8.landing-distance-density-altitude-correction. Landing Distance and Density Altitude Correction
Published landing distance data in the Pilot's Operating Handbook (POH) or Airplane Flight Manual (AFM) is computed for a new airplane, a perfectly executed approach and landing, a hard-surfaced level runway, and standard atmospheric conditions (sea level, 15°C, 29.92 inches Hg). Real-world conditions almost never match these assumptions, so the pilot must apply corrections — most importantly for density altitude — before committing to a runway.
Why density altitude matters on landing. As density altitude (DA) increases, the air becomes less dense. Three things happen simultaneously on approach and rollout:
- True airspeed (TAS) increases for a given indicated airspeed (IAS). The airplane is flown across the threshold at the same IAS shown in the POH, but its actual speed over the ground (in still air) is higher.
- Touchdown groundspeed is higher, so the energy that must be dissipated by brakes and aerodynamic drag during rollout increases.
- Aerodynamic drag and propeller drag (if go-around is initiated) are reduced, lengthening both the air distance over the 50-ft obstacle and the ground roll.
As a rule of thumb, landing distance increases roughly 3.5–4% for every 1,000 ft increase in density altitude. A landing requiring 1,200 ft at sea level on a standard day can easily exceed 1,600–1,800 ft at a 5,000-ft DA airport on a hot afternoon, even before considering wind, slope, or runway surface.
Computing density altitude. Density altitude is pressure altitude corrected for nonstandard temperature. The shortcut formula is:
DA = PA + (120 × (OAT − ISA temp))
where ISA temp at the field = 15°C − (2°C × PA/1,000).
Example: Field elevation 4,500 ft, altimeter 30.12, OAT 32°C.
- Pressure altitude ≈ 4,500 − (30.12 − 29.92) × 1,000 = 4,300 ft.
- ISA temp at 4,300 ft ≈ 15 − (2 × 4.3) = 6.4°C.
- DA ≈ 4,300 + 120 × (32 − 6.4) ≈ 4,300 + 3,072 ≈ 7,372 ft.
Using the POH landing distance chart. Most POH charts are entered with pressure altitude and OAT (the chart applies the density-altitude correction internally), then corrected sequentially for:
- Weight — lower landing weight shortens distance; higher weight (within limits) lengthens it.
- Wind — subtract for headwind component, add a penalty (often 10% per 2 knots, capped) for tailwind.
- Runway surface and slope — dry grass typically adds 15–20%; an upslope shortens landing roll, a downslope lengthens it.
- Runway condition — wet, contaminated, or icy surfaces dramatically increase ground roll.
Procedure for the pilot.
- Obtain field elevation, current altimeter setting, OAT, surface wind, and runway data from ATIS/AWOS or the chart supplement.
- Compute pressure altitude and density altitude.
- Enter the POH landing distance chart at the correct PA/OAT line; read the total distance over a 50-ft obstacle and the ground roll.
- Apply weight, wind, slope, and surface corrections in the order the POH specifies.
- Apply a safety factor. The FAA strongly recommends multiplying the corrected POH distance by 1.5 (a 50% safety margin) for normal operations. Some operators require 1.67 (60%) or 1.82 (80%) for transport-category and Part 135 operations; private pilots should adopt a comparable margin.
Example. A POH gives a landing distance over a 50-ft obstacle of 1,400 ft at sea level, standard day, maximum landing weight. At a density altitude of 6,000 ft on a calm day, applying the 4% per 1,000 ft rule of thumb:
Corrected distance ≈ 1,400 × (1 + 0.04 × 6) = 1,400 × 1.24 = 1,736 ft.
Applying the FAA 1.5 safety factor: 2,604 ft of usable runway is the minimum length the pilot should accept. If the runway is 2,500 ft long, the pilot must select a different airport, wait for cooler temperatures, reduce landing weight, or accept the risk only with a clearly identified mitigation.
Approach speed considerations. Always fly the POH-published indicated airspeed for the approach — do not add speed for high density altitude. The IAS already accounts for the lower air density; adding a knot "for the family" floats the airplane and consumes runway. However, recognize that the higher TAS means the runway sight picture and closure rate will look unfamiliar — flare timing must be referenced to height above the runway and rate of closure, not airspeed alone.
Go-around capability. The same density altitude that lengthens landing roll also reduces climb performance. Before crossing the threshold, the pilot must verify that, if a go-around becomes necessary, the airplane can clear obstacles in the departure path at the computed DA. On hot-and-high days, an apparently routine landing decision is also a takeoff-and-climb decision.