MANUFACTURER: Sikorsky Aircraft Corporation
(Sikorsky) Model S76A, B, and C Helicopters SUBJECT: Airworthiness Directive 2010-26-09
SUMMARY: This amendment supersedes an existing
emergency airworthiness directive (EAD) for the specified Sikorsky
model helicopters. The EAD requires inspecting the LITEF Attitude
Heading and Reference System (AHRS) unit of the navigation system
to determine if it is at a Mod Status "18.' If either AHRS unit is
at Mod Status "18,' the EAD requires installing placards on the
instrument panel to prohibit single pilot instrument flight rule
(IFR) and single pilot night flight and reducing airspeeds to 120
knots indicated airspeed (KIAS) if both autopilots uncouple during
instrument meteorological conditions (IMC) or night flight. The EAD
also requires inserting minimum crew and airspeed limitations into
the Limitations section of the applicable Rotorcraft Flight Manual
(RFM) to limit the minimum flight crew to 2 pilots for night flight
and IFR flight and to reduce airspeed to 120 KIAS if both
autopilots uncouple during IMC or night flight. This amendment
contains the same requirements but draws the appropriate
distinctions between IFR and IMC as used in the intended operating
limitations. Also, unlike the EAD, this AD states the airspeed must
be reduced to 120 KIAS if both autopilots uncouple during IMC or
night flight. Further, we are removing the limitation contained in
the Active Temporary Revisions relating to pilots keeping their
hands and feet near the flight controls. This AD was prompted by
the need to supersede the EAD to state the distinction between IFR
and IMC as used in the operating limitations and to reduce the
airspeed to 120 KIAS if both autopilots uncouple during IMC or
night flight. The actions specified by this AD are intended to
implement operating limitations based on an anomaly in the AHRS
related to the 26 volt AC inverter that could result in a
decoupling of both autopilots and to prevent loss of control of the
helicopter during IMC and during night flight.
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