MIL-STD-704F Department of Defense Interface Standard: Aircraft Electrical Power Characteristics

The standard "MIL-STD-704F Department of Defense Interface Standard: Aircraft Electrical Power Characteristics" developed by the United States Department of Defense describes the requirements and characteristics of aircraft electrical power supplied at the input terminals of electrical utilization equipment. Standards MIL-HDBK-704-1 through 8 define test methods and procedures for determining compliance of airborne utilization equipment with the electrical power characteristic requirements defined herein. Electromagnetic interference and voltage spikes are outside the scope of this standard.

For the purposes of this standard, the following terms and definitions apply:

• Aircraft electrical power systems: An aircraft electrical power system consists of a main power supply, an emergency power supply, power conversion equipment, control and protection devices, and an interconnection network (such as wires, cables, connectors). Main power is obtained from aircraft generators driven by the aircraft propulsion engines. Emergency power is obtained from batteries, engine bleed air, independent auxiliary power units, ram air-driven generators, or hydraulic-driven generators.
• Crest factor: It is the absolute value of the ratio of the peak to the RMS value for each half cycle of the voltage waveform.
• Current: Alternating current (AC) is the RMS value for half a cycle measured between successive zero crossings of the fundamental frequency component. Direct current (DC) is the instantaneous value.
• Current modulation: Current modulation is the difference between the maximum current and the minimum current. Percentage current modulation is the ratio of the current modulation to the average (average for DC, RMS value of the fundamental for AC) multiplied by 100 over a one-second period.
• DC voltage: Steady-state DC voltage is the time average of the instantaneous DC voltage over a period not exceeding one second.
• Distortion: AC distortion is the RMS value of the AC waveform excluding the fundamental. In a DC system, DC distortion is the RMS value of the alternating voltage component in the DC voltage.
• Distortion factor: AC distortion factor is the ratio of AC distortion to the RMS value of the fundamental component. DC distortion factor is the ratio of DC distortion to the DC steady-state voltage.
• Distortion spectrum: The distortion spectrum quantifies AC or DC distortion in terms of the amplitude of each frequency component. The distortion spectrum includes harmonic and non-harmonic components of the waveform, as well as components resulting from amplitude and frequency modulation.

In this standard, the aircraft electrical power system requirements are specified as follows:

• Aircraft electrical power system performance: The aircraft electrical power system shall provide electrical power with the specifications specified in this standard at the terminals of the utilization equipment during all operations of the power system.
• Electrical power supply characteristics: The characteristics of the electrical power measured at the output terminals of an unregulated power supply or the regulating point of a regulated power supply shall be within the limits specified in the applicable detailed specification of the power supply. It is the responsibility of the airframe manufacturer or modifier to provide distribution and protection network to the terminals of the utilization equipment while maintaining the power characteristics specified in this standard.
• Protective devices: Protective devices must operate independently of control and regulating equipment.

In this standard, aircraft handling equipment requirements are specified as follows:

• Power compatibility: The usage equipment shall be compatible with the power specifications specified herein. The usage equipment shall not require electrical power of a better quality than that specified herein. The usage equipment shall comply with applicable aircraft specification requirements for control of electromagnetic interference and voltage spikes caused by lightning, electromagnetic pulses, and power switching. Electromagnetic interference and voltage spikes are not covered by this standard.
• Operation: When supplied with electrical power having the specifications specified herein, the aircraft operating equipment shall provide the level of performance required by its detailed specification for each operating condition. Operation of the operating equipment shall not cause the power characteristics at its terminals to exceed the limits specified herein. Operation of the operating equipment under any electrical system operating condition shall not have an adverse effect on the electrical system or cause the electrical system to fail.
  • Normal operation: The equipment in use must provide the performance level specified in its detailed specification.
  • Abnormal operation: The use equipment shall be allowed to deteriorate or lose function unless otherwise specified in its detailed specification. The use equipment shall not be damaged or cause an unsafe condition. The use equipment shall automatically return to full performance when the normal operation of the electrical system is restored.
  • Transfer operation: The utilization equipment may not be required to operate under the transfer condition unless a performance level is specified in its detailed specification. The utilization equipment shall automatically return to the specified performance when normal operating characteristics are restored.
  • Emergency operation: The operating equipment must provide the performance level specified in its detailed specification when such performance is necessary for flight or safety.
  • Start-up operation: The equipment in use must provide the performance level specified in its detailed specification when performance is required during the start-up operation.
• Power failure: Loss of power (AC or DC) or loss of one or more AC power phases to any terminal of the utilization equipment shall not result in an unsafe condition or damage to the utilization equipment.
• AC power usage: Utilization equipment requiring more than 0,5 kVA of AC power shall be configured to use three-phase steady-state balanced power. The load unbalance of individual utilization equipment shall be within the limits shown in the figure. The load unbalance of utilization equipment with a total load greater than 30 kVA shall not exceed 3,33 percent of its total three-phase load. Single-phase AC power shall be used only on a line-to-neutral basis. AC power shall not be rectified on a half-wave basis.

In this standard, external power supply requirements are specified as follows:

• External electrical power sources shall provide power with the characteristics specified in this standard at the power input terminals of the aircraft electrical handling equipment. To allow for a steady-state voltage drop between the aircraft external power receptacle and the aircraft handling equipment terminals, the voltage at the external power receptacle shall be:
  • AC system 113 to 118 volts (0 to 5 volt AC voltage drop)
  • 28 VDC system 24 to 29 volts (0 to 2 volt DC voltage drop)
  • 270 VDC system 260 to 280 volts (0 to 10 volt DC voltage drop)

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