Document ID: chunk:federal_register_of_legislation:F2009L01698:body:0:p2
Version: federal_register_of_legislation:F2009L01698
Segment Type: other
Provision Reference: 
Character Range: 2725–5389

2009.

Background:  Makila 1A and 1A1 ECUs incorporate a backup control law that fixes N1 (gas generator speed) at 65% when at least two of the three N2 (power turbine speed) signals are lost.  The intent is to limit the maximum speed attainable by the power turbine in the event of a failure of the shaft between the engine and the main gearbox that could result in collateral damage to the N2 speed probes.

             Several occurrences of 65% N1 backup activation remain unexplained despite detailed investigation.  It is postulated that the events may have been due to corruption of the engine N2 speed signals by short perturbations, for example by electromagnetic interference.  The potential therefore existed for a hazardous condition in which both engines on the same helicopter were simultaneously affected.

             To address this risk, Turboméca introduced modification TU250, which affects the CS board in the ECU and allows recovery from the 65% backup mode if the loss of N2 speed signals is determined to be temporary.  Incorporation of modification TU250 was mandated by AD/MAKILA/9 (EASA AD 2007-0144).

             The installation of TU250 CS boards, however, has resulted in a few occurrences of erratic engine behaviour, in the form of unexpected N1 variations and/or illumination of the "GOV" warning light.  The conclusions from an investigation by Turboméca are that these malfunctions are due to a lapse of quality control in the varnishing process applied to the boards, and that only boards in a specific serial number range, as defined under "Applicability" and referred to below as the "suspect batch", are affected.

             Turboméca has addressed this latest concern in two steps which provide first a near-term and then a long-term solution.  The two steps are described in the referenced mandatory service bulletins (MSB).

             The first step, described in MSB 298 73 0809, recommends that no helicopter has CS boards from the suspect batch in both engines.  Boards with more than 200 hours of trouble-free operation are exempt because service experience has shown that the malfunctions potentially induced by this manufacturing discrepancy are most likely to occur early in the lives of the boards.

             The second step, described in MSB 298 73 0810, recommends that ultimately all TU250 CS boards in the suspect batch, regardless of time in operation, be replaced with TU250 boards not from the suspect batch.

   This Directive makes the replacement of TU250 CS boards in the suspect batch mandatory.

James Coyne
Delegate of the Civil Aviation Safety Authority

4 May 2009