Slow Flight Under the Current ACS Standard, the Stall Warning Confusion That Still Fails Checkride Candidates, and What Minimum Controllable Airspeed Actually Means Today

The FAA’s Airman Certification Standards (ACS), which replaced the Practical Test Standards around 2015, changed what correct slow flight looks like on a checkride. The new standard requires demonstrating minimum controllable airspeed without sustained stall warning activation - a meaningful departure from the continuous-horn approach many pilots were trained under. Candidates who learned to the old standard are failing checkrides because of this gap.

What Did the Old PTS Standard Require for Slow Flight?

Under the Practical Test Standards (PTS), slow flight was largely defined by slowing until the stall warning activated, then holding flight right at that edge. The horn blaring, buffet beginning - that was accepted as correct execution. Most instructors taught students to park it there: horn on, stall warning continuous, maintain altitude and heading.

This was the standard for years. A lot of training materials, ground school content, and some active instructors still reflect it.

What Does the Current ACS Require Instead?

The ACS slow flight task requires establishing and maintaining flight at a speed where any further reduction in airspeed, any increase in angle of attack, any increase in load factor, or any reduction in power would result in an immediate stall. That is the definition of minimum controllable airspeed.

Critically, the ACS specifies this is accomplished without a sustained stall warning. The stall warning activating briefly during a configuration change or while transitioning down to slow flight speed is acceptable. But once established, you should be just above the activation threshold - not sitting on it.

That word “sustained” carries significant weight in this task.

Why Did the FAA Change the Standard?

There are two clear reasons, and both hold up under scrutiny.

The first is desensitization. Flying for extended periods with a continuous stall warning trains pilots to tune it out. The stall warning - whether it’s a horn, a light, or buffet feedback - exists to signal something urgent. Ten minutes of continuous activation during a practice maneuver teaches the nervous system to treat that signal as background noise. That is a safety problem with real consequences.

The second is task separation. Slow flight and the stall series are two distinct tasks evaluating two distinct skill sets. Slow flight tests aircraft control at reduced airspeeds: maneuvering, altitude and heading maintenance, configuration management, coordination in a low-energy regime. The stall series tests recognition of stall onset and prompt recovery with minimum altitude loss. Conducting slow flight while sitting in the buffet blurs both tasks together. The ACS keeps them separate intentionally.

How Do I Set Up Slow Flight Correctly?

The setup sequence is straightforward. Choose a traffic-free practice area, clear with two 90-degree clearing turns, note your altitude, then begin slowing.

Reduce power while holding altitude with back pressure as airspeed bleeds off. Add flaps progressively - the examiner may specify a configuration (flaps up, approach flaps, or full flaps) or ask you to select one and justify it. Have an answer ready for that second scenario.

Your target airspeed is roughly 5 to 10 knots above the published stall speed for your configuration. In most light trainers with full flaps, that puts you around 60 to 65 knots indicated. The goal is to be slow enough that a small pitch-up or further power reduction would activate the warning - but not actually sitting on it.

What Should I Expect the Airplane to Feel Like?

Control response becomes sluggish. Pitch and roll inputs produce the desired effect, but later than expected. The airplane handles like it’s wading through resistance. This is normal, and part of what the examiner is assessing is whether you’ve spent enough time here that it doesn’t surprise you.

Power requirements increase substantially. In a Cessna 172 with full flaps at slow flight speed, you may be running significant power just to hold altitude. This surprises students who are used to reducing power to slow down. At the bottom of the speed envelope, that relationship reverses - you need power to maintain altitude, sometimes a lot of it. Add it proactively rather than chasing altitude after you’ve lost it.

Trim requires continuous adjustment. Every configuration change and every power setting change requires a trim update. If you haven’t trimmed adequately, the airplane will pitch down aggressively the moment you relax back pressure.

What Maneuvers Will the Examiner Ask For?

Once established, expect turns, climbs, descents, and configuration changes.

Turns are limited to 30 degrees of bank under the ACS. At 30 degrees of bank, stall speed increases by approximately 8%. At 45 degrees, the increase exceeds 40%. Stay at or below 30 degrees, and stay coordinated throughout. Adverse yaw at slow airspeeds is substantial with every aileron and power input. The ball will swing visibly if your feet aren’t active on the rudder pedals - and your examiner is watching it.

For climbs, add power carefully and watch airspeed closely - adding power tends to pitch the nose up, which bleeds more airspeed toward the activation threshold. Descents work in reverse. Both require anticipation.

Configuration changes during slow flight require simultaneous management of pitch, power, and trim. Adding flaps initially produces a pitch-down moment, then a pitch-up as lift increases. Practice these transitions until they are smooth and predictable.

What Causes the Most Common Slow Flight Failures on Checkrides?

Five problems account for most slow flight failures.

The stall warning horn. Candidates trained under the old PTS establish slow flight with continuous stall warning activation. The examiner notes the discrepancy or asks the candidate to describe the current standard and gets an answer that reflects outdated training. Know that the ACS requires minimum controllable airspeed without sustained stall warning, and be able to state it clearly.

Altitude loss. Students lose altitude and try to recover with pitch, which bleeds more airspeed and triggers the horn. The root cause is almost always delayed or insufficient power application. Add power early - before the altitude is already gone.

The ball. Sloppy rudder work that goes unnoticed in cruise is immediately visible in slow flight. If your feet aren’t continuously active, the ball drifts. Examiners look directly at the inclinometer throughout this maneuver.

Configuration transitions. Some candidates execute slow flight cleanly in a single configuration but struggle when the examiner calls for changes between flaps settings. Each transition changes lift distribution and pitch attitude. Practice them deliberately until each one is predictable.

The oral. The examiner asks what minimum controllable airspeed means, and the candidate gives an answer based on old PTS language or one that doesn’t reflect the ACS rationale. Prepare specifically for these questions: What does the ACS require for slow flight? Why is bank limited to 30 degrees? What does each flap configuration change do to the airplane? Why is slow flight in the certificate requirements at all?

Why Does Slow Flight Training Actually Matter?

Slow flight is not a maneuver pilots perform intentionally in normal operations. It’s in the certification requirements because pilots encounter that speed regime unintentionally - and those are the moments that become accidents.

Turning base to final and losing more airspeed than expected. Flaring for landing and bleeding more speed than the runway length was accounting for. Executing a go-around from short final while slow, still configured for landing, and climbing toward obstacles. These are real scenarios. Slow flight training builds the muscle memory, power management instincts, and control-feel familiarity that allows a pilot to respond correctly in those moments rather than freeze.

That is precisely why the ACS wants these skills clean and distinct from stall recognition training - because blurring the two undermines the practical value of both.

Where Can I Find the Current ACS Slow Flight Standard?

The FAA publishes the Airman Certification Standards for every certificate level free at faa.gov. Download your level, locate the slow flight task, and read the standards, knowledge, and risk management columns. If anything there doesn’t match what you’ve been taught, bring the document to your instructor.

Also read the slow flight chapter in the Airplane Flying Handbook (FAA-H-8083-3B), also free at faa.gov. Read it alongside the ACS task description - the two documents reinforce each other and together give the clearest picture of what the current standard expects.

Key Takeaways

• The ACS replaced the PTS around 2015 and changed the slow flight standard: minimum controllable airspeed is now demonstrated without sustained stall warning activation.
• Brief stall warning activation during transitions is acceptable; continuous activation once established is not.
• Bank is limited to 30 degrees in slow flight; stall speed increases approximately 8% at 30 degrees and over 40% at 45 degrees.
• The five most common checkride failures in slow flight are: sustained horn activation, altitude loss from inadequate power, uncoordinated rudder work, poor configuration transitions, and weak oral answers.
• The practical purpose of slow flight training is preparation for inadvertent low-speed situations - base-to-final turns, landing flares, and go-arounds - not intentional cruise at minimum speed.