Wacky Tachs

The majority of tachometers on the market are mechanical meters, which translate the engine RPM from a cable into a meter reading of how fast your engine in running — and they don’t always work right. The tachometer should monitor the speed of your engine, and keep time (in some cases) of the number of hours that the engine has run. Unless you happened to be lucky enough to have experienced a tachometer problem in flight training, the indication provided by a malfunctioning tach can be confusing. In some cases, it may cause you to do the wrong thing, at the worst time!

Example: You have just been cleared for takeoff at a large metropolitan airport. You use light power to taxi on to the runway, then release the brakes and smoothly advance the throttle to the firewall. Your engine RPM rise to redline — as you would expect — but then you hear a screech, and your tachometer suddenly reads 3000 RPM, several hundred RPM above the redline!

WHAT DO YOU DO? You have to immediately determine if this is a real overspeed situation or an instrument error…

  • LISTEN to your engineif the prop was really spinning at 3000 RPM, the tips would be crackling from breaking the sound barrier, and your engine would be screaming from the extra strain.
  • LISTEN to the instrument – If you can hear the instrument howling, chances are that it is an instrument problem, and not an engine problem.

TACH TROUBLESOverspeed Reading
CONSTANT SPEED OPS: If you see a higher than normal RPM and retard the throttle or prop control to get the RPM down to the correct range, you will have just inadvertently placed your engine in over-square operation. Your manifold pressure is now greater than your prop RPM. That’s not the best situation for low speed operations … or takeoffs!

CONSTANT AND FIXED PITCH PROPS: You have just reduced your engine power due to a false reading. Translation: If your normal redline is 2500 RPM, but your tachometer indicates 3000 and you decide to reduce power / prop speed, you will reduce your engine power by 25 to 40%! Depending on the length of runway, you may have just bought yourself a very bad day — you could have to abort the takeoff, or you may go off the end of the runway, trying to pull your plane into the air at slightly more than half-power! Neither of these options is particularly pleasant, which is why you need to be familiar with how tachometers fail.

IN CRUISE: Continue to fly on to the next airport, and use your judgment and the sound of the engine to guide your prop control and engine speed settings. If you think about it, you know what full power and normal cruise power sound like. You also know the speeds your plane will fly at, using different engine power settings. USE this knowledge to allow you to safely complete the flight.

IN TAKEOFF: You lose your tachometer. Consider a prompt return to the airport for repairs. While you know the sounds of your engine and the speed at which your plane normally flies, you are starting off in the wrong condition – at full power and speed, with no way to tell where you are setting the engine up for flight. … or whether or not the tach reading is the symptom of a much larger problem.

BOTTOM LINE: If you are ready and know what a tachometer failure or a drive wire or cable failure look and sound like, you won’t be lost when your tach fails. Armed with this, you will be able to make the right decision, and avoid a bad situation when a tach failure occurs in your life.