One of the most critical aspects of aerospace EMI testing is that performed on the power lines of the equipment. The tests include various power line voltages, frequencies (for AC power), ramps, dropouts, surges, spikes, ripple, and the like. This blog will examine the characteristics of aircraft electrical power as specified in MIL-STD 704 and DO-160 Section 16.
First, about what is considered power. MIL-STD 704F, paragraph 6.10.1 states, “An aircraft electrical system is composed of a variety of power components (generation, conversion, inversion, control, power distribution, power management devices, etc.) that provide power to aircraft buses and utilization equipment terminals.”
The input power to be tested is that which comes from the aircraft power buses, typically either 400 Hz or variable frequency power (e.g., 350-800 Hz), or 28 VDC, or 270 VDC. This does not include any power which is not provided directly from the aircraft bus, such as one device providing 12 VDC to another, or 5 VAC lighting for indicators, or even if a 28 VDC line is used but provided by an intermediate supply or device. These “power lines” are typically localized and have been filtered and conditioned before routing to the destination. As such, they are less affected by other equipment on the electrical bus, which can create power line anomalies. Additionally, components and subsystems not directly tied to the aircraft power bus will have their own requirements, which are likely less severe than those required by MIL-STD 704F or DO-160.
The electrical power onboard aircraft can be subjected to a wide range of effects. When running on ground power and engaging the onboard power sources, the transition can cause a dropout or a sudden shift in voltage, frequency, inrush currents, and other similar issues. When I am a passenger on an aircraft and listen to the 400 Hz whine, see the lights blink, and hear the whining oscillate, I wince, knowing what all the electronics have just experienced. The intent is to limit any transitional changes or dropouts to 50 mS or less. This is where performing the dropout transient tests can provide information on how the equipment responds.
Initial power-on may result in significant current surges due to the input capacitance of all equipment on the bus. For this reason, current inrush tests are performed to ensure that the current demand will not create issues such as overcurrent from the supply, tripped breakers, or other capacity problems. As power generation increases, voltage may rise slowly, depending on the criticality of the equipment and the resulting category. For example, Category B and Z of DO-160 Section 16.6.1.5 require equipment to survive between 10 VDC and 20.5 VDC for up to 35 seconds.
As equipment loads and demands change, these can create voltage surges (the removal of a heavy load) or sags (the initiation of a heavy load). Other equipment on the bus must operate normally during these events, unless otherwise specified and agreed upon by both the manufacturer and the purchasing agent.
The standards define normal power characteristics as their range of voltage, and for AC power, their frequency range. For example, in DO-160, normal DC power may operate from 22.0 to 30.3 VDC and be considered normal. For 400 Hz, voltages can vary from 100 VAC to 122 VAC, and from 390 to 410 Hz. However, in emergency situations, the 28 VDC lines may drop to 18 VDC, and the AC power may vary from 360 to 440 Hz. In these cases, equipment may need to operate without interruption. There are several abnormal operating parameters that also need to be addressed.
Knowing the criticality of the equipment is important in determining the criteria and level of compliance needed. MIL-HDBK 704-1, paragraph 4.5.1, provides four examples of performance requirements for various equipment. These range from the most critical “Flight Critical Computer and Flight Displays” to the least critical “Coffeepot” (however, arguments may be made for the critical nature of coffee). In this section, the standard helps define the level of operation that may be appropriate for each device. In their example, the Flight Computer shall provide 100% full performance during normal, abnormal, and emergency electrical operations, and shall not shut down due to power failures lasting up to 50 ms. However, the coffeepot must operate under normal conditions, which is a given. But the unit may shut off and is not required to return to normal operation automatically after abnormal or emergency operations, or during and after power transfers and dropouts. Once the emergency passes, a fresh pot of coffee may be necessary for those who just dealt with the situation, but it will require manual intervention to initiate.
