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I didn’t get flying last weekend, as the altimeter was out getting its required biannual calibration. I got it back during the week, and reinstalled it today. I was very happy to find no leaks in the static system after reinstalling the altimeter, as there are roughly two dozen different connections in the system, so it is a nightmare to track down any leaks. I also did a pitot system leak check while I was at it, and found that there was a small leak. It turned out to be a loose quick disconnect fitting where the pitot line from the “round dial” airspeed indicator connected to the main pitot manifold. The line was perhaps a bit short, so it was pulling on the connector. I couldn’t get it to snap into place in the fitting, so I replaced it with a longer new piece of pitot line, and the next leak check was perfect.

I’m using quick-disconnect fittings sold by SteinAir. There are quick to hookup, but I find them a bit sensitive to too short line. I’ve also found that if you disconnect and reconnect a line end too many times it stops making a good seal so you need to replace that section of line.

An ideal pitot system has no leaks, and there is air flowing in the system only when the airspeed changes. When the airspeed increases, the pressure at the entrance to the pitot tube increases, which compresses the air in the pitot system. Air flows in from the pitot tube until the whole system is at the same pressure, then the air flow stops. When the airspeed decreases, the pressure at the entrance to the pitot tube decreases, and the higher pressure air inside the pitot system expands and flows out the pitot tube until the whole system is at the same pressure once again.

Air flowing in a system requires a pressure difference to drive the flow - it flows from high pressure to low pressure. The amount of pressure difference required depends on the speed of the flow, the diameter and length of the piping (plus many other factors). The effect of a pitot leak depends on how large the leak is, and where it is in the system. Small leaks close to the pitot tube have negligible effect, as the length of the piping between the pitot tube and the leak is small. In fact, most pitot tubes have a small leak built in, as they have a water drain near the entrance to the pitot tube. There is only a short distance between the entrance to the pitot tube and the water drain, and the cross section is large, so only a very tiny pressure drop is created by the flow to replace the air lost through the water drain. Leaks near the instrument panel have a larger effect, as the air must flow through many feet of piping, which requires a larger pressure difference between the pitot tube and the leak to drive the flow.

I don’t have an easy way to calculate the effect on the airspeed accuracy of the leak I found. The indicated airspeed for a given power setting doesn’t seem to changed much since the last time I did a pitot system leak check, so I suspect the effect of the leak was minimal.

I did get flying today, which was much appreciated. I checked out all the major systems and avionics, to be sure the aircraft is ready for the cross country flying season.

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