Specifying a Ground Power Unit — 400 Hz, 28 VDC, and the Figures a TOR Usually Leaves Out

Most ground support equipment enquiries that reach us state the requirement in a single line — "ground power unit, 90 kVA, one unit."

That line looks complete and cannot be ordered against. A kVA figure states the size and says nothing about what kind of power leaves the machine, what it connects to, or where the figure has to hold. The consequence shows up at tender evaluation: one bidder offers a unit that draws from airport mains, another offers one with its own diesel engine, the prices are far apart, and purchasing cannot compare them — even though both answered "90 kVA" exactly as the TOR asked.

This is the set of figures that actually has to be stated, referenced to standards anyone can check.

1. Start from the power the aircraft needs, not the power the unit makes

MIL-STD-704 is the US Department of Defense standard for the characteristics of electric power as delivered at the input terminals of the equipment using it. It covers voltage, frequency, phase, power factor, ripple, maximum current, electrical noise and abnormal conditions. The systems it sets out are:

• 115/200 V AC at 400 Hz (the 200 V figure indicating three-phase)
• 28 V DC nominal

What tends to be overlooked is that MIL-STD-704 also permits alternative systems: variable frequency systems, where frequency varies across roughly 360–800 Hz, and double-voltage systems at 230/400 V, 400 Hz. Where a fleet mixes types and generations, assuming everything is 115/200 V 400 Hz is a risk worth closing while the requirement is still being written.

On the European side, EN 2282:1992 specifies the characteristics of electrical power supplied to the terminals of equipment installed in the aircraft, and states explicitly that it applies to both on-board and ground systems. Its structure separates the 115/200 V 400 Hz AC system, the 28 V DC system and special systems, and includes a test method for voltage spikes generated by the equipment itself.

2. The specification has to hold at the aircraft connector, not at the machine

This is the clause that changes acceptance testing most, and the one Thai tender documents most often omit.

ISO 6858:2017 specifies the electrical output characteristics and interface requirements between an aircraft and ground support electrical supplies, covering external power generation whether provided as a central source or at the point of use. The characteristics it sets — nominal 28 V DC, and three-phase 400 Hz AC at either 115/200 V or 230/400 V — are measured at the aircraft attaching connector, at the point the standard identifies in its Figure 1.

The difference between "measured at the unit's terminals" and "measured at the aircraft connector" is the voltage lost in the supply cable and connector. That loss is real and depends on cable length, conductor size and connector condition. A unit that tests cleanly at its own output can still fall short where it matters if the cable run is longer than the design assumed.

Two practical consequences:

1. State the supply cable length actually needed, rather than accepting whatever standard length is offered. The distance from the stand's power point to the receptacle differs by aircraft type.
2. State where acceptance testing will measure. If it is not written down, the bidder will measure wherever its unit passes.

ISO 6858:2017 also includes safety requirements, but says plainly that it does not address performance and safety issues under regional control, and excludes requirements for ground traffic control purposes such as towing points, identification and warning lights. Those come from the operator's and the airport's own regulations — "complies with ISO 6858" does not cover them.

3. 400 Hz only, or 28 VDC as well

The first question, before any model is looked at.

Some types need 400 Hz AC alone; some need 28 V DC as well for particular tasks; and there are units that supply both from one machine. Getting this wrong means either buying a second unit that was not needed, or — worse — a unit that cannot serve part of the fleet at all.

The quickest check is to work through the documentation for each type that must be supported and tabulate what each one requires, rather than relying on what one engineer is used to.

4. Output and outlets — what pushes the size up is usually not the aircraft

A common misreading is that larger aircraft require larger units.

What moves a requirement up to the 180 kVA class is usually how many aircraft must be powered at once, and how many outlets are needed — not the size of any single one. An operator with several stands preparing aircraft simultaneously should size from the busiest hour of real operations, then choose between one high-output unit with multiple outlets and several smaller units. That is a trade between unit cost, how freely equipment can be moved, and what happens when one unit goes in for maintenance.

5. Power source: airport mains, or an engine on board

This splits the market into two families whose costs and constraints differ sharply.

• Drawing from airport mains requires a frequency converter to produce 400 Hz from the airport's distribution supply. It suits stands with ground power run to them: lower running cost, no exhaust, no engine noise.
• A diesel-engined unit is for where no ground supply reaches — remote stands, temporary operating areas, or as backup when airport distribution fails. The trade is fuel cost, engine maintenance and fuel storage requirements.

Many operators need both, for different reasons. Writing both into one tender line and then comparing prices is comparing things that are not the same thing.

6. Tow tractors: always read the two figures together

For tow tractors, selection starts from the heaviest aircraft to be moved, then reads two figures in combination:

• The tractor's own weight — what puts the tyres down onto the apron
• The drawbar pull the manufacturer declares

Either figure alone misleads, because declared pull cannot be delivered if there is not enough weight to stop the tyres spinning first. Conventional tow tractors come in a series of weight classes precisely so the unit can be matched to the aircraft actually operated, without over-buying.

7. What we need in order to quote comparably

Send these four and we will shortlist the models that meet the requirement and return a line-by-line specification comparison:

1. The aircraft types to be supported — all of them, not the most frequent one
2. The power required — 400 Hz only or 28 VDC as well, and at what output
3. The installation form — towable, fixed, or trailer-mounted
4. Quantity and delivery timing

If a TOR or an earlier enquiry already exists, send it as it stands. We will quote against your own clause numbers so purchasing can check it line for line.

Where the quoted specifications come from

From the manufacturers' own datasheets, with the source named on every page of the catalogue — measured values rather than marketing copy. The standards referenced are ones that can be checked: MIL-STD-704, ISO 6858, ISO 1540, SAE ARP5015, EN 2282, and MIL-STD-461 where interference control is specified.

Specifications change by sub-model and option, so the manufacturer's current datasheet always governs; we can supply it in full.

Our role in this work

Sahawatthanakit (1988) represents no GSE manufacturer exclusively and does not manufacture. Our role is sourcing and model selection for the operating unit, which is exactly what allows us to compare models across European, US and Asian manufacturers on the merits of the job rather than on what we happen to stock.

We are a registered vendor with Thai Aviation Industries (TAI), with that scope extended to Ground Support Equipment.

Send the aircraft types and how the equipment will be used to 02-096-2118 or 081-866-8368 and we will return a line-by-line comparison.