Introduction of the B757-300 series aircraft required the RB211-535E4 engine to be rated to a higher thrust. The maximum take off thrust rating of the engine was revised from 39610 lb (E4) to 42540 lb (E4B). The increased thrust was achieved by increasing the rotating assembly speeds by increased fuel flow into the engine. The net effect was increased thrust but also increased rotational speeds and temperatures.
EGT is a measure of the temperature at the inlet to the Low Pressure Turbine (LPT) and is a reflection of the temperature in the combustion section and the High Pressure Turbine (HPT).
Due to EGT problems on the RB211-535E4B engine; Rolls Royce have issued several modifications (Service Bulletins) to introduce revised EGT system ballast resistors on the E4B rated engine to optimise the indicated EGT margin and to maintain indicated EGT within the operating limits.
TCX have incorporated these modifications in an attempt to reduce the number of EGT exceedance events in the B757-300 fleet.
It has now come to light that although the engines passed the rolls Royce test cell checks, both engines have very low N3 margin meaning that when full take off power is used, under certain conditions there is an increased risk that N3 will exceed the operating limits.
TCX continue to suffer from both EGT and N3 exceedance events on the B757-300 aircraft; mainly when full take off power is used during down route departures from certain airports (high temp / Short / obstacle avoidance).
Engines suffering an exceedance may need to be inspected following the event.
This can be determined by reference to the EICAS exceedance page, noting the magnitude and duration of the exceedance, then referring the Boeing Fault Isolation Manual (FIM).
TCX have incorporated EGT modifications where applicable in an attempt to reduce the number of EGT exceedance events in the B757-300 fleet.
TCX have introduced a revised operating procedure for the B757-300, the APU is now used to deliver air to the air conditioning packs during the take off phase of flight; reducing the air taken from the engine and reducing the EGT / demand on the engine.
TCX are working closely with Rolls Royce to understand the problem. TCX have supplied Rolls Royce with engine parameter data that has been analysed.
Initial findings from this exercise have indicated that relative to the norm, the Intermediate Pressure System (Intermediate Pressure Compressor [IPC] and Intermediate Pressure Turbine [IPT]) speed is slow and the HP speed is fast. This is indicative of either HP Compressor Deterioration and / or IP Turbine efficiency deterioration.
Borescope inspection reports indicate that the condition of the HP Compressors appears satisfactory. Gas path cleaning (compressor wash) of AB in March 06 had no significant long-term effect on EGT or N3. Therefore it is most likely the underlying problem is IPT deterioration.
Boeing Fleet Newsletter 63, 16th October 2006