Air France AF447 and my experience with frozen pitot tubes
The final report on the 2009 AF447 crash is out today. I didn’t read the entire report but most of the blame was placed on faulty pitot-tubes, as had been expected. There is also a fair amount of discussion about the pilot’s actions but the head of the French BEA stated that “the same situation could have occurred with a different crew on board”.
I can appreciate that statement and the difficulty the AF447 pilots were in. Airline pilots are practically human robots, almost everything they do is a programmed procedure. That’s not to say there aren’t many times they have to exercise sound judgment but when it comes to basic flying procedures, they aren’t supposed to have to figure things out. When the pitot tubes became frozen the AF447 crew was presented with conflicting warnings and information that made no sense. The cockpit warnings and information weren’t reflective of the aerodynamic condition that the airplane was in, nor was it a situation the crew had been adequately trained to deal with. How do you react when one indication contradicts another? My condolences go out to the families of the passengers and crew.
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I had a frozen pitot tube once. I was flying an older Seneca to Europe at the end of the northern hemisphere winter. I had refueled in Kuujjuaq and was on my way to Iqaluit. In Iqaluit avgas (100LL) is available in drums only and you have to buy the entire drum. I was hoping to buy just one drum of avgas in Iqaluit but my fuel consumption looked to be a bit high. Not wanting to buy a second drum for an additional 10 gals or so I decided to throttle back to a more conservative power setting, hoping to save a bit of gas between CYVP and CYFB.
Image courtesy of Great Circle Mapper.
I knew I shouldn’t have throttled back, one of the points I often have to explain about doing ferry flights over the North Atlantic is that many times you won’t be able to go to max range power settings in piston engine airplanes because the engine temperatures will fall below the green arc due to the extreme cold. As it was the temps in my Seneca didn’t fall below the green arc but only a few minutes after I had throttled back the left engine oil pressure started to drop and the left engine oil temperature started to rise. Both needles stabilized before going out of the green arc but I knew something had happened nonetheless.
My guess was that the left oil cooler had congealed or frozen. The oil inside the cooler got so cold that it would no longer flow through, thereby blocking the cooler. The oil cooler bypass valve opened so I was running the left engine essentially with no oil cooler. This wasn’t a dangerous situation in itself as long as the oil temperature and pressure stayed in the green arc, but not fun either.
As I approached Iqaluit the weather was beautiful and clear but surface temperatures were in the -30C range. I had been delayed a bit in Kuujjuaq and was arriving in Iqaluit about 20 minutes after dark so I decided to set up for the ILS approach. My Seneca had a nice autopilot and I decided to fly a coupled approach. I was flying a stabilized approach at 95 kts IAS, one notch of flaps and was passing through about 1,600 feet when the airspeed dropped to about 90. I added some power.
I should note that the pitot heat was ON. I fly light airplanes like you would a large airplane, meaning I turn the pitot heat ON when I go on the runway and OFF when I leave the runway.
A few moments after I added power the airspeed dropped again, this time significantly. My immediate thought was that I lost the cranky left engine. I grabbed the yoke, disconnected the autopilot and shoved both throttles way up. I looked at the airspeed and it had now dropped dangerously low to around 65 knots. If you stall a twin engine airplane on one engine, especially close to the ground, you most likely won’t live. So I did what I was trained to do. I yanked both throttles back and pushed the nose down.
I yanked the throttles back because I thought at the time the airplane was only developing power on one engine, which would be extremely dangerous in a stall.
I pushed the nose down hard. It was dark but I could see the runway. I only had just over a thousand feet of altitude.
The airspeed kept dropping and I realized something wasn’t right. I was reacting to an indication of low airspeed but there were no other stall indications. No stall warning, no aerodynamic buffet, nothing. I glanced over at the GPS and saw the groundspeed indicating something like 130 knots. By now my airspeed indicator was flat at ZERO.
Realizing that I was not in a stall but had a frozen pitot tube, I added power and raised the nose to a nose-level attitude. I was only about 3 miles from the runway and I flew the rest of the way by eyeballing the nose attitude, guestimating the power setting and glancing at the GPS groundspeed, making sure to keep it up quite high.
Iqaluit has a long runway and I was able to make a reasonable landing. The entire “upset” (if you want to call it that) only lasted maybe 10 seconds, maybe less. Much like the AF447 crew, I reacted in a programmed way but the situation was not what was indicated. First I reacted to what I thought was an indication of losing an engine (unusual drop in airspeed during an autopilot coupled approach) and then I reacted to what I thought was a stall. My problem was easy to identify, as soon as I realized my initial responses didn’t produce the expected result I was able to identify the real problem, the frozen pitot tube. Sadly, the AF447 crew was in a much more complicated and dire situation (for one, they weren’t able to see the ground and fly by visual reference, I was).
On the rollout the pitot tube started to unfreeze. There was never any loss of power on the left engine, that was just my initial reaction based on the indicated loss of airspeed and earlier indications (which had persisted) of high oil temp and low oil pressure. There was also never a loss of airspeed, just a frozen pitot tube.