A National Aeronautics and Space Administration sientist wrote the National Transportation Safety Board recently that "some engineers would question" whether the DC10 should have been returned to service before the effects of wind gusts on the plane's engine support pylon are better understood.
The letter, from W.C. Walton Jr., NASA's principal dynamacist, was released today as the safety board's hearing continued into the May 25 DC10 crash here that killed 273 persons.
The letter was dated July 19, six days after the DC10 was returned to service by the Federal Aviation Administration. The McDonnell Douglas jumbo jet had been grounded for 37 days while investigators studied problems with the plane's pylons.
Pylons are the metal structures that hold the engines to the wing.
Walton testified today that, in his view, "gust loads are improperly understood."
His letter, released after he testified, said that "there is the possibility that . . . loads from gusts have been significantly understimated. Some engineers would question a return to operational status with structure that has shown a propensity to crack if it is also known that there is uncertainty about the . . . loads."
Walton said in an interview later that he personally shared that view.
Wind gusts can occur while an airplane is airbourne that put unusual pressures on various parts of the aircraft. Such gusts can occur without warning in clear air, and are a factor manufacturers must consider when they design an airplane.
In his letter, Walton wrote that "it would be particulary difficult to acquire flight data during the relatively short period of developmental flight tests for the rarely occuring gusts of intensity" approaching the limits an aircrat is designed to withstand.
The pylons that Walton studied were not on the plane that crashed, but were damaged ones discovered during inspection of other DC 10 following the crash.
Walton was assigned to help the safety board evaluate the damage to a United Airlines DC10 that had severe cracking in the spar web, the large plate across the top of the pylon. On that United plane, the web was virtually detached from the rest of the pylon and had a large hole in it.
The FAA, with help from outside consultants, satisfied itself that the damage to the United pylon occurred because of faulty quality control during manufacturing. Similar but less severe problems were found in pylons of 36 DC10s. All those pylons were produced during a six-month period after McDonnell Douglas had moved its pylon production line from one facility to another.
M. Craig Beard, chief of engineering and manufacturing for the FAA's Los Angeles office, disagreed with Walton's letter. "I think our analysis methods have been good enough to handle the [gust load] question," Beard said. He said the FAA is convinced the spar web problem is traceable to manufacturing defects.