ACS tolerances

The Private Pilot Airplane ACS (FAA-S-ACS-6C, effective May 31, 2024) anchors slow-flight tolerances around an airspeed band just above stall warning, altitude, heading, and bank limits. The configuration is "as specified by the manufacturer or as required for landing."

The tolerances at a glance

Parameter	Target	Tolerance	Reference
Airspeed	Just above stall warning	+10/−0 KIAS	VII.A.S3
Altitude	Entry altitude	±50 ft	VII.A.S3
Heading	Selected heading	±10°	VII.A.S3
Bank in turns	Up to 30°	±10°	VII.A.S3, S4

Plus structural requirements:

• Clear the area before beginning — VII.A.S1
• Establish slow flight in the recommended configuration — VII.A.S2
• Maintain coordinated flight throughout — VII.A.S3
• Recover to cruise flight — VII.A.S5

Airspeed +10 / −0 KIAS

Asymmetric band: you can be 10 knots fast (above the established slow-flight speed, no stall risk), but zero margin on the slow side (slower means stall warning continues or the airplane stalls).

The target airspeed is the airspeed just above the stall warning — typically 5–10 KIAS above V_SO. The exact value depends on the airplane and configuration; the standard isn't a fixed number but a relative position to stall warning.

Altitude ±50 ft

Tighter than the ±100 ft standard used in steep turns (V.A.S5) and ground reference maneuvers (V.B.S3). The closer tolerance reflects the precision expected in a regime where altitude is the primary index of pilot control.

In slow flight, altitude management is challenging because power is the primary altitude control and small power changes produce noticeable climb or sink rates. Anticipate and trim.

Heading ±10° (and bank up to 30° in turns)

VII.A.S3 anchors heading at ±10°. In coordinated wings-level slow flight, this should be straightforward — but with active rudder management for the high-AoA condition, even straight-and-level can drift if rudder is uncoordinated.

VII.A.S4 requires a coordinated turn at up to 30° bank without inducing stall. Steeper banks raise stall speed (V_S × √n; at 30° bank, n = 1.15, so stall speed rises by about 7%). The maneuver demands the pilot anticipate the increased stall margin loss in the turn and manage AoA accordingly.

Coordinated flight

The ball stays centered. At high AoA + full power, left-turning tendencies are exaggerated — significant right rudder is required just to hold heading. Uncoordinated slow flight is a setup for inadvertent stall-spin (cross-ref to IV.G slip-vs-skid and to VII.D spin awareness).

Configuration freedom in the standard. The ACS allows the manufacturer's recommended slow-flight configuration. Most trainers use full landing flaps, gear down (if applicable), and prop full forward. Specific power setting varies — the standard is the configuration that produces the target airspeed, not a specific RPM.