The flurry of interest triggered by last week’s news of a typical test issue—discovered two months earlier 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 re-engined Airbus.
Airlines are counting on the lower fuel burn of the A320NEO, and the competing Boeing 737 MAX, to stave off the effects of an inexorable rise in operating costs. Furthermore, the fiercely competitive battle over engines for the NEO between Pratt’s PW1100G and CFM International’s alternative Leap-1A, has put both powerplants 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 and the eagerness with which Pratt is pushing itself back into the mainstream jet engine business with the geared turbofan. According to figures from the engine makers and Airbus, CFM is currently slightly ahead. The Leap-1A has been selected for just over 800 A320NEO family aircraft, or around 34% of the firm aircraft orders. The PW1100G has been selected for around 750 aircraft, or 32%, while the engine selection for a further 800 A320NEOs on firm order is up for grabs.
As a result, the players are keenly aware of the potential impact of any slip-up during development, even though the initial phases of engine and airframe test programs are frequently interrupted by events of the type that hit the PW1100G in May. In the incident reported last week, 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-Next Generation Product Family.
The engine, which eventually went on to amass 110 hr. of test time 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 red line, if the engine goes above it you don’t get a major surprise. So we run the engine pretty hard,” says Saia, who adds that some company-unique tests are also “aggressively” run to build up experience with the engine in a relatively short amount of time.
The testing on engine 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 to the first stage of the HP turbine,” 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, so we noted distress on the platform [at the base of the vane]. There was some heat distress/discoloration on the surface that 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 deal with the overheating.
“So we had the choice of going in and making the modification, or continuing with the test. Although we were not happy we had this condition, we also elected not to go in and make the modification because we didn’t want to lose schedule—so we continued to run. We had a unique way of monitoring which uses pressure measurements to get a feel for how the part has changed over the testing,” says Saia.
On this basis, Pratt therefore elected to continue running the engine. “So we completely ran the fan, LP compressor and LP turbine as expected except for one point—which was the overtemp test.” This test runs the engine 300-400F above red line, and relative to the highest expected thrust rating of 33,000 lb. for the engine on the A321, is equivalent to around 39,000 lb., or 20% more thrust.