The following is a reprint of a story first published on the now-defunct Ninety Nines list serv.
What would you do if your airspeed indicator slowed 80 knots and then, suddenly, moved to 140 knots, and you weren’t doing anything?
What Is This?
By Pat Gregory
The visibility was greater than 10 miles, and a bit windy with light turbulence around 3000 feet. I stayed at 5500 until I passed the Livermore Airport and headed Southwest for the Calaveras Reservoir where I would report in to the RHV tower. There was some fog in the Santa Clara valley, although RHV and Livermore were reporting clear skies.
As I approached the corridor behind Mt. Hamilton, I noticed my airspeed starting to decline. It continuously dropped and when it reached 80 kts, I thought, carb ice. So, I pulled the carb heat on and waited for things to clear up.
It didn’t.
I knew I was about 10 miles out of RHV, and had already descended to 3500 feet by that time. I also knew that Livermore was closer, so I diverted there to land. I called the tower controller and said I was losing airspeed and wanted to land there. He cleared me to land any runway and cautioned me that there was wind shear reported in the vicinity.
Meanwhile, I was trying to get as much altitude as I could so I’d have some chance of gliding there if the engine quit. After my 180 turn toward Livermore, my airspeed started increasing. And it kept on increasing, even though I was climbing. I eased the throttle back a bit. No effect, so I eased it back some more. Still the airspeed increased. I was doing 140 kts . By now, I was close enough to the Livermore airport to land, so I throttled back some more and started my descent to land.
I flew a normal pattern and landed without incident. I was able to taxi to transient parking.
The tower asked me to call, which I did immediately. Then I called my mechanic and left a message, and got a ride home, leaving the plane on the ground at Livermore.
My mechanic has checked out the plane and found nothing amiss. I also had the avionics shop check it over because my pitot-static check had just been completed a few days earlier. They also found no problems. I was able to fly the plane back to RHV with the engine and all instruments working perfectly.
I have filed an ASRS report and discussed this with several experienced CFIs. I suspect I may have experienced some severe wind shear. There was wind shear being reported at Livermore during that time. Another possible explanation is temporary blockage of the pitot port that may have cleared up when I executed the 180 degree turn toward Livermore Airport.
Here’s what Pat has to say about herself:
I’ve been flying since 1989 after I stopped skydiving. It was my way of staying in the air. With over 1000 hours and an instrument rating, I’ve been checked out in taildraggers and had a floatplane lesson.
I bought my beloved C172, Becky, in 1991 and learned how to do my own maintenance (all that is legal for me to do myself). I keep her hangared at RHV and try to fly every week. My favorite local place to fly is Hollister where I get the best chocolate milkshakes west of the Mississippi.
I’ve flown as far east as Maine and Daytona Beach, as far north as Fairbanks, and as far south as San Diego, although most of that was in Susan Larson’s C182 (Mikey).
reprinted with permission from the author.