Those who feel unsafe during a flex/assumed temperature takeoff... then how do you feel like takeing off on a 35°C day?
Jet engines are "pressure" limited up to a certain temperature, up to which they are certified to give max rated thrust. This certain temperature is sometimes called flat rate temperature, or FRT. Either you use N1/EPR/IEPR values, these parameters are designed for pilots to keep the thrust at the max limit. We look up the values calculated by engineers, and use these in day to day life.
If OAT is above this FRT, the engine is limited in temperature increase. And even though we still use N1/EPR/IEPR/..., the idea behind them, the rules to calculate these values, change. That's the whole reason that makes this story possible. Assumed is not an invention or anything, it's been a part of aircraft jet engines since day 1. The higher the OAT above the FRT, the lower the thrust you are allowed to get from it. We might not see that directly, it is calculated in the N1/... values you use. In a way, you always do a kind of derated takeoff anyway above FRT (even though some might not realise this). If you would even try to get all thrust out of the engine, we are operating it outside it's limits. THAT is dangerous.
The FRT's I know for jet engines nowadays are in the 15 to 25°C region. So any day OAT is above this temperature, you are sort of derating anyway. Reading this thread I get the scary feeling some people think they always get the max thrust out of it...
So what's the difference? Assuming a temperature of 20° on a 10° day, or doing a takeof on a sunny day with 35°C if your FRT is 25°C. The first one is actually safer since real density is higher, giving slightly better "general" performance than on a 35°C day.
The "you have some safety margin extra with full thrust".... Yes, you have it that day, but if you want to fly on a 35°C day, to me, you consider that specific takeoff as being a safe thing to do that day, and the day after, and the day after,... so why can't you use assumed day in day out? It's the same thing.
Last edited by BraceBrace; 21st May 2008 at 04:25.