Understanding Today’s Antenna Complexities
With up to 20 antennas per aircraft, greater connectivity means more things can go wrong
Aircraft antennas perform a broad range of functions on modern aircraft, from conveying voice and data to helping locate the aircraft via the GPS network. There are generally at least two radio antennas on an aircraft as well as a GPS receiver; a VOR (omni-directional radar-ranging) antenna; and an ELT (emergency location transmitter) antenna, not to mention additional antennas for other systems.
For example, a modern aircraft such as the Boeing 787 has more than 20 antennas protruding from its fuselage, including the systems mentioned above, but also others for satellite communications, marker beacons and weather radar, and ultra-high-frequency DME (distance-measuring equipment) antennas—transponder-based radio navigation technology that measures range and distance by timing the propagation delay of UHF radio signals. The 787 also features antennas for its instrument landing system, wireless local area network, air traffic control and traffic collision-avoidance system, terminal cellular system, automatic direction-finder and crew wireless LAN unit.
Aircraft, like everything else, are becoming more connected, and that connectivity means increased complexity and greater possibilities for electronics systems to go wrong.
With all these systems contained on the exterior of aircraft, it is not surprising that antennas are subject to considerable wear and tear, and sometimes fail. “Antenna repair is becoming a more and more critical area,” says Jose Pinho, engineering and quality manager at the components workshop at Portugal’s TAP Maintenance and Engineering. Repairs to antennas fall into three main categories—electrical and electronic, mechanical and cosmetic. For electrical and electronic repairs, the device may have failed or have been removed during a troubleshooting sequence, explains Pinho. Depending on the type of antenna, the TAP shop normally can return it to service. “Testing of antennas is achieved by measuring how much power is effectively radiated by the device,” he says.
In terms of mechanical failure, antennas are subject to being bumped by all sorts of objects, from carts and trolleys to birds. This leads to cracks, bumps and nicks to the fairings and structures in the immediate vicinity of the radiating elements, Pinho says. Repairing this damage may involve physically intervening in the immediate envelope of the antenna, thus affecting gain (efficiency). In that case, an anechoic chamber may be used for testing. “This is not only an expensive installation, but each test becomes very time-consuming,” says Pinho.
In terms of cosmetic damage, paint can be chipped or become discolored in the antenna area from the effect of the slipstream, ice and water. In addition, when liveries change there may be a need to refinish antennas. “The paint used [on antennas] must be a special one, and its application must be extremely calibrated,” Pinho notes. “This means special paint-application equipment, and on top of that, the anechoic test becomes necessary again, too.”
Lightning, Bird or Stone?
Lightning strikes also are a major issue when it comes to antenna maintenance, say technicians at MRO Lufthansa Technik (LHT). “VHF antenna systems are subject to lightning strikes,” says Timo Alexnat, shop manager for avionics at LHT. Repairs vary widely, depending on the type of antenna and location on the interior or exterior of the aircraft.
In general, the harsh environment that antennas are exposed to on the aircraft exterior is an issue. “The main exterior passive antennas are damaged by erosion and stones being thrown up during aircraft touchdown, and we experience many antennas damaged by lighting,” confirms Frank Dersewski, senior component and production engineer at LHT. “Antennas are also operating in a cold environment, so mechanical parts can fail. And that also goes for the electronic systems in active antennas.”
Damage caused to antenna systems by lightning includes small and larger holes, or partial melting of the system due to the energy of the lightning. External antennas are prone to lightning strikes because they act as conductors. Active antennas such as those for the internet contact electronic components—with amplifiers inside—that are subject to failure in the interior of the aircraft.
Mechanically operated external antennas, meanwhile, such as weather radar antennas, feature gears and motors that may fail due to erosion. These systems also may be damaged by bird strikes. Occasionally, external antennas may have damage caused by ramp rash (accidents on the ground). It is standard practice at LHT to carry out a visual inspection of antenna systems when an aircraft comes into the shop. It may be possible to identify faults in an antenna based on the performance of the equipment connected to it—the VHF transceiver, for example.
Measurement tools for antenna repair at LHT include voltage measurement devices and a special antenna room that is sealed from outside electrical interference and simulates the aircraft fuselage. Mechanical repairs may involve replacing bearings or motors in mechanical systems. Motor control systems—electronic parts—may also need to be replaced. Functional testing of antenna electronics systems helps to determine if there is a fault. Passive antennas that have been subject to serious damage from a lightning strike may need to be scrapped. Cosmetic damage can be repaired with repainting.
Because of the demands of antenna repair and the range of systems involved in today’s aircraft, an MRO must have a range of specialties including mechanical, electrical and electronics repair skills. “On older aircraft, it is more likely that we see the need to make repairs to antennas due to wear and tear,” says Lufthansa Technik technician Andre Jahnke.
Retrofits to aircraft are being carried out to fit new entertainment and internet-based systems that require new antennas. This includes a large program to retrofit aircraft in the Lufthansa fleet.
At LHT, information and expertise gained during the antenna repair process is eventually fed back to the OEM. Some repair procedures are specially developed to extend the life of the antenna as far as possible before it is completely replaced. New techniques for testing weather radar antennas are also being developed. “Ultimately, we try and share our experience with the OEM because we have a large fleet, so can pass on useful information,” says Dersewski.
Thales, meanwhile, has developed aircraft antenna systems that are intended to be maintenance-free. Its latest FlytLINK communications and safety technology operates using Iridium Certus broadband services over a network of 66 satellites that cover 100% of the globe, including poles and oceans. FlytLINK uses this network to provide highly reliable, mobile and essential voice, text and web communications for pilots, crews and passengers, including real-time weather and provision for Wi-Fi. Its safety features include optional flight-data streaming, push-to-talk voice, ACARS (Aircraft Communications Addressing and Reporting System) and other embedded safety services. Operationally, FlytLINK enhances in-air reporting, service logging, flight crew scheduling, aircraft monitoring and other operational service needs. It also supports electronic-flight-bag pairing, real-time weather, active aircraft tracking, secure pilot and crew Wi-Fi access, and enhanced calling.
Keeping A Low Profile
While these types of services are becoming more common, in Thales’s case they are being transmitted by an unusual type of antenna. The system operates via an encapsulated, low-profile antenna in which the entire device mounts directly on the skin of the aircraft without the necessity of filler plates or spacers. “As a result, antenna replacement is intended to take place in less than 1 hr.,” says Robert Squire, Thales director, marketing for Iridium NEXT. “The embedded antenna becomes a consumable assembly.” The system is serviced via a network of Thales partners globally and by the OEM. Repairs to FlytLINK require technicians to gain access, disconnect the single feed cable and remove the mounting screws. Installation simply reverses this process, says Thales. The idea is to make the MRO process for the antenna as straightforward as possible.
One type of damage that this new antenna design may avoid is caused by bird strikes. A unique characteristic of the Thales FlytLINK antenna is that because of its very low profile—less than 2-in. in height—it is not necessary to subject it to a bird-strike test at the OEM. Bird strikes are deemed to be a particular damage risk for large and protruding antenna structures.
This does not mean that FlytLINK escapes the multitude of other tests deemed necessary for new antennas. “All Thales satellite communications antennas are subject to rigorous testing for extremes of heat, cold, cooling, humidity, pressure, depression and vibration, which are typically the source of antenna damage,” says Squire. At Thales, the company simulates all these environmental characteristics for a new antenna as part of the test regime.
FlytLINK relies on a number of new proprietary technologies, Squire adds. “The specific FlytLINK antenna technology is a Thales trade secret. However, it is based on sophisticated multi-element, electronically steered, phased-array algorithms,” he tells Inside MRO.