What Is MIL-STD-704?

MIL-STD-704 sets the electrical power interface for equipment on military aircraft. It describes the power the aircraft supplies, in normal operation and when things go wrong, so that equipment designers know exactly what their power input must handle. It covers the normal voltages and frequencies, and the transients and abnormal conditions, such as power interrupts and surges, that equipment must tolerate without damage or unacceptable disruption.

The standard defines the power quality at the equipment’s input terminals. It does not test equipment itself. The test procedures are in a companion handbook, MIL-HDBK-704, split into parts for each power type, which equipment makers use to show that their product works across the conditions the standard describes.

The current revision is MIL-STD-704F, issued in 2004 and updated by a change notice in 2016. Earlier revisions, back to 704A, still appear in contracts for older aircraft, and the limits differ between them, so the revision matters.

Military aircraft use several kinds of power, and 704F covers each. The steady-state figures below are for normal operation at the equipment input; abnormal and emergency conditions widen them.

400 Hz AC lets aircraft use lighter transformers and motors than 50 or 60 Hz mains, which is why it remains the standard aircraft AC frequency.

The heart of MIL-STD-704 is a set of operating conditions, each with its own voltage and timing limits:

Airborne computers and displays draw from a power bus that varies with engine, generator and load conditions. Designing to MIL-STD-704, or protecting the input to its requirements, keeps the system stable through those variations. In practice that means a wide-range DC input, transient suppression, and hold-up capacitance or a small energy store to ride through the 50 ms transfer interrupt without rebooting.

Equipment also needs to define its own behavior. The standard expects specified performance through normal and transfer conditions, safe behavior during abnormal conditions, and no damage from power failure. Mission computers often add power-loss protection for their storage, so a data write is not corrupted if power drops.

AC-powered equipment has further limits to meet, such as current distortion, power factor and inrush current, so that it does not upset the aircraft supply for other users. DC equipment has limits on inrush current and the ripple it reflects back onto the bus.

Civil aircraft equipment is tested to RTCA DO-160, whose Section 16 covers power input in a similar way. Military ground vehicles use MIL-STD-1275 for their 28 V DC supply, which has different spikes, surges and starting dips because vehicle alternators and batteries behave differently from aircraft generators. Our guide to MIL-STD-1275 covers the vehicle standard.

Many rugged power supplies are designed to meet both 704 and 1275 so one computer can be fitted to aircraft and vehicles. Our overview of US military standards shows how the power standards fit alongside MIL-STD-810 and MIL-STD-461.

Several systems in our rugged mission computers range list MIL-STD-704 compliance for their power input, usually alongside MIL-STD-1275 and MIL-STD-461. They suit airborne roles such as ISR payload processing on crewed and uncrewed aircraft, as well as vehicle installations. Our wider military computers range covers panel PCs and displays for defense platforms, where the power input specification is checked against the platform before selection.

We supply mission computers with MIL-STD-704 and 1275 power inputs, and integrate and test each system before it ships. Tell us the aircraft or vehicle and the power standard your program calls up, and we will shortlist systems with matching evidence.