Slow Flight and the ACS Rewrite - the Stall Warning That Has to Stay On

The 2016 Airman Certification Standards (ACS) fundamentally changed how slow flight is graded: the stall warning must activate and remain active for the entire maneuver. Many pilots - and instructors - never got the full picture of this change, and some have failed checkrides not for poor altitude control or sloppy coordination, but because the horn was silent.

What Changed When the FAA Replaced the PTS With the ACS?

Under the old Practical Test Standards (PTS), slow flight was demonstrated at an airspeed where “any further increase in angle of attack, increase in load factor, or reduction in power would result in an immediate stall.” That language sounded precise but left significant room for interpretation. Pilots could fly several knots clear of the stall warning and still satisfy the spirit of the standard.

The ACS replaced that subjectivity with a single objective requirement: the stall warning device must activate and remain active throughout the entire maneuver. In a Cessna 172, that means the horn is on - not approaching, not intermittent. On. If it’s silent, the airspeed is too high, and a thorough DPE is listening for it.

The change serves a deliberate purpose. You cannot argue with a horn. The old language allowed pilots to technically comply while missing the entire point of the exercise.

Why Does Angle of Attack Matter More Than Airspeed?

An aircraft doesn’t stall because of a specific airspeed - it stalls when the angle of attack exceeds the critical angle, the point where airflow over the wing separates and lift collapses. As you slow down, you must continuously increase angle of attack to maintain lift.

Stall warning systems are calibrated to a specific angle of attack, not a specific airspeed, and are designed to activate roughly 5 to 10 knots before the 1-G stall speed. Slow flight places you precisely at that angle - sustained. You are operating near the limit of the wing’s capability, controlled and deliberate.

This is why “slow flight” can’t be done effectively at any airspeed “in the ballpark.” The horn marks the specific angle of attack the FAA requires you to work at.

What Is the Region of Reversed Command?

In normal cruise flight, pulling back pitches the aircraft up and produces a climb. Adding power increases speed. Those responses are reliable and consistent.

In slow flight - at high angle of attack near the stall warning - those instincts break down. Pulling back increases drag faster than it increases lift, causing the aircraft to decelerate. Without a simultaneous power addition, the aircraft moves toward a stall rather than a climb. In the region of reversed command, speed is managed primarily with power, not pitch.

This isn’t abstract aerodynamics. On short final with a high, slow approach, the instinct to pull back to arrest a sink rate can accelerate the problem instead of solving it. Slow flight exists specifically to teach pilots how to recognize and manage this regime before they encounter it at fifty feet on final - where there is no recovery.

How Do You Establish Slow Flight to ACS Standards?

Clear the area first. Two 90-degree clearing turns in opposite directions, or a single 180. The high nose attitude in slow flight significantly reduces forward visibility, so confirm the area is clear before the airspeed starts dropping.

Reduce power smoothly and increase back pressure to hold altitude as airspeed bleeds off. The nose will rise noticeably - in a Cessna 172, expect a pitch attitude several degrees above the cruise reference. When the stall warning activates, that’s your target. Hold it there.

Add power as needed to maintain altitude. At this angle of attack, induced drag is substantial, and some aircraft require close to full power to sustain level flight in the slow flight regime.

What Are the ACS Tolerances for Slow Flight?

• Altitude: ±100 feet
• Heading: ±10 degrees
• Bank angle (in turns): ±10 degrees
• Airspeed: stall warning remains active - there is no numerical tolerance

That last point is the critical one. Unlike other maneuvers with explicit knot-based airspeed bands, slow flight is graded by the presence of the stall warning, not a specific number on the airspeed indicator. If the horn goes quiet, you’re too fast, regardless of what the gauge reads.

What Mistakes Do Students Make in Slow Flight?

Going too fast is the most common failure. The stall warning activates, the student gets uncomfortable, and they unconsciously add a small buffer. The horn goes quiet. They believe they’re doing slow flight - they’re not. The fix is straightforward in theory and harder in practice: trust the horn. If it goes silent, reduce power slightly and increase back pressure until it reactivates.

Poor rudder coordination is the second failure point. At high power settings and low airspeed, left-turning tendencies - P-factor, torque, slipstream, and gyroscopic precession - are amplified. Without active right rudder input, the ball rolls right and the aircraft develops a left slip.

Crossed controls at high angle of attack near the stall warning are a precursor to an incipient spin. Examiners are watching your feet as closely as your altitude. The examiner is not just watching the VSI.

Should You Practice Slow Flight With and Without Flaps?

The ACS requires demonstration in both configurations, and they fly differently enough that each needs deliberate practice.

Clean configuration (no flaps) is harder. Stall speed is higher, the nose attitude is more extreme, and the aircraft is less pitch-stable. Small inputs produce larger pitch changes. Many students underestimate this configuration because they’ve only practiced the flaps-extended version and assume the two feel similar.

Flaps extended activates the stall warning at a lower airspeed and requires a less extreme nose attitude. The maneuver feels more stable. Add flaps in stages - one increment at a time - and let the aircraft stabilize before extending further. Going to full flaps all at once shifts trim significantly and makes altitude control harder than it needs to be.

On carbureted engines - Cessna 172s, most Pipers - monitor for carburetor icing. You’re running more power in slow flight than in a power-off descent, which reduces risk, but know your aircraft and your conditions.

What Is the Correct Recovery Sequence From Slow Flight?

The sequence matters, and the order is specific:

1. Pitch forward first - reduce angle of attack before anything else
2. Add full power - once angle of attack is reduced
3. Retract flaps in stages - as airspeed builds, not all at once

The most common mistake is reversing steps one and two: pulling power back while still at slow flight airspeed. Removing thrust while the aircraft is at high angle of attack near the stall warning removes what was sustaining lift. Any pitch confusion in that moment can produce a secondary stall - and secondary stalls happen on checkrides more often than examiners like to acknowledge.

Commit it to muscle memory before the checkride: attitude first, then power, then flaps in stages.

Key Takeaways

• The 2016 ACS requires the stall warning to activate and remain active for the entire slow flight maneuver - a silent horn means you’re too fast, not that you’re doing it well
• Slow flight teaches the region of reversed command, where speed is managed with power rather than pitch - the same aerodynamic regime encountered on every short final
• Coordination is critical: amplified left-turning tendencies at high power and low airspeed demand active right rudder; crossed controls near the stall warning are a spin precursor
• Practice both clean and flaps-extended configurations - they feel meaningfully different, and both appear on the checkride
• Recovery sequence is non-negotiable: pitch forward first, then full power, then flaps in stages - reversing the order risks a secondary stall during recovery