The media coverage of the “start-up” issues with the PW1100G-JM on the A320neo family seems a bit of overkill to neutral observers. Of course, the airlines are pushing for additional compensation and Pratt & Whitney isn’t saying much or fighting back, as it is never good to disagree with customer. As a result, we’re seeing one side of the story a bit more than the other.
A Very Strong EIS
The media’s focus on the extended start-up times has nearly masked the incredible performance of this new engine. To date, and only a few months in, the engine is already at a >99% dispatch reliability rate. An unprecedented level of reliability for a new engine. Confidence in the engine was echoed yesterday at Airbus Innovation Days by Fabrice Bregier who noted the engine is exceeding some of its promises. Pratt & Whitney promised strong reliability out of the box, and has already achieved it. This should be capturing the headlines for the GTF engine that has been in the works for more than 20 years.
In addition, the engine is meeting or exceeding its performance objectives, and is surpassing targets for fuel efficiency and reliability. Relative to the A320 ceo, the GTF engine powered neo is saving about 100 gallons of fuel every flight hour, so the typical airplane in service is saving 1,000 gallons of fuel every day (~€7,381 in daily savings with FRA Jet A pricing). When was the last time that a new engine met all of its performance targets at EIS, and didn’t require a performance recovery package after EIS? Let’s simply say that one would need to go back a long way into history to find an EIS this good. The engine, other than the start-up time issue, is performing superbly.
Aeroturbopower, in this article, analyzed the impact on Indigo of the longer start-up times, and found virtually no impact on the airline’s schedule. This is, of course, contrary to what is being stated publicly. Yes, the current GTF does take too long to start, but no, it won’t break either the airline or its ability to put money in the bank. Airlines have padded schedules so much the four minutes currently needed at startup does not seem to matter.
The Challenge
The challenge for any engine start is the differential thermal gradient in the compression system that can result in a slight rub “or kiss” between the rotating blades and the case. For quick airline turnaround operations there is not much of an effect from the temperature gradient. But when an engine sits for 1.5 to 2 hours, a longer, a motoring (or cooling) procedure is required prior to startup.
The GTF start-up procedure, like for most newer jet engines, consists of two steps — “motoring to cool” the engine – in which the shaft is turned pneumatically or electrically (depending on airplane and engine type) to begin rotation of parts. This motoring clears out the thermal gradient from the engine core. The second step, the engine start itself, injects fuel into the combustor and ignites the mixture, accelerating the engine to its ground idle and taxi configuration. Typically, both processes on a V2500 engine today takes around 75-100 seconds per engine.
On the GTF, the motoring cycle is not a fixed period as on the V2500 or CFM-56, but is variable and controlled by the engine electronic control to optimize the cooling sequence. Today, with the current GTF engine configuration, the cooling sequence can reach 2 minutes per engine or an impact of 4 minutes if engines are started sequentially. This is longer than in service engines and has been a situation that the initial customers have been experiencing. Pratt has recognized this early on and has been aggressively developing a hardware and software modification to address the situation.
The Fix
Pratt & Whitney has engineered a fix, which is already incorporated into new build engines, with appropriately modified software coming in June.
Once incorporated, depending on the engine condition, the motoring time for a GTF can be slightly less or slightly longer, but in the same range as today’s engines. For those engines already in service, the repair is well inside the engine, requiring disassembly of the compressor module. This is not a simple software change in the engine.
Bottom Line
No doubt that the initial longer start-up time and handful of software nuisance warnings were teething problems that are being quickly overcome. Soon the focus should turn to this long awaited engine that is meeting all its performance commitments and demonstrating mature reliability levels within its first 100 days of service.
