Wednesday, September 9, 2009
Blocked Pitot Tubes
When accessible, pitot tubes and static ports should be checked in every preflight
The seriousness of pitot blockages is reflected in NTSB investigations, such as a lengthy one completed earlier this year. On May 15, 2005, Midwest Airlines Flight 490 (a Boeing 717-200) was flying in night instrument conditions from Kansas City, Mo., to Washington, D.C., with 80 people on board. When the plane was at FL230 (with a clearance to climb to FL270), the autopilot was on and the airspeed was 300 knots. The closest weather cell was 20 to 25 miles away, and the crew members felt they didn’t need to turn on airplane anti-icing because the outside temperature was still too warm to require it.
The crew got an alert regarding the rudder system, then the airplane pitched down about 20 degrees. Investigators figured that the warning was triggered when the rudder-limiting system’s pitot tube iced over. Icing had also accumulated on the other pitot tubes, preventing the air-data system from accurately figuring the airspeed. The captain remembered hearing the autopilot-disconnect warning signal. When the pitch down occurred, the captain was still the pilot flying, but the first officer began assisting on the controls. Investigators found that the pilots weren’t coordinated in their control inputs. The airplane continued in a steep dive, which the first officer felt was “almost beyond recovery.” Both pilots recalled saying, “Up, up, up,” during the initial descent, and noted that the airplane didn’t respond to control inputs at first and that the flight controls felt very heavy. The first officer thought that the airplane lost at least 5,000 feet of altitude during the first descent. The captain reported that the elevator response was “not normal” and that he wasn’t getting the amount of response he expected from the flight control inputs. At times, he’d get little response from the elevator control inputs, but then it would quickly change to “a lot” of response, unlike anything he had experienced in training or actual flight.
The airplane then pitched up; the first officer stated that he told the captain to push forward on the control wheel, and assisted him in doing so. As the airplane pitched up, the airspeed decreased to about 190 knots. At that point, the autothrottles weren’t engaged, and the first officer increased the engine power. The captain said that while he was trying to recover the airplane, he attempted to maintain a level pitch attitude and tried to level the wings, but altitude control was unobtainable.
The airplane entered another dive. The first officer said that he had been trying to keep the airspeed away from the stall speed and the overspeed red zone. The captain stated that the airspeed changed instantaneously from low to high—at points, it became greater than 400 knots—with an overspeed warning in literally seconds. The airspeed went from the bottom of the airspeed indicator to the top so quickly that the captain couldn’t visualize the airplane doing so.
As the pilots began to recover from the event, the captain elected to divert to Kirksville Regional Airport in Kirksville, Mo. He began making arrangements with ATC and briefed the passengers while the first officer continued to fly. The first officer had the captain reengage the autopilot, and they continued using the autopilot until they were on approach. The entire event and recovery occurred in IMC. The NTSB found that there were five pitch cycles, lasting a total of eight minutes and bringing the plane as low as 10,600 feet and as high as 23,300 feet. Although the accuracy of recorded airspeeds is uncertain, the NTSB said they varied from a low of 54 knots to a high of 460 knots during the incident.
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