A Coke-bottle-shaped container called a combustor can and a finely machined piece of metal called a spacer have sullied the name of history's most successful commercial jet aircraft engine.
With the leaves on the verge of turning New England golden, the officers and men of the Pratt & Whitney Group of United Technologies Corp. are asking themselves if they can do it better. They build the JT8D engine, which powers two-thirds of the commercial jetliners flown by U.S. airlines.
Before Aug. 22, the term JT8D was the property of the aviation fraternity. No longer.
That day, in Manchester, England, one of nine combustor cans on a British Airtours Boeing 737 cut loose and shot through the engine covering and a thin maintenance access panel into a wing fuel tank as the plane attempted a takeoff. The pilot braked to a halt on the runway, but 55 people died in the fire that followed.
Fifteen days later in Milwaukee, a Midwest Express McDonnell Douglas DC9 crashed shortly after takeoff, killing all 31 people aboard. While there are more questions than answers about that accident, the pilot's problems apparently started when a spacer let loose from the interior of the right-side engine on the two-engine plane and punched a hole in its covering, taking a number of other engine pieces with it. The other engine subsequently suffered an undetermined power loss and it too was damaged.
"This is a tough thing for a company to go through," said Pratt & Whitney President Arthur E. Wagner. "These have certainly been tragic occurrences. But we'd like to have a little more light and less heat . . . . You end up in what is a tragedy for people and a problem for your company, reputation-wise . . . . You want to make sure there is nothing you overlooked, and take steps to reduce problems in the future."
The accidents involving the JT8D not only trouble Pratt & Whitney, but renew questions about the vigor of the government's guarantor of aviation safety, the Federal Aviation Administration. How loud should it ring the alarm? When? Should it concentrate on engine design or on airline maintenance?
Every time an engine fails or is shut down in flight, the FAA must be told. Computer printouts on JT8D incidents dating back to 1978 are 1 1/2 inches thick, but in almost every case the plane landed safely. The records show everything from ingested birds to oil filter leaks.
Combustor cans had let loose from JT8Ds four times before Manchester, according to the FAA, but never with catastrophic results. Over the years there were a number of Pratt & Whitney advisories to airline maintenance directors on how and when to inspect combustor cans and how often to replace or repair them. This is a normal part of commercial aviation and applies across the board to airplanes and engines. As experience is gained, it is shared.
FAA specialists read the advisories and study the incident reports. If they find an alarming trend or something that must be fixed to avoid catastrophe, they are supposed to back up the advisories with the force of federal regulation, an Airworthiness Directive, which is mandatory for U.S. carriers.
Combustor cans were addressed for the first time in an FAA Airworthiness Directive after the Manchester accident.
There is considerable preliminary evidence to suggest that an inspection of the combustor cans on the British Airtours flight would have been prudent based on engine performance records by previous flight crews and as suggested in Pratt & Whitney advisories. The FAA has no control over British flight crews or maintenance.
Before Milwaukee, spacers had flown off JT8D shafts 15 times, according to the FAA, but never with results catastrophic to the airplane. The FAA is considering its first Airworthiness Directive on the spacer, but so far has limited itself to sending a lower-priority GENOT, or general notice, to its chief airline inspectors.
Arthur Pidgeon, manager of the FAA's aircraft certification office in Burlington, Mass., which is responsible for engines, said of the previous spacer problems, "The FAA did not consider it serious enough to issue an Airworthiness Directive. We have under consideration right now mandatory action."
Pratt & Whitney is proud of its JT8D, and with justification. More than 13,000 of the engines have been sold. While Pratt & Whitney officials concede that statistics are no solace to those involved in an accident, they want to be sure the statistics are understood: the JT8D makes about 8 million takeoffs a year. Almost all spacers and combustor cans stay put.
Selwyn D. Berson, Pratt & Whitney's executive vice president for commercial products, said, "We can design engines, and make recommendations, but we can't police it. The FAA decides who is qualified to repair the engines. We set the requirements."
Midwest Express' maintenance records show that all recommended and required milestones have been met, federal investigators said.
A jet engine consists of three sections -- a compressor, a combustor and a turbine.
Air coming through the intake is forced through a series of compressor stages consisting of one rotating and one static set of blades that look something like fan blades. Spacers, mounted on the shaft of the engine, separate the compressor stages and rotate with the shaft.
After the air is compressed, it is sent to the combustor cans, where it is combined with jet fuel and ignited. The resulting gases are forced through another set of blades, called a turbine, and exhausted. The exhaust gases spin the turbine, which turns the shaft, which drives the compressor.
Airline engines are supposed to be built so the plane will survive if there is an engine failure. Two principles apply:
First, if it can be anticipated that a part might fail, then it must be shown that the failure will be "contained" to the engine casing and will not damage the rest of the aircraft. It is expected that fan blades will sometimes fail, and Pratt & Whitney engineers run hundreds of tests in their laboratories here to demonstrate that, if they do so, the failure will be contained.
Second, if it is impossible to contain the failed part, then the part must not fail. Spacers and some other rapidly rotating parts are simply too large and spin too swiftly -- up to 8,000 rpm -- to be contained. Spacers are expected to last the life of the engine, but are subject to regular inspections.
Combustor cans, which reach temperatures of 1,700 degrees Fahrenheit, must be inspected, repaired or replaced on a regular schedule that is supposed to catch trouble before there is a failure.
There are two kinds of spacers in JT8Ds, and that has become a bit of an issue in Milwaukee. The older type has a removable sleeve; the newer is a single part. All the spacer failures have occurred on engines with the removable sleeve.
Pidgeon said that corrosion sometimes develops under the removable sleeves. Corrosion weakens metal and can cause a spacer failure. When engines receive their regular inspections, the area under the sleeve must be checked.
"From an engineering design point of view, both sleeves are perfectly airworthy," Pidgeon said. "It's when you don't maintain the removable sleeve that you have problems."
Pratt & Whitney spokesman Phil Giaramita said, "We have tried to suggest to our customers that they ought to move" in the direction of replacing removable sleeves. "We have allowed them to decide for themselves." So has the FAA.
The cost per spacer is about $3,300 and there are 10 spacers per engine. Giaramita said only about 900 engines still have the removable sleeves.
Tim Hoeksema, president of Midwest Express, said the one-piece spacer has been installed on the two engines Midwest Express has overhauled in its 16-month history, but the older kind is still used in the two other engines on the two-airplane line.
"We have asked the FAA if in their opinion we should change those spacers immediately, and their official recommendation is that, per their formal documents, that should be considered when the engine is overhauled," he said.
Investigators do not know why the pilot in Milwaukee was unable to fly the plane after the engine failed. The subsequent problem in the other engine might have arisen because the aircraft got into a position that disturbed the normal flow of air into the engine inlet. If airflow is perturbed, a jet engine tries to reverse itself and stalls.
Despite its troubles, officials of Connecticut's largest employer say there has been no loss of airline confidence in Pratt & Whitney engines and no slowdown in orders. In the next decade, Berson said, the worldwide market for engines, spares and parts will be $71 billion. He would not project Pratt & Whitney's share.