Airworthiness Notice - D006, Edition 1 - 23 September 2003

Eurocopter as 350: Investigation of Flight Control Concerns

The purpose of this Airworthiness Notice is to update aircraft owners/operators, maintainers and pilots of AS 350 rotorcraft on the investigation that is presently being conducted with regard to concerns expressed with the flight control characteristics of the AS 350 when the hydraulic system pressure is lost.

(Reference: Airworthiness Directive CF-2003-15 and its revisions)

System Architecture

As operators of the AS 350 are aware, this aircraft has a single hydraulic system to lighten control forces and allow the aircraft to be flown at speeds where manual control loads are excessive. Loss of the hydraulic system is considered possible and therefore the aircraft has been shown during certification to have adequate handling qualities when in the reversionary manual control mode. However, at high speed the loads are considered excessive, and a safety unit comprising an accumulator, non-return valve, and solenoid valve, were installed on each servo. The accumulator charge allows the pilot to reduce speed safely to where the manual control forces are more manageable. When this speed is achieved, the pilot is expected to select the cut-off switch on the collective, which vents all residual accumulator pressure evenly, and transitions to manual controls.

Airworthiness Concern

The Transportation Safety Board of Canada (TSB) provided a preliminary briefing of a recent fatal accident which had an unusual flight path as captured by an aircraft tracking system providing aircraft position data at 30 second intervals and corroborating eye witness statements. The TSB reported a communication from the pilot to a dispatcher that a hydraulics failure had occurred and the aircraft was being repositioned. TSB has concentrated their investigation on control system abnormalities and manipulation of the hydraulic system.

In addition to the above, Transport Canada was informed by an operator of uncommanded lateral input occurring during a maintenance ground run where the accumulators were depleted. Given the possibility of a control problem contributing to the fatal accident and the reported service difficulty, Transport Canada elected to issue Airworthiness Directive (AD) CF 2003-15. This AD is intended to identify control system abnormalities prior to flight, by expanding the pre-flight flight control checks and minimizing the potential for in-flight uncommanded control inputs, by ensuring the hydraulic system is not depressurized for non-emergency reasons. Also, as an added precaution, this AD informs the pilot to land as soon as possible to reduce exposure to a control anomaly.

Service History

Since the issuance of this Airworthiness Directive, there have been two operators reporting control anomalies. These can be categorized as follows:

  1. During depressurization of the accumulators, as required by the pre-flight check imposed by this Airworthiness Directive, an uncommanded cyclic movement occurs. The uncommanded control movement is generally lateral and to the control stop, but fore and aft movement and/or smaller amplitudes have also been observed, with a reported force of approximately 10 to 20 pounds. This has occurred with both SAMM (Goodrich) and Dunlop servos.
  2. Upon re-pressurization of the hydraulic system, there is an uncommanded cyclic movement similar in character to the movement described in para 1 above. This has occurred on the SAMM (Goodrich) servo.
  3. Instance of flight control rate limiting was encountered in flight where the pilot experienced control restrictions with hydraulic power available. The control restriction was characterized as only being able to move the control at a maximum rate, regardless of how hard the pilot forces the cyclic, but did not restrict the maximum control displacement available.

A meeting was held with the French Airworthiness Authority (DGAC), Eurocopter, Transportation Safety Board, Ontario Ministry of Natural Resources and Transport Canada to discuss the details of the accident and discuss reported service difficulties. At this meeting Eurocopter provided information, which addresses anomalies identified as follows:

  1. Uncommanded movement when system is being depleted: Eurocopter ground test demonstrated that an uncommanded movement is possible when one lateral accumulator is depleted and the other charged. Based on this test, Transport Canada concluded that a similar result may be expected in flight.

Mitigating Factors:

The above uncommanded input is prevented in-flight when the pilot follows the Aircraft Flight Manual (AFM) procedure, which indicates that following a hydraulic failure, the aircraft is slowed promptly to a specified speed and the hydraulic cut-off is activated. By activating the cut-off, any unbalanced force as a result of asymmetrical residual accumulator pressure is avoided. If the hydraulic cut-off is not activated, then the sustained asymmetrical pressure may occur if the accumulator depletes at a different rate until the residual pressure is depleted through normal movement of the flight controls. The force identified by Eurocopter test would be approximately 5 daN (11 pounds).

1. Uncommanded movement when the system is being pressurized: Eurocopter confirmed that there is an identified service issue related to a product improvement made to SAMM servos to improve sealing. The modification had an adverse effect causing increased drag on the servo input lever-centering pin. Upon pressurizing the system or after re-pressurization of the hydraulic system, the centering pins retract at different times (one servo is pressurized when the other is still in bypass) and an uncommanded movement can occur. The problem is reported in Eurocopter Lettre-Service 1446-67-00 dated, Marignane, 21.02.00. If this problem occurs, Eurocopter recommended application of WD40 lubricant.

Mitigating Factors:

This problem will be identified when the system is pressurized on start and during the pre-flight check since the hydraulic pressure has been depleted and is re-pressurized prior to flight maintenance action (WD40) required prior to flight. It is Transport Canada's position that the application of WD40 lubricant may not ensure that the pins will continue to allow proper function of the bypass upon re-pressurization of the hydraulic system. Nevertheless, the existing AD does not allow the pilot to shut the hydraulic system off in-flight unless responding to an emergency such as a hydraulic problem. It is Transport Canada's understanding that when the pilot chooses this action to address an in-flight emergency, then the system would not be re-pressurized in-flight. To ensure that this is understood, this Airworthiness Directive is presently being revised to specifically prohibit reactivating the hydraulic system in flight.

2. Flight control rate limiting when pressurized: This control anomaly did not result in any uncommanded control movements but did affect handling qualities. Hydraulics off controllability was unaffected.

Transport Canada Continuing Airworthiness Investigation

Transport Canada, in cooperation with the DGAC, have undertaken to continue to examine the servos that have been removed as a result of uncommanded movement to ensure that there are no hidden failure modes. Also, TC intends to conduct a review of all flight control failure modes related to the control systems, and associated in-flight handling qualities in various flight regimes and environmental conditions.

The three servos from the accident aircraft (Dunlop) were bench-tested and torn down, and no faults were found. A servo related to uncommand lateral movement (SAMM) was also bench-tested and torn down, and no anomalies were detected. The servo associated with flight control rate limiting when pressurized (Dunlop) was investigated with the assistance of Dunlop, and their findings concluded that the servo had significant wear that was beyond what is normally experienced when nearing its overhaul period. The reason for this condition has not been identified.

Training

Presently the Airworthiness Directive restricts operation of the aircraft with the hydraulics off except in an emergency situation. This restriction has prevented recurrent training in this flight condition. Handling qualities are significantly different enough that a lack of training may adversely affect safety. Transport Canada determined that current type-rated pilots could forgo training for a limited period to allow for sufficient time to conduct an investigation.

Transport Canada Position

It is Transport Canada's position that the aircraft can be operated safely as long as pilots operate the aircraft in accordance with the restrictions of the referenced airworthiness directive and its revisions.

For Minister of Transport

M.J. Eley
Director, Aircraft Certification

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