scope:

Introduction

As a necessary minimum, all aircraft electrical equipment and systems should be tested in a laboratory and should comply with the requirements of BS 3G 100: Part 4, Section 2 before being installed in an aircraft. If this is done, and all the installations are well designed and properly implemented, then it is probable that the aircraft as a whole will be relatively quiet and free from interference.

Unfortunately, however, such ideal conditions cannot always be met. Equipment and systems such as those bought 'off the shelf' and from overseas may never have been subjected to any EMC testing, and it may also be that meeting the requirements of BS 3G 100 incurs unacceptable penalties with regard to weight, size, cost or installation problems, and as a consequence degraded system performance or aircraft operational limitations may have to be accepted.

Furthermore, even when the requirements of BS 3G 100 are met in the laboratory, unsatisfactory performance of equipment or systems may still occur on the aircraft due to a number of reasons including:

(a) The impracticability of installing an ideal system, with adequate cable segregation and separation, adequate screening and ideal bonding and grounding conditions.

(b) The need for equipment and systems to be installed at positions which are not the most suitable for EMC.

(c) The need for aerials and feeders to be sited at positions which are not optimum from an EMC aspect.

(d) The situation of equipment within unshielded parts of the aircraft (e.g. within radomes, in parts of cockpits, or behind non-metallic sections) which may be subjected to radiated fields considerably in excess of those encountered within shielded metallic hulls, as called for in BS 3G 100 testing.

(e) Present limitations of BS 3G 100.

It is therefore necessary to have some means of controlling or limiting the interference which may arise. This can generally but not always be done by the use of suppressors or filters mounted in or on the equipment itself, or by taking suitable protective measures on the installation and the interconnecting wiring, or by blanking or reducing the operational bandwidth.