Zed, We Have a Bug!

I’ve never had an unexpected stall warning sound off, and honestly, that’s a scenario that gives me chills just thinking about it. Low and slow, the last thing any of us want to hear is the horn blaring when we weren’t expecting it. The time and altitude it takes to recognize an approaching stall and recover are likely longer than we’d like to admit. That’s why making sure the stall warning system is functioning properly is critical to ensure the earliest possible warning.

For those Cirrus aircraft with the lever-switch type lift detectors, the stall warning is simple to check during preflight or postflight. But the older aircraft that use the small hole in the right wing’s leading edge require a different approach. Testing those systems takes special tools and a bit more care because they’re vulnerable to very different environmental influences.

After a recent experience with a failed stall warning system, I couldn’t resist the Men in Black reference in this article’s title, “Zed, we have a bug!”.  It was the first thing that came to mind when we confirmed the cause of the failure. More importantly, it reminded me just how urgent it is to check these systems regularly, maybe even at every postflight inspection. I know that sounds excessive, but hear me out.

How It Works

The stall warning system operates on a simple principle: when the angle of attack increases beyond a certain point, the small hole in the leading edge moves from sensing positive pressure to sensing a vacuum. That change is transmitted through a ¼-inch diameter nylon tube to a pressure switch mounted in the cabin center console. Any leak or restriction in that line changes the pressure/vacuum the switch “sees” and, therefore, how the system responds.

But not all restrictions are created equal. A partial blockage might delay the pressure change reaching the switch, giving you a late warning or none at all before the stall. A complete blockage can trap pressure in the line, keeping the horn silent when it should sound or trapping a vacuum that keeps it on continuously.

One recent customer reported that their stall warning stayed on almost the entire flight, regardless of airspeed. That symptom pointed toward a restriction in the line, with the usual suspects being water, bugs, or a mud dauber nest. The fix is straightforward: blow out the line.

The Bug Hunt

To clear the line safely, we disconnect it from the pressure switch first as shop air can easily damage the switch if it’s still attached. We apply shop air to the line’s open end at the switch, and it will vent at the port on the wing’s leading edge. Out of curiosity, we sometimes try to catch whatever comes out. Unfortunately, whatever it was exited with enough force that it surprised the tech and left a dandy red mark in the palm of his hand. We never did find the remains but feel certain it was a bug.

Blockages like that can happen anytime, often with no warning to the pilot. You’d think the odds of a bug hitting the exact spot of the stall warning port would be slim, but it happens more often than you’d expect. Washing the airplane doesn’t help either; that little hole seems to attract water like a magnet. The problem is that this system doesn’t always give any obvious indication it’s failed unless you take a minute to test it.

Easy Field Test

If you want to “calibrate” your own check, you’ll need a simple tool: a baby nasal aspirator, the classic rubber bulb type. Very high tech.

1. Turn on the aircraft master switch so you can hear the stall warning.
2. Stand at the right wing and, if needed, open the copilot door for better sound.
3. Squeeze the bulb and hold it.
4. Insert the tip gently into the stall warning port, then release the bulb.

As the bulb reinflates, it creates a small vacuum, simulating the low-pressure condition that activates the stall warning. You should hear the horn sound.

 

A quick preflight check will uncover any recent activity from the local mud dauber maintenance crew. A postflight check can reveal if you’ve had an in-flight insect ingestion. Either way, it’s a small step that could save your hide someday.

Final Thoughts

The stall warning system isn’t glamorous or sexy like an AOA, and it rarely gets much attention. In a proper world, one should only hear a stall warning during training or ground testing. We could go fly and do some stalls to check the warning system, but a simple ground check is much safer and more cost effective. It may be over the top but doing this simple check a part of your preflight and/or post flight is a great way to ensure you don’t have a bug.

© 2025 Paul New