Hiccups in the testing of Pratt's engine for the A320 are 'routine'
The flurry of interest triggered by the recent news of a typical test issue discovered two months previously on a Pratt & Whitney for the was a timely reminder of just how sensitive the industry has become in the run-up to the debut of the reengined .
Airlines are counting on the lower fuel burn of the A320NEO, and the competingMAX, 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 , 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.
“So we had the choice of going in and making the modification, or continuing with the test. . . We elected not to make the modification because we didn't want to lose schedule. [But], 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.
“We completely ran the fan, LP compressor and LP turbine as expected except for one point—the overtemp test.” This 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 about 39,000 lb., or 20% more thrust.
“We decided we'd leave it for the last test in case there was any distress. . . .When we were done, we had distress on the vane well beyond what was operationally acceptable. We had notified Airbus [of the schedule risk of] some vane damage—but it was a conscious decision.”
In the meantime, modifications were made to engine No. 3, the HP system stress test unit next in line. “We were able to make the modification before we went to test. It took us a couple of weeks to validate the root cause and identify corrective actions. It was quite simple, we just added a few cooling holes at the root of the vane.” Rather than use extra cooling air, Saia explains, the existing air is redistributed.
“We ran No. 3 at 5% above rotor red line and successfully demonstrated it.” Pratt classes the event as a success for the Block 1 phase as it unearthed the need for a revision before it reached the final certification standard stage. “We go into a test not wanting to over-cool a part. You want to do enough to let it live in the environment, but nothing more,” Saia adds.
One of the earliest to begin the next phase is engine No. 5, the first Block 2 unit now under assembly. “It will go to test in early October but is still on its schedule,” insists Saia. The program includes margin for a break between Blocks 1 and 2. “We know we are going to find areas to modify, and that certification testing will start in October and go through June 2014. We deliver engines to Airbus late in June or early July .”