Electrical power generation and distribution

This category covers generators and starter-generators, transformer rectifier units, inverters, contactors, bus bars, power distribution panels and the control units that manage them. Batteries have their own guide, the wiring that carries the power has another, and the engine that turns the generator is under propulsion. What is bought here is capacity and behaviour under failure, in that order.

The load analysis comes first

For transport category airplanes the rule is explicit: the required generating capacity and the number and kinds of power sources must be determined by an electrical load analysis, and the generating system must be designed so that power sources function properly independently and in combination, and so that no failure of any source creates a hazard or impairs the remaining sources supplying essential loads.¹ Every serious conversation in this category starts from that analysis. Bring yours, with the growth allowance you actually expect, and ask suppliers to quote against it. A supplier who asks to see the load analysis before quoting is a supplier who has done this before.

What to specify, and what to check

Ask for continuous rating at the ambient temperature and altitude of the installation, and ask what derating applies. Ask for the overload profile and how long the unit will hold it. Ask what the fault current contribution is and how coordination with the protective devices was verified. Ask what happens on generator loss: which buses shed, in what order, and how the essential loads are held. Ask for the environmental qualification categories and the electromagnetic compatibility evidence, and ask whether the unit has been qualified in the orientation and cooling condition your installation gives it. Where the item is approved as an article, a technical standard order authorisation is a design and production approval,² and the FAA says plainly that it is not an approval to install and use the article on an aircraft.³ The installation approval is a separate document and a separate cost.

Reliability arrives as a number or not at all

Ask for the mean time between unscheduled removals from fleet data, not the predicted figure from the reliability model. Ask for the overhaul or inspection interval and what the shop visit costs. Ask what the spares lead time is today and what it was a year ago. Ask what the obsolescence plan is for the semiconductors inside the unit, because the power electronics generation will end before the airframe does.

Where the public record can be checked

Airworthiness directives are legally enforceable rules the FAA issues when an unsafe condition exists in a product and is likely to exist or develop in others of the same type design, and they apply to appliances as well as aircraft, engines and propellers.⁴ Search the part numbers you are being offered before you commit, and read what the directive required and how quickly the fix was available. That record shows what went wrong and what was done about it. It does not show how the supplier treated the operators who were grounded, which is a question for those operators.

Questions that sort suppliers

Ask for the load analysis assumptions the quote is built on. Ask for fleet reliability data with the fleet size attached. Ask for the qualification report and the installation limitations. Ask what the repair network is and where the nearest approved shop sits.