Slow Flight and the ACS Update That Changed the Maneuver Nobody Realized Had Changed

Slow flight changed when the FAA Airman Certification Standards (ACS) replaced the old Practical Test Standards (PTS) - and many pilots and instructors never got the memo. Under the ACS, slow flight must be performed without activating the stall warning horn. If the horn sounds, you’ve gone too far.

What Changed With the ACS and Why It Matters

Under the old PTS, slow flight meant flying at minimum controllable airspeed with the stall warning horn continuously sounding. Instructors taught it that way. Students passed checkrides that way. That definition stuck for years.

The ACS defines slow flight differently: establish and maintain an airspeed at which any further increase in angle of attack, any increase in load factor, or any reduction in power would result in an immediate stall - without activating the stall warning.

That last clause is the change. Flying with the horn on is now a failure point on the checkride, not the target.

Why the FAA Made This Change

The old approach trained pilots to react to a sound rather than understand their margin from the stall. The stall warning horn isn’t always reliable - in banked turns stall speed rises, with ice accumulation the horn may activate late or not at all, and at high density altitude the airplane can feel sluggish well before any warning sounds.

The ACS version requires pilots to fly just at the edge of the stall envelope without crossing it. That demands a deeper understanding of angle of attack and stall margin - not just listening for a horn.

How to Set Up the Maneuver Correctly

Clear the area first. Two 90-degree clearing turns or one 180 - done thoroughly and deliberately. The examiner is watching how you scan for traffic while setting up the maneuver.

Configure with flaps at approach setting for most training aircraft; some examiners request full flaps as well. Check with your designated pilot examiner (DPE) in advance. Extend flaps in increments, slowing to within the white arc before each notch.

If flying a carbureted engine, apply carb heat during the slowdown. It’s a small step that signals situational awareness to the examiner.

What Airspeed Are You Actually Targeting?

Slow flight is not a fixed number - it shifts with weight, bank angle, configuration, and density altitude. In many common training aircraft, minimum controllable airspeed without triggering the stall warning runs roughly 5 to 8 knots above the point where the horn would activate in the clean configuration. With full flaps, that number changes.

The ACS tolerance is plus 10, minus 0 knots from your target airspeed. You can be 10 knots fast. You cannot be slow - slow is where the horn lives.

Know your airplane’s numbers before the checkride. Practice until you can find that airspeed consistently without hunting for it.

The Power Problem Most Students Don’t Anticipate

Pulling power to slow down, getting the flaps in, and then watching the altimeter drop - this is the most common early stumble. Flying on the back side of the power curve with flaps extended requires more power than most students expect to hold altitude.

In a Cessna 172 or Piper Archer, practice the configuration before the checkride and know the approximate power setting that holds altitude. Showing up without that number is avoidable.

Use trim. Holding constant back pressure through the entire maneuver is not airmanship - it’s compensating for a poorly configured airplane. Get the trim set so the airplane wants to fly there, then manage with small inputs.

How to Handle Turns in Slow Flight

When the examiner asks for turns, bank angle becomes a stall margin problem. Increasing bank increases load factor, which raises stall speed, which shrinks your margin to the warning horn.

Keep bank angles shallow - 20 degrees is a solid target. The ACS tolerates plus or minus 10 degrees of bank. As you enter the turn, the nose will tend to drop; manage back pressure carefully without over-rotating.

Rudder coordination is critical. Adverse yaw is pronounced at low airspeed. An uncoordinated entry swings the nose the wrong direction and can push you into the stall warning - or worse. Ball centered, always.

Altitude tolerance through the maneuver is plus or minus 100 feet.

How to Exit Slow Flight Without Losing Altitude

The exit sequence matters as much as the maneuver itself: power first, then flaps incrementally.

Add power to stop any sink rate and begin accelerating. Then retract flaps one notch at a time, letting the airspeed respond before pulling the next notch. Yanking flaps to full-up at slow flight airspeed removes lift faster than the airplane can accelerate, and the altitude you’ve held carefully through the whole maneuver can disappear in the final seconds.

What the Oral Exam Will Cover

Expect the examiner to ask about the relationship between angle of attack and stall, why the ACS version of slow flight excludes the stall warning, and how load factor affects stall speed. These aren’t trick questions - they’re checks that the pilot understands the maneuver, not just the procedure.

Download the ACS from faa.gov - it’s free - and read the slow flight task line by line. Have an instructor ask you every question in the knowledge column out loud before the checkride.

Why Slow Flight Matters Beyond the Checkride

Every approach and landing is slow flight. Short final in the landing configuration is slow flight. Stretching a glide in the pattern when energy management goes wrong requires knowing the stall margin. Carrying minimum approach airspeed over the fence on a short-field landing requires knowing that airplane at low speed, intimately.

The pilot who truly understands slow flight isn’t staring at the airspeed indicator - they’re flying by feel and by sight, glancing at instruments to confirm. Controls behave differently near minimum controllable airspeed: heavier, more responsive to pitch inputs. The airplane communicates differently down there. The ACS is trying to develop pilots who hear that communication, not pilots who wait for a horn.

Key Takeaways

• The ACS requires slow flight without the stall warning horn. If the horn activates, the maneuver is failed.
• Slow flight is not a fixed airspeed - it changes with weight, configuration, bank angle, and density altitude.
• Airspeed tolerance: plus 10, minus 0 knots. There is no margin below the target.
• In turns, load factor increases stall speed - keep banks shallow (20 degrees) and rudder coordinated.
• Exit the maneuver power first, then flaps incrementally. Treat the exit with the same discipline as the maneuver itself.