Overview

The Civil Aviation Safety Authority (CASA) issued Airworthiness Directive (AD) S-76/73 for the Sikorsky S-76 Series Helicopters in 2004. This directive was introduced to address safety concerns regarding the wiring of the inverters in these helicopters, particularly in relation to the autopilot system. The AD was issued under subregulation 39.1(1) of the Civil Aviation Safety Regulations 1998. The policy objective behind this AD is to prevent the disengagement of both autopilots following a No. 2 DC generator failure, which could lead to loss of control during instrument meteorological conditions (IMC). The AD requires specific tests and modifications to the wiring to ensure the No. 1 inverter is connected to the No. 2 DC primary bus and the No. 2 inverter to the DC essential bus. If these modifications are not made, an airspeed limitation placard and corresponding annotation in the Aircraft Flight Manual must be implemented.

Scope and Application

The Airworthiness Directive (AD) issued under the Civil Aviation Safety Regulations 1998 applies specifically to Sikorsky S-76 Series Helicopters, namely the S-76 A, B, and C models, that are equipped with dual channel autopilots and dual inverters. The directive mandates specific actions to address an identified unsafe condition related to the wiring of the inverters. It applies to a defined set of serial numbers for each helicopter model, which are detailed in the directive. The AD is issued by a delegate of the Civil Aviation Safety Authority (CASA) and is applicable across the Commonwealth of Australia. The directive requires operators to perform a test to determine the wiring configuration of the inverters and, if necessary, modify the wiring as specified to ensure that the No. 1 inverter is wired to the No. 2 DC primary bus and the No. 2 inverter is wired to the DC essential bus. Additionally, operators must install a placard and annotate the Aircraft Flight Manual (AFM) with a specific airspeed limitation if the No. 1 inverter is found to be wired to the DC essential bus. The directive outlines specific timelines for compliance with these requirements and specifies when the placard and AFM annotation should be removed after the wiring modification is completed. The AD became effective on 28 April 2004, and its applicability is strictly limited to the specified helicopter models and serial numbers, with no exclusions or exemptions stated within the directive itself.

Key Provisions

The Airworthiness Directive (AD) in question applies specifically to Sikorsky S-76 Series Helicopters with dual channel autopilots and dual inverters installed. The directive mandates certain actions to address an unsafe condition identified in these aircraft, which can be found in the requirement section (paragraphs 1-3). The key requirement is to determine if the No. 1 inverter is wired to the DC essential bus, as outlined in paragraph 1 of the requirement section. If the test specified in Sikorsky Aircraft Corporation Alert Service Bulletin (ASB) No. 76-24-14A, Revision A, dated 9 October 2003, reveals that the No. 1 inverter is indeed wired to the DC essential bus, then the directive mandates the installation of a placard near the airspeed indicator with the limitation "Maximum IMC Airspeed 120 KIAS" and an annotation of this limitation in the Operating Limitation section of the Aircraft Flight Manual (AFM). Additionally, the directive requires modifying the electrical wiring so that the No. 1 inverter is wired to the No. 2 DC primary bus and the No. 2 inverter is wired to the DC essential bus, in accordance with the Accomplishment Instructions, paragraph 3.C. of the ASB. The removal of the aforementioned placard and AFM annotation is required after the electrical wiring has been modified. The AD imposes obligations on the owners and operators of the affected helicopters. They must ensure that the specified operational test is carried out to determine if the No. 1 inverter is wired to the DC essential bus. If the test results indicate that the wiring configuration is unsafe, then the required modifications must be made to the electrical wiring and the airspeed limitation placard must be installed and annotated in the AFM. These actions must be completed within the specified timeframes outlined in the compliance section of the AD. Failure to comply with these requirements may result in the unsafe condition not being addressed, which could potentially lead to loss of control of the helicopter during instrument meteorological conditions (IMC) operations. Failure to comply with the AD may result in civil or criminal penalties. Under the Civil Aviation Act 1988, a person who contravenes an AD may be subject to a penalty of up to $22,200 for an individual and up to $111,000 for a body corporate. Additionally, the AD may be used as evidence of an offence under the Civil Aviation Regulations 1988, which could result in further penalties. It is important to note that the AD is enforced by the Civil Aviation Safety Authority (CASA), and non-compliance may result in the suspension or revocation of the aircraft's certificate of airworthiness, which would prevent the aircraft from being flown until the required actions have been taken.