Understanding Turbomeca DEECU Systems
By Greg Napert
Question — What do the Arriel, Arrius, have in common? Answer — More than you think!
For starters, they are two major geographical features in the Pyrenees Mountain Range in the South of France.
But this isn't a geography lesson, so for our purposes, they are Arriel 2 and Arrius 2 turboshaft engines produced by Turbomeca in France.
Also, both of these engine models were introduced into service with digital engine controls.
(Incidentally, Turbomeca names all of its engine models after major geographic features in the Pyrenees).
Turbomeca now considers the latest models of digitally controlled turboshaft engines it manufactures "third generation" turbine engines. According to Bill Mizell, Training Manager for Turbomeca Engine Corporation in Grand Prairie, TX, "The Makila, was the first turboshaft engine that Turbomeca designed which used an automatic electronic control. This is an electronic control box using analog type technology. This system works very well, but the natural evolution was to move to digital electronic controls."
He explains, "Many people today use the term FADEC, which stands for Full Authority Digital Engine Control. The Arrius 1A, was the first of Turbomeca's new generation digitally controlled engine to come on the scene. It doesn't have Full Authority, however, it utilizes a manual backup.
Turbomeca doesn't refer to its digital technology as FADEC, however, as the controls are not technically "Full Authority."
"Turbomeca is very adamant about this," explains Mizell. None of the controls on its engines are referred to as FADECs. FADECs technically require two or more channels so that there is redundancy in the system. Instead, Turbomeca uses the acronym DEECU, which stands for Digital Engine Electronic Control Unit. Turbomeca's DEECUs have only one channel and have a mechanical backup — an actual mechanical linkage between the cockpit and the engine. All Turbomeca engine electronic control designs are currently DEECUs so that in the event of an electronic failure, the pilot can revert to manual control."
Mizell continues, "This term FADEC has become one of these buzz words people use to talk about any kind of electronic engine control. But it's important to realize that the DEECU is not full authority, it's single channel and uses a mechanical linkage for backup.
"Having said that, some of the Helicopter manufacturers continue to use the term FADEC when referring to our equipment as installed on their helicopters."
The two Turbomeca products in the medium helicopter, medium engine power class which have new generation DEECU controls are the Arriel 2 and the Arrius 2 which can be grouped together in terms of their technology.
The Arrius 2 engine is an improvement over the Arrius 1 which uses a digital control unit somewhat larger in size than the Arrius 2 units. In both cases, they are using Digital Engine Electronic Control Units. They may be lighter now, but they all work in essentially the same way.
Mizell says the Arriel and the Arrius are very different in terms of horsepower and design, but they still use very similar control units.
Electronic controls have not only made the engine easier to operate, it has allowed the engine manufacturer to increase horsepower output. It does this by enhancing the ability to control the engine during power changes and acceleration and prevent surge.
Mizell explains, "The nice thing about the digital electronic control system is that by using the various assorted sensors on the engine, the control system knows more about what's going on with the engine than a mechanical system does.
"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
(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.
The objective with a helicopter is to keep the main rotor rotating at a more or less constant speed. The main rotor is connected to the free turbine, or the N2 speed. Rotor speed and N2 speed are directly proportional. So, that means the objective will be to keep the N2 speed more or less constant, or within the range dictated by the helicopter manufacturer.
(Step 3) Checking the datum
To do that, it needs to check for a pre-programmed value (called datum). The computer compares actual conditions to the datum and makes a decision as to whether we need to increase or decrease fuel flow. It makes this comparison electronically, and it then sends that signal as to whether it needs to increase or decrease it to the next decision point.
But it's not quite this easy. The datum, in addition to having some pre-programmed parameters, is altered by various operating conditions. For example, the position of the collective pitch, or what we call anticipation, biases that decision. That is to say, we've told the computer we want a certain fuel flow to maintain a specific N2, but if I happen to be moving the collective pitch, that's going to affect the rotational speed — so, I have a collective pitch position signal transmitted electronically to the computer saying "you need to bias that quantity of fuel flow I just asked for based on rotational speed to also make allowances for the load (increased or decreased pitch).
Then, there's an N1 speed signal coming from the other engine, which is compared to the N1 speed on this engine, which makes a load sharing decision as to what needs to be done to fuel flow to keep the N1s matched. Keep in mind that the N1 is directly related to Power on these engines.
So, the computer (DEECU) takes all of these things into consideration, and it makes a decision as to what it wants to do to N1 to get the N2 speed that's required based on these conditions.
N1 is the gas generator speed. So now, we're sending a datum as to how much N1 we want to an N1 datum selection memory in the computer. It then takes this information and compares it to the selected control mode in step 1 to check for compatibility. If these numbers are compatible with the control mode, it passes on the information. This information is called the Raw N1 Datum. In other words, this is what we want N1 to do without taking any limitations into consideration.
(Step 4) Checking the limits
The next step for the computer, is a verification step in the decision process. It checks what the pilot is asking for against limits. It looks at the torque limit and the amount of torque. Also checked is the N1 speed, the control lever position, and it also calculates standard day information to correct for atmospheric conditions. The computer then takes the value that is lowest, the limit, or the raw datum, and passes it on through the logic process. For instance, if what I'm asking for is not as high as the limit, that data will go on through. If what I'm asking for is higher than the limit, it will only let the limit go through.
From this point, the signal is referred to as the "limited N1 signal."
(Step 5) N1 control
The limited N1 signal then goes to N1 control. Remember that the N1 is what needs to be changed in order to control N2. You can think of the N1 control and N2 control as governors, but in this case they are electronic memory. In any case, this N1 control takes this limited N1 signal and compares it to the actual N1 — it checks to see if actual N1 speed is higher or lower than the requested value. This determines if we need more or less fuel flow to increase or decrease the speed of the N1.
(Step 6) Fuel flow selection
Having determined what it needs to do, the N1 control sends a signal to the fuel flow selection. The fuel flow selection is made based on the amount of fuel flow requested, the control mode selected, and whether or not we're starting the engine. In every case it checks the control mode, making sure the selections are compatible with what we're requesting. If we're starting the engine, the DEECU will send a different fuel flow signal than if it's operating. Having verified the amount of fuel flow requested and insuring that it is compatible with the control mode selected, it passes the signal on to the next step.
(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.
One last thing to remember — as the engine manufacturer, Turbomeca provides a wide variety of indications and options on its engines. But ultimately, the helicopter manufacturer chooses which ones to make available on their aircraft. So, all of the functions that we discussed may or may not be available on a particular helicopter.