By Ashley Halsey III
Washington Post Staff Writer
Sunday, July 11, 2010; C01
A business jet crossed into the potentially dangerous wake turbulence of a United Airlines flight bound for Europe on Tuesday in what the Federal Aviation Administration initially recorded as the 23rd operational error this year by the region's air traffic controllers.
The FAA acknowledged that a mistake caused the smaller plane to violate safe-distance requirements, but the agency said that after further review, the incident was downgraded from an "operational error" to a less serious "proximity error."
Turbulence left by a massive Boeing 777, which weighs up to 328 tons on takeoff, can be likened to waves created by a large ship moving through still water. It ripples through the air well after the plane has passed. And just as the wake of a ship can capsize surrounding boats, air turbulence can endanger nearby aircraft and, in the most-serious incidents, cause them to crash.
Wake turbulence from a Boeing 767 landing at Mexico City's airport was cited by investigators as the cause of a 2008 Learjet crash that killed Mexico's interior secretary and 15 other people.
"Wake turbulence encounters are a very real concern and one of the few separation standards we have that are based on science," said an FAA staff member familiar with Tuesday's incident. The staff member asked not to be named for fear of losing his job.
"For the layman, the controller's mistake put the [small plane] at risk of encountering dangerous wake turbulence left behind by a departing heavy jet," the staff member said. "The two aircraft were not in danger of colliding. The real issue is the controller forgetting to turn aircraft."
An internal FAA document, the Administrator's Daily Alert Bulletin, distributed to air traffic control supervisors Thursday said the small plane "crossed the course" of the United flight, violating the required separation between aircraft of 1,000 feet in altitude and three miles in distance.
The ripple effect from the Boeing 777 is so great that FAA regulations are even stricter to protect small aircraft from its "wake remnant." Minimum distances are five miles behind, 2,500 feet on either side and 1,000 feet below.
The Cessna Citation 550, with a maximum takeoff weight of about 15,000 pounds, came within 2.79 miles and 800 feet of the larger jet.The second look
In response to an inquiry, the FAA on Friday underscored its belief that the air space violation did not involve a most-serious, "category A" operational error, as initially described in an agency internal document.
"After reviewing initial information, data and radar tracks, the FAA determined that no operational error occurred," the FAA said in a statement. "The two aircraft maintained nearly 100 percent of required separation standards. The review indicated the air traffic controller directed the smaller aircraft away from the wake turbulence of the Boeing 777 aircraft."
FAA supervisors who review errors are permitted to downgrade a controller's mistake to a "proximity error" if the no-fly separation zone is violated by less than 20 percent. But under FAA rules, they are prohibited from doing so if the encounter involves wake turbulence.
Air traffic controllers guide an average of more than 2,000 flights a day in and out of the region's four major airfields: Dulles International, Reagan National, Baltimore-Washington International Marshall Airport and Andrews Air Force Base. In the first six months of this year, those airfields recorded more operational errors than in all of 2009.
In one incident, on June 28, a United flight with Rep. F. James Sensenbrenner Jr. (R-Wis.) on board narrowly avoided colliding with a 22-seat business jet as the airliner was being guided toward a landing at National. Onboard collision-avoidance systems warned the pilots that they were headed for a midair encounter, and the United pilot took evasive action.
The FAA dispatched a "tiger team" of investigators to Potomac Terminal Radar Approach Control (TRACON) in Warrenton after that incident, the 22nd operational error recorded at that facility this year. TRACON controllers are responsible for all aircraft until they reach about 21,000 feet, where they are handed off to teams that direct long-haul routing.The maneuvers
The following account of Tuesday's encounter between the Cessna Citation and the United flight is drawn from commercially available flight-tracking information, internal FAA documents and information provided by the agency:
About the time that the United jet -- with 250 aboard -- pulled away from the gate at Dulles and rumbled toward the runway, the Cessna was in the process of lifting off at BWI, about 45 miles northeast.
It was just after 5:30 p.m., and the BWI tower handed off control of the Cessna to a controller at Potomac TRACON. The Cessna pilot, bound for Stafford County, was told to go west and then southwest.
It is routine for controllers to have southbound flights skirt the eastern edge of the Dulles airspace, integrating those planes into the flow of aircraft to and from Dulles for relatively brief periods.
Dulles planes break out of that flow either to climb to their cruising altitude or to make their final approach for landing.
The big United jet took off from Dulles toward the north at 5:43 p.m., banking to the right as it began to climb and prepared to turn toward the east and Germany.
Then the controller directing the Cessna made a mistake. The Administrator's Daily Alert Bulletin contains this description:
"The controller planned on clearing [the Cessna] direct to a fix within the . . . airspace, however the instruction was not issued and [the Cessna's] last assigned heading took the aircraft into another sector's airspace."
The controller recognized the mistake and made a pair of course alterations to direct the Cessna away from the turbulence. That loss of required separation was recorded by TRACON.
"Wake errors are always considered more severe than non-wake errors," said the FAA staff member with knowledge of the incident. "The wake is unpredictable. Unlike the risk of collision, where a pilot can do whatever he can to avoid the other aircraft because he can see it, with wake turbulence, you cannot see it. You don't know you've hit it until you've hit it. That's why we place so much emphasis on avoiding the areas that may have the most potential for wake."