Geared Turbofan engine too hot to handle?
Teething problems for Pratt & Whitney's new narrowbody engine
Pratt & Whitney’s struggles to deliver correctly functioning Geared Turbofan engines for Airbus A320neo aircraft is a reminder that aircraft development is always fraught with risks. Airbus clearly signposted in January that 2016 neo deliveries would be back loaded into the second half of the year. It is targeting 56 aircraft this year but as of the end of July has only delivered 11, so it has its work cut out in Toulouse. Where has it gone wrong?
The first airlines to fly the A320neo complained to Pratt & Whitney that the Geared Turbofan (GTF) PW1100G engine has a much longer delay before it can be restarted than the industry standard of less than one minute. All engines need a cooling cycle between uses, however the GTF is requiring a three minute delay which is increasing the aircraft’s time at the gate, which in turn negatively impacts the airline’s punctuality and profitability.
After the engines are shut down, rising heat causes a temperature differential between the top and bottom of the engine, which causes a slight bowing in the rotor as the metal expands at different rates. If the engine is started when the bowing is at its worst, a harmonic vibration can develop. Matthew Bromberg, President Pratt & Whitney’s aftermarket division said that “the engine will bounce a little, to the point where it can eat into the seals that are surrounding the blades.”
Pratt & Whitney has developed a three-phase fix. Firstly it has installed software onto all engines currently flying which measures how long the engine has been cooling for, and if needed causes it to spin slowly when started, hence forcing the three-minute start up. This is not an acceptable long term solution though so two technological alterations are being made. A coating is being applied to the engine blades which will strengthen the engine’s third and fourth shaft bearings, thus preventing the harmonic vibration and a coating will be put on some of the blades that will improve the sealing function on the compressor. The first new engines with the fix should be fitted onto aircraft this month. Airbus is reported to have 25 engine-less A320neos parked at Toulouse so this fix cannot come soon enough. Pratt & Whitney will upgrade the engines currently in service over the summer at a time convenient to the airlines.
When the idea of a re-engined narrowbody was espoused, Airbus was quick to highlight that it was a low risk programme because the only significant development required was on a new engine. However, the engine is arguably the most technologically complex, and safety critical part of an aircraft. The geared turbofan is pushing technological boundaries for Pratt & Whitney. It is therefore unsurprising that it has encountered some stumbling blocks. The company is quick to point out though that the engines have a 99% dispatch reliability rate and are meeting thrust, fuel burn and noise expectations.
Whilst the responsibility for fixing the issues lies with Pratt & Whitney, it is Airbus that customer’s hold responsible. Qatar Airways was the planned launch customer but refused to take delivery of the aircraft and has already said it will be seeking compensation. Lufthansa and Indigo may well follow suit. How this will be borne by Airbus and Pratt & Whitney remains to be seen. The engine problems also highlight how aircraft programmes are on a learning curve which is why airlines are often want deliveries slots some way in the future in order to ensure the aircraft is mature. For Airbus of course the fix is also necessary part of achieving FY16 guidance.
These teething problems with the engine are also likely to raise concerns about how the Geared Turbofan will perform through its life cycle. The new gearbox technology means that Pratt & Whitney is not able to depend on its wealth of its existing engine data in order to manage the aftermarket cycle. It was also one of the risk areas that drove Rolls-Royce’s decision not to participate in the programme. However, lower maintenance costs was one of the selling points of the engine.
The A320neo is also offered to customers with the LEAP engine from CFM (GE and Safran) which has more conventional two shaft architecture. The first of these aircraft was delivered to Turkey’s Pegasus Airlines in late July CFM has now accelerated the LEAP development which will power some of the A320neos and all of the competing new B737MAX aircraft. The LEAP engine is said to have a start up time of 50 seconds so is unaffected by this issue.
Pratt & Whitney developed the PW1100G with the aim of re-entering the narrowbody engine market which has been dominated by GE. Whilst the engine has achieved this aim, its problems entering service mean that GE seems set to maintain its leading position and reputation for consistency in the narrowbody market.
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