10.takeoff-and-landing-performance. Takeoff and Landing Performance
Takeoff and landing are the two most performance-critical phases of flight. The majority of general aviation accidents involving runway overruns, obstacle strikes, and loss of control occur during these phases — frequently because the pilot failed to compute (or honestly apply) book performance numbers. Chapter 10 of the PHAK treats takeoff and landing distance as the product of multiple atmospheric, configuration, and technique variables that the pilot must understand before takeoff.
Forces and the Takeoff Roll
During the takeoff roll, the airplane accelerates from rest until it reaches lift-off speed (V_LOF), typically 1.1 × V_S1. Acceleration depends on the difference between thrust and the sum of drag plus rolling friction:
- a = (T − D − μ(W − L)) / m
Where μ is the rolling friction coefficient (≈0.02 for paved, dry runway; 0.05–0.10 for turf or soft surfaces). Because acceleration is a function of excess thrust, anything that reduces thrust available (high density altitude, reduced manifold pressure, propeller inefficiency) or increases drag and friction (soft surface, tall grass, gear in trailing-edge mud) lengthens the takeoff roll dramatically. A useful rule of thumb: takeoff distance varies roughly with the square of the lift-off groundspeed.
Variables Affecting Takeoff Distance
- Density altitude. The single most important atmospheric factor. As DA rises, both engine power and propeller thrust decrease, and the true airspeed required to achieve V_LOF increases. A common approximation: takeoff distance increases about 10% per 1,000 ft of density altitude.
- Aircraft weight. Higher weight increases V_S1 (and thus V_LOF), increases rolling friction, and reduces acceleration. A 10% weight increase can increase takeoff distance by approximately 20%.
- Wind. Headwind shortens takeoff distance; tailwind lengthens it. Rule of thumb: each 10% of lift-off airspeed in headwind reduces ground roll by about 19%; each 10% in tailwind increases it by about 21%.
- Runway surface and gradient. Soft, wet, or grass surfaces increase rolling friction. An upslope gradient adds a gravity component opposing acceleration; a 1% upslope can increase ground roll by 2–4%.
- Runway condition. Standing water, slush, or snow add fluid drag and reduce friction.
- Flap setting. Manufacturer-recommended takeoff flap setting reduces V_LOF and shortens ground roll, at the cost of a shallower initial climb.
Computing Takeoff Distance
Use the POH performance chart for the specific airplane. Procedure:
- Enter the chart with pressure altitude and outside air temperature to find density-altitude corrected ground roll and total distance over a 50-ft obstacle.
- Apply weight correction (interpolate between weight columns).
- Apply wind correction (typically subtract X% per knot of headwind, add a larger X% per knot of tailwind).
- Apply surface correction per the POH notes (e.g., +15% for dry grass).
- Add a personal safety factor — the FAA and most flight schools recommend at least 50% additional distance over the book number for non-paved or non-standard conditions.
Example
POH ground roll at sea level, 15 °C, 2,300 lb: 800 ft. Field is at 4,000 ft pressure altitude, 25 °C (DA ≈ 6,000 ft), no wind, dry grass. Apply +60% for DA (6 × 10%) and +15% for grass: 800 × 1.60 × 1.15 ≈ 1,472 ft. With a 50% personal safety factor: 2,208 ft required.
Landing Performance
Landing distance comprises the air distance (from 50 ft AGL to touchdown) and the ground roll (touchdown to full stop). Touchdown speed is typically 1.3 × V_S0. Like takeoff, landing distance is sensitive to density altitude, weight, wind, surface, and gradient — but with one important inversion: high DA does not reduce braking effectiveness, only the true groundspeed at touchdown, which still increases stopping distance because kinetic energy varies with the square of groundspeed.
Key landing variables:
- Approach speed control. Excess speed at the threshold is the leading cause of overruns. Each 10% of excess approach speed increases landing distance roughly 20%.
- Flap configuration. Full flaps reduce V_S0 and approach speed, shortening both air and ground distances.
- Braking and surface. Wet, contaminated, or grass surfaces can double ground roll. Hydroplaning becomes possible above approximately 9 × √(tire pressure in psi) knots.
- Wind. Same headwind/tailwind ratios apply as for takeoff.
Regulatory and Practical Application
14 CFR 91.103 requires that, before flight, the pilot in command become familiar with all available information concerning that flight, including runway lengths and takeoff and landing distance data for any airport of intended use. For a runway to be legally and operationally adequate, it must accommodate the corrected book distance plus an honest safety margin. Treat POH numbers as the minimum a new airplane achieved with a test pilot on a perfect day — your real-world airplane and your real-world technique will not match those numbers.