Factors Affecting Takeoff and Landing Performance

The take-off and landing distances in a flight manual were measured on one kind of day: a stated mass, a paved, level and dry runway, standard temperature and the published technique. The real day is almost never like that. The aerodrome may be high, the afternoon hot and humid, the runway sloping or covered in grass, and each of these changes the distance the aeroplane needs.

The flight manual charts take mass, pressure altitude, temperature and wind as inputs. Slope and surface are handled either in the charts or by correction factors, which for performance Class B aeroplanes are standardised. Wind, the other large input, is covered in wind components and crosswind limits. Knowing which way each factor acts, and roughly by how much, lets a pilot see when a runway that looks long enough is not.

Why distances change

A take-off distance depends on two things: the speed the aeroplane must reach and how quickly it accelerates to it. A landing distance depends on the speed at touchdown and how quickly the aeroplane can be stopped. Every factor in this article works through one or both.

The wing responds to indicated airspeed, so the rotation, lift-off and approach speeds read on the airspeed indicator do not change with altitude or temperature. What changes is the true airspeed, and so the groundspeed, behind them. Acceleration is the net force along the runway, thrust minus drag and rolling resistance, with any slope component of the weight added or subtracted, divided by the mass.

Mass affects both. A heavier aeroplane lifts off at a higher speed and accelerates more slowly, so as a rule of thumb a 10 per cent increase in mass lengthens the take-off distance by about 20 per cent. On landing it arrives faster with more kinetic energy, and the landing distance grows by roughly 10 per cent.

Density altitude

Density altitude is the altitude in the International Standard Atmosphere at which the actual air density would be found: pressure altitude corrected for non-standard temperature. It is not shown on any instrument. The usual approximation adds about 120 ft for every degree Celsius above ISA and subtracts the same below it.

A worked example shows how quickly it grows. At an aerodrome 5,000 ft above sea level with a QNH of 1003 hPa, the pressure altitude is about 270 ft higher, 5,270 ft, allowing 27 ft per hPa. ISA temperature there is about +4.5 °C. On a +30 °C afternoon the deviation is about ISA +25.5, worth some 3,060 ft, so the aeroplane performs as though it were at about 8,300 ft.

A high density altitude hurts twice:

• Higher true airspeed for the same indicated speed. The aeroplane must reach a higher groundspeed before lift-off, and it arrives for landing with a higher groundspeed and more energy to dissipate, so both the take-off run and the landing roll lengthen.
• Less thrust or power. A normally aspirated piston engine loses roughly 3 per cent of its sea-level power for each 1,000 ft of density altitude, and its propeller has less air to work on. Jet thrust also falls with density.

Flight manual charts are entered with pressure altitude and outside air temperature, never with the aerodrome elevation, and the chart applies the density effect itself. Calculating density altitude separately is useful for understanding and as a cross-check.

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Hot and high operations

Hot and high operations, also called high airfield or high altitude airport operations, bring these effects together. The take-off run is longer, and the climb after it is weaker: thrust is lower, and the same indicated climb speed means a higher true airspeed and groundspeed, so the gradient over the ground is flatter just where the terrain is often close. On transport aeroplanes this can make the take-off climb-limited rather than field-length-limited, and the one-engine-inoperative second-segment requirement can reduce the maximum take-off mass sharply on a summer afternoon.

On landing, the higher true airspeed at the same reference speed means more kinetic energy, a longer ground roll and more work for the brakes, which can limit the landing mass through brake energy. Aircraft limitations also apply: the Airbus A320, for example, is limited to a maximum runway altitude of 9,200 ft and a mean runway slope of ±2 per cent.

Piston aeroplanes have extra concerns. A turbocharged engine holds its rated power up to its critical altitude, but the airframe still suffers the aerodynamic effects of thin air. A normally aspirated engine at full rich mixture is over-rich at a high density altitude, which is why the flight manual's high-altitude take-off procedure normally calls for leaning. A day colder than standard works the other way, giving denser air, more power and shorter distances.

Humidity

Water vapour is lighter than dry air, with about five-eighths of its density at the same pressure and temperature, so humid air is less dense and the density altitude is higher than temperature and pressure alone suggest. The vapour also displaces oxygen, so a piston engine loses some power. The effect on density is small compared with that of temperature, but charts assume dry air: on a hot, humid day the aeroplane will accelerate and climb a little worse than the chart shows. FAA material calls the worst combination high, hot and humid: high elevation, high temperature and high humidity, together with low pressure, produce the highest density altitude.

Runway slope

On a sloping runway a component of the weight acts along it. Uphill, it opposes the acceleration on take-off and helps the deceleration on landing; downhill, it does the reverse. So:

• an upslope lengthens the take-off and shortens the landing;
• a downslope shortens the take-off and lengthens the landing.

For Class B planning, when the flight manual has no slope data, the runway slope correction increases the take-off distance by 5 per cent for each 1 per cent of upslope and the landing distance by 5 per cent for each 1 per cent of downslope. No credit is taken for the helpful direction: a downslope on take-off and an upslope on landing are ignored, which adds margin. Class A flight manuals give slope corrections directly, within the runway slope limits of the type, ±2 per cent for the A320.

The FAA's Chart Supplement publishes the runway gradient when it is 0.3 per cent or more.

Runway surface corrections

A grass surface increases rolling resistance, so the aeroplane accelerates more slowly on take-off, and it gives poorer braking friction on landing. Wet grass is worse on both counts. A wet paved runway barely affects the take-off acceleration, but it reduces braking and lengthens the landing.

The runway surface correction factors used for Class B, where the flight manual gives no better data, are:

EASA guidance adds that very short wet grass on firm soil can be slippery enough to increase landing distances by as much as 60 per cent, a factor of 1.6, and recommends the wet factor of 1.15 whenever the wetness of the grass is in doubt.

Light aeroplane handbooks, especially in the United States, often express the grass correction differently, as a percentage of the ground roll only, for example "increase distances by 15 per cent of the ground roll figure". The airborne part of the distance is unaffected, so the percentage must not be applied to the total distance over the 50 ft obstacle. Standing water, slush, snow and ice are contamination and need specific flight manual data (see wet and contaminated runways).

Class B distance factors

For a commercial Class B flight (see performance classes and data), the corrections and the dispatch factors are applied together, and they multiply rather than add:

1. Read the gross distance from the flight manual chart at the actual mass, pressure altitude, temperature and wind. The wind is factored to no more than 50 per cent of a headwind component and no less than 150 per cent of a tailwind component, unless the chart already includes the factoring, as the CAP 698 charts do.
2. Apply the surface and slope corrections.
3. Apply the dispatch factor: for take-off, × 1.25 against the take-off run available when there is no stopway or clearway; for landing, the distance must fit within 70 per cent of the landing distance available, a factor of about 1.43, plus 15 per cent if the runway may be wet.

A worked take-off: a gross take-off distance of 500 m, from dry grass (1.2) with a 2 per cent upslope (1.1), becomes 500 × 1.2 × 1.1 = 660 m, and × 1.25 = 825 m of take-off run required. A worked landing: a gross landing distance of 560 m requires 560 ÷ 0.7 = 800 m of landing distance available on a dry runway, and 800 × 1.15 = 920 m if it may be wet.

These multipliers are Part-CAT rules for commercial air transport. The performance rule of Part-NCO, NCO.POL.110, requires in general terms that the performance be adequate for the flight and the aerodromes used. The theory examinations use the Class B method, and the reasons for the margins apply to any take-off or landing.

The Koch chart

The Koch chart is a nomogram in the FAA Pilot's Handbook of Aeronautical Knowledge for estimating the effect of density altitude without an aircraft-specific chart. A straight line drawn from the aerodrome temperature on one scale to the pressure altitude on another crosses two further scales: the percentage increase in take-off distance and the percentage decrease in rate of climb, both compared with sea-level standard conditions. The percentages are applied to the aeroplane's sea-level standard-day figures.

The chart is an estimate for planning. It is useful for seeing at a glance how quickly a hot day at a high aerodrome erodes the margins, but the aeroplane's own flight manual charts are authoritative. A similar check comes from the density altitude rule of thumb: at 3,000 ft pressure altitude, where ISA is +9 °C, a temperature of 29 °C gives ISA +20 and a density altitude of about 3,000 + 20 × 120 = 5,400 ft.

Library articles are written for study and exam preparation. They do not replace your aircraft's approved documentation, your operator's procedures or the regulations themselves.