Airbus Flex Thrust

[granard]

How do pilots decide on the temp to put into the MCDU for flex thrust take off

[demomonkey]

There are two principle methods depending on the airline;

Automated: 'A computer' either onboard or somewhere in the ether connected via ACARS will generate a FLEX temp based on the prevailing conditions and limitations (e.g. from the ATIS, AIS etc). This method is very opaque to pilots as 'what happens under the covers' is completely unknown. It's a good idea to do a 'does that sound right for these conditions' sanity check on this data.

Manual: Using paper based performance manuals specific to an airport, runway and even intersection. This gives the pilot a maximum and actual figures but because it involves alot of this number divided by that number multiplied by the first number you thought of... has HUGE potential to contain errors and therefore should be cross-checked by both pilots. Boeing manuals are much easier in my limited and humble experience for manual performance calculations.

DM...

[Max Angle]

Well using a well laid out manual just requires reading across from a weight to the flex and subtracting any corrections (QNH, Anti-ice) so it's primary school maths really and not too tricky.

[potkettleblack]

Don't know what manuals the earlier poster was using but ours are very straight forward. Adjust for the bits and pieces noted by Max Angle and then check that the adjusted flex temp and V speeds are within limits and hey presto.

[demomonkey]

Ok, maybe I was being a little too flippant, but my point was that you have to do the various adjustments at the bottom of the page. The Boeing format you don't even have to do that.

Not saying it is hard, its just more cumbersome than the people in Seattle's approach. However, I am not saying one vendor's approach is any better than the other.

[FlightDetent]

Originally Posted by demomonkey

people in Seattle's approach. However, I am not saying one vendor's approach is any better than the other.

And then there are the other vendors who will supply performance [AFM based] and obstacle data incorporated in the tables; the graphical and logical layout may differ. The computer-driven solution is both much safer and effective and cheaper. Effective training must be conducted for line crews to benefit fully, but same applies for the paper version except that that one requires some user mathematics.

FD (the un-real)

[h3dxb]

Granard: To understand , U need to know what flex thrust is.
Every rated engine has a maximum thrust, depending on construction let say 100%, to reach this thrust, U go on the upper limits (thermal and mechanical wise). A today engine is flatrated to 38deg C. That means till 38deg OAT at SLA U can get this 100% easy. Normally U don't need this thrust. To safe lifetime of the engines, U make now so called flex TO. Let's say for TO on RWY XY at AP XYZ U have 4Km . Lets say yr liftoff is after 3km, yr thust U need is 75%. Now You flex the engine: U tell the FADEC the OAT is eg. 78deg C.
The FADEC reduces the Fuel flow so, that the calculated engine limits will never reached for this OAT , U have only 75% and U safe the engine. The actual Flex temp depends on RWY lenght and temp+pressure ALT. I remember this is based on engines manufacture and flight ops from the operator. U wanna safe money U flex, wanna Fun TOGA

Hope this helps

[szot]

Rule of Thumb for Reduced / Flex and Max / TOGA Take-Off

N1 reading is the only gauge that verifies the engine is producing expected thrust. "EPR is useless"

N1 for Reduced / Flex is minimum of 85% - normally 86% to 88%

N1 for Max / TOGA is minimum 90% - normally 92% to 97%

Reduced / Flex rated thrust should never be below climb power, if you receive a take-off thrust setting lower than climb, use climb power setting for take-off.

[barit1]

Reduced / Flex rated thrust should never be below climb power, if you receive a take-off thrust setting lower than climb, use climb power setting for take-off.

While I know this is SOP for many/most operators, there is no technical reason you cannot take off using a flex thrust below MCL. In fact, it's an excellent way of preserving engine life - the flex TO serves as a warmup cycle for the more demanding climb rating. It's just foreign to the typical pilot's training and experience.