A variometer is an instrument that measures how quickly an aircraft is climbing or descending. While the concept sounds simple, the variometer is far more sophisticated than a basic altitude gauge. It detects changes in air pressure in real time and translates them into a rate of vertical movement, typically displayed in knots or meters per second. The instrument is most closely associated with gliding, where finding and staying in rising air is the entire basis of unpowered flight.
Variometer vs. Vertical Speed Indicator
Powered aircraft carry a similar instrument called a vertical speed indicator (VSI). Both devices measure the same basic thing: the rate of change in altitude. The difference lies in how pilots use them and how sophisticated the readings need to be. In a powered airplane, the VSI is a reference tool. Pilots glance at it to confirm they’re holding level flight or maintaining a steady climb or descent. It doesn’t need to be particularly fast or nuanced.
In a glider, the variometer is the single most important instrument in the cockpit. Glider pilots monitor it almost continuously, because their ability to stay airborne depends on detecting columns of rising air called thermals. A basic VSI would tell a glider pilot that the aircraft is climbing, but it wouldn’t distinguish between a real thermal and a temporary altitude gain caused by the pilot pulling back on the stick. That distinction matters enormously when you have no engine to fall back on.
How the Basic Mechanism Works
At its core, a mechanical variometer relies on a sealed reference chamber (essentially a small flask or reservoir) connected to the outside atmosphere through a narrow tube called a capillary. When the aircraft climbs, the outside air pressure drops. The air trapped in the chamber is now at a higher pressure than the air outside, so it flows outward through the capillary. When the aircraft descends, the reverse happens and outside air flows in.
The instrument measures the pressure difference across this capillary. Because the airflow through the narrow tube is proportional to that pressure difference, the variometer can calculate the vertical speed of the aircraft. A diaphragm sensor inside the instrument deflects in response to the pressure difference, moving a needle on the display. Climb shows on one side, descent on the other, with the center position meaning level flight. The size of the capillary and the volume of the reservoir together determine the instrument’s response time, with a smaller reservoir or wider capillary producing faster readings.
This simple type requires no electrical power, which makes it a reliable backup that works regardless of battery condition.
Total Energy Compensation
The basic variometer described above is called an “uncompensated” variometer, and it has a significant flaw for glider pilots. If a pilot pulls the nose up, the glider temporarily climbs by converting speed (kinetic energy) into altitude (potential energy). The uncompensated variometer reads this as a climb and the needle swings upward, even though the glider hasn’t found any rising air. Pilots call this false signal “stick lift” or a “stick thermal,” and chasing it is a classic beginner mistake.
A total energy (TE) compensated variometer solves this problem by accounting for both altitude and airspeed changes. The idea is that a glider in still air is a closed energy system: any altitude gained by pulling up comes at the cost of lost airspeed, and the total energy stays roughly the same. The TE variometer cancels out these exchanges and only responds when the glider’s total energy actually increases or decreases, which means the air itself is lifting or sinking. As glider pioneer Helmut Reichmann explained, a total energy variometer shows changes in the combined potential and kinetic energy of the sailplane, making it far more useful for locating thermals.
Most modern sailplanes come equipped with TE-compensated variometers. The compensation is typically achieved through a specially shaped probe mounted on the aircraft’s fin. This probe is designed so that its pressure reading naturally incorporates airspeed effects, feeding the variometer a corrected signal. Very few powered aircraft use total energy variometers, since their pilots care about actual altitude change, not air mass movement.
Netto Variometers
Total energy compensation removes the effect of the pilot’s control inputs, but the variometer still shows the glider’s own sink rate mixed in with any air movement. Every glider, even in perfectly still air, descends at a predictable rate that increases roughly with the square of its airspeed. A netto variometer takes compensation one step further by subtracting this known sink rate from the reading.
The result is a display that shows only the vertical movement of the surrounding air mass. If the netto variometer reads zero, the air around the glider is neither rising nor sinking. If it reads positive, you’re in lift. This is especially valuable when flying between thermals at high speed, because a basic variometer would show a strong descent (due to the glider’s high-speed sink rate), masking moderate lift that a netto variometer would reveal. The compensation works by bleeding a small calibrated flow of air through the system, proportional to airspeed squared, which offsets the glider’s expected performance at that speed.
Audio Feedback
Glider pilots spend much of their time looking outside the cockpit, scanning for visual cues like cloud formations and other circling gliders. Staring at an instrument dial isn’t practical when you’re also navigating and watching for traffic. Electronic variometers solve this with audio output: a series of tones that change in pitch and rhythm based on what the air is doing.
In a climb, the variometer produces intermittent beeps. The faster you climb, the higher the pitch and the more rapid the beeping. In sinking air, the tone shifts to a low, steady, descending pitch. A pilot can instantly tell, without looking at the panel, whether they’ve flown into a thermal or are losing altitude. This audio feedback becomes second nature and is one of the most distinctive sounds associated with soaring flight.
Modern Flight Computer Integration
Today’s variometers have evolved well beyond standalone instruments. Modern units from manufacturers like LXNav integrate GPS, moving map displays, final glide calculators, and flight recorders into a single device. A final glide calculator uses the variometer’s data alongside wind information and GPS position to determine whether the glider has enough altitude to reach its destination, accounting for expected sink rates and any lift along the way.
These systems can also store and display terrain databases, record flights in a standardized format for competition verification, and communicate with collision-avoidance systems. Some units accept input from portable navigation devices for additional route planning. Despite all this technology, the core function remains the same: telling the pilot, in real time, what the air around them is doing. The variometer reading is still the number glider pilots watch and listen to more than any other.