ACS tolerances

The Private Pilot Airplane ACS (FAA-S-ACS-6C, effective May 31, 2024) anchors instrument-descent tolerances around airspeed, heading, and level-off precision at the target altitude. The tolerances are identical to the constant-airspeed climb (B) — same numerical bands, same structural requirement.

The tolerances at a glance

Parameter	Target	Tolerance	Reference
Descent airspeed	Target (typically cruise or descent speed)	±10 KIAS	VIII.C.S2
Heading	Selected heading	±10°	VIII.C.S2
Level-off altitude	Target altitude	±100 ft (level-off precision)	VIII.C.S2

Plus the structural requirement:

• Establish a constant-airspeed descent solely by reference to instruments — VIII.C.S1

Airspeed ±10 KIAS

Same standard as the climb (B). The descent is constant-airspeed; the pilot holds the target speed throughout. ASI is primary for pitch.

The target airspeed varies by airplane and POH. Some airplanes use cruise speed for descent; others use a published "descent speed" that's intentionally faster than cruise to increase descent rate without overspeed risk. Use the briefed value.

Heading ±10°

Same standard as VIII.A and VIII.B. HI is primary; small bank corrections handle drift.

In descent, left-turning tendencies are weaker than in the climb (lower AoA, lower power). Less right rudder is required. Pilots transitioning from VIII.B to VIII.C sometimes over-rudder out of habit; the result is right-heading drift.

Level-off altitude ±100 ft

Identical standard to the climb (B). The level-off occurs within ±100 ft of the target altitude.

The 10% anticipation rule applies here as it does in the climb — lead by 10% of the descent rate. The numerical anticipation is the same; only the direction changes (lead below the target in a climb; lead above the target in a descent).

Solely by reference to instruments

VIII.C.S1 requires the maneuver without external visual references. Same standard as VIII.A.S1 and VIII.B.S1.

What's the same as B; what's different

Same as B:

• ASI primary for pitch.
• 10% anticipation rule for level-off.
• Pitch-and-power-together transition.
• Heading and airspeed tolerances.

Different from B:

• Power reduced rather than increased to establish the regime.
• Trim direction (nose down for descent rather than up for climb).
• Left-turning tendencies less aggressive.
• Pitch attitude visibly nose-down rather than nose-up on the AI.
• VSI shows negative rather than positive.

The same task, different power setting and direction. A pilot who's clean on the climb (B) finds the descent (C) intuitive. Train B first; C is the mirror application of the same discipline.