Understanding Turbomeca DEECU systems

"The mechanical system makes certain assumptions and those assumptions are adjusted into the control. For example, a mechanical control doesn't know operating temperatures or compression ratios, so it can't react to these parameters. The electronic control is able to more efficiently operate the engine and provide faster response times, smoother acceleration and deceleration, and higher power without the occurence of a surge due to its ability to react to changing conditions," he says.

Additionally, the digital engine control has contributed to better maintenance practices as more information is available for the technician to use in order to better maintain and troubleshoot the engine.

Understanding the electronic control system
Operation of the electronic control units on all of the Turbomeca engines are very similar. To understand the digital logic of the control unit, Mizell explains that it's important not to over-analyze the electronics. Instead, it's more important to try to understand the decision and logic processes, as well as the inputs and outputs that the computer uses to make decisions.

The following will be an explanation of the basic logic process followed by the digital electronic engine control unit used on the Arrius and Arriel engine models.

Although the Control System Operation schematic referenced here is for the Arriel 2, it is very similar on the Arrius 2.

Mizell explains, "This is a flow chart that represents the decision process that the computer goes through to make up its mind how much fuel flow to give the engine in order to control the rotation speed of the engine. The rotational speed of the engine (N1) is directly proportional to power."

Please refer to the Control System Operation illustration on the following page as Bill Mizell explains the DEECU thought process:

(Step 1) Control Mode Selection
There are certain things the pilot can tell the computer it wants the computer to do to the engine through various controls in the cockpit. These are: Stop, Idle, Transition from Idle to Flight, And Flight.

The pilot can also tell the computer he/she wants to operate this engine automatically, or manually. The pilot can eliminate the computer control on purpose (such as for training). The pilot can transition from automatic to manual control of the fuel control unit and back. The pilot can also tell the computer what amount of power will be the maximum it will allow him to pull on the selector such as; Max. OEI(one engine inoperative) rating, Intermediate OEI rating, and Max Take-Off rating. The pilot can also tell the computer they want to fly normally, or they can choose a training mode and command the computer to fly in either single or twin engine operation. You can actually tell the computer that you're going to pretend you are operating in an OEI condition, and you want all the instruments to do everything they would do during an actual OEI situation.

The computer will decelerate one of the engines back to an idle point so that you're flying on only one engine. While it appears you are pulling the Max power out of the operating engine, in reality you're not actually pulling that power, and you don't use any of the allotted OEI operating time. The computer even gives the pilot more options for training. For example, on the Arriel 2S1, you have an OEI 30 second rating and in the event of an actual OEI situation on takeoff and landing, the maximum amount of power you use is basically limited to 30 seconds continuous use — if you use that power, the engine will have to be removed for inspection. For training purposes, however, we tell the computer that we're going to train for OEI 30-second power. This allows us to simulate OEI 30-second power with all of the visual indications on the instrument panel, yet you're really only pulling Max Takeoff power. Max Takeoff has a 5 minute limit, so if you obey the 30-second limit for OEI training, you remain well within engine limitations.

Throughout the entire thought process, the DEECU continues to check the control mode to ensure that the decisions it makes are compatible with the control mode selected.

Understanding Turbomeca DEECU Systems

By Greg Napert

November 1998

(Step 2) N2 Control
The first decision the computer makes is to determine if the N2 speed is where it's suppose to be and if it is controlling the fuel flow in order to insure the N2 is where it's suppose to be.