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How It Works: Stall horn

Warning a pilot to take action

Stall warning horns come in many sizes and flavors. Their operation can be as simple as low air pressure sucking on a plastic reed next to the cockpit that makes noise, or as sophisticated as triggering an electronic switch that sounds a warning. Arapahoe Aero at Centennial Airport near Denver had two types in the house on a recent visit: a Cirrus with a tab or “lift detector” that lifts to sound an electronic horn, and a Cessna 172 with just a hole in the wing. Both accomplish the same task; that is, making an annoying noise in the cockpit five to 10 knots before an aerodynamic stall occurs, warning the pilot to take action.

Preflight
Two systems seen most commonly are the hole in the wing and the lift detector system. The hole in the wing is a more basic device. When a low pressure area is created around the hole by a high angle of attack, either a plastic reed sounds in the cockpit as air passes over the reed, or the low pressure triggers a switch to operate a horn in the cockpit.

Stall warning horns come in many sizes and flavors. Their operation can be as simple as low air pressure sucking on a plastic reed next to the cockpit that makes noise, or as sophisticated as triggering an electronic switch that sounds a warning. Arapahoe Aero at Centennial Airport near Denver had two types in the house on a recent visit: a Cirrus with a tab or “lift detector” that lifts to sound an electronic horn, and a Cessna 172 with just a hole in the wing. Both accomplish the same task; that is, making an annoying noise in the cockpit five to 10 knots before an aerodynamic stall occurs, warning the pilot to take action.

When low pressure occurs around the hole and reed system, such as at a high angle of attack, it creates suction that can do two things, depending on the system. In a Cessna it vibrates a plastic reed in the cockpit behind a speaker baffle in the cabin. The speaker is at the upper right corner of the cockpit near the wing leading edge. There is no electricity involved.

The “lift detector” stall warning horn uses a tab that can be lifted during the preflight walk-around to sound the stall horn and requires an electrical system to operate. The lift detector does not react during normal airflow over the wing. When the angle of attack increases such as during an approach to a stall, disrupted airflow raises the tab and that sounds the electronic buzzer.

Alton Marsh

Alton K. Marsh

Freelance journalist
Alton K. Marsh is a former senior editor of AOPA Pilot and is now a freelance journalist specializing in aviation topics.

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