Evolution of the lightning zoning guideline and coming challenges for lightning protections of unconventional platforms

The lightning zoning is a cornerstone in the upstream
lightning hazards analysis, in the purpose to define the threat
at rotorcraft and at system levels, for both lightning direct and
indirect effects.
From 2014 to 2018, the regulatory working groups SAE-AE-
2 and EUROCAE WG31 have reviewed the lightning zoning
document. A new issue was released (ED-91A or
ARP5414B), as the result of the analysis of in-service
experience (location and type of damage and failures), and
leading to a better adaptation of the standard to the reality for
the various types of aircraft including rotorcraft. This zoning
of rotorcraft was a common work between five Helicopters
manufacturers based on an initial proposal of Airbus.
The main modification consists in the suppression of the
lightning zone 2 (high probability of sweeping strike) of the
airframe and of the zone 1 (high probability of attachment) on
the nose. As the airframe is mainly in zone 3 (low probability
of attachment), there is a need to consider the local effect of
the protrusions (antennas, FLIR cameras...) on the zoning,
and the possible creation of local zones 3 direct attachment
(peak up to 40KA) or zone 1 (peak up to 200 KA).
That work shows consequently the need to have a prediction
tool to support the assessment of the local threat and in
particular to evaluate the effect of protrusions on the
attachment risk. It is mentioned in ED-91A/ARP5414B that
there is a need for validation. Such an activity is obviously
essential for new shapes as VTOL with multi-rotor
configurations for which experience is rather absent.
Existing electromagnetic tools can be used, but still need
validation for a better prediction. The final goal would be to
have a validated means of compliance or at least as an
intermediate step a help for the design of protections and the
certification phases.

The first part of the paper provides an overview in terms of in
service experience and zoning tools based on Airbus
experience. The second part reports the results obtained on a
complete Helicopter with the Airbus electromagnetic
numerical tool for primary attachment locations prediction,
the limitations of such a prediction tool and the
recommendations in terms of utilization of the tool and
associated protection policy.