Plane & Pilot
Wednesday, September 9, 2009

Blocked Pitot Tubes

When accessible, pitot tubes and static ports should be checked in every preflight

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.


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