Indicated airspeed with pitot cover in place?

[RobertE]

I took off without removing the pitot cover yesterday.  I got what I thought was a normal indication of airspeed while proceeding down the runway and only realized my error when I was getting 100 knots indicated and 140 knots groundspeed while flying into a headwind.  My guess now is that an indicated 75 knots was probably closer to 80 or 85.  Indicated airspeed upon my return seemed a bit low but not terribly so.  The pitot cover is one of those rubber tube types that is sealed.  No air could have leaked in via a hole for the grommet that holds the flag because that hole is outside the seal.

So how did this happen?  I must say it's caused me to question the utility of the "airspeed alive" portion of the takeoff checklist.  I've got an M20J, if that's relevant.

[Parker_Woodruff]

All your pitot system does is measure a difference in pressure. Think about what happens if your pitot tube freezes over as you climb or descend.

Strange on the takeoff roll, though.  I'm going to have to think about what could have caused that...

[John Pleisse]

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[N601RX]

Did it go back to zero as soon as the plane stopped?  If your cover completely sealed the hole you could get a reading due to pressure buildup from a temp change.

[RobertE]

Quote: N601RX

Did it go back to zero as soon as the plane stopped?  If your cover completely sealed the hole you could get a reading due to pressure buildup from a temp change.

[stevesm20b]

I did the same thing once. My airspeed seemed normal during the groundroll. Soon after becoming airborne was when I noticed the airsrspeed indicator was way off. My pitot cover has a very small hole in the tip, so a small amount of air would bleed through, just not enough to allow the indicator to work properly.

[knute]

[...]

[Piloto]

Sorry Knute, the forum switched my identity again

Your airspeed indicator is actually a differential pressure gage with one side connected to the pitot tube and the other side to the static port. Assuming the pitot cover is air tight the pressure stored in the pitot would be higher than the one on the static port (ambient pressure). As you climb the differential pressure (Pitot pressure - ambient pressure) would increase thus showing an increase in air speed. This behaviour is similar to what you see when the pitot is ice-up. The speed that your indicator showed is kind of high for the event. I would check the static ports balance (left & right). Make sure they are not blocked or disconnected.

José