Originally Posted by
WhiteKnight
How do you know that you are still good without doing the calculation again. You could be field length limited. i don’t understand how you can possibly know? What about when youchange the temp from 20 to 35 degrees? Is it still good, where is the limit?
Because the TO speeds are based on an assumed temperature of whatever your flex is. At least for Airbus (see getting to grips with performance), and I’m sure it’s the same for other manufacturers as well.
So if your flex is 75°, your indicated TO speeds assume a much higher ground speed.
I’m no performance engineer, so please correct me if I’m wrong: With a flex of 75, your performance calculation assumes max engine thrust (TOGA) at 75° and takes into account ground speed at 75°. So any increase in OAT, up to your flex temperature shouldn’t change your performance. The only difference is that the lower your OAT is, the more safety margin you have due to the lower ground speeds and less distance used. An OAT of 75° should give you TOGA with the same performance figures. Although an OAT of 75° is probably above the aircraft certification, so you might not be able to calculate this.
And the only way to be sure is to try a calculation with 35° yourself. Or even higher temperatures.
For info: The company I work for prohibits the use of flex if the flex temp is less than OAT+5°. I believe it’s because the temperature sensor at the engine might measure a different temperature than your OAT sensor and thus your TO performance might be invalid. TOGA is required in that case.