Slow Flight, the Two Thousand Seventeen ACS Revision, and the Stall Warning Horn That Should Not Be Ringing

The 2017 Airman Certification Standards (ACS) revision changed the definition of slow flight for the private pilot checkride, and many students - and instructors - are still training to the old standard. Under the current ACS, slow flight must be performed just below the stall warning threshold, not at it. If the horn is ringing, you are flying the pre-2017 version of the maneuver.

What Did the 2017 ACS Change About Slow Flight?

For decades, minimum controllable airspeed was taught by slowing the aircraft until the stall warning activated, then holding it there. The horn became the target - an audio cue that told pilots they were in the right place. That approach produced pilots who normalized an alarm designed to demand their attention.

The 2017 ACS redefined slow flight as flight performed at an airspeed at which any further increase in angle of attack, any increase in load factor, or any reduction in power would result in a stall buffet or unmistakable stall warning. The operative word is any further - meaning you must be positioned just below that threshold, not inside it.

This distinction is not subtle. A stall warning horn active during slow flight means the maneuver is being flown to an outdated standard.

Why Did the FAA Make This Change?

The concern is conditioning. Training repeatedly with the stall warning running teaches the nervous system that the horn is background noise - expected, normal, not urgent. That is precisely the opposite of what the warning system is designed to do.

The FAA’s position with the updated standard: learn to fly with precision just below the warning threshold. Understand where the edge is. Feel how the airplane changes at low airspeed. But do not desensitize yourself to the alert that exists to get your immediate attention.

The new standard is also more demanding to execute. When the horn is ringing, it tells you where you are. Flying just below the threshold - without triggering the warning - requires more precise pitch management, more precise speed control, and sharper attention to control feel and aircraft sound.

How Do You Find the Right Airspeed for Slow Flight?

The specific target airspeed matters less than knowing your aircraft. Here is a reliable method to find your reference point:

1. Climb to a safe altitude in clean configuration.
2. Slow the aircraft gradually and note the exact airspeed when the stall warning first activates.
3. Set your slow flight target 5 to 8 knots above that activation point - enough margin to maneuver without crossing into the warning zone.
4. Repeat the process in each flap configuration you may be asked to demonstrate. Stall warning activation airspeed changes with flap setting.

In a Cessna 172, expect slow flight to fall somewhere in the 45 to 50 knot range depending on configuration, but verify this with your specific aircraft.

What Are the ACS Tolerances for Slow Flight?

The private pilot ACS sets the following performance standards for slow flight:

• Airspeed: within ±10 knots of the target
• Heading: within ±10 degrees in straight flight
• Altitude: within ±100 feet in level flight

These numbers define the outer limits of acceptability. An examiner is also watching for smooth, purposeful control inputs; coordinated flight with the ball centered; and whether corrections happen before tolerances are exceeded - not after. And yes, they are listening for the stall warning horn.

How Do Clearing Turns Factor Into Slow Flight on a Checkride?

Before any slow flight demonstration, FAA guidance requires two 90-degree clearing turns covering at least 180 degrees of arc. The purpose is to visually confirm the airspace around the aircraft is clear of traffic before slowing down and focusing inside the cockpit.

Students rush this step consistently. A quick shallow S-turn that barely scans the forward hemisphere is not a clearing turn - and examiners know the difference. Scan above, below, and to both sides. This procedure exists because pilots maneuvering at altitude have collided with aircraft they never saw. It is not a formality.

How Do You Manage Turns in Slow Flight?

Turns are where altitude loss most commonly occurs during slow flight practice. Banking increases load factor, and increased load factor raises stall speed. A 30-degree bank - the maximum the ACS requires - at a slow flight airspeed carries real consequences if power and back pressure are not managed together.

As you roll into the bank, increase back pressure and add power as needed to maintain altitude. Both inputs work together; letting either one slip will produce altitude loss or an airspeed bleed toward the warning zone. A common error is relaxing back pressure during rollout - fixating on the roll while forgetting to hold the pitch, which allows the airspeed to drop at exactly the wrong moment.

What Else Should You Know Before the Checkride?

Carburetor heat: On carbureted engines, apply carb heat before reducing power for slow flight. Low-power operation over extended periods, especially in moist air, creates conditions where carburetor ice can form. Your Pilot’s Operating Handbook specifies when to use it - follow it.

Exiting slow flight: Apply full power first, then retract flaps in stages as airspeed builds. Know the maximum flap extension speed for your aircraft - 85 knots in many Cessna 172 variants. Never allow airspeed to exceed that limit with flaps extended. The sequence is airspeed first, then flaps.

Why Does Slow Flight Matter Beyond the Checkride?

Every landing approach is flown in or near slow flight territory. Final approach airspeeds are deliberately close to stall speed, with margin for safety and gusts. The flare takes the aircraft slower still.

Pilots who have genuinely internalized what their aircraft feels like at the low end of the speed range - not just tolerated a horn, but actually felt how control forces and responsiveness change - develop calibrated instinct. They can sense two knots of difference through the controls. They hear it in wind noise. They feel the aircraft going soft before any warning activates.

That instinct is what responds when something goes wrong on final: an unexpected gust, a steeper bank than intended, a passenger shifting weight. The 2017 ACS revision is aimed at producing that kind of pilot, not just a different airspeed on a data sheet.

What Should You Do Right Now?

If you are in primary training: Ask your CFI directly which standard you are being trained to. Reference the 2017 Airman Certification Standards. If there is any uncertainty, look it up together. The private pilot ACS is a free download from FAA.gov. Slow flight is covered in Area of Operation 7.

If you are in checkride prep: Take a solo flight to evaluate your slow flight honestly. Set up the maneuver and listen. If the horn is running, you are flying the old standard. Adjust pitch slightly, add enough power to hold altitude, and find the zone just below the warning. That is where the current standard places the maneuver.

If you are certificated and due for a flight review: The ACS governs initial certification, but the skills behind it do not expire as best practice. Your CFI or designated examiner may hold you to the current standard regardless of when you earned your certificate.

Key Takeaways

• The 2017 ACS redefined slow flight as flight just below the stall warning threshold - not at it. An active stall warning horn during slow flight does not meet the current standard.
• The change was deliberate: conditioning pilots to normalize the stall warning reduces its effectiveness as a critical alert.
• Determine your aircraft’s stall warning activation airspeed, then target 5 to 8 knots above that point for slow flight practice.
• Turns in slow flight increase load factor and raise stall speed - manage back pressure and power together through every bank.
• The private pilot ACS is a free FAA download. Area of Operation 7 covers slow flight. Read the actual standard before your checkride.