Aircraft Electrical Power

Standardized power interfaces play a crucial role in any aircraft to be airborne. MIL-STD-704 Aircraft Electrical Power Characteristics stands as a beacon, defining the compatibility between airborne utilization equipment and aircraft power systems.

In this blog, we help you understand the particulars of MIL-STD-704, its components, and the comprehensive requirements it sets for both AC and DC systems.

Understanding MIL-STD-704

MIL-STD-704 is a United States Military Standard that outlines the standardized power interface between military aircraft and their equipment and carriage stores. Covering a spectrum of parameters including voltage, frequency, phase, power factor, ripple, maximum current, and abnormal conditions such as overvoltage and undervoltage, this standard ensures seamless interoperability and performance in the demanding conditions of military aviation.

MIL-HDBK-704: A Guide for Compliance

Complementing MIL-STD-704 is MIL-HDBK-704, a handbook providing essential guidance on test procedures to determine compliance with the aircraft electrical power characteristics specified in MIL-STD-704 revisions A through F. Broken down into eight parts, this handbook serves as a comprehensive resource for demonstrating the compatibility of utilization equipment with the established standards.

Components of the Electrical System

The MIL-STD-704 defines the basic aircraft AC system at 115/200 volts, 400 Hz, with the larger figure indicating three-phase power. Variable frequency systems are allowed to switch between 360 and 800 Hz. For double voltage systems, the frequency remains the same at 230/400 volts. Secondary power systems can be variable frequency or double voltage, with an option to use 115 volts, 60 Hz, for running commercial off-the-shelf products expecting mains electricity.

On the DC side, the system runs at either 28 volts or 270 volts, providing a versatile power distribution framework.

MIL-STD-704 Requirements

MIL-STD-704 establishes stringent requirements to ensure the compatibility and performance of aircraft electrical power systems. These requirements encompass a range of parameters:

  • Power Factor: Ensures the alignment of real power with the apparent power in the system.
  • Voltage: Specifies the voltage characteristics at the utilization equipment power terminals.
  • Phase Unbalance: Ensures uniformity in the distribution of three-phase power.
  • Frequency: Sets the standard for the frequency of the power system, allowing for variable frequency within defined limits.
  • Maximum Current: Establishes limits on the maximum current that the power system can provide.
  • Ripple: Defines the acceptable levels of electrical ripple in the power supply.
  • Abnormal Conditions: Addresses scenarios such as overvoltage and undervoltage, providing guidelines for operation under abnormal circumstances.
  • Electrical Noise: Specifies limits for electrical noise to ensure signal integrity and reliability.

Test Requirements

Both the aircraft power system and the utilization equipment undergo rigorous testing to demonstrate compliance with MIL-STD-704. Aircraft testing ensures that the electric system power characteristics adhere to the standard under all operational conditions, typically defined in the aircraft detail specification. Utilization equipment, on the other hand, is tested for compatibility and compliance with electric system power characteristics using methods defined in MIL-HDBK-704-1 through 8, depending on the power system type.

MIL-STD-704 stands as a cornerstone in ensuring standardized and interoperable electrical power systems. As aircraft systems evolve and technology advances, the commitment to rigorous testing and compliance with MIL-STD-704 remains instrumental in safeguarding the performance and reliability of airborne utilization equipment.