1 The natural sink of the glider 3 Changes in altitude caused by the glider slowing down or speeding up.  (e.g., if the pilot pulls back on the stick, the glider slows down and gains  altitude as speed is converted into height.) 2 External up (lift) or down (sink) drafts cause the glider to gain or  lose altitude (e.g., thermals) A glider pilot does not actually want a variometer to display  this type of altitude change because every time the pilot  slows down, that pilot will think the glider is in a strong thermal.  Stick pulled back 3,120 ft 50 knots 3,000 ft 80 knots Figure 4-24.  Flight maneuvers that display altitude changes on an uncompensated variometer. cavity changes, minute alterations in the shape of the  membrane occur. As a result, electrical resistance in the  embedded resistors changes. These changes in electrical  resistance are interpreted by a circuit board and indicated  on the variometer dial as climb or descent. Many electrical variometers provide audible tones, or beeps,  that indicate the rate of climb or rate of descent of the glider.  Audio variometers enhance safety of flight because they make  it unnecessary for the glider pilot to look at the variometer to  discern the rate of climb or rate of descent. Instead, the pilot  can hear the rate of climb or rate of descent. This allows the  pilot to minimize time spent looking at the flight instruments  and maximize time spent looking outside for other air traffic.  [Figure 4-25] Some variometers are equipped with a rotatable rim speed  scale called a MacCready ring. This scale indicates the  optimum airspeed to fly when traveling between thermals  for maximum cross-country performance. During the glide  between thermals, the index arrow is set at the rate of  climb expected in the next thermal. On the speed ring, the  variometer needle points to the optimum speed to fly (STF)  between thermals. If expected rate of climb is low, optimum  interthermal cruise airspeed is relatively low. When expected  lift is strong, however, optimum interthermal cruise airspeed  is much faster.  [Figure 4-26]  A MacCready ring has single  4-13 Figure 4-25.  When an electric variometer is mounted to the glider,  a non-electric variometer is usually installed as a backup. 30.0 29.9 29.8 I 0 0 F E T I 4 5 6 7 9 2 0 8 3 CALIBRATED TO  20,000 FEET ALT OFF GO L N AV NET AVG PULL PUSH HW DIST ALT PUSH 0 2 4 6 8 10 -2 -4 Acceleration UNITS PUSH N 30 60 E 120 150 STEER FOR S 210 240 W 300 330 STEER RADIO FOR ON ON 160 40 140 60 120 80 20 100 knots 0 2 2 4 4 6 6 8 8 10 10 knots winter 5 0 6 0 70  $_{ }$8 0 9 0  1 0 k n o t s $_{ }$S A L T O 0 2 2 4 4 6 6 8 8 10 10 2 0 4 6 8 10 2 4 6 8 10 20 18 16 14 12 10 0 2 2 4 4 6 6 8 8 10 10 knots winter 5 0 6 0  7 0 $_{ }$8 0 9 0 1 0 k n o t s S A L T O 0 2 2 4 4 6 6 8 8 10 10 2 0 4 6 8 10 2 4 6 8 10 20 18 16 14 12 10 Non-electric Electric Current climb rate Climb rate during last circle