Hi all,

 

Although I'm a seasoned MD-11x pilot, I've never used a flex temp to de-rate my takeoffs. I understand that you would input a higher temperature to "trick" the engines into not running at say 103% N1. Could anyone provide me a description of what temperature entered equals what N1?

 

It is nice to power down the runway at max, and have a ridiculous climb rate, but its not necessarily the most realistic procedure.

Hey Chris,

 

It's a lot more complex than that. I'm no expert on the matter. I believe the FADEC will adjust the limits according to ambient conditions even without FLEX, so to say a certain FLEX temp will give you a certain N1 would be off the mark. As an example (numbers are figurative), on a standard (29.92 / 15C) day, it might give you 100%. On a 30.10 / 20C day, it may give you 103%. From my understanding (note that disclaimer), using a FLEX temp of 30 on either day will result in a different N1 value.

 

For accuracy, you're going to want something like TOPCAT. It will run the values through performance tables to give you the appropriate FLEX temps to enter. It will give you optimum values specific to individual runways and weather conditions as well.

 

I think FDX uses Flap 25 takeoffs for their MD-11s (something I got off the interwebs, so it could very well be incorrect). That would factor into what derate and FLEX temps would be available for the particular runway.

 

Even if what I wrote above is not as accurate as what's desired, the point of FLEX isn't to hit a particular N1. Rather, the point is to use the lowest possible power setting for the given scenario: runway length/slope, ambient temp, pressure, aircraft weight, and so on.

If you're just using the MD-11 and no other commerical jet, there are detailed Excel charts available in the AVSim library for free:http://library.avsim.net/search.php?SearchTerm=md-11+flex+temperature&CatID=root

Kyle pretty much summed it up perfectly, It's not about hitting a certain N1 or EPR.

 

Also a good thing about Flex/assumed temps compared to a derate take off is you are not Vmcg limited in the case of an engine failure and still have full TOGA thrust at your disposal if needed.

 

 

Available thrust is goverened by a few factors, QNH, field elevation and ambient temp. Regarding temperature, the hotter it is outside the less thrust the engines can produce.

 

You can test this by sitting with the engines at idle, switch the OAT to -20c note the N1 and the switch the temp to +20c and have another look.

 

It's all about keeping the EGT down, extending engine life and reducing maintenance cost's. Been a while since I flew the PMDG MD11 however I can remember on very hot day's departing at MTOW using TOGA out of places like Quito and Mexico the EGT would be right on the redline, sometimes even exceeding.

 

Regards

I have found [from going back to the md11 cockpit] that if the topcat calculatesa margin of 100m, but I usually rotate well before that.

 

 

The Margin calculated by TOPCAT is based on one engine inoperative conditions, during a normal takeoff, you would normally rotate at 2000ft/600m earlier, provide you correctly meet all the assumptions for the takeoff.

 

Best regards

Palle