(Step 7) Limiting the flow
The fuel flow signal is then checked against fuel flow limitation. The amount of fuel flow limitation is based on the air temperature, the air pressure, the compressor pressure (P3) and the N1 speed. All of those are sensors on the engine that are communicating with the computer to calculate fuel flow limits based on atmospheric conditions. And the computer also takes into account the compression ratio (P3 - compressor outlet versus P0 - atmospheric pressure).
(Step 8 and 9) Metering valve control and limits
Based on those functions, it calculates what the limit of the fuel flow will be and makes a decision about what to do with the metering valve. The computer then sends a metering valve position signal to the metering valve control. To review, this is done to change to establish the N1 speed required, without exceeding limits, to get the N1 speed that has been calculated and is needed to rotate the N2 speed and, thus, the main rotor at a particular rotational speed.
(Step 10 and 11) One Engine Inoperative calculations
The computer also, by looking at the atmospheric conditions and rotational speeds, has a memory for how long we operate in the One Engine Inoperative (OEI) mode, how much power we used while in this mode, and how many times we used OEI power. All of these factors are in affect if the engine can be allowed to continue to operate, and if the engine requires maintenance. Certain OEI ratings require that the engine be removed and inspected (30-second ratings, for example). Operating in other OEI ratings, such as the two-minute rating, allow the engine to accumulate a specific amount of time during a given TBO period at that rating before the engine requires inspection and possible maintenance. The computer records the time at these ratings, as well as the number of events, and puts that in memory so that you can interrogate the computer using the instrument panel or laptop computer. It also turns on certain lights in the cockpit for the operator to let them know when they are using OEI power.
More reliable maintenance
Mizell says, "The beauty of the digital control system for maintenance is with all this processing power, the computer is also capable of performing an engine performance checks for you. It can also do your cycle counting for the gas generator and free turbine.
The computer is also capable of fault detection. The computer constantly monitors the entire engine, and its systems, and the internal operation of the computer itself. If there are fault codes, it identifies that fault, records it, and sends an indication of that fault to the cockpit. And, certain kinds of fault codes are available as a maintenance aid and can be downloaded to tell us what has malfunctioned. For example, if a temp sensor went bad and we lost our temperature signal, it would tell you in the cockpit, or through an external monitoring unit.
This system also makes flying the aircraft much easier than in the past. Gone are the days of having to calculate density altitude to determine maximum allowable engine power. Now, the computer does this for the pilot.
How it does this is as follows:
For every density altitude, there is a different N1 speed representing a certain power limitation (let's say Max takeoff power). Max takeoff power at sea level will be one speed of the gas generator, but at a higher altitude, Max takeoff power is reduced, therefore the N1 rotation speed is reduced. So, we have what we refer to as a N1 biased signal to the cockpit. The cockpit indication will always be the same regardless of the density altitude. In operation, every time that needle comes up to that red line, the engine will be at Max take-off power, regardless of what it really is — it could be 100 percent N1 today and 99 percent tomorrow, but because the computer biases the indication, the cockpit indication will always be the same. This reduces the workload for the pilot as they don't have to calculate the density altitude — the computer takes care of this.
The main thing that technicians need to remember about the DEECU is that it is still a machine, and you still have situations on the engine that will occur that the computer won't know about. It won't know, for instance, that the engine has an oil leak or that the engine is consuming oil at a certain rate, or that the engine's making metal. There are other common everyday things that happen to an engine that require monitoring. That's why you have a periodic and daily inspections. The DEECU only serves as one additional tool. It is interesting to note, however, that in some cases, the DEECU has widened the interval between periodic inspections. For instance, your first periodic inspection is 450 hours on the Arriel 2S1 engine.