Did you ever notice the large (usually in red) placard located near the tail rotor on most rotorcraft? Up until recent years, it was my opinion that this was a general statement pertinent to the overall machine. Contrary to popular beliefs in the fixed wing world, helicopters are aircraft too!
Manufacturers and outfitters of rotary wing aircraft face many unique challenges when it comes to selecting and installing avionics. Helicopters, like their fixed wing counterparts have numerous and often multiple types of missions. One day the flight log may reveal transporting VIPs to a company board meeting and the next day flying wildlife management biologists over a stretch of desert. When it comes to equipment selection it soon becomes clear that one package will not accomplish all the requirements.
In the United States the Federal Aviation Administration regulations governing rotor wing aircraft are found in either Federal Air Regulations (FAR) Part 27 for normal category helicopters and Part 29 governs those machines used in a transport capacity.
The minimum required flight deck equipment for a normal category rotorcraft includes an airspeed indicator, altimeter, and a magnetic direction indicator.
Machines that fall into the transport category also require the above mentioned equipment along with a clock, free air temperature indicator, vertical speed indicator, nontumbling gyroscopic attitude, and rate of turn displays with a capability to operate at least 30 minutes independently of normal electrical power.
Operators of the classic Bell 47 with a primary mission of herding cattle might find the minimum required instruments are adequate to fulfill the intended work scope. Airframe manufacturers catering to the VIP transport industry are more likely to produce equipment with an avionics suite that will rival those found in top-end business jets. In fact the AB139 was the first aircraft certified with the Honeywell EPIC flight deck. After all, when the chief executive of a Fortune 100 qualifier gets off the corporate Gulfstream and needs to get from the airport to the company headquarters, a machine that will provide a “continuation of the mission” is preferable to giving the CEO a “helicopter ride.”
Technology in the world of rotary flight is advancing at the same rate as elsewhere in the aviation industry with electro mechanical devices being replaced by those run by software programs. Global positioning systems (GPS) from a simple handheld up to dual units interfaced through flight management systems (FMS) are now the most frequent means of position determination. Many technicians have already expanded their toolboxes to include a PC to allow the viewing of manufacturers’ technical manuals or interact with various airframe and engine electronic control units (ECU) as a means of fault finding or trend monitoring.
Historically the word “avionics” has been associated with radio communications, auto flight, and navigation; however, the most basic definition is “electronics as applied to aviation.” Many maintenance technicians have gotten in the habit of contacting the “avionics person” anytime a black box with more than two wires could be a culprit.
Standard repair practices are often included in maintenance documents and are an excellent source of precautions, as well as techniques to assist in most troubleshooting projects. The utilization of sophisticated test equipment is in many cases not needed when determining the serviceability of a line replaceable unit (LRU). In fact the majority of troubleshooting at the flight line level can be accomplished with a basic volt ohm meter (VOM). Even with the Honeywell EPIC system, the aircraft inputs are analog (variable voltage, frequency, or current), digital (such as an ARINC 429 data bus), or discrete (ON = 11-14v DC, OFF = 0-3v DC) and all can be tested either by using the central maintenance computer contained within EPIC or as mentioned easily procurable test equipment.
Any time maintenance is planned it is paramount to remember electrical or electronic systems do have the potential to be deadly, either to themselves, or worse, to an ill-prepared technician. It is therefore imperative to be familiar with any cautions and warnings listed in manufacturers’ documentation as well as knowledge of any equipment being checked.
Electrical bonding is an area that is frequently misunderstood and requires constant attention. One primary function is to protect the structure, equipment, and personnel from the hazards associated with lightning strikes and subsequent discharges. By maintaining adequate bonding, a stable path for the equalization of electrical charges is ensured.
There are different classifications and methods to ensure static charges can be safely dissipated. As an example, the means for bonding antennas to airframe will differ from the methods and resistance specifications providing the normal current path between other types of equipment and structures. In fact Bell Helicopter addresses six different classes of bonding procedures in its Electrical Standard Practices Manual.
The hardware used in the electrical bonding process should be chosen carefully. Considerations should include mechanical strength, amount of current to be conducted, compatibility with surrounding materials, and of course the ease of installation and removal. It is best to replace any components used in bonding with those specified by the manufacturer.
Surface preparation for bonding connections on rotorcraft is another area to be addressed. Different methods are needed based on the component to be bonded as well as the aircraft structure used for the electrical connection. When working with aluminum, extreme care should be used in the cleaning process to avoid the removal of excess metal and special attention should also be used in the selection of abrasive materials used in surface preparation. For example, using steel wool on aluminum components can result in galvanic action that will produce corrosion. It is, however, important to remove all organic materials including paint, anodizing, or other chemical treatments that may impair electron flow. Manufacturer’s or installation agent’s recommendations should always be consulted prior to undertaking a project that may have an impact on static buildup.
Antenna installations are another challenging endeavor on rotary wing aircraft. Once again composite materials such as fiberglass may impede proper bonding, and devices that are in need of a ground plane may operate at less than optimum performance levels. Fortunately there are several products on the commercial market that may provide a remedy for these concerns.
Certain types of adhesive metal sprays can be applied to otherwise nonconductive surfaces and facilitate an improved radiation pattern for many antennas. Choosing the proper or best location on the airframe is another important step in ensuring optimum performance from systems that depend on radio waves.
One such report was from an operator of a Bell Model 407. In this case the communications radio appeared to have a limited transmit range, but strangely enough only when talking to a station in front of the helicopter. When flying away from a station the range was normal. This determination was made after the receiver transmitter and antenna had each been replaced with no improvement. The problem could never be duplicated while on the ground. As it turned out the communications antenna was located on the top of the machine, behind the main rotor. By communicating with the manufacturer it was determined that the preferred antenna location was under the center belly and after the change was made, the problem was alleviated. Apparently the main rotor, when operating, was having a negative impact on the radio transmission that had to pass through the blades to reach stations in front of the machine, while stations behind had a non-hindered reception.
Coax cables are the most frequent instrument of choice for conducting radio frequency (RF) type signals up to 1 gigahertz (GHz). Carriers of RF energy are unique conductors and should be dealt with accordingly. Many coax transmission lines found in helicopters have a 50-ohm impedance value. These cables consist of a center conductor wrapped in an insulator, which is then surrounded by a wire braid or a shield. Finally a protective sheath covers the entire conductor.
The length of most coax cables has to be calculated based on the distance of the antenna to the receiver transmitter. Once this is determined, the wavelength of the RF signal has to be factored in and the cable is cut in incremental lengths of the wavelength. This is why it is common to find coils of coax stowed in out-of-the-way locations.
The installation of RF carriers also requires some consideration. Like all conductors, bend radius and chafing precautions are essential. Exposure to heat is one factor that can reduce the life of a coax antenna cable. In addition the method of attachment can be an issue. When clamps are used, they should secure the cable without compressing the sheath and insulation. As this cable is impedance critical, when the relationship of the center conductor to the surrounding shield changes, so will the capacitive properties and likewise the impedance value. Lacing cord has been used by many manufacturers to secure coax cables and it is not uncommon to see plastic wire ties. When using wire ties, the pull tension is critical as to not compress the cable.
Wiring and Connections
Wire terminations and mechanical connections are the weakest part of any electrical circuit and as a result, many electrical discrepancies in helicopters are a result of corrosion, stress, and vibration at these connections. Correct practices and tooling are essential to ensure continued airworthiness. Manufacturers install wiring to provide minimum interference with reliability being the paramount concern.
In recent years the FAA has been paying significant attention to studies conducted on aging aircraft. At mandated intervals various types of inspections are conducted to determine the structural integrity and functionality of various components. Aircraft wiring for the most part has been immune to the stringent criteria applied to the rest of the machine. It has been learned that insulation may break down as a result of age or perhaps as a result of exposure to heat, vibration, and even FOD.
In a recent contract awarded to Texas Aviation Services in Fort Worth involving extensive rework to Black Hawk helicopters, one requirement is to connect each and every wiring harness throughout the aircraft to a device utilizing a PC and diagnostic software which will subject every single wire and coax to an insulation check, measure voltage drop, and check for complete circuit integrity. Are these extreme measures? Yes! Will they ever filter down into general aviation? Probably.
Many helicopters use single wire circuits with the fuselage supplying the common return path. Special attention is required to make sure a proper electrical connection is made to the structure. This is important considering wires are made of copper, terminations are often cadmium plated, and steel hardware is used to connect the assembly to aluminum structure. I wonder what might happen if moisture gets in?
I have come to realize that, all in all, helicopters are aircraft too. And in regard to the placard next to the tail rotor, it is without a doubt, good advice!