Frozen Pitot Tubes, Then and Now…
(All you never wanted to know about pitot tubes)
- “(Recife Brazil, June, 2009) Air France flight AF 447 was carrying 228 passengers and crew when it disappeared off radar screens after encountering turbulent weather on June 1, four hours into its flight from Rio de Janeiro to Paris. Investigators have said there were “inconsistencies” with speed readings prior to the crash, raising speculation that speed sensors on the A330-200 jet, known as pitot tubes, may have iced up, feeding wrong information to the cockpit and confusing the pilots as they hit a storm.”
- “(Aviation Week, May, 2011) Investigators into the crash of Air France flight 447 have located and recovered the memory unit of the aircraft’s flight data recorder… messages sent out immediately before the crash indicated that pitot tubes had sent erroneous speed data, most likely caused by icing.“
It was the summer of 1965 when a co-worker and I decided to take flight instruction out at what was then Palwaukee Airport in Wheeling, Illinois. George Priester owner of the airport and Priester Aviation, operated a fleet of Piper J-3 Cub aircraft. It was a very basic plane… no electronics, crank start (the propeller was the crank!). It had tandem seating for two and an old fashioned stick on the floor to control the elevator and ailerons. We fondly referred to them as “paper” Cubs because the frame was covered with a sturdy canvas skin. It was often described as a plane that could just barely kill you. You could get it in any color as long as it was yellow.
The flight instruments consisted of “needle, ball, and airspeed,” an altimeter and a compass, none of which required external power of any kind. The airspeed indicator sampled air pressure provided by a pitot tube to give its reading. A pitot tube is a device that is exquisite elegance in it basic simplicity. It is a small tube, usually mounted on the wing of an aircraft that simply samples the airflow entering the tube. That air pressure is mechanically translated to the airspeed dial. Again, it requires no outside power source except for the pressure of the air entering the tube. All aircraft use them; from the 1937 Piper Cubs to the 2011 Boeing 787’s just starting to roll off the production line today.
I was an earnest student but definitely would not have been at the top of my class had there been any competitive grading system. Nick, my instructor, was a taskmaster and he sensed my lack of confidence but did nothing to encourage me. I didn’t like him and I was sure he didn’t like me. One evening we were shooting take-offs and landings when he observed I was altogether too fixed on the instrument panel, sparse as it was.
“Don’t focus on the instruments,” he yelled (In a Piper Cub you always had to shout, but somehow I felt he would yell at me no matter where we were). Well of course I was looking at the instruments. Especially the airspeed indicator. If you let your airspeed drop too low the plane would literally fall out of the sky… in flight terminology a “stall.”
“Learn to fly by feeling what the plane is doing, don’t stake you life on a pitot tube!” With that he took business cards out of his shirt pocket and used them to cover the needle, ball, and airspeed indicators. The rest of the evening we shot touch and goes (landing touchdown, followed by an immediate take off) with the instruments covered. I cursed him silently, I perspired, I did the best to control my racing pulse rate, but I did learn to fly the plane by sensing the outside noise and how the controls felt as I applied pressure to the rudder pedals and stick.
Well time sure flies when you’re having all that fun and several months later we were in mid-winter. I had switched instructors, completed my first solo flight, and was flying at least 50% of the time by myself. It was a bitter cold day when I rented a Cub for a solo. The hard frozen ground crunched under my feet as I walked around the plane for preflight inspection… oil, fuel, leading edges, control surfaces, pitot tube, skin (well… paper), all appeared in perfect order. Once in the cockpit, the lineman spun the prop to get me started. Remember, the Piper Cub had no battery. I set the altimeter and gave the engine extra time to warm up before doing the engine preflight. I turned the plane into the wind and did the engine run-up then taxied out to the runway, checked all directions for other air traffic and took off. No radio, no control tower, strictly VFR (visual flight rules), see and be seen was the order of the day at Palwaukee Airport in 1965.
The dense cold air gave the plane extra lift and I was climbing easily over the high tension power lines a mile or two beyond the runway. It always seemed a bit strange to me that high tension power lines would lay just beyond the end of the runway but hey, I had no idea of which came first, the power lines or the airport. I started to throttle back from take-off power when I glanced at the airspeed indicator. Zero! Theoretically the aircraft and I should be falling to the ground in a full stall. It took me a split second, but I very quickly remembered crabby ol’ Nick… “Fly by feeling what the plane is doing.”
“I can do that!” I smiled to myself. The plane continued to climb easily in the cold air, the controls felt solid and responsive, the engine and outside noise was normal for a climb. It was really quite simple; the airspeed indicator was wrong. My eyes, ears and the seat of my pants told me plane was behaving in a completely normal fashion, I checked again for traffic and executed a left pattern for an immediate return to the field. Each time I glanced at the airspeed indicator it was the same. Zero! No problem. I landed uneventfully, taxied to a tie-down, and shut down the engine. Once out of the cockpit I again inspected the pitot tube visually. It was perfect from all outward appearances. In the flight office I reported a frozen pitot tube and gave them the tail number of the aircraft. They asked if I wanted another plane, but I declined. As I walked back to my car that day I had my first positive thought about Nick.
Now I’m no expert, but I’m pretty sure you cannot fly an Airbus A330 “by feeling what the plane is doing.” Never-the-less, if Nick is still around, I’m sure he’s more crotchety than ever, reading the news reports and muttering to himself, “Don’t stake your life on a pitot tube!”
God bless Nick… and the 228 souls aboard Air France Flight 447.