For commercial airline Passenger Electronic Devices, see this topic

Canadian CH-148

(Canadian Press, September 2013)

Electromagnetic interference concerns are hampering progress on a Canadian Air Force project.

According to The Canadian Press, the Canadian government has refused to accept four CH-148 Cyclone test helicopters currently parked at the Canadian Forces facility in Shearwater, N.S., on the basis that they are “non-compliant.” Designed by Sikorsky Aircraft Corporation, the twin-engine CH-148 Cyclone helicopters are slated to replace the CH-124 Sea Kings, which have been in operation since 1963.

However, the Canadian news outlet reports, defense sources with intimate knowledge of the program have elaborated on the public report, saying that certain flight systems, including a computer that runs the engines, are not considered sufficiently shielded against powerful electromagnetic waves. Earlier this year, the directorate of airworthiness at the Department of National Defence and the Canadian Armed Forces cited electromagnetic compatibility, electromagnetic vulnerability and electromagnetic interference concerns in its decision to impose flight restrictions on the CH-148 Cyclone helicopters.

“Each of them [the concerns] are potential show-stoppers,” a defense source, who asked for anonymity, told The Canadian Press. “The vulnerability depends on the frequency and the strength of the signal. You have the potential of losing your instruments and not knowing where you are, and having to take visual cues from outside your aircraft to get down safely.”

Air Force engineers and government officials are increasingly skeptical regarding whether or not the shielding problem can be sufficiently remedied in a reasonable amount of time. Initially scheduled to be ready for service in 2008, only a few Cyclone helicopters have thus far been delivered for testing. The project is five years behind schedule and over-budget.

“The aircraft was not designed from the ground up with this kind of shielding in mind,” the defense source told The Canadian Press. “Military aircraft, the skin of military aircraft, [is] sometimes embedded with a fine copper screen or mesh to prevent the intrusion of electromagnetic interference.” The CH-148 Cyclone helicopters, however, are based on a less-rugged civilian design.

Spacecraft incidents

The following examples are extracted from "Electronic System Failures and Anomalies Attributed To Electromagnetic Interference", R D Leach & M B Alexander, NASA Reference Publication 1374, July 1995, available from http://trs.nis.nasa.gov/search.jsp.

Range safety interference

In an incident related to the range safety system on a Saturn vehicle, the range safety receivers detected a low-level signal from somewhere in the KSC (Kennedy Space Centre) vicinity. This signal was not always present. The possibility of signal mixing was considered and all radio stations and mobile transmitters in the area were investigated - no spurious signals detected. KSC and MSFC (Marshall Space Flight Centre) EMC personnel worked all one night trying to solve the problem. At one point soon after the signal suddenly stopped, one of the MSFC EMC personnel stepped outside the small trailer used to house test equipment, noticed daybreak had occurred and searchlights surrounding the vehicle had turned off. On his request, they were turned on again and the unwanted signal reappeared. Further investigation revealed the searchlights were carbon arc lamps that produced a broadband radio frequency (RF) signal and the lamp reflectors beamed the signal directly to the range safety antennas on the vehicle.

RAU Transient Susceptibility

The remote acquisition units (RAU’s) are data transmission interfaces between Shuttle Spacelab payload experiments and experiment controllers on the ground. An RAU is designed to shut down if its voltage drops below a certain level for a certain time period. During checkout of Spacelab payloads at KSC, RAU’s experienced numerous shutdowns when various items of equipment were turned on or off. MSFC EMC personnel were called to conduct a problem investigation that determined the test setup used for preflight tests of Spacelab payloads did not accurately simulate fuel cell capacitance and line impedance. A representative capacitance was used in the proper location, power-up problems ceased, and testing proceeded unhindered. Solving this problem produced a valuable understanding of susceptibility of RAU’s to transient signals. Another result of this investigation was the formulation of an EMI test requirement to insure that RAU’s do not fail due to operation of experimental equipment.

Wake Shield Experiment

The wake shield was an experiment, towed behind STS-60, launched February 3, 1994, to create a better quality vacuum than was available within the orbiter. Failure occurred because the small satellite used in the experiment could not be deployed due to EM1 with its attitude control system. The EMI was caused by inductive coupling (crosstalk) between the unshielded attitude control sensor cable and the power bus of the spacecraft. The control systems cable was redesigned and shielding added. This was an unpleasant lesson learned at the cost of a failed experiment.

Military incidents

These examples come from MIL-STD-464C, "ELECTROMAGNETIC ENVIRONMENTAL EFFECTS REQUIREMENTS FOR SYSTEMS" Appendix A, and from NASA RP-1374 above.

Aircrews have reported severe interference to communications with and among flight deck crew members. UHF emissions in the flight deck environment caused interference severe enough that crews could not hear each other for aircrew coordination. This problem poses a serious hazard to personnel with the potential for damage to, or loss of, the aircraft and aircrew during carrier flight deck operations.

Aircraft on approach to carrier decks have experienced interference from shipboard radars. One such problem involved the triggering of false "Wheels Warning" lights, indicating that the landing gear is not down and locked. A wave-off or preflight abort could occur due to this EMI induced condition.

Aircraft systems have experienced self-test failures and fluctuations in cockpit instruments, such as engine speed indicators and fuel flow indicators, caused by sweeping shipboard radars during flight-deck operations. These false indications and test failures have resulted in numerous unnecessary pre-flight aborts.

An aircraft lost anti-skid braking capability upon landing due to RF fields from a ground radar changing the weight-on-wheels signal from a proximity switch. The signal indicated to the aircraft that it was airborne and disabled the anti-skid system.

High-powered shipboard radars have caused interference to satellite terminals located on other ships, resulting in loss of lock on the satellite and complete disruption of communication. The interference disables the satellite terminal for up to 15 minutes, which is the time required to re-establish the satellite link. Standoff distances of up 20 nautical miles between ships are required to avoid the problem.

A weapon system suffered severe interference due to insufficient channel selectivity in the receiver’s front end. Energy originating from electronic warfare systems and another nearby “sister” channelized weapon system (operating on a different channel but within the same passband) coupled into the victim receiver and was “processed,” severely degrading target detection and tracking capability. Installation of an electronically tuned filter immediately after the antenna countered the off-channel interference problem by: 1) eliminating receiver front-end amplifier saturation and 2) reducing overload of the system processor with extraneous in-band signals. Note: this may refer to a documented "Friendly Fire" incident in the first Gulf War, when an RAF Tornado aircraft was shot down while returning to its base by a Patriot missile, defending the area.

The effects of lightning can cause physical damage to personnel and equipment. In one of numerous documented lightning incidences, lightning appeared to enter a Navy aircraft nose, travel down the right side, and exit on top of the right vertical tail. The pilot suffered from flash blindness for 10-15 seconds. Upon regaining his vision, the pilot noticed all cockpit electrical power was gone. After another 15 seconds had elapsed, all cockpit electrical power returned on its own, with no cockpit indications of any equipment malfunction.

In another case, lightning attached to the nose pitot tube, inducing transients that damaged all 28 volt DC systems. The pilot, disoriented, broke out of a cloud bank at 2000 feet above the ground, at 600 knots and a 45 degree dive. Nearly all cockpit instruments were dysfunctional – compass, gyrohorizon, and so forth. A secondary effect occurred but was not uncovered for several months. The lightning current path that carried the direct effects lightning current did what it was supposed to do, but the path was not inspected on landing. Over 800 man-hours were expended to correct electrical (28 volt DC) problems but no effort went into inspecting for direct effects damage to ensure the lightning protection system was intact. The rigid coax from the front of the radome to the bulkhead had elongated and nearly torn away from its attachment point at the bulkhead due to magnetic forces involved. This damage reduced the effectiveness of the designed lightning protection. Another secondary effect was the magnetization of all ferrous material which caused severe compass errors. The entire aircraft had to be degaussed.

When an aircraft was flying in clouds during a thunderstorm, the pilot was unable to transmit or receive on the communications radio. Further investigations were performed with the most reasonable conclusion that the radio blanking was caused by electrostatic discharge. Several incidents were also reported where pilots and ground crews received shocks due to static discharges from aircraft canopies. These incidents occurred on the carrier deck after the aircraft had been airborne for several hours.

Static discharges from the canopy were shocking pilots on a fighter aircraft during flight. Charges accumulating on the outside of the canopy apparently induced a similar charge on a conductive finish that was on the inside of the canopy. When a discharge occurred on the outside of the canopy, the internal charge discharged to the pilot’s helmet. Proper grounding of the conductive finish on the inside of the canopy fixed the problem.

An aircraft had a small section of the external structure made of fiberglass. Post-flight inspections required personnel to get in close proximity to this non-conductive structural component. On several occasions, personnel received significant electrical shocks which caused them to fall from ladders and be injured. Corrective action was easily accomplished by applying a conductive paint to the surfaces exposed to airflow and personnel contact.

A fighter aircraft was experiencing severe degradation of the UHF receiver when flying in or near clouds. Investigation revealed that the aircraft was not equipped with precipitation static dischargers. Installation of these devices solved the problem.

U.S.S. Forrestal

In 1967 off the coast of Vietnam, a Navy jet landing on the aircraft carrier U.S.S. Forrestal experienced the uncommanded release of munitions that struck a fully armed and fuelled fighter on deck. The results were explosions, the deaths of 134 sailors, and severe damage to the carrier and aircraft. This accident was caused by the landing aircraft being illuminated by carrier based radar, and the resulting EMI sent an unwanted signal to the weapons system. Investigations showed that degraded shield termination on the aircraft allowed the radar frequency to interfere with routine operations. As a result of this case, system level EMC requirements were revised to include special considerations for electroexplosive devices.

PIONEER Remotely Piloted Vehicle (RPV)

PIONEER was an RPV with nondevelopmental item (NDI) status. It was essentially a non-military specification item that relied on COTS hardware. Because of this, the Navy anticipated EMI problems during a test flight in January 1987 aboard the U.S.S. Iowa. The RPV pilot, using a portable remote control box, experienced a series of uncommanded control transfers between his remote control box and another used by a student pilot. These uncommanded signals caused loss of control and a crash landing. There were also instances during other RPV flights of anomalous signals and switching. Subsequent investigation found that the remote control boxes received EMI from HF communication transmitting antennas located aboard the Iowa that coupled into the box due to inadequate shielding and cable termination. Utilizing improved cables and cable connectors, proper shielding, filters, and internal logic changes corrected the problem. The HF signal caused other problems with the RPV internal systems, but eventual modifications also corrected these. Generally, use of COTS hardware makes the hardening of electronics necessary to protect them from the very harsh EME found aboard Navy vessels.

B-52 Stability Case

When op-amp-based flight control systems were first added to the B-52 bomber autopilot stability augmentation system, use of the HF radio resulted in the uncommanded activation of all rear empennage flight control surfaces. The wiring system, which had not been changed, was found to be susceptible to HF. This was a case of using new (at that time) technology and introducing a potentially dangerous problem that had not existed before using the new technology.

UH-60 Blackhawk Case

An Army Sikorsky UH-60 Blackhawk helicopter, while flying past a radio broadcast tower in West Germany in 1987, experienced an uncommanded stabilator movement. Spurious warning light indications and false cockpit warnings were also reported. Subsequent investigation and testing showed that the stabilator system was affected by EMI from high intensity radiated fields (HIRF). The Blackhawk has a conventional mechanically linked flight control system with hydraulic assist. The stabilator system, however, uses transmitted digital signals (fly-by-wire) to automatically adjust its position relative to control and flight parameters. These digital signals are highly susceptible to HIRF. When the Blackhawk was initially designed, the Army did not routinely fly near large RF emitters. The Navy version of the Blackhawk, the SB-60 Seahawk, however, has not experienced similar EMI problems because it is hardened against the severe EME aboard modern ships. Despite the Army identifying several hundred worldwide emitters that could cause problems and instructing its pilots to observe proper clearance distances, between 1981 and 1987 five Blackhawk helicopters crashed and killed or injured all on board. In each crash, the helicopter flew too near radio transmitters. The long-term solution was to increase shielding of sensitive electronics and provide as a backup some automatic control resets.

AH-64 Apache Helicopter

The Apache helicopter was designed primarily as an airborne antitank weapon to provide quick-strike capability against tanks and armored vehicles. The Apache is essentially an electronic device with multiple electric and electronic systems that control navigating and fighting abilities. During early missions, pilots complained that HIRF signals interfered with electronics. In one case, EMI triggered an overspeed condition that could have resulted in double engine failure. Subsequent reports showed that the aircraft was susceptible to low-level emitters such as commercial microwave, television, and airport and missile radar. At one point, the Army concluded that the Apache should not be used on aircraft carriers. After determining that the source of unwanted signals was coupling in the I/O cables, the shielding was redesigned; but not before many costly aircraft were built with EMI deficiencies.

F-16 Flight Controls

An F-16 fighter jet crashed in the vicinity of a Voice of America (VOA) radio transmitter because its fly-by-wire flight control system was susceptible to the HIRF transmitted. Since the F-16 is inherently unstable, the pilot must rely on the flight computer to fly the aircraft. Subsequently, many of the F-16's were modified to prevent this type EMI, caused by inadequate military specifications on that particular electronics system. This F-16 case history was one of the drivers for institution by the Federal Aviation Administration (FAA) of the HIRF certification program. (Another VOA HIRF case occurred in 1984 near Munich, Germany. A West German Tornado fighter crashed after flying too close to a powerful VOA transmitter.)