13.3 Cruise, Descent, and Landing Performance

ACS PA.I.F.K1 does not end at brake release. Cruise, descent, and landing charts are the same skill: enter the real atmosphere and the real weight, read TAS, fuel flow, and landing distance, then refuse to substitute the airspeed indicator for a planning instrument.

TAS, the 2 percent rule, and the cruise chart

True airspeed is how fast you move through the air mass. Indicated airspeed is what the ASI shows. At altitude the air is thinner, so the same dynamic pressure (same IAS) means a higher true speed. PHAK Chapter 8’s planning approximation is explicit: add about 2 percent to calibrated airspeed for each 1,000 feet of altitude. At 6,000 feet, 110 KCAS is about 12 percent faster, or roughly 123 KTAS, on a standard day. That rule is for a quick estimate. Temperature off ISA moves TAS further. The number you put on a nav log comes from the AFM/POH cruise table or a flight computer using pressure altitude and OAT.

The table below is a teaching cruise table, not a real AFM. Same airplane, same 2,400 rpm, standard temperature, no wind.

TAS rises. Fuel flow at a fixed rpm usually falls a little because the engine is swallowing less air (and you have leaned). Specific range — nautical miles per gallon — improves. That is why a normally aspirated airplane often goes farther up high, until the climb cost or the wind says otherwise.

Range is not endurance

PHAK separates the two with care.

• Endurance is time. Maximum endurance is the condition of minimum fuel flow, which sits at minimum power required — the bottom of the power-required curve. You stay aloft longest, not farthest.
• Range is distance. Maximum range is the condition of maximum speed per fuel flow, which PHAK places at maximum L/D. You cover the most ground per gallon.

Holding a fast cruise because “higher TAS means more range” is backwards if that TAS came from a high-power setting well above L/D max. You are converting fuel into speed you do not need. Holding a very slow speed to “save gas” can put you on the back side of the power curve, where more power is required to go slower in level flight (region of reversed command). The AFM cruise table, not a vibe, picks the rpm and mixture for the mission.

Trap: using IAS as TAS for fuel. The ASI is how you fly the airplane. The nav log uses TAS, then groundspeed after wind. If you plan a 180 NM leg with 110 KIAS as if it were TAS when the chart says 123 KTAS, every subsequent number is wrong: time, fuel, and the remaining-fuel versus remaining-distance check. Mix that error with a headwind you also treated casually and the arrival fuel is a surprise. The unsafe picture is inconsistency — IAS on the log, TAS in your head, GPS distance on the tablet — not a single conservative rounding. 14 CFR 91.151 still wants day VFR reserve of 30 minutes (night 45) at normal cruise. A log built on IAS is not how you prove that reserve.

Worked TAS / fuel example. Teaching table at 6,000 feet: 117 KTAS, 7.5 gph. No-wind groundspeed is 117 kt. A 180 NM still-air leg is 180 / 117 ≈ 1.54 hours and 1.54 × 7.5 ≈ 11.6 gallons, plus the 91.151 reserve at that same cruise. If you had used 110 IAS as TAS, you would have logged 1.64 hours and 12.3 gallons — and you would still be wrong when the wind came in, because the wind triangle also wanted TAS. Plan with the chart. Fly the IAS the chart assumed for that power setting.

Landing — the takeoff factors, plus speed and the flare

PHAK: the critical landing is high gross weight, high density altitude, and unfavorable wind, plus runway slope and surface. You touch down at a published indicated speed, but TAS is higher when density is low, so the groundspeed is higher and the rollout grows. PHAK’s altitude rule of thumb for landing distance is about 3½ percent more distance per 1,000 feet of altitude. At 5,000 feet the minimum landing distance is about 16 percent longer than at sea level.

Speed is a squared problem. PHAK: a 10 percent excess landing speed causes at least a 21 percent increase in landing distance. A 10-knot tailwind also adds about 21 percent. Floating because you were fast, then touching long, is how a “plenty of runway” airport becomes a go-around — or an overrun.

Aerodynamic drag (hold the nose up, spoilers on bigger airplanes) helps only down to about 60–70 percent of touchdown speed. After that, brakes do the work. Water, slush, rubber deposits, and ice cut braking effectiveness. A downslope plus a tailwind plus a wet surface is how a landing chart’s dry, level, zero-wind number becomes fiction.

Hydroplaning — PHAK’s formula and units

PHAK Chapter 11: dynamic hydroplaning is the tire riding on a thin sheet of water instead of the runway. Minimum hydroplaning speed in knots is nine times the square root of the main-gear tire pressure in psi:

Vp ≈ 9 × √P

PHAK’s own example: tire pressure 36 psi, √36 = 6, Vp = 54 knots. A trainer tire at 25 psi is 9 × 5 = 45 knots. That is minimum speed to start dynamic hydroplaning. Once it starts, it can continue well below that speed. Standing water, not just “a little rain,” is the setup. Grooved runways help; they are not a promise. The knowledge-test number is PHAK’s 9, with knots and psi. (The Airplane Flying Handbook cites test data nearer 8.6; do not mix the two on a PAR item that is quoting PHAK.)

Contaminated runways and the go-around

A contaminated runway is water, slush, snow, ice, or loose debris deep enough to affect acceleration or braking. Takeoff acceleration suffers; landing brakes may do almost nothing. If the AFM has no wet or contaminated data, treat the dry chart as optimistic, not conservative. Hydroplaning plus a tailwind plus a downslope is a rejected landing, not a longer float.

A go-around is a climb from a dirty configuration, often below Vy, sometimes below Vx, with residual sink. Retract flaps on the AFM schedule, maintain a safe airspeed, and accept that high density altitude and high weight make the initial climb anemic. The go-around decision has to come early enough that the remaining pavement and the remaining excess thrust still exist. Stretching a bad approach to “save the landing” is how you use the last of both.

Scenario: Riley’s fast, wet arrival

Riley is high and fast on final at a 5,000-foot density-altitude airport, 10 knots above the AFM approach speed, runway wet, 8-knot tailwind because the other end has a displaced threshold Riley did not want. PHAK already added about 16 percent for altitude, 21 percent for the extra speed, and about 21 percent for a 10-knot-class tailwind — and that was on a dry chart. Tire pressure is 36 psi; 54 knots is the PHAK hydroplaning gate, and Riley’s TAS on short final is not far from it. Riley’s job is not to prove the flare. Riley’s job is to go around while there is still climb and still sky, then land into the wind on a runway that matches the chart.

The stabilized approach and energy management

Every approach-and-landing Task in ACS Area IV opens with the same knowledge element — "a stabilized approach, including energy management concepts" — and Task IX.B repeats it for the emergency approach. It is the single most repeated knowledge element in the ACS, so it is worth stating precisely.

A stabilized approach means that by a defined gate on final, the airplane is established on the intended flight path, at the target airspeed, in the landing configuration, with a constant rate of descent and only small corrections required. For light airplanes flying a visual approach, a commonly taught gate is 300 to 500 feet AGL. If the approach is not stabilized at the gate, the correct action is a go-around, not a salvage attempt.

Energy management is the reason the concept exists. The airplane arrives with two kinds of energy — potential (height) and kinetic (airspeed) — and the landing works only if both reach roughly zero at the same place. Pitch and power are the two controls that trade them:

The forward slip to a landing

ACS Task IV.M is the forward slip to a landing, and it is the tool for the "high with excess altitude" case when you do not want more airspeed.

A forward slip is flown with aileron toward the desired direction of drift and opposite rudder, holding the longitudinal axis at an angle to the flight path. The airplane's ground track is unchanged while the fuselage is presented broadside to the relative wind. That deliberate misalignment produces a large increase in parasite drag, so the airplane descends steeply without accelerating — which is exactly the point. It dissipates altitude, not energy stored as speed.

Distinguish it from a sideslip, which is used for crosswind landings: there the longitudinal axis stays aligned with the runway and the slip cancels drift.

Short-field and soft-field approach and landing

Section 13.2 covered the takeoff halves of these Tasks. ACS Tasks IV.D (soft-field approach and landing) and IV.F (short-field approach and landing) carry the same three knowledge elements as every other Area IV landing Task — stabilized approach and energy management, the effect of atmospheric conditions on landing performance, and wind correction technique — but the procedures differ, and the written test asks which one you are flying.

Short-field approach and landing. The problem is stopping in the least distance over an obstacle. Fly the AFM approach speed exactly (often near 1.3 × VS0) with the AFM flap setting, on a stabilized approach aimed at a chosen touchdown point. Extra speed is the enemy: the 10-percent-speed / 21-percent-distance rule above is precisely why floating past the aim point ruins the number. Touch down at minimum controllable airspeed at or just beyond the aim point with little or no float, then — per the AFM — retract flaps to put weight on the wheels and apply maximum braking short of a skid, holding aft elevator for aerodynamic braking until the airplane slows.

Soft-field approach and landing. The problem is the opposite: the surface, not the length. Touch down as slowly and as gently as possible and then keep the weight off the nosewheel. Use a normal approach, then carry a small amount of power through the roundout to cushion the touchdown in a nose-high attitude, hold full aft elevator after the mains touch so the nosewheel settles last, and keep rolling — braking on soft ground is what digs the wheels in or noses the airplane over. Retracting flaps here works against you, since you want the wings still carrying weight.

The one-line discriminator the test wants: short field brakes hard and stops short; soft field brakes as little as possible and keeps moving.

Two cautions the test asks about. First, the airspeed indicator may be unreliable in a slip, because the static port is no longer in undisturbed air — fly attitude and use the AFM-noted error. Second, some AFMs prohibit or limit slips with full flaps because of a tailplane blanking or fuel-unporting concern; that limitation is in the AFM, and it is a limitation rather than a technique preference.