Slow Flight, the Two Thousand Seventeen ACS Revision, and Why the Stall Warning Sounding During This Maneuver Is Now a Checkride Bust

The 2017 Airman Certification Standards (ACS) changed the slow flight standard in a way that still catches students off guard: under the ACS, the stall warning horn must not sound during your slow flight demonstration. Instructors trained under the old Practical Test Standards (PTS) often taught the opposite - that horn-on meant you were doing it right. That mismatch between training and the current standard is causing checkride failures that have nothing to do with a student’s actual flying ability.

What Changed Between the PTS and the ACS?

Under the old Practical Test Standards, slow flight was defined as flight at Minimum Controllable Airspeed (MCA). In practice, that meant flying right to the edge - stall warning sounding, maximum control deflection, controls mushy. If the horn wasn’t on, many instructors told students to slow down more. Horn on meant you were on target.

The ACS, which the FAA rolled out between 2016 and 2017, replaced that definition entirely. Under the ACS, slow flight is now defined as flight at an airspeed where any further reduction in speed, or any increase in load factor, would cause an immediate stall - without activating the stall warning.

Those three words - without activating warning - are in the ACS task description for a reason. This is not a footnote change. It is a different standard.

Why Did the FAA Change the Slow Flight Standard?

The FAA’s reasoning comes down to stall and spin accident prevention. Regularly training pilots to fly with the stall warning sounding as a normal part of flight may condition them to ignore it. In the training environment, the horn becomes background noise that signals success. In the real world, that same horn might mean you are at 500 feet over a neighborhood with your airspeed decaying toward a departure stall. The horn should trigger an immediate corrective response - not a mental note that says good, I found MCA.

The ACS moved slow flight away from the warning zone precisely to preserve the stall warning’s urgency as a cue.

How Do I Set Up Slow Flight to the ACS Standard?

The target is the airspeed band just above where the stall warning activates. This is still genuinely slow flight - throttle back, flaps extended, controls mushy, airplane working hard. You are simply not in the warning zone.

In a Cessna 172, depending on configuration and weight, that band is approximately 54 to 58 knots. The exact number will vary by aircraft. You need to know your specific airplane.

Setup steps:

1. Reduce power and let airspeed decay toward your target band
2. Add flaps incrementally as you slow (many instructors recommend flaps 10° first, then 20°)
3. Add just enough power to maintain altitude once established
4. Establish wings-level, coordinated flight in that band

The most common mistake in the setup is over-slowing. Students trained to find MCA push right past the ACS target because their muscle memory says fly until the horn comes on. They end up deep in stall warning territory thinking they nailed it.

What Are the ACS Tolerances for Slow Flight?

The ACS tolerances are specific:

• Airspeed: ±10 knots
• Altitude: ±100 feet
• Heading (straight and level): ±10 degrees
• Bank (during turns): ±10 degrees

The ±100-foot altitude tolerance sounds reasonable until you experience how much power input is required to hold altitude at these speeds. Large throttle corrections for small altitude deviations. Corrections have lag. Students fall into an oscillating cycle that slowly walks them outside the tolerances.

The fix is anticipation. If your vertical speed indicator is showing more than 50 feet per minute in the wrong direction, correct now. Don’t wait until you’ve lost 80 feet and then try to salvage it.

How Do I Handle Turns in Slow Flight?

A 30-degree banked turn increases load factor to approximately 1.15 Gs. That raises the stall speed. In slow flight - already operating near the stall - that increase can push you into stall warning territory if you’re not prepared.

The technique: add a small amount of power before entering the turn to compensate for the increased load factor. Keep banks shallow. The ACS permits up to 30 degrees of bank, but 15 to 20 degrees is more manageable, especially early in training. Smooth entry, smooth rollout, coordinated throughout.

Watch airspeed during the turn. If it starts decaying, add power or reduce bank immediately. Under the ACS, once the stall warning sounds during your slow flight task, you have lost the standard.

How Do I Handle Configuration Changes During Slow Flight?

Some examiners will ask for flap changes while you are established in slow flight. Retracting flaps is the more demanding change - when you remove flaps, you lose lift and the airplane wants to settle.

The technique is to add power simultaneously as you retract flaps to replace the lost lift. Retract incrementally, not all at once. Be ready to make a small pitch adjustment to hold airspeed. The ACS evaluates whether you can execute these changes while remaining coordinated, within altitude tolerances, and without allowing airspeed to decay into stall warning territory.

What Should I Know About Coordination in Slow Flight?

Adverse yaw is pronounced at slow airspeeds. Adding aileron to enter a turn produces a strong yawing tendency. Students who haven’t developed rudder feel tend to fly slow flight in a constant state of slight skid or slip. The ball wanders.

The examiner watches coordination throughout the entire slow flight task. A centered ball, coordinated turns, and smooth rudder inputs are not bonus points - they are the baseline. Slow flight is a fundamentals check. If coordination is sloppy at cruise, slow flight makes it obvious.

One useful habit: if altitude is bleeding and you’re about to add power, pause for one second first and check the ball. A slightly uncoordinated airplane is an inefficient airplane. Cleaning up the coordination sometimes recovers a small amount of altitude and airspeed before you even touch the throttle.

How Do I Know If I Was Trained to the Wrong Standard?

If your instructor has been teaching you to intentionally activate the stall warning during slow flight, have a direct conversation before your practical test. Ask specifically: Are we flying to the ACS standard? Are we targeting airspeed above stall warning activation?

If there is any uncertainty, download the Airman Certification Standards directly from faa.gov - it is a free PDF. Pull up the slow flight task and read it together. The language is unambiguous.

Some Designated Pilot Examiners (DPEs) will test this in the oral portion. They may ask what airspeed you are targeting and why. If you answer minimum controllable airspeed without clarifying that this means above stall warning activation under the ACS, expect a follow-up. Know the distinction before you sit down for the oral.

Four Practice Habits That Actually Help

1. Know your exact stall warning activation airspeed. Not approximately. Get up to a safe altitude with your instructor, configure for slow flight, and slowly reduce airspeed until the horn first sounds. Note that indicated airspeed. Your ACS slow flight target is above that number.

2. Practice the entry and the recovery equally. The ACS evaluates both directions. Entry: power reduction, incremental flaps, power addition to hold altitude. Recovery: power up, flaps up incrementally, trim, climb. Neither direction should be an afterthought.

3. Connect slow flight to the traffic pattern. What you are doing at altitude is a controlled version of what you are doing on final approach at 300 feet above the ground - low airspeed, configuration changes, power adjustments, small margins for error. When that connection clicks, slow flight stops feeling like an abstract checkride maneuver and starts feeling like practice for something real.

4. Watch the VSI constantly. Don’t wait for altitude loss to correct. Catch the trend early and make small corrections. Anticipation is the skill that separates pilots who hold altitude in slow flight from pilots who oscillate through the tolerances.

Key Takeaways

• Under the 2017 ACS, the stall warning must not sound during a slow flight demonstration - this is the opposite of what the old PTS required
• Target the airspeed band just above where your aircraft’s stall warning activates; in a Cessna 172 that’s approximately 54–58 knots depending on configuration
• ACS tolerances are ±10 knots airspeed, ±100 feet altitude, ±10 degrees heading, and ±10 degrees bank in turns
• Anticipate, don’t react - slow flight requires small, early corrections, not large salvage inputs
• If you’re unsure which standard you’ve been trained to, download the ACS from faa.gov and read the slow flight task before your checkride