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Flight Performance - Level 3

Airplane Performance -Section 1

 

EFFECT OF TEMPERATURE AND ALTITUDE ON AIRPLANE PERFORMANCE

The figures published in the Flight Manual for the performance capabilities of a certain model of airplane are always related to standard atmosphere (29.92 inches of mercury at 15 C at sea level). However, only rarely will the airplane actually operate under conditions that approximate standard atmosphere. Any increase in temperature or altitude means a decrease in the aircraft's optimum performance.

Air density decreases with altitude. At high elevation airports, an airplane requires more runway to take off. Its rate of climb will be less, its approach will be faster,   because the true air speed [TAS] will be faster than the indicated air speed [IAS] and the landing roll will be longer.

Air density also decreases with temperature. Warm air is less dense than cold air because there are fewer air molecules in a given volume of warm air than in the same volume of cooler air. As a result, on a hot day, an airplane will require more runway to take off, will have a poor rate of climb and a faster approach and will experience a longer landing roll.

In combination, high and hot, a situation exists that can well be disastrous for an unsuspecting, or more accurately, an uninformed pilot. The combination of high temperature and high elevation produces a situation that aerodynamically reduces drastically the performance of the airplane. The horsepower out-put of the engines decrease because its fuel-air mixture is reduced. The propeller develops less thrust because the blades, as airfoils, are less efficient in the thin air. The wings develop less lift because the thin air exerts less force on the airfoils. As a result, the take-off distance is substantially increased, climb performance is substantially reduced and may, in extreme situations, be non-existent.

Humidity also plays a part in this scenario. Although it is not a major factor in computing density altitude, high humidity has an effect on engine power. The high level of water vapor in the air reduces the amount of air available for combustion and results in an enriched mixture and reduced power

Mountain airports are particularly treacherous when temperatures are high, especially for a low performance airplane. The actual elevation of the airport may be near the operational ceiling of the airplane without the disadvantage of density altitude. Under some conditions, the airplane may not be able to lift out of ground effect or to maintain a rate of climb necessary to clear obstacles or surrounding terrain.

Density altitude is pressure altitude corrected for temperature. It is, in layman terms, the altitude at which the airplane thinks it is flying based on the density of the surrounding air mass.

Too often, pilots associate density altitude only with high elevation airports. Certainly, the effects of density altitude on airplane performance are increasingly dramatic in operations from such airports, especially when the temperature is also hot. But it is important to remember that density altitude also has a negative effect on performance at low elevation airports when the temperature goes above the standard air value of 15 C at sea level. Remember also that the standard air temperature value decreases with altitude.

In order to compute the density altitude at a particular location, it is necessary to know the pressure altitude. To determine the latter, set the barometric scale of the altimeter to 29.92" Hg and read the altitude.

Density altitude can be calculated for any given combination of pressure altitude and temperature, by using the circular slide rule portion of a flight computer.

TAKE OFF PERFORMANCE CHARTS

Your Airplane Flight Manual publishes information, usually in chart or table form, on the take-off performance of a specific model of airplane. As a pilot, you should familiarize yourself with these charts/tables, to be able to predict how your airplane will perform under varying atmospheric conditions and you should refer to these charts/tables whenever there is any doubt that the takeoff conditions may not be sufficient for the performance capabilities of the airplane. In addition, it is important to remember that the charts/tables for any particular airplane were compiled from performance figures of factory new equipment in optimum conditions. Any typical general aviation airplane, with considerable time on both airframe and engine, will have a poorer performance potential than that predicted by the charts/tables. In addition, under-inflated tires, dragging brakes, dirt on the wings, etc., will also affect performance negatively.

If after calculating density altitude and checking the tables, it appears that the take-off run will require more runway than is available, you, as pilot-in-command, have several alternatives. You can lighten the load, if possible, or you can wait until the temperature decreases. Generally, the most critical time for flight operations when the temperature is very hot is from mid-morning through mid-afternoon. This is especially true at high elevation airports, but even at lower elevations, aircraft performance may be marginal. Aircraft operations should, therefore, be planned for early morning or late evening hours.

It is important to remember that in taking off from airfields that are at high elevation, you should use as a reference the same indicated airspeed that you would use during take-off from an airfield at sea level. It is the true airspeed and groundspeed that is affected by the increase in elevation and temperature.

CLIMB PERFORMANCE CHARTS

Your Airplane Flight Manual also publishes data for climb performance. The maximum, or best rate of climb, is the rate of climb which will gain the most altitude in the least time and is used to climb after take off until ready to leave the traffic circuit.

Many Airplane Manuals also publish charts for cruise climb. Cruise climb, or normal climb, is the climb airspeed used for a prolonged climb. The chart indicates the fuel used, time required to reach altitude, and still air distance covered in order to reach various altitudes when climbing at a certain indicated airspeed with various power settings.

CRUISE PERFORMANCE CHARTS.

Performance figures for cruise at gross weights are also given in most Airplane Flight Manuals. These charts show the fuel consumption, true airspeed, endurance and range that may be expected when cruising at a certain altitude with the engine being operated at normal lean mixture at various combinations of rpm and MP settings (to give a required % of power).

LANDING PERFORMANCE CHARTS

Perfect landings are usually preceded by deliberately planned, and well executed approaches. Correct approach speeds are important. Your Airplane Flight Manual for any particular model of airplane recommends the speeds to use on approach with various flap settings. These airspeeds should always be used.

The factor of weight is important in determining landing speed. All airplanes stall at slower airspeeds when they are light. A lightly loaded airplane, landing at the same airspeed that is used when it is heavily loaded, will float before touchdown to dissipate the excess energy, thus extending the landing distance. If the Owner's Manual does not publish a table of approach speeds as a function of reduced weight, a rule of thumb is to reduce the calibrated approach airspeed for the maximum weight of your airplane by one-half of the percentage of the weight decrease. If for example, the airplane weight is 20% below maximum, the calibrated approach airspeed would be decreased by half of that, or by 10%

On some airplanes, the manufacturer may require a particular approach speed for all weights because, during certification flight testing, it was found that for stability and control reasons, or for go-around safety, a fixed airspeed is required. Always comply with the manufacturer's recommendations.

Since there is some loss in the quality of braking action on the grass of a sod runway, the ground roll after landing can be expected to be longer than it would be on a hard surface runway.

Density altitude affects the landing performance of an airplane as greatly as it affects take-off performance. High temperature and high elevation will cause an increase in the landing roll because the true airspeed is higher than the indicated airspeed. Therefore, even though using the same indicated airspeed for approach and landing that is appropriate for sea level operations, the true airspeed is faster, resulting in a faster groundspeed (with a given wind condition). The increase in groundspeed naturally makes the landing distance longer and should be carefully considered when landing at a high elevation field, particularly if the field is short.

Some Airplane Flight Manuals contain performance charts and tables, which relate landing distance to density altitude. Pilots should develop the habit of referring to these charts/tables in order to anticipate the distance that will be required to safely land their airplane under various conditions of flight.

CLICK HERE for a more detailed and advanced review of Take-off and Landing performance of airplanes, on unsloped and sloping runways.


The material for this section is reproduced from the publication, FROM THE GROUND UP, with the permission of its copyright owner, Aviation Publishers Co. Ltd. No further reproduction is authorized, in any print, electronic or other form of media, without the prior consent of the publisher at http://www.aviationpublishers.com . Any questions regarding this portion of the website should be directed to Dr. Claudius Carnegie. Questions regarding the publication, FROM THE GROUND UP, should be directed to the publisher at info@aviationpublishers.com.

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Updated: May 03, 2008