AIRBORNE POWER - November 02nd, 2016

MIL-STD-704F Introduction & Overview

This blog is about giving a general overview of MIL-STD-704 for Airborne systems, may be useful for apprentices, graduates or Junior designers who have started out in the Aerospace sector.

At ELEK-TECH ELECTRONIC SYSTEMS here in the UK we design our products to meet such requirements ensuring piece of mind for our customers and end users.

MIL-STD-704F (inc. MIL-HDBK-704-8): AIRCRAFT ELECTRIC POWER CHARACTERISTICS

This standard establishes the requirements and characteristics of aircraft electric power provided at the input terminals of electric utilisation equipment. MIL-HDBK-704-1 through -8 defines test methods and procedures for determining airborne utilisation equipment compliance with the electric power characteristics requirements defined herein. Electromagnetic interference and voltage spikes are not covered by this standard.

MIL-STD-464C: INTERFACE STANDARD- ELECTROMAGNETIC ENVIRONMENTAL EFFECTS REQUIREMENTS FOR SYSTEMS

This standard establishes electromagnetic environmental effects (E3) interface requirements and verification criteria for airborne, sea, space, and ground systems, including associated ordnance.  
 
MIL-STD-461F: REQUIREMENTS FOR THE CONTROL OF ELECTROMAGNETIC INTERFERENCE CHARACTERISTICS OF SUBSYSTEMS AND EQUIPMENT

This standard establishes interface and associated verification requirements for the control of the electromagnetic interference (EMI) emission and susceptibility characteristics of electronic, electrical, and electromechanical equipment and subsystems designed or procured for use by activities and agencies of the Department of Defence (DoD).

Such equipment and subsystems may be used independently or as an integral part of other subsystems or systems.

This standard is best suited for items that have the following features: electronic enclosures that are no larger than an equipment rack, electrical interconnections that are discrete wiring harnesses between enclosures, and electrical power input derived from prime power sources. This standard should not be directly applied to items such as modules located inside electronic enclosures or entire platforms.

The principles in this standard may be useful as a basis for developing suitable requirements for those applications. Data item requirements are also included.

General Requirements for systems operating from 28VDC Supply

Normal operation
Normal operation occurs when the system is operating as intended in the absence of any fault or malfunction that degrades performance beyond established requirements. It includes all system functions required for aircraft operation. Normal operation characteristics shall be in accordance with figures 13, 15 and table IV.
Utilisation equipment shall provide the level of performance specified in its detail specification.

Abnormal operation
Abnormal operation occurs when a malfunction or failure in the electric system has taken place and the protective devices of the system are operating to remove the malfunction or failure from the remainder of the system before the limits for abnormal operation are exceeded. The power source may operate in a degraded mode on a continuous basis where the power characteristics supplied to the utilisation equipment exceed normal operation limits but remain within the limits for abnormal operation. Overvoltage and under voltage values shall be within the limit of figure 14.
Utilisation equipment shall be permitted a degradation or loss of function unless otherwise specified in its detail specification, shall not suffer damage or cause an unsafe condition and shall automatically resume full performance when normal operation of the electrical system is restored.

Emergency operation
Emergency operation occurs following the loss of the main generating equipment when a limited electric source, independent of the main system, is used to power a reduced complement of distribution and utilization equipment selected to maintain flight and personnel safety. The DC steady state voltage in emergency operation shall be between 16 and 29 volts.
Utilisation equipment shall provide the level of performance specified in its detail specification when such performance is essential for flight or safety.

Electric starting operation
Electric starting operation is a specialised case of normal electric system operating conditions where the normal voltage limits may be exceeded due to the high electric demand. The DC voltage in electric starting operation shall be between 12 and 29 volts. Electric starting of an auxiliary power unit (other than battery starts) is a normal operating function and is not included.
Utilisation equipment shall provide the level of performance specified in its detail specification when performance is essential during the starting operation.

Transfer operation
Transfer operation occurs when the electric system transfers between power sources, including transfers from or to external power sources. Transfer operations are considered to be a normal operation. Under conditions of bus or power source transfers, voltage and frequency shall not vary between zero and normal operating limits for longer than 50 milliseconds. A normal transient may occur upon completion of a transfer.
Utilisation equipment may not be required to operate under the transfer condition unless a level of performance is specified by its detail specification. Utilisation equipment shall automatically resume specified performance when normal operating characteristics are resumed.

Grounding
All electrical power input terminals, including AC neutral and DC negative terminals, shall be electrically isolated from case ground. Equipment chassis shall not be used for power returns.

Polarity or phase reversal
DC equipment shall not be damaged by reversal of positive and negative connections. Employing a positive physical means to prevent phase or polarityreversal shall also fulfil this requirement.

Multiple input terminals
Equipment having multiple input terminals for connection to more than one power source shall isolate the inputs from each other so that one power source cannot supply power to another. AC inputs shall not be paralleled. DC inputs shall be protected with blocking diodes if they are paralleled.

MIL-STD-704F Requirements

MIL-STD 704 A-F COMPATIBILITY

Relevant MIL-STD-464C/461F requirements:

EM Interference (EMI)
The unit shall meet the EMC emissions requirements defined in MIL-STD-461F (Tests: CE101, CE102, RE101 and RE102)

EM Vulnerability (EMV)
The unit shall meet the radiated EMC susceptibility requirements defined in MIL-STD-461F for all applicable tests (Tests: CS101, CS114, CS115, CS116, RS101 and RS103)
The unit shall meet the radiated EMC susceptibility requirements defined in tables 5 & 6 of the 464C and limited to a max average field strength of 200V/m.

EM Pulse (EMP)
The system shall meet its operational performance requirements after being subjected to the EMP environment. This environment is classified and is currently defined in MIL-STD-2169. This requirement is applicable only if invoked by the procuring activity. Compliance shall be verified by system, subsystem, and equipment level tests, analysis, or a combination thereof.

Electro-static discharge (ESD)
All electrical and electronic devices that do not interface or control ordnance items shall not be damaged by electrostatic discharges during normal installation, handling and operation. The ESD environment is defined as an 8 kV (contact discharge) or 15 kV (air discharge) electrostatic discharge. Discharging from a 150 pF capacitor through a 330 ohm resistor with a circuit inductance not to exceed 5 µH to the electrical/electronic subsystem (such as connector shell (not pin), case, and handling points). Compliance shall be verified by test (such as AECTP-500, Category 508 Leaflet 2).

Lightning
The system shall meet its operational performance requirements for both direct and indirect effects of lightning.

Modular Aircraft Power

We offer a range of fully protected MIL-STD-704F Electronic Payload Modules that can be used to build up a complete Modular Airborne Power system architecture rapidly combined with Health prognostics and intelligent control. We use the latest high power density COTS power bricks to rapidly develop & deploy systems with low risk and cost.

Dependant on power requirements we can provide small lightweight distributed portable power units and battery charge management systems with intelligent control for small unmanned system applications.  Our clever approach in combining innovative cooling and high efficiency techniques allows us to put more power into smaller places than has been previously demonstrated.