What is Flat Rated Engine ?

First of all you have to know and understand this:

There is a relation between the ambient temperature and the thrust produced by an engine; this relation is a linear relation, where the thrust increases with a decreasing ambient temperature.

So, if you draw a diagram with thrust on the verical axis and ambient temperature on the horizontal axis, you'd see more or less a straight line from the top left to the bottom right corner.

Something like this:

This means that at very high ambient temperatures an engine is not producing a lot of thrust. You can see this in Arizona for instance, where a plane has a far lower performance due to the high temperatures.

For instance a fully loaded B727 on a take-off from Phoenix will hardly climb at all after take-off during the summer.

The opposite is true as well; in very low temperatures, the engine will produce a huge ammount of thrust.

The same B727 taking off from Anchorage in the winter will now look a much more performant plane!

However, you've probably heared of Newton, haven't you? How about his famous law:

ACTION = REACTION??

Thrust is definately an action, isn't it? But where is the reaction? Well, it is in the plane's structure: the internal strenght of the engine, the screws connecting the engine to the wing, the wing itself, etc., etc.

If the action is huge, the reaction equally has to be huge, resulting in the need for very strong and thus heavy components! However, once installed these components are there to stay, even if the ambient conditions are such that they do not require such strong components, simply because the action is not that big (Phoenix in the summer). In that case they represent nothing more then a weight penalty, decreasing the plane's performances even further!

That's why the manufacturer introduces a flat rating. The engine is limited to produce a certain maximum thrust even if the temperature is very low, so there is no need for those very strong and heavy components. This is also the thrust the manifacturer is talking about when he's talking about for instance his 100,000 lds. engine. If this engine would not be flat rated, he would not be able to tell you this figure; remember the relation between thrust and temperature I've told you about?

On the diagram, this maximum thrust can be represented by a horizontal line cutting off the upper left part of the diagram.

If the ambient temperature is high, no influence is sensed on the thrust produced.

If the temperature is low, the thrust produced is reduced to the maximum rated thrust

The actual thrust vs. temperature diagram looks therefore something like this:

The point on the temperature scale corresponding with the intersection (X) of the thrust line with the max rated thrust line is called the flat rated temperature of the engine.

Ambient temperatures above the flat rated temperature result in a thrust reduction due to natural ambient limitations, temperatures below the flat rated temperature result in a constant thrust due to mechanical limitation of the engine.

(actually, the thrust slightly reduces as well, but the reason for that is very complicated, since I have to start talking about 4 other parameters infuencing thrust as well)