MIL-STD-704F for Test Engineers: The Conditions Your AC Source Must Actually Reproduce

Aviation & 400 Hz · 4 min read · 45 cited facts

MIL-STD-704F does not describe how to test equipment. It describes how the aircraft's electrical power behaves — and that distinction is the reason so many test benches get specified wrongly. Your equipment is not being asked to meet the standard. It is being asked to survive an aircraft bus that does, including the parts of that bus behaviour a clean laboratory supply will never produce on its own.

So the question for a test engineer is not whether a source is 'MIL-STD-704F compliant'. It is whether the source can reproduce every condition the standard permits the aircraft to present. Those are different requirements, and only the second one gets you through qualification.

The six power types

The standard covers 115/200 V three-phase 400 Hz AC, 115 V single-phase 400 Hz, 115 V variable-frequency AC across 360–800 Hz, 115 V 60 Hz for COTS support, 28 VDC and 270 VDC. A test programme rarely needs all six, but it usually needs more than one — and the DC clauses are routinely scoped late, after the AC source has already been bought.

Normal operation is the easy part

Parameter	Value	Clause
Steady-state voltage (L-N)	108.0–118.0 V RMS	Table I
Frequency	393–407 Hz, modulation 4 Hz	Table I
Distortion factor	≤0.05	Table I
Crest factor	1.31–1.51	Table I
Peak voltage	±271.8 V	Table I

Normal 400 Hz AC operation, MIL-STD-704F Table I

Any reasonable programmable source will sit inside that envelope. The crest factor and distortion figures are worth noting though: they constrain waveform quality, not just amplitude, and a source that meets the RMS specification while producing a poor waveform is not reproducing the bus.

The transients are what size the instrument

Parameter	Value	Clause
Normal AC transient	surge 180 V for 10 ms; sag 80 V for 10 ms	Figure 3
Abnormal AC overvoltage	V = 124.6 + 2.77/t for 0.05 ≤ t ≤ 6.925 s	Figure 4
28 VDC transient	50 V for 12.5 ms; sag 18 V for 15 ms	Figure 13
270 VDC transient	330 V for 20 ms; sag 200 V for 10 ms	Figure 16

Transient and abnormal conditions

Abnormal conditions are a curve, not a number

The abnormal AC overvoltage limit is defined as V = 124.6 + 2.77/t over 0.05 to 6.925 seconds, settling to 125 V sustained. That is a time-varying envelope, and reproducing it needs a source that can sequence voltage against time with millisecond resolution — not one that can merely reach the endpoints. The same is true on the DC side, where the 28 V abnormal limit follows V = 31.38 + 0.931/t.

The DC bus is a second programme

Parameter	Value	Clause
28 VDC steady state	22.0–29.0 V, ripple ≤1.5 V	Table IV
270 VDC steady state	250.0–280.0 V, ripple ≤6.0 V	Table IV
Emergency 28 V	16–29 V	§5.3.2.3
Electric starting	12–29 V	§5.3.2.4

DC steady-state limits

Electric starting takes the 28 V bus down to 12 V. Emergency operation takes it to 16 V. These are the conditions where hold-up behaviour in the DUT is genuinely exercised, and they need a DC source that can be driven down as precisely as it can be driven up.

What this means for specifying a source

AC capability to at least 180 V RMS line-to-neutral, with ±271.8 V peak headroom
Three-phase output with phase-angle control, since phase difference is specified at 116–124°
Frequency range covering 360–800 Hz for variable-frequency platforms, plus 60 Hz for COTS
Programmable df/dt — the standard permits 250 Hz/s normal, 500 Hz/s abnormal
Millisecond-resolution envelope sequencing, not just setpoint stepping
DC capability to 50 V for the 28 V bus and 350 V for the 270 V bus, with ripple injection

Specifying a 704F bench?

Send us the power types and clauses your programme has to satisfy. We will come back with the source and load capability that covers them.

Talk to an engineer