Types of Altitude
Knowing the glider’s altitude is vitally important to the pilot for several reasons. The pilot must be sure that the glider is flying high enough to clear the highest terrain or obstruction along the intended route. To keep above mountain peaks, the pilot must be aware of the glider’s altitude and elevation of the surrounding terrain at all times. Knowledge of the altitude is necessary to calculate TAS.
Altitude is vertical distance above some point or level used as a reference. There may be as many kinds of altitude as there are reference levels from which altitude is measured, and each may be used for specific reasons.
The following are the four types of altitude that affect glider pilots. [Figure 4-19]
- Indicated altitude—altitude read directly from the altimeter (uncorrected) after it is set to the current altimeter setting (QNH) in the Kollsman window. Indicated altitude can be used for maintaining terrain/ obstacle clearance and estimating distance to glide over the terrain without benefit of lift.
- True altitude—true vertical distance of the glider above sea level (known as MSL). This altitude is measured above a standard datum due to the shape of the earth (often expressed in this manner: 10,900 feet MSL, 5,280 feet MSL, or 940 feet MSL). Airport, terrain, and obstacle elevations found on aeronautical charts are expressed as MSL (true altitudes). With proper local altimeter setting (QHN), indicated and true altitude are synonymous.
- Absolute altitude—vertical distance above the terrain, above ground level (AGL). An altimeter set to the proper pressure reading (QFE setting) indicates zero feet at touchdown. It is referred to as QFE. The METAR weather report in the remarks section lists this setting as sea level pressure (SLP) and is expressed in millibars (mb). Absolute altitude is very important when flying low and determining when an out landing must be accomplished. An out landing is a glider landing not at the intended airfield but required when lift is insufficient to remain aloft.
- Pressure altitude—altitude indicated at which the altimeter setting window (barometric scale) is adjusted to 29.92 is the standard datum plane, a theoretical plane where air pressure (corrected to 15 °C or 59 °F) is equal to 29.92 “Hg. Pressure altitude is used for computer solutions to determine density altitude, true altitude, TAS, etc.
- Density altitude—pressure altitude corrected for nonstandard temperature variations. Density altitude is a yardstick by which we can reference the density of air. The wings of the glider are affected by the density of air because they use air molecules to generate lift. Thinner air restricts fuel burn and lowers compression in the engine thereby reducing the power output. Thinner air necessitates an increased AOA to generate the same amount of lift. High density altitudes decrease towplane (and self-launching combustion powerplant) power, so expect much longer takeoff runs during tows, slower climbs, and the higher TAS results in longer landing rollouts due to higher ground speeds.
When conditions are standard, pressure altitude and density altitude are the same. Consequently, if the temperature is above standard, the density altitude is higher than pressure altitude. If the temperature is below standard, the density altitude is lower than pressure altitude. The density altitude determines the gliders performance and greatly affects the performance of towplanes and powerplants in self-launching gliders.