Following the separation of the IP turbine disc from the drive arm, the engine behaved in a manner that differed from the engine manufacturer’s modelling and experience with other engines in the Trent family, with the result that the IP turbine disc accelerated to a rotational speed in excess of its design capacity whereupon it burst in a hazardous manner.
On 9 December 2010, in conjunction with the release of the Trent 900 IPTOS as advised in Rolls-Royce NMSB RB.211-73-AG639, Airbus released service bulletin A380-73-8011 to operators of Trent 900 equipped A380 aircraft. This bulletin required the IPTOS to be installed across the Trent 900-equipped fleet.
On 3 December 2010, Rolls-Royce released NMSB RB.211-73-AG639, advising Trent 900 operators of the introduction of a revised standard of engine management software that featured an IP turbine overspeed protection system (IPTOS).
The IPTOS was intended to detect engine conditions with the potential to lead to an IP turbine over speed. In response, IPTOS would shut down the engine before the IP turbine disc reached its critical burst speed. Shaft breaks and disc separation, such as occurred in VH-OQA can occur for mechanical reasons such as component fatigue, an over torque being applied to the shaft or a manufacturing defect, or by localised heating such as from an oil-fed fire. During the course of the investigation into the No.2 engine failure in VH-OQA, the ATSB was provided a detailed summary of the IPTOS protection system, which works on the following logic:
The first element arms the system, and is based on detecting a prescribed rate of temperature change of turbine cooling air at the front (TCAF) or rear (TCAR) of the IP turbine. Such rates of change indicate that a fire has developed within the engine that may lead to localised heating of the IP turbine disc or shaft.
Once armed, if an abnormally high rapid rate of compressor deceleration is detected, a shaft break or disc separation is indicated and the EEC (engine electronic controller) will instantly shut off the fuel, open all the bleed valves and close the variable stator vanes.
Flight crew are alerted to an IP shaft failure through a flight deck annunciator alert that raises the message ‘ENG FAIL-SHAFT FAILURE’.
Rolls-Royce reported that the engine EEC software upgrade that included the IPTOS functionality was incorporated across the Trent 900 fleet by 6 December 2010.
On 13 December 2010, EASA issued airworthiness directive AD: 2010-0262 in respect of modifying the Trent 900 EEC software by incorporating the IPTOS logic, as detailed in Rolls-Royce NMSB RB.211-73-AG639. The airworthiness directive required all Trent 900 engines to be modified within 10 flight cycles.
A full copy of EASA AD: 2010-0262 is available at: ad.easa.europa.eu/ad/2010-0262
ATSB assessment of Rolls-Royce, Airbus and EASA safety action
The ATSB is satisfied that the action taken by Rolls-Royce, Airbus and EASA adequately addresses the safety issue in respect of the risk of an IP turbine overspeed and burst. Therefore the ATSB makes no recommendation in relation to this issue.
