Qantas Airways says the latest high thrust variants of the Rolls-Royce Trent 900 engines powering its Airbus A380 aircraft can be used for only 75 flights before needing replacement.

The disclosure was made in Qantas' affidavit against Rolls-Royce, which it alleges may have been "misleading or deceptive" when it proposed its Trent 900 engines to the carrier.

Qantas' statement of claim says engines with a "B mod" or "C mod" high pressure (HP)/intermediate pressure (IP) support structure need replacement after operating 75 take-offs at the 72,000lb maximum thrust level Rolls-Royce sold to Qantas.

Qantas cannot operate Trent 900 engines with an earlier "A mod" HP/IP support structure, the affidavit says.

"Were the aircraft to be used with a full or commercially viable payload at the required thrust for take-off from [Los Angeles International Airport] LAX, the engines would need to be replaced after 75 such take-offs," Qantas General Manager Engineering and Maintenance Adrian Verkerk tells Qantas' lawyers in the affidavit.

Qantas is the only A380 operator using the 72,000lb Trent 972 variant. Applying the maximum thrust exposes the engine to 540 psi at P30, the affidavit says. P30 defines a point in the engine where the pressure is measured.

A Rolls-Royce presentation sent to Qantas on 18 November says the Trent 900's HP/IP support structure would be subject to "high severity" if the engines were operated to 540 psi at P30 on more than 75 flight cycles.

The affidavit identifies a minimum of five "B mod" or "C mod" engines that operated more than 75 take-offs with maximum thrust. A Qantas spokesman could not specify the exact number of engines that had exceeded those parameters.

It is "typically necessary" for Qantas to apply the maximum take off thrust of 72,000lb from LAX if a flight carries the maximum payload of 60,900kg, the affidavit says.

Maximum thrust is needed due to a high fuel load for the 14h flight, relatively short runways at LAX, and increased headwinds during departure times that accommodate Sydney airport's curfew, the affidavit says.

Rolls-Royce advised Qantas in a presentation on 12 November to apply a minimum 8% derated take-off thrust in the 72,000lb variant, the affidavit says.

"When planning take-off thrust, when TOGA [take off go around] is not required... a minimum Tflex of 37 degrees C for a 72K rating and 34 degrees C for a 70K rating, or a minimum take-off fixed derate of 8% (72K) or 4% (70K) should be used," the affidavit quotes the presentation.

This thrust derate was made in order to "reduce the engine pressure ratio in the 'P30' area of the engine and therefore increase the life of the oil transfer tubes within the HP/IP support structure", the affidavit says.

Qantas' claim says a defect rendered "an oil supply tube in the HP/IP support structure susceptible to breaking or cracking during high severity exposure and leading to the possibility of an oil leak and a resulting oil fire in the engine".

On 2 December the Australian Transport Safety Bureau identified fatigue cracking within a stub pipe, which feeds oil to the HP/IP bearing structure, as the likely cause of the 4 November uncontained engine failure on a Qantas A380.

For Qantas to abide by Rolls-Royce's derated take-off thrust guidelines, A380 flights LAX would carry only 80 passengers due to payload restrictions.

In 2000, Rolls-Royce told Qantas that the list price of a Trent 900 rated up to 76,000lb is $12.85m. Qantas originally selected the 70,000lb Trent 900 variant on 16 February 2001, the affidavit says.

However, Airbus advised Qantas on 19 March 2007 of a specification change notice (SCN) that said an increase in design weight brought the A380's maximum zero fuel weight from 361 tonnes to 366 tonnes.

That same day Qantas increased its thrust requirement for the Trent 900 to 72,000lb. "The increased thrust rating was therefore necessary to support Qantas's payload requirements on critical routes," the affidavit says of a SCN Airbus and Qantas signed.

The 540 psi value at P30 was chosen as a divider between "typical" and "high severity" operations, the affidavit quotes Rolls-Royce as saying in a letter. This value represents "a balance between the distribute tail of the non trans pacific routes running our and the start of the tail of the trans pacific routes".

Qantas and Rolls-Royce agreed on terms to permit the airline's A380 fleet to return to service after a three-week grounding, the affidavit says. Those terms included not operating any "A mod" standard engine or any engine that had been subjected to a "high severity" operation until the HP/IP support structure was replaced.

When contacted, Rolls-Royce's spokeswoman says: "We continue to work closely with Qantas on operational and commercial matters but it would be inappropriate to comment further at this stage."

Source: Air Transport Intelligence news