Assumed Temp?

Question

What does assumed temperature do? Can any one please explain the concept?

Answer 1

In modern day aircraft, the takeoff performance is expressed in terms of weights. For example, with a given thrust setting, the computer will determine the weight at which the aircraft can be to satisfy the requirements. ( Runway length, air density etc) If the max weight the computer tells you is more than the actual weight of the aircraft you can use that thrust setting. For example Thrust at assumed temp 50 is 120,000. Thrust at assumed temp 45 is 135,000. If your airplane weighs 130,000 lbs you can use Assumed Temp 45 but not 50.

Generally assumed temps are 5 degrees apart. 50, 45,40 etc

Assumed temp 50 will always produce less thrust than assumed temp 40. This is because the computer knows that the hotter the air, the less dense it is so in turn the less power the engine can produce. If the temperature is 30 degrees, you obviously cant use Assumed Temp 25, or 20. You have to use 30 and over.

Using an assumed temperature increases engine life, saves a whole lot of gas, and doesn’t give the passengers such a rush. ( Although it’s quite enjoyable)

You also might want to know, that at some airport, Phoenix for example if the temperature exceeds a certain point most aircraft cant takeoff. The airlines have to pay extra $$$ to the manufacturer to get the paperwork to determine performance. Most airlines don't even bother getting the performance of their aircraft in the 60 degree Celsius range.

Note: all the weights and temps I said are out of the hat. Perhaps an airline pilot can give us some of his numbers??

Answer 2

WHY USE AN ASSUMED TEMP?
Using "assumed temperature" saves engine wear and extends life as only the minimum power/thrust setting necessary for that weight at that temperature is used for takeoff. Always using max power, which results in high temperatures inside the engine, leads to engine wear.

HOW WE FIGURE IT:
At each altitude and temperature, there is a max percentage that can be used of the total power/thrust the engine can give. At lower, cooler temps and alts you can get 100%. Hotter or higher conditions will restrict the amount of power/thrust available per performance data.

Additionally, for a given temp and altitude (air density factor) there is a correlating maximum weight the wings can lift into the air. At lower, cooler temps and alts you can expect to be limited only by the manufacturers max allowable takeoff weight. Hotter or higher conditions will restrict the weight of the plane for take off.

In figuring "assumed temp", I simply take the weight of the aircraft and see where that fall in the takeoff data chart. (The max allowable already being considered.) These charts have power settings, weights and temp levels all graphed out. The correlating temperature for the aircrafts actual weight may be the max allowable in the 30, 40 or even 50 degrees celsius range of the chart. We then use that associated temperature for our weight to find the related power setting, which may be up to 16% below the max available. (Max is found by looking at performance data for the actual temp/alt. It may not always be 100%.)

I then "assume" that we are operating at that higher temperature for the purposes of setting our takeoff power, thereby using the lowest possible power setting to enable us to safely takeoff at our weight for that flight.

There are situations where we are not permitted to use such a reduced power take off like very short or contaminated runways (wet, icy, snowy ect.)

Answer 3

Charles has the basic concept down correctly. When we get our final paperwork, it includes a selection of thrust settings for takeoff. The biggest "de-rate" as it is usually referred to that we can use for our weight is the lowest thrust setting for the conditions and is often "AT50" (assumed temp 50). In the event of a short runway or high weight we may be AT35 or even (horrors) full TO power.

Less than max settings for takeoff are used for a couple of reasons. First, we track them and provide the data to the engine maker, the less stress we put on the engine, the cheaper our engine contracts. Second, it saves fuel.

Answer 4

Nothing to add except perhaps it's easier to think of it like this. Thrust is set depending on the temperature as described by Charles. If you want to take off with a reduced thrust, you can assume a higher temperature and then check to see if you still have enough runway to takeoff. If you do, off you go a bit quieter for the neighbors, if not, it's pedal to the metal.

Answer 5

As an airline pilot, Charles has it pretty correct.

Answer 6

i dont know
may be in gets converted into kelvin

Answer 7

????

http://www.b737.org.uk/assumedtemp.htm