Let's say I have some LRU device that requires some DC voltage to be powered on. Are there any general requirements in aviation/aerospace that say how accurate that supply voltage must be in order to be allowed on a plane?


To be more specific, consider an LRU that takes 28V on the input and provides it to a DC-DC converter. I would like to know how accurate that 28V being provided, needs to be.

  • $\begingroup$ For the updated question, the LRU (if it's a cots equipment) probably has established specs for its input power. 704E/F etc. so the spec data sheet should contain the info. If not, it's kind of a tricky subject. Are you trying to develop a power distribution system? It would help receive even more practical answers. $\endgroup$ Jan 12, 2017 at 18:51
  • $\begingroup$ @GürkanÇetin Thanks, that's a good bit of information for when dealing with a specific LRU. Also, no I am not trying to develop a power distribution system. I am just asking a general question here as to whether or not there is a global set of requirements for input power that applies to all LRUs (outside of the spec data sheet for a particular LRU). $\endgroup$
    – Snoop
    Jan 13, 2017 at 12:25
  • $\begingroup$ Sorry for noob question, but could you define "LRU" ? Are you talking about this ? If so, what type of equipement can that be ? That's unclear to me. $\endgroup$
    – kebs
    Jan 13, 2017 at 14:22
  • $\begingroup$ @kebs No problem, I'd be happy to help. As you suggested, an LRU is a line-replaceable unit and that's exactly what I am talking about in this question. Line replaceable units are parts or modules of an aircraft, things like fuel gauges or inertial measurement devices that contribute to operating the aircraft. $\endgroup$
    – Snoop
    Jan 13, 2017 at 14:47

2 Answers 2


Short answer:

  • Operational voltages on the 28 V bus may range from 22 V to 30 V.
  • Abnormal voltages on the 28 V bus may occur from 0 V to 60 V.

The detailed requirements depend on the manufacturer, the type of plane and the jurisdiction.


US military standard MIL-STD-704(F) Aircraft Electrical Power Characteristics defines electrical properties of military aircraft equipment. For instance the 28 V DC bus is specified as:

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A similar standard should exist for commercial aircraft, unless they just apply MIL-STD-704 as well.

Europe: ISO 1540

In Europe there is ISO 1540:2006 Aerospace -- Characteristics of aircraft electrical systems. This standard differentiates by the actual power source:

  • Category A: TRU (transformer)
  • Category B: engine driven DC generator
  • Category R: regulated DC power from active converter

Category A and B are specified identically for their DC characteristics. Category B is the worst case for AC ripples. Category R is the best case, as it is already regulated. Nevertheless, connected equipment needs to cope with the worst case, that is category B (engine driven generator).

ISO 1540 specifies the 28 V DC bus characteristics (summarized) as:

  • 28 V DC bus - Normal operation: 22 V - 30 V
  • 28 V DC bus - abnormal/onset: 0 V - 60 V

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  • $\begingroup$ Very nice summary. Discusses the power supply inputs to the LRUs present on the voltage bus exactly like I was asking for. Thank you very much. $\endgroup$
    – Snoop
    Jan 13, 2017 at 12:18
  • $\begingroup$ Cat-Z can go upto 80V for 100ms $\endgroup$
    – Naib
    Aug 31, 2019 at 15:36

It would depend on whether you are connecting directly to the main DC bus (28V presumably) or being powered by a DC/DC converter.

DC/DC converters are required to meet TSO C71 Airborne Static ("DC to DC") Electrical Power Converter in a certified installation.

Many LRUs I have seen are designed to operate on a DC input voltage from 10 VDC - 32 VDC

If the main power bus is AC, you would need to have an AC to DC converter to provide the DC power.

If you are trying to test the LRU, it should have a specified input voltage range.

  • $\begingroup$ That document you posted refers to the output of the DC-DC. I want to know about the input power supply voltage to the LRU. Maybe I should have been more specific. $\endgroup$
    – Snoop
    Jan 12, 2017 at 15:18
  • $\begingroup$ For example, if the DC-DC on the LRU takes in 28V... I want to knowhow accurate does that 28V have to be? $\endgroup$
    – Snoop
    Jan 12, 2017 at 15:19
  • $\begingroup$ Your LRU should probably accept anything from 10 VDC to 32 VDC as an input unless are being powered by a converter that will provide you with 28 VDC. That converter would need to meet TSO C71 for its output power (which is your input) $\endgroup$ Jan 12, 2017 at 15:40
  • $\begingroup$ I think there is a lot of useful information here but no DC-DC at the LRU input, it's just an LRU being tested with a bench-top power supply. So that's why I am asking about the voltage that goes into the LRU. $\endgroup$
    – Snoop
    Jan 12, 2017 at 15:56
  • $\begingroup$ So you are testing an LRU - are you trying to just verify it is functional or are you trying to verify the full range of input voltage for the LRU? $\endgroup$ Jan 12, 2017 at 16:14

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