In part one of our avionics troubleshooting article, we focused on the different types of avionics systems and how they differ from architectural and maintenance viewpoints. We also discussed, in general terms, getting the information we need to help us find the system/LRU that is at fault. Here in part two, we will concentrate on tips to help isolate the system/LRU at fault. We will cover the most common avionics systems in the field.
Air Data Systems– Air data systems can come with an air data computer (ADC) or sensor, and one or more indicators. If the helicopter has an electronic flight instrument system (EFIS), the air data information can be displayed there. The helicopter’s pitot-static system is also involved and must be considered part of the system for troubleshooting purposes.
Ask this first: What failure flags/warning messages are in view in the cockpit? A true computer/sensor failure will have multiple indications in the cockpit. A failure of the pitot-static system will also cause multiple failures to be displayed and should be checked out before going any further. Many of today’s ADCs/sensors talk different languages (e.g., Airborne Radio Incorporated (ARINC) 429, Avionics Standard Communications Bus (ASCB) and analog). If all these outputs have failed, then it is most likely the ADC/sensor. If only certain failure flags/messages are in view, then it can still be the ADC/sensor, but it can also be the indicator/display or the wiring between the ADC/sensor and the display.
If the helicopter has a dual system installed, a quick way to verify the ADC/sensor is to take the suspect LRU and connect it on the good side. If the problem follows the LRU, that is your culprit. If it does not, then the LRU is good and additional troubleshooting is in order. A wiring check of the LRUs mounting rack and connector are in order.
Attitude Heading Reference Systems (AHRS) – An AHRS, as its name implies, is an LRU that provides the displays in the cockpit with attitude and heading information. Depending on other systems installed in the helicopter, it may also provide attitude and heading information to those systems. This system is typically installed as a dual system, with one for the pilot’s side and one for the copilot’s side. For the heading function there will also be a flux valve or flux gate for each system to detect the horizontal portion of the earth’s magnetic field for heading data. All AHRS provide an output of magnetic heading. They do not provide true heading.
The key questions to ask when troubleshooting AHRS are these: During what part of the flight did the problem occur? Did it appear on both sides of the cockpit or just one? Was the helicopter in a particular maneuver? Was the problem constant or did it come and go? Did the problem show itself only when another system was turned on or the flight crew keyed their radio? Did the problem show itself on the ground and then go away with weight off wheels/skid? Were both attitude and heading data indicating a problem or only one of them?
One of the most common discrepancies with an AHRS is a heading split between the pilot’s and copilot’s sides. Some things to consider that can cause the heading split are area magnetic interference, aircraft magnetic interference, the flux valve and its associated wiring, flux valve calibration and the attitude heading reference unit (AHRU) itself.
Area Magnetic Interference– The earth’s magnetic field can be disturbed enough to cause a heading split if a tug, power cart, hangar, steel-reinforced ramp or blast fence is in close proximity to a flux valve. When troubleshooting this problem, move the helicopter to an area away from these kinds of metal structures and see if the problem clears.
Aircraft Magnetic Interference – What are generally referred to as “hard iron errors” are the result of ferrous metals being within one meter of the flux valve. A hand-held compass can be used to detect the ferrous material by moving it above the flux valve and looking for heading changes on the compass.
Flux Valve and Associated Wiring – Flux valves do not fail often. Having said that, and realizing that a helicopter is a large vibration producer, either the flux valve itself or its associated wiring could become loose. Check the flux valve for mounting integrity in that all mounting screws are tight. Also check to see that no damping fluid is leaking from the flux valve. Check the flux valve connectors for corrosion and to see that the shielding wiring is good.
Flux Valve Valibration – Errors in flux valve mounting/calibration can cause heading splits. When performing a compass swing, ensure that you have an accurate heading reference available. In a dual system, perform the compass swing on both systems at the same time.
Attitude Heading Reference Unit(AHRU) – The AHRU is seldom the cause of heading splits, but you can do the following to be sure:
1. Make sure that the heading split is occurring when the AHRU is slaved to the flux valve.
a. Using the heading of the standby compass as a reference, compare the headings of both the pilot’s and copilot’s AHRS against the heading of the standby compass, and write the headings down.
b. Swap the pilot’s and copilot’s AHRUs.
c. Using the standby compass as a reference, compare the headings.
2. If the AHRUs are good, the heading split on a specific side should not change. The AHRU is good.
Electronic Flight Instrument System (EFIS) and Engine Indicating Crew Alerting System (EICAS)– Today’s newer helicopters might have an EFIS, an EICAS or both. For clarification, the EFIS has a primary flight display (PFD) and a multifunction display (MFD). The EICAS stands alone. Most of these systems have built-in test (BIT), so troubleshooting is not difficult. The displays are usually identical and interchangeable, so again you can take a suspect LRU and plug it in on the good side to see if the LRU works or not.
It is helpful to understand what is generated within the display itself and what comes from another system. For example, the MFD can display navigation mapping, weather radar, terrain awareness warning system (TAWS) and other data. If range rings are displayed, they are part of the MFD architecture and do not come from another system. The actual map data, weather data or TAWS data does, in fact, come from the applicable system.
The PFD’s main display is attitude and heading. The compass card and attitude sphere are generated by the PFD. The actual attitude and heading data comes from the attitude and heading source. Knowing what is being displayed and where it originates can save you time in system troubleshooting.
The EICAS display of engine data, like the PFD, has an internal and external component. The engine and rotor speed displays are generated in the EICAS, while the input to drive the displays comes from the engines and gearbox.
Flight Director (FDs)– Some helicopters have a single or dual flight director installed. They might be called by different names, but they are still lateral and vertical navigation tools for the pilot. In essence, the FD receives inputs from a host of other systems and generates a computerized steering command for the pilot. It is important to remember cause and effect. The attitude and heading source will have a pronounced effect on the FD, as will the ADC or sensor. FDs might also use a combination of lateral, longitudinal and vertical accelerometers as damping terms in proving computer-generated steering commands. These might come from an AHRS or stand-alone accelerometers. Know which FD modes use accelerometer inputs and where the accelerometer input is coming from — this is a big help in troubleshooting this system. If your FD has a localizer and glideslope mode, don’t forget that these are not air data modes, but radio modes. That brings another sensor into the mix. Before removing the FD for a problem, make sure there were no failure flags or EICAS failure messages in view that could be the cause of the FD problem.
Autopilot (AP) – Many helicopters today have APs installed. Many helicopters might also have a stability augmentation system (SAS) installed as part of the AP or instead of an AP. SAS is a hands-on system that helps the pilot fly the helicopter. The AP is a hands-off system wherein the pilot has set the AP to fly the helicopter and once it is engaged, his or her hands are off the controls and his or her feet are on the floor. He or she is now an avionics system manager (scary thought). The AP is a stabilizing platform wherein the pilot sets a lateral and vertical reference and engages the autopilot which now works to keep the helicopter at those references.
APs get blamed for everything because they are probably the most misunderstood system on the helicopter. When the pilots go for their type rating on the aircraft, they rarely (if ever) have to demonstrate proficiency on the AP. Hence, it becomes misunderstood. Like the FD, the AP relies on other system inputs for it to do its job. In a helicopter that has both a FD and an AP, the pilot can usually couple the FD to the AP, and we now have a system that provides automatic navigation and stability.
A common discrepancy I have seen is that the “AP will not fly the FD command properly.” In a dual system, we know we can put a suspect box on the good side and see what happens, so we will leave that right where it is. On a single installation, we don’t have that luxury so we have to do a bit more work. The first thing is to ask the pilots is: Could they fly the FD command manually? If the answer is yes and the AP could not fly the command, you have an AP problem. If the pilot could not manually fly the FD command, then the AP certainly cannot do so and you have an FD problem. Got it?
Since the APs are actually flying the helicopter, they are connected to the rigging through control rods and bell cranks and hydraulic systems. Here, the wrong tension on the rigging or an incorrect set control rod will manifest itself as an AP problem. Again, cause and effect come into play. If you truly understand the inputs the AP needs to do its job, you will be less likely to blame it when another system is at fault.
That is all we are going to cover in this article. I hope that you will it useful in troubleshooting your avionics when required. If there are other systems for which you would like some tips, let me know and I will see about doing another article at a later date.