Nyxair SF34 at Carlisle on Aug 30th 2019, lost control of propeller rpm
Last Update: August 13, 2020 / 15:29:13 GMT/Zulu time
The AAIB released their final bulletin concluding the probable cause of the incident was:
During the climb after takeoff, the left engine condition lever did not respond to the commander’s inputs and he was unable to adjust the propeller rpm on the left engine.
The condition control cable had suffered damage due to chafing against the accessory gearbox, which most likely affected the correct operation of the cable. Although chafing protection was fitted to the cable, it was not in the correct location and would not have provided the intended protection.
The AAIB analysed:
Chafing of the condition control cable where it passes over the accessory gearbox is a known condition and the aircraft manufacturer published non-mandatory SB 340-76-027 in 1988 to mitigate this problem with the addition of chafing protection. It also published optional SB 340-76-044 in 2016 to increase clearance between the condition lever and the accessory gearbox.
Despite the presence of convolex tubing on ES-NSD’s left engine condition control cable, the aircraft’s maintenance records did not contain any reference that SB 340-76-027 had previously been embodied. Possible explanations could include that the cable, or entire left engine assembly, had previously been installed on another aircraft having SB 340-76-027 embodied; or, that at some point in the past, maintenance personnel had added chafing protection without formal embodiment of the SB.
SB 340-76-027 described the precise location in which the chafing protection should be added to condition control cable to prevent chafing against the accessory gearbox. The chafing protection on the condition control cable from ES-NSD was installed at an incorrect location, and was adjacent to, rather than coincident with, the area where chafing damage was most likely to occur. It was not established why the chafing protection was installed in this location but there are several possible explanations: the position of the chafing protection may have been measured from an incorrect datum, for example the end of the rigid metal cable sleeve shown in Figure 5; if the cable-ties were not sufficiently tight, the chafing protection may have slipped from its original position; or, it may have been installed without reference to SB 340-76-027. Following this incident, the operator requested instructions from the manufacturer to perform SB 340-76-027 on all of its Saab 340s and introduced inspections of the cable in its maintenance programme.
In addition to the visible damage, the operator suggested that the cable had broken in the large bend near the HMU gearbox. The cable was not physically examined but it was considered possible that the cable had also suffered an internal failure at this point, which was not externally visible in the photographs provided. But the manufacturer considered that the substantial chafing damage evident in the photographs would have been sufficient to cause problems with cable operation and could have accounted for the lack of response from the condition lever during the incident.
It was not established if SB-340-76-044 had been embodied on ES-NSD but it is likely that if the standoff bracket had been in its original (pre-SB) position, it would have been visible on the left edge of Figure 3. Therefore, it was considered likely that SB-340-76-044 had been embodied.
This article is published under license from Avherald.com. © of text by Avherald.com.
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