How an Altimeter Work

Summary

This video explains the function and operation of a barometric altimeter, a critical instrument in aircraft. It discusses how the altimeter works by measuring static pressure, its calibration using barometric references like QNH and QFE, and its role in determining altitude or height. The video also covers the impact of atmospheric pressure changes, the use of different altimeter types, and practical considerations like calibration tables, instrument errors, and alternate static sources. Key concepts such as altitude, height, and elevation are clarified, and the video provides insights into how pilots use the altimeter to ensure accurate readings during flight.

Takeaways

• 😀 The altimeter is a key flight instrument that uses static pressure to measure the vertical distance of an aircraft relative to a reference level.
• 😀 Barometric altimeters, radio altimeters, and GPS altimeters are the three main types, with the barometric altimeter being the most commonly used in aviation.
• 😀 A barometric altimeter measures altitude by comparing the static pressure at the aircraft's location to the standard atmospheric pressure at sea level (29.92 inHg).
• 😀 The barometric altimeter is calibrated to measure altitude relative to the mean sea level (MSL) under standard conditions, but it can be adjusted for different conditions.
• 😀 The vertical distance measured by the altimeter can be referred to as altitude, height, or elevation depending on the reference level used (MSL, ground level, etc.).
• 😀 The altimeter uses an aneroid capsule inside the instrument to measure changes in atmospheric pressure, which causes the capsule to expand or contract and move the indicator needles.
• 😀 Pilots need to adjust the barometric reference (QNH) on the altimeter for accurate altitude readings, as the standard atmospheric pressure can vary from location to location.
• 😀 When the aircraft needs to measure height above the ground rather than MSL, the altimeter can be adjusted to use the airport pressure (QFE) as a reference.
• 😀 The Colesman window in the altimeter allows pilots to adjust the barometric reference to QNH or QFE, which is essential for accurate altitude or height readings.
• 😀 The altimeter uses three needles to indicate altitude: one for tens of thousands of feet, one for thousands of feet, and one for hundreds of feet. Some altimeters also include a flag for easy reference of 10,000 ft.
• 😀 When the static port gets blocked, the altimeter reading will freeze, and an alternate static source can be used to restore functionality, though calibration adjustments may be necessary.

Q & A

What is the role of the altimeter in aircraft?

The altimeter measures the vertical distance between the aircraft and a specified reference level, usually mean sea level. It provides altitude readings critical for navigation and flight safety.

How is a barometric altimeter connected to the aircraft's systems?

A barometric altimeter is connected to the pitot-static system, receiving only static pressure information from the static port, which helps determine the altitude by measuring atmospheric pressure.

What are the different types of altimeters?

There are three main types of altimeters: barometric altimeters (which use atmospheric pressure), radio altimeters (which use radio waves to measure height above the ground), and GPS/GNSS altimeters (which use satellite data for altitude determination).

Why is the term 'vertical distance' used instead of 'altitude' or 'height' in altimeter readings?

The term 'vertical distance' is used because it can refer to different measurements depending on the reference point. If measured from mean sea level, it is altitude; if measured from the ground, it is height; and from a ground point to mean sea level, it is elevation.

How does a barometric altimeter work to measure altitude?

A barometric altimeter uses the fact that static pressure decreases with altitude. It compares the measured static pressure with a standard reference pressure (29.92 inches of mercury) to calculate altitude. If pressure is higher than standard, the altitude reading will be lower, and vice versa.

What happens inside the altimeter when an aircraft changes altitude?

Inside the altimeter, an aneroid capsule filled with a constant pressure of 29.92 inches of mercury expands or contracts based on changes in static pressure. This movement of the capsule controls the gears that move the altimeter needles to display the current altitude.

Why do pilots need to adjust the altimeter during flight?

Pilots need to adjust the altimeter to account for changes in atmospheric pressure, which can affect the accuracy of the altitude reading. Adjustments are made using the QNH (mean sea level pressure) or QFE (airport pressure) to ensure correct altitude measurements.

What is the difference between QNH and QFE in altimeter settings?

QNH refers to the pressure at mean sea level and is used for altitude readings relative to sea level. QFE is the pressure at the airport level and is used to measure height above the airport. Pilots adjust the altimeter to either QNH or QFE depending on whether they need altitude or height information.

What is the function of the Colesman window on an altimeter?

The Colesman window allows the pilot to adjust the barometric reference (QNH or QFE) in the altimeter. It is calibrated in inches of mercury or hectopascals and is adjusted using a knob labeled 'barrow adjust' to ensure accurate altitude readings.

What are some of the common errors that affect altimeter readings?

Common errors include mechanical wear, static port blockages, position errors due to aircraft maneuvers, and aircraft configuration changes. Calibration tables are used to correct for minor errors, but static port blockages can cause the altimeter to freeze at a certain altitude.