Blocked Pitot Tubes

In addition to participating in the French government's investigation, the NTSB began looking into two incidents in which erroneous airspeed and altitude indications were reported on Airbus A330 airplanes. An incident on June 23, 2009, involved a Northwest Airlines flight en route from Hong Kong to Tokyo. Another incident, on May 21, 2009, involved Brazil's TAM Airlines Flight 8091, which was en route from Miami to Sao Paulo, Brazil. Initial reports indicated that the flight crew noticed an abrupt drop in outside air temperature, which was accompanied by St. Elmo's fire and followed by the loss of the air-data reference system (ADRS), disconnection of the autopilot and autothrust, and loss of primary airspeed and altitude information. The crew used backup instruments; after about five minutes, primary data was restored. The flight landed safely in Sao Paulo.

Whenever they're accessible, pitot tubes and static ports should be checked for obstructions as part of every preflight. Blockages may be due to moisture (water or ice), dirt, insects, debris or failing to remove protective covers. The airspeed indicator is a differential pressure gauge or electronic device that measures the difference between pitot (dynamic or impact air pressure) and static pressure. When the aircraft is stopped on the ground, the pressures are equal and the airspeed indicator shows zero airspeed. When the aircraft moves, the pressure in the pitot line becomes greater than the static pressure, and the instrument begins to register airspeed. If changes in dynamic pressure can't be transmitted in the pitot line because of a blockage, then the airspeed indicator has only what's trapped inside to compare with static pressure. In a descent, static pressure increases, causing the airspeed indicator to register a decrease in airspeed. In a climb, as the static pressure decreases, the trapped dynamic pressure becomes stronger in comparison to the static pressure, so the indicator shows airspeed as increasing.

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.

The NTSB concluded that the probable cause of this incident was the loss of reliable airspeed indication due to ice accumulation on the air-data/pitot sensors. Also contributing were the flight crew's improper response to the erroneous airspeed indications, the lack of coordination during the initial recovery of the plane to controlled flight, and icing conditions.

A single-engine Cirrus SR22 was en route from Tucson, Ariz., to Englewood, Colo., with just the pilot on board. He was climbing from 15,000 feet to 16,000 feet to avoid thunderstorms and snow showers. The SR22 was in clouds when the airspeed indication on the PFD became "hash marks." The pilot overrode the autopilot, initiated a descent and turned on the pitot heat. Shortly thereafter, the airspeed indication returned. The pilot told investigators that he sensed he was in a descent and pulled back to stop the descent and slow the airplane. Then, the attitude indicator became unusable. The pilot activated the parachute system before the airplane descended into trees, receiving substantial damage; the pilot's injuries were minor. The NTSB determined that the probable cause of this accident was the pilot's failure to activate pitot heat while flying in the clouds and visible moisture, resulting in pitot tube contamination and the subsequent loss of air data for the PFD. Contributing to the accident were the icing conditions and the pilot's spatial disorientation.

A single-engine Cessna 172P was flying from Bear Crik Airstrip in Tioga, Pa., for a day/VFR local flight with a pilot and three passengers. After takeoff, there was no airspeed indication, so the pilot elected to divert to Wellsboro, Pa. After landing, the pilot removed mud from the pitot tube, refueled the airplane and decided to return to Bear Crik. After departure, the airspeed indicator again failed. Upon landing, the airplane bounced and touched down again about 1,000 feet from the approach end of the grass runway. It then became airborne again, touching down after another 500 feet. The nosewheel came off and the gear strut dug into the ground, causing the airplane to nose over. The pilot told investigators, "As the airport has a 50-foot obstacle at one end, I elected to land downwind. I made a low and slow approach. Upon landing, I found I was too fast!" Runway 3 was 1,600 feet long by 100 feet wide. Wind was reported from 170 degrees at nine knots. Examination of the airplane's pitot tube by an FAA inspector revealed that it was blocked with mud.

The NTSB determined that the probable cause of this accident was the pilot's improper in-flight decision to land downwind with an inoperative airspeed indicator and his improper landing flare.

Peter Katz is editor and publisher of NTSB Reporter, an independent monthly update on aircraft accident investigations and other NTSB news. To subscribe, write to: NTSB Reporter, Subscription Dept., P.O. Box 831, White Plains, NY 10602-0831.