Date: 201209
Local time of day: 1801-2400
Location: ZZZ.Airport, US
Flight conditions: VMC
Light: Daylight
Aircraft: Cessna 150
Operator: Personal
Operating under: Part 91
Flight plan: None
Mission: Training
Flight phase: Initial Climb
Airspace: Class D ZZZ
ATC / advisory: Tower ZZZ
Aircraft component: Pitot-Static System
Component problem: Malfunctioning
Reporter organization: Personal
Reporter function: Pilot Flying; Instructor
Reporter qualification: Flight Crew Commercial; Flight Crew Flight Instructor; Flight Crew Instrument; Flight Crew Multiengine
Human factors: Confusion; Human-Machine Interface
Anomaly: Aircraft Equipment Problem Critical
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Overcame Equipment Problem; Flight Crew Returned To Departure Airport; General Declared Emergency
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: This was the very first flight for my new primary student. I was flying and demonstrating the student's first takeoff; on Runway 31L. At [3;700] FT; this is the shorter of the two runways at the airport. Our original plan was to fly the Blake Departure to take us west of the airport. When North Flow procedures are in effect; Runway 31L is normally used for westbound departures and 31R for eastbound departures. We were cleared for takeoff from Runway 31R and instructed to fly the Blake (westbound) Departure. This essentially was a clearance to takeoff from 31R and then turn left and depart through the departure path of 31L; which was also an active runway at that time. In years of flying/instructing at the airport; I've never received a takeoff clearance from one runway that included departure instructions to cross the flight path of the other active runway. Lined up on 31R I queried the controller about the departure clearance. The same confusing clearance was delivered once again. I explained to my student that sitting on an active runway was not the place to sort out departure confusion. The weather was VFR; we had a takeoff clearance and could climb straight out until the departure issue was sorted out. I proceeded with the takeoff. The airplane had been trimmed for the proper takeoff setting. Power application and acceleration seemed normal. At 45 KIAS the airplane became airborne; a bit sooner than expected and began to climb. (Vs 1G for the airplane is about 47 KIAS) I instinctively lowered the nose to increase airspeed toward 60 KIAS in the climb. My first thought was that the wind; which had been down the runway at 7 KTS; had increased. As the airplane continued its initial climb; airspeed increased slightly; but not as much as I expected. The engine was smooth and appeared to be producing full power. Flaps were up and the airplane was very controllable but 'felt' like it had more drag than it should have. By the time I analyzed all of these possibilities; there was insufficient runway remaining to abort the climb. As the plane was controllable I continued to climb at 100 FT per minute past the airport perimeter. I notified Tower that I had a problem and would need to stay in the pattern rather than depart to the west as had been the original plan. The airplane was under control; albeit in a very shallow climb; but airspeed was still hovering around 50 KTS. I began to suspect that I might have an airspeed indicator issue; but wanted to hold off on further troubleshooting until I reached a safe altitude. I concentrated on flying a track that would clear obstacles and get me to pattern altitude safely. At 600 FT and about 2 miles from the airport I turned crosswind and declared an emergency with Tower. Nearing the downwind leg I found a large hospital protruding into my climb path. The airplane's rate of climb also seemed to diminish. Maneuvering around the hospital; the airplane eventually made it to pattern altitude. I notified Tower of my intent to stick with a landing at the airport. Next I switched on my iPad to read a GPS ground speed off of my charting application (Foreflight). Ground speed was 90 KTS. This essentially confirmed that I had a pitot issue and a safe landing was executed on Runway 31L at the airport. In training circles; a blocked pitot is often discussed. The most common example used ad nauseam represents a fully blocked pitot situation where the airspeed indicates zero airspeed. Had I seen very little or no airspeed during the takeoff roll; I would have instinctively aborted the takeoff. The partial failure in this case was far more insidious. I had just enough indicated airspeed to continue with the takeoff and commence the initial climb. The 'drag' I felt on the airplane during the subsequent climb was imagined. It stemmed from my aversion to allowing airspeed to decay further and resulted in an attempt to climb in a cruise attitude. A contributing factor may have been the preceding confusion of a takeoff clearance that included counter intuitive departure instructions.




Synopsis: C150 instructor pilot experiences an airspeed indicator malfunction during takeoff resulting in a low airspeed; but interpreted as excessive drag. Once safely airborne the iPad is turned on and GPS ground speed is compared with indicated airspeed to confirm that the airspeed indicator is erroneous.