ACS tolerances

The Private Pilot Airplane ACS (FAA-S-ACS-6C, effective May 31, 2024) anchors instrument straight-and-level tolerances around the three primary parameters — altitude, heading, and airspeed — plus the structural requirement of maintaining the maneuver solely by reference to instruments.

The tolerances at a glance

Parameter	Target	Tolerance	Reference
Altitude	Entry altitude	±100 ft	VIII.A.S2
Heading	Selected heading	±10°	VIII.A.S2
Airspeed	Cruise / entry airspeed	±10 KIAS	VIII.A.S2

Plus the structural requirement:

• Maintain straight-and-level flight solely by reference to instruments — VIII.A.S1

Altitude ±100 ft

Same standard as steep turns (V.A.S5), ground reference maneuvers, and the visual basic maneuvers. The notable change is how you hold it — without the visual horizon, the altimeter and AI together carry the load that visual cues normally provide. Trend detection (VSI starting to move) gives early warning before the altimeter moves measurably.

Heading ±10°

Same standard as visual maneuvers (V.A.S5, V.B.S3). Without a visual heading reference, the HI is the primary instrument, supported by AI bank and turn coordinator.

A common bust pattern: the pilot focuses on altitude and lets heading drift unnoticed. The scan must include all three primary parameters in cycle.

Airspeed ±10 KIAS

Anchored to the entry / cruise airspeed. In trimmed cruise, airspeed drift signals that something else has changed — typically pitch (which changes drag and airspeed) or power. The ASI is a diagnostic instrument as well as a primary one.

Solely by reference to instruments

VIII.A.S1 requires the maneuver to be flown without external visual references. The view-limiting device is the apparatus; the discipline is using only what the panel tells you.

The DPE evaluates by observing:

• Scan execution — eye movement across the panel (visible to a safety pilot from the right seat).
• Correction quality — small, deliberate inputs vs. large reactive ones.
• Trim management — does the pilot trim after corrections, or hold control inputs?

A pilot who frequently shifts the view-limiting device or steals glances outside has failed S1 even if the numerical tolerances hold.

The tolerances are the same as visual; the difficulty is different. With visual reference, the horizon makes attitude obvious — pitch drift is visible at the wings, bank is visible at the cowl. Without it, you have to interpret indications. The maneuver is straight-and-level; the skill is the interpretation.