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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. 

## Variometer 

Variometer instruments measure the vertical ascent or descent of the local air mass and glider combined and displays that information as speed. The principles of operation are similar to the altimeter. The variometer is viewed as a critical part of the glider’s instrument display, giving the pilot information on performance of the glider while flying through the atmosphere. The variometer depends upon the pressure lapse rate in the atmosphere to derive information about rate of climb or rate of descent. A non-electric variometer uses a separate insulated 

tank (thermos or capacity flask) as a reference chamber. The tubing is plumbed from the reference chamber through the variometer instrument to an outside static port. [Figure 4-20] 

<img src='content_image/534522.jpg'>

A variometer with tubing is connected to an outside static port is uncompensated. The internal hairspring mechanism determines sensitivity of the variometer. The variometer has a very rapid response due to the small mass and lightweight construction of the moving parts. [Figure 4-21] 

Pressure differences between the air inside the variometer/ reference chamber system and the air outside of the system tend to equalize as air flows from high pressure areas to low pressure areas. When pressure inside the reference chamber is greater than the pressure outside, air flows out of the reference chamber through the mechanical variometer to the outside environment. When air pressure outside the reference chamber is greater than pressure inside, air flows through the variometer and into the reference chamber until pressure is equalized. The variometer needle indicates a vertical descending air mass or sink which is falling air that forces the glider to lose height. Figures 4-22  and 4-23  illustrate how the variometer works in level flight and while the glider is ascending. In addition, Figure 4-24  illustrates certain flight maneuvers that cause the variometer to display changes in altitude. 

Electric-powered variometers offer several advantages over the non-electric variety. These advantages include more rapid response rates and separate audible signals for climb and descent. 

Some electric variometers operate by the cooling effect of airflow on an element called a thermistor, a heat-sensitive electrical resistor. The electrical resistance of the thermistor changes when temperature changes. As air flows into or out of the reference chamber, it flows across two thermistors in a bridge circuit. An electrical meter measures the imbalance across the bridge circuit and calculates the rate of climb or descent. It then displays the information on the variometer. 

Newer electric variometers operate on the transducer principle. A tiny vacuum cavity on a circuit board is sealed with a flexible membrane. Variable resistors are embedded in the membrane. When pressure outside the 

