That could help explain a central mystery of the fiery accident - why the fail-safe system did not prevent the Gulfstream jet from reaching takeoff speed while its tail flaps were locked.
Federal investigators have said the May 31 crash took place after the twin-engine plane barreled down a Massachusetts runway at 190 m.p.h. - well above takeoff speed - but failed to get airborne. It skidded off the runway and burst into flames.
The investigators said it appeared that the rear flaps, known as elevators, had not been unlocked. Planes with locked elevators cannot get lift.
The experts, with the National Transportation Safety Board, also said it appeared that the plane's two pilots, both with many years of service, had failed to do the required preflight check to make sure that the elevators were unlocked.
In the five-paragraph letter, Gulfstream reminded pilots that they must unlock the plane's "gust-lock" device before takeoff. The device secures the tail elevators when the jet is parked, to lessen possible damage from wind gusts.
The letter warned the pilots of about 2,000 Gulfstream jets now in service not to assume that the gust-lock device is turned off and that the elevators are free-moving just because the pilots can move the throttle.
In fact, the letter said, "throttle movement is not an absolute indicator of the gust-lock system for any Gulfstream model."
Signed by Mitchell A. Choquette, director of customer support and field service, the letter reminded pilots to "check all flight controls" before each takeoff to ensure that wing flaps and tail elevators are moving freely.
The NTSB, in a preliminary report in June, said inspection of the wreckage showed that the tail elevators appeared to be locked. The report did not address the functioning of the fail-safe system.
The NTSB is not expected to complete its investigation and declare an official cause until next spring.
Savannah, Ga.-based Gulfstream declined to make public the full text of the technical letter, though spokesman Steve Cass summarized it. The Inquirer obtained a copy from a jet owner.
The letter's contents were first reported Wednesday by Bloomberg News. While the letter never explicitly referred to the May 31 accident, it was the second time since the crash that Gulfstream has reminded pilots about the importance of pre-takeoff checks.
In an interview Wednesday, John Hansman Jr., a professor of aeronautics and astronautics at Massachusetts Institute of Technology, said it appeared the pilots of Katz's plane made a simple error that had the effect of defeating the fail-safe system.
Under correct procedure, Hansman said, they should have turned off the gust-lock and then started the engines.
Instead, he said, they apparently started the engines first and only belatedly tried to turn off the gust device.
He said they apparently tried to turn off the lock as the plane rolled down the runway. The hydraulic system, powered up by the running engines, may have defeated that, he said.
The NTSB's report noted that while the elevators appeared to be locked, the gust-lock handle was found in an "off" position at the crash scene.
Reacting to the letter, Steven M. Janos, a professional pilot rated for the jets, said he always followed the same procedure: releasing the gust lock first, then starting the engine.
"My understanding is that if you start the engine, you will not be able to release the gust lock," he said Wednesday.
Hansman said he was unwilling to criticize the fail-safe system.
"It's very hard to design a safety system where someone can't find some way around an interlock," he said.
Cass, the Gulfstream spokesman, said he, too, saw no failure of the fail-safe system. "I think it's premature to jump to that conclusion," he said. "We're still waiting for the report back from the NTSB and that report . . . will give a causation."
The accident at Hanscom Field in Bedford, outside Boston, killed Katz, 72, only four days after he had won an auction to resolve ownership of The Inquirer, the Philadelphia Daily News, and Philly.com.
After making a fortune in law, billboards, parking, and sports, Katz became one of the region's biggest charitable givers, pledging millions to Temple University, Dickinson University Law School, and other beneficiaries.
The crew that day, highly experienced on that plane, was pilot James P. McDowell, 61, and Bauke De Vries, 45, who together had amassed the equivalent of two full years in the air.
Along with Katz and the pilots, the accident took the lives of three passengers - Susan K. Asbell, 68, Marcella M. Dalsey, 59, and Anne B. Leeds, 74 - and a flight attendant, Teresa Ann Benhoff, 48.
The passengers, friends of Katz's, had joined him for the day's travels. The group had flown to the Boston area for the day to meet with the historian Doris Kearns Goodwin for a charitable event and were heading back that evening.