Civilian and military aircraft require reliable, high-performance power supplies to ensure safe operation in a variety of environmental conditions. Created in 1959, MIL-STD-704 has and continues to set the standard for aircraft power sources. It underwent several revisions over the last few decades to correct errors and standardize test methodologies. The current revision “F” was issued in March of 2004 and updated in December 2016. Read on to learn more about MIL-STD-704 requirements and why they’re crucial for ruggedized power supplies.
What is MIL-STD-704?
MIL-STD-704 is a military specification that defines and outlines the requirements for compatibility between the airborne utilization equipment (power supplies) and the aircraft power system (including the external sources for the power system).
It also establishes the characteristics that the power system needs to provide at the utilization equipment power terminals. The standard covers multiple areas for AC and DC systems including:
- Power factor
- Voltage
- Phase unbalance
- Frequency
- Maximum current
- Ripple
- Abnormal conditions (e.g. over or undervoltage)
- Electrical noise
What Are MIL-STD-704 Requirements?
MIL-STD-704 states that aircraft electrical power systems should offer electric power at the utilization equipment terminals with the characteristics specified in this standard during all operations of the power system. The requirements encompass aircraft electric power systems, aircraft utilization equipment, and external power source requirements.
Aircraft Electric Power System Requirements
An aircraft electric power system consists of a main power source, emergency power source, power conversion equipment, control and protection devices and an interconnection network (wires, cables, connectors, etc.). The main power is derived from aircraft generators driven by the aircraft propulsion engines. Emergency power is derived from batteries, engine bleed air, independent auxiliary power units or ram air, or hydraulically driven generators.
The airframe modifier or manufacturer must provide the protection network and distribution to utilization equipment terminals while maintaining the power characteristics specified in the standard. Furthermore, all protective devices must function independently of regulation and control equipment.
Aircraft utilization equipment must accept power with the characteristics as specified in the standard to generate power at their output terminals in accordance with their own detailed specifications. The utilization equipment is typically a ruggedized power supply which provides reliable electrical power for airborne systems such as radar, sonar and computer processors, etc.
Aircraft Utilization Equipment Requirements
MIL-STD-704 also covers the requirements for aircraft utilization equipment. Below you’ll find an overview of all these requirements.
Power Compatibility
The equipment used in a military aircraft must be compatible with the power characteristics specified in the MIL-STD-704 standard. The standard sets the input requirements for military aircraft power supplies, compliance to the standard ensures compatibility and optimal performance between the source and utilization equipment. Such equipment shall not require a power of better quality than specified by the standard.
The utilization equipment shall also be compliant with the requirements of the applicable aircraft specifications for the control of electromagnetic interference and voltage spikes caused by power switching, lightning, and electromagnetic pulses. MIL-STD-704 does not cover voltage spikes and electromagnetic interference.
Operation
Besides power compatibility, another crucial requirement of aircraft utilization equipment is that it shall not cause the power characteristics at its input terminals to exceed the limits specified in the standard, as well as providing the level of performance required by its detailed specification for each operating condition.
Regardless of the operating conditions of the electrical system, the utilization equipment should not negatively affect the electrical system or cause it to malfunction.
The operation modes addressed in MIL-STD-704 are: normal operation, abnormal operation, transfer operation, emergency operation and starting opoeration.
Power Failure
As per MIL-STD-704, any utilization equipment that loses power (AC or DC) or one or more phases of AC power must not result in an unsafe condition or damage to that equipment.
AC Power Utilization
MIL-STD-704 also provides more specifications for utilization equipment using AC power as per below:
- For equipment requiring more than 0.5 kVA of 400 Hz AC power, the configuration shall be designed to utilize three-phase steady-state balanced power.
- Utilization equipment whose total load is greater than 30 kVA should have a load unbalance no greater than 3.33% of its total three-phase load.
- Single-phase AC power should be used only on a line-to-neutral basis.
- AC power should not be half-wave rectified.
External Power Source Requirement
When aircraft are not airborne, external sources are used to supply the power system. These sources shall supply power to the external power receptacles. To allow for steady state voltage drops between these receptacles and the utilization equipment terminals the voltages shall be within the following ranges:
- AC system 113 to 118 volts (AC voltage drop of 0 to 5 volts)
- 28 VDC system 24 to 29 volts (DC voltage drop of 0 to 2 volts)
- 270 VDC system 260 to 280 volts (DC voltage drop of 0 to 10 volts)
With external sources compliant to the above, the ruggedized power supplies within the aircraft can continue operation when the aircraft is on the ground.
Test Requirements
Both the aircraft power system and the utilization equipment shall be tested to ensure compliance with the standard. Military aircraft are tested to demonstrate that the electric system power characteristics are within the limits of the standard throughout all operating conditions. The aircraft test requirements are typically defined in the aircraft detail specification.
Utilization equipment shall be tested to demonstrate compatibility and compliance with the electric system power characteristics. The applicable test methods to be used are defined in MIL-HDBK-704-1 thru 8, depending on the type of power system (AC single or three phase, 60Hz, 400Hz or variable frequency, DC 270V or 28V). The test requirements for the utilization equipment shall be in accordance with the equipment’s detail specification.
Ruggedized Power Supplies
Viable Power can design and build custom AC-DC Power Supplies and DC-DC Power Supplies to meet the requirements of the latest, or any of the previous revisions of MIL-STD-704. If you are looking for a rugged MIL-STD-704 compliant aerospace power supply, Viable Power’s team of experts can assist you. Request a proposal or contact us for more information on our custom power supply design services for the military aerospace sector.
