing a hot and high departure. If any of these conditions do exist then a TOGA thrust setting must be used. Flex is also not recommended with a tailwind. Flex assumes a higher outside air temperature than actually exists, therefore you never input a Flex TO Temp lower than the outside air temperature. The computers command the engines into using less thrust for the takeoff than they would otherwise do with a TOGA thrust setting. Why do this? Well a TOGA thrust setting pushes the engines to the limit of their capabilities, and though they are designed to take the strain it will age them more quickly so maintenance and replacement costs will be higher. Using a lower thrust setting prolongs the life of the engines and reduces the cost. The thrust that results must not be reduced by more than 25 % of the full rated takeoff thrust. The PSS Airbus has a default setting of 42 deg C which means that if you leave it at that the computers will command the engines to provide thrust which would be achieved from a TOGA takeoff with that outside air temperature. One effect of using a FLEX takeoff is that you will need to use more of the runway to take off. This is why you must only use TOGA thrust on a short runway. For many airlines the rule is that the FLEX TO TEMP must not be higher than 60 (ISA+45 at msl) even though the MCDU can go to 999 degrees C. The calculation is done as follows: Enter runway analysis with actual weight and wind. If actual weight is not shown, and interpolation is not desired, use the next highest weight shown in the analysis. The following was from one or other of my manuals: "Read the maximum temperature shown in the box, or else interpolate Apply temperature corrections as appropriate to determine the Corrected Temperature (CT). CT = maximum temperature + correction. If CT is higher than OAT and TREF, use CT as flexible temperature, limit CT to ISA+45

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Hi Peter,In addition to Rob's excellent reply, here are two more things you can try:1. If you are flying the A340, search for a340perf.zip here at the Avsim file library. It contains some tables that can be used for calculating the flex temp in the A340.2. Try this tool:http://www.flightxpress.de/files/0604/Tools/TPC.exeIt gives takeoff speeds and Flex temp settings for given runway length, weight etc (but only at sea level as far as I understand).Best,/Bj

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But...all that said, it's really a moot point with the PSS Scarebus panels, as flex EPR/N1 is grossly miscalculated by the PSS MCDU. It should never be possible to see a lower takeoff power setting at TOGA than at FLEX...yet I see that regularly. And a 30 deg C flex-down should produce much, much larger decreases in takeoff power settings than the PSS panel sets. I've given up on flex takeoffs using the PSS panel and just stick to TOGA, where I can get semi-consistent performance.RegardsBob ScottATP IMEL Gulfstream II-III-IV-V L-300Washington, DC

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Hi Peter,That's bizarre, I tried downloading it again now, and it worked just fine; filename a340perf.zip...But it is available at Herv

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