Recording tach time

I have been reading some posts on Hobbs vs Tach. The reality is that, if you are looking to ever sell your aircraft or want a true time, a Recording Tach is the way to go. Hobbs meters give a relatively false time about the Engine/Airframe. If I am doing trouble shooting on an aircraft that has to be run-up for a 1/2 hour then repaired, then run-up for another 1/2 hour thats an hours time. Same goes for oil changes, a run-up before and after is to be done. (if the maintenance facilty is doing it right). A lot of time can be put on a hobbs meter just for maintenance work. If I were an aircraft owner, I would have a recording tach installed. If anyone is interested in a recording tach let me know, I can install a tach with a field approval, since there is not an STC for the cirrus yet. With reqards to the airswitch, If you have the money to spend on the labor to install an airswitch and dont mind your wings being taken off of your plane to run the wiring through the plane then this will be right for you. Have a good one guys!!

Jim: Which tach are you using?

The airswitch just goes on the pitot line behind the panel - no wing removal needed!!

The EI tach is probably the best on the market for the price and looks.

You still have to run the wiring inside the wing.


I would be much interested if anyone has installed the EI in a Cirrus? Any first hand experience? Keeps the rest of us away from the bleeding edge!

No, you do NOT have to run any wiring inside the wing. The airswitch is a small pressure switch that attaches to the pitot line - which is ALREADY installed from the pitot tube to the altimeter - the pressure switch taps into this pressure line behind the instrument panel. Then there is a short bit of wire from the switch to the hourmeter, which in our plane is installed just below the existing Hobbs meter.

The CORRECT way of installing the airspeed switch would be to install the switch out on to the pitot tube itself inside the wing for proper actuation of the pressure switch. Tapping off the pitot line behind the instrument panel is not the correct way of installing the switch. If the individaul who installed the switch just “T” off the line you are also creating a vacuum across that line at lower airspeeds and indication from the switch probably does not occur when it is supposed to.


Absolute bollocks. "creating a vacuum across that line at lower airspeeds "??? Meaningless techno-babble.

There is, essentially, no air movement inside the pitot line - it is a closed system except for the opening at the end of the pitot tube itself. The pressure at all points inside the pitot line is exactly the same, no matter how many tees or junctions there are, providing there are no leaks.

Airspeed indicator errors at low airspeeds are due to changes in the angle of attack, and consequent mis-alignment of the pitot tube with the relative airflow. This is of no consequence to an airswitch, which is simply adjusted to switch at a speed somewhat below the dirty stall speed.

I did make one error in my previous post, where I said the altimeter was connected to the pitot line. I meant of course to say airspeed indicator.


I agree with you 100%, as does my mechanic. A T-junction to a pressure switch will have absolutely no effect on the airspeed indication. Pressure is pressure - it is still a closed system downstream of the pitot tube.

Locating the wires to the Hobbs may be an issue, as might altering the factory-built setup in that regard at all. As I mentioned, an old post indicated that the airswitch modification had been done to an SR20: N5841. The>Windy at Palwaukee airport show this airplane on their aircraft list. I have sent an email to them inquiring about the modification. I’ll post back here when/if I get a response. Stay tuned…