In true medical fashion, I would like to request of you all 2nd and 3rd opinions about the cause of an Airspeed Indicator (AS) problem I developed last week.

 

The problem: I am getting erroneously low (by 20-40kts) AS readings. This came about suddenly, and has never been a problem before.

 

The history:

 

First, you must know that the plane (L-IVP) has 970hrs TT and has always had a very accurate AS, matching very closely GPS readings (adjusting for wind and altitude) over the last 10 years.

 

It is my habit to always verbally call out "airspeed alive" as I accelerate down the runway for takeoff--the AS usually becomes active at about 40kts or so.  Recently, however, the AS did NOT become active at the usual spot down the runway during a takeoff run, despite a normal runup and full power settings. Just as I pulled the power to abort the takeoff, the AI seemed to bump to life.  I exited the runway and regrouped. I shut down and inspected everything externally--all looked good. Oh well, perhaps I had inadvertently left the pitot cover on, and it later blew off as I ran down the runway, I thought (no dice, as it turns out, as I later found the cover safely stowed in the baggage compartment). Sooo, I tried another takeoff with the same result--this time the AS became active, but far down the runway, and I aborted again.

 

The local A&P did a thorough inspection of the pitot system (all A-OK), and did a full test of the AS with a meter attached to the pitot tube showing that indicated AS was the same as the metered setting on the calibration device through a full normal range of speeds up to VNE. He also "set" an airspeed with the pressure on the pitot and clamped it off, making sure there was no decay in the indicated airspeed, which might have indicated a leak in the pitot system. He also disconnected the pitot system within the cockpit and reverse blew some air back out toward the pitot tube, on the outside chance a bug or something had temporarily blocked the system.  All appeared normal, so I went out today to do another test flight.

 

Results: I ran down the runway today once again after normal runup at full power. Once again, NO active AS at the usual point. I decided to proceed with the takeoff referring to the GPS speed.  After takeoff and climbing into the pattern at 120kts (GPS) the AS started swinging back and forth between 70 and 88kts--it never stabilized and I could not even retract the gear, indicating that the squat switch did not think I had enough airspeed to trip the solenoid. I brought the plane around the pattern and landed without incident, using the GPS speed indication + a good margin for safety. The AS throughout the flight was always significantly below normal reading by anywhere from 15-40kts.

 

So, we have here a situation in which the AS tested out fine on the ground, and bad in the air. Any idea why?

 

The mechanic is stumped. I am stumped. One opinion has suggested that the gear speed switch diaphragm may have ruptured, but wouldn't that result in a decay in indicated speed with a test device putting clamped air pressure on the pitot tube? 

 

Help and ideas appreciated.

Doug Johnson