August 05, 2013
Credit: Pratt & Whitney
The flurry of interest triggered by the recent news of a typical test issue discovered two months previously on a Pratt & Whitney PW1100G for the A320NEO was a timely reminder of just how sensitive the industry has become in the run-up to the debut of the reengined Airbus.
Airlines are counting on the lower fuel burn of the A320NEO, and the competing Boeing 737 MAX, to stave off an inexorable rise in operating costs. This, along with the fiercely competitive battle over the NEO between Pratt's PW1100G and CFM's alternative Leap-1A, has put both engines in the spotlight. Launched ahead of the Leap, the PW1100G is further down the development path and is due to power the A320NEO for its first flight in October 2014. The first Leap-1A, by comparison, does not start up on the test stand until September.
Adding to the pressure is the finely poised market balance between the two engine options. According to figures from the engine makers and Airbus, CFM is slightly ahead. The Leap-1A has been selected for roughly 800 A320NEO-family aircraft, or 34% of the firm aircraft orders. The PW1100G has been selected for around 750 aircraft, or 32%, while the engine selection is up for grabs for an additional 800 A320NEOs on firm order.
As a result, the players are keenly aware of the potential impact of any slip-up during development, even though the initial phases of test programs are frequently interrupted by events of the type that hit the PW1100G in May. In the latest incident, one of the four Block 1 configuration test units, engine No. 2, was conducting low-rotor stress tests at Pratt's West Palm Beach, Fla., site, when engineers noticed “some distress on the inlet guide vane to the first stage of the high pressure [HP] turbine,” says Bob Saia, vice president for the next-generation product family.
The engine, which eventually went on to amass 110 hr. during the first test phase, was used primarily for stress tests of the fan, low-pressure (LP) compressor and LP turbine. “When we lay out Block 1 testing, the objectives are first to validate the overall design, and then to clear key characteristics of the design for certification reports,” says Saia. Part of this involves taking measurements while running the engine up to 5% above red line (maximum permitted operating levels) “to show structural integrity. The airworthiness directives are written so even if you clear the red line, if the engine goes above it you don't get a major surprise. So we run the engine pretty hard,” says Saia.
Testing on No. 2 therefore included runs at 105% speed. “Early in the test program, in the first 10 to 15 hours, we noted some distress on the inlet guide vane,” says Saia. The vanes are stationary airfoils that align the flow of gases from the combustor exit into the HP turbine. The vane “sees all the hot gas coming out of combustor,” he adds.
“We have cooling in all four stages of vanes and blades and noted distress on the platform [at the base of the vane]. Some heat distress/discoloration on the surface indicated [the part] was running hotter than predicted at the outer diameter.” Saia says it was immediately obvious that adjustments would be required to the cooling system to solve the overheating.